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A Hard Oxide Semiconductor with A Direct and Narrow Bandgap and Switchable p–n Electrical Conduction Sergey V. Ovsyannikov,* Alexander E. Karkin, Natalia V. Morozova, Vladimir V. Shchennikov, Elena Bykova, Artem M. Abakumov, Alexander A. Tsirlin, Konstantin V. Glazyrin, and Leonid Dubrovinsky Chemical bonding in inorganic solids partly predetermines and interconnects their elastic and electronic properties; metals are usually plastic, semiconductors are brittle, and insulating ceramics may be rather hard. Overplaying the combination of these properties could greatly extend the applicability of materials for industrial use. Here we report an oxide semiconductor (perovskite-type Mn2O3) having a narrow and direct bandgap of 0.45 eV and a high Vickers hardness of 15 GPa. All the known materials with resembling electronic band structures (e.g., InSb, PbTe, PbSe, PbS, and InAs) play crucial roles in the semiconductor industry, e.g., in IR detectors, thermal image detectors, photodiodes or photo-electromagnetic detectors, and in other devices. The perovskite-type Mn2O3 comprising nontoxic, earth-abundant and inexpensive elements, is much more incompressible and stronger than the above semiconductors, and unique combination of its electronic and mechanical properties makes it particularly promising. Furthermore, we show that applied pressure (stress) can reversibly or irreversibly (in dependence on pressure value) ‘switch’ the electrical conduction type of perovskite-type Mn2O3. Development of semiconductor technologies is closely related to advances in materials science and engineering. Modern technological challenges require designing and creating novel unique and “smart” materials. Newly fabricated semiconductor Dr. S. V. Ovsyannikov, E. Bykova, Dr. K. V. Glazyrin, Prof. L. S. Dubrovinsky Bayerisches Geoinstitut Universität Bayreuth Universitätsstrasse 30, D-95447 Bayreuth, Germany E-mail: [email protected] Dr. A. E. Karkin, N. V. Morozova, Prof. V. V. Shchennikov Institute of Metal Physics Russian Academy of Sciences Urals Division GSP-170, 18 S. Kovalevskaya Str., Yekaterinburg 620219, Russia E. Bykova Laboratory of Crystallography Universität Bayreuth D-95440, Bayreuth, Germany Prof. A. M. Abakumov Electron Microscopy for Materials Research (EMAT) University of Antwerp Groenenborgerlaan 171, B-2020 Antwerp, Belgium Dr. A. A. Tsirlin National Institute of Chemical Physics and Biophysics Akadeemia tee 23, 12618 Tallinn, Estonia Dr. K. V. Glazyrin Petra III, P02, Deutsches Elektronen Synchrotron Notkestr. 85, 22607 Hamburg, Germany
DOI: 10.1002/adma.201403304
Adv. Mater. 2014, DOI: 10.1002/adma.201403304
systems with advanced properties or unusual electron band structure features may, in turn, lead to emergent industrial applications. The overwhelming majority of existing industrial semiconductor devices is based on solid solutions, doped modifications, and heterostructures of a number of basic semiconductors that are distinguished by electronic, chemical, mechanical and other properties. Extending the list of the basic semiconductors remains one of primary goals for semiconductor science and technology. For instance, it is worth mentioning recent advances in fabrication of multifunctional hard wide-bandgap semiconductors, like A 3IV N 4 (AIV – Si, Sn, Ge, Zr, Hf) with tunable optoelectronic properties,[1–4] comparable with those of In1−xGaxN.[5] High-pressure high-temperature (HP-HT) synthesis is known to be a powerful tool for preparation of new unique materials and phases of which properties can be very different from those of their ambient pressure analogues. In this work we examined the properties of two high-pressure polymorphs of manganese oxides, perovskite-type ζ-Mn2O3,[6] and marokitetype Mn3O4[7] (may be labeled as γ-Mn3O4 after hausmannite, α-Mn3O4 and cubic spinel, β-Mn3O4). The γ-Mn3O4 polymorph was discovered several decades ago;[8] it was found to crystallize in the orthorhombic CaMn2O3 (marokite)-type structure,[9,10] and to show anti-ferromagnetic ordering below 210 K.[11] Very recently a recoverable at ambient conditions ζ-Mn2O3 with a double-perovskite-type structure with intricate electronic configuration, Mn2+(Mn3+)3(Mn3.25+)4O12 has been revealed.[6] The ζ-Mn2O3 polymorph has a triclinic distortion (space group F1, # 2, P in the standard settings) (Figure 1a), and fine details of its 1 crystal structure are yet to be studied. Both polymorphs, perovskite-type ζ-Mn2O3 and marokite-type γ-Mn3O4 are considerably denser than the ambient-pressure α-Mn2O3 and α-Mn3O4 phases,[12–15] and therefore, they could demonstrate new physical characteristics for these oxides. Since the magnetic properties of these polymorphs were recently reported,[6,11] in the present work we investigate their optical and electronic transport properties by means of near-infrared and optical absorption spectroscopy, by Raman spectroscopy and by measurements of the electrical resistivity. The case of ζ-Mn2O3 appears to be more spectacular, and, in addition, we examine its mechanical properties by means of compressibility study and Vickers hardness testing. Furthermore, we probe the evolution of the electronic transport properties of ζ-Mn2O3 upon compression. The temperature dependencies of the electrical resistivity were investigated on three bulk samples of pure ζ-Mn2O3 (Figure 2a) synthesized at slightly different HP-HT conditions in the stability region of this phase.[6] These curves unambiguously suggest an intrinsic semiconducting
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conductivity are nearly equal. For intrinsic semiconductor this fact means the equivalence of hole and electron mobilities, and hence, from these data we cannot determine the carrier concentration and mobility. The magnetoresistance (MR) effect in ζ-Mn2O3 looks as a conventional parabolic function of the magnetic field, B (Figure 2d).[16] Using a standard expression, MR = A1(µB)2 (where, µ is the carrier mobility) and assuming the coefficient A1 to be about 1,[17] we find the carrier mobility µn,p ≈ 123 and 106 cm2/(V s) at 309.1 K and 349.2 K, respectively. Then, from the resistivity data we can estimate the Figure 1. The crystal structures of: a) the perovskite-type Mn2O3 and b) orthorhombic Pbcmintrinsic carrier concentration (1/ρ ≈ niµ円e円, Mn3O4. (a) corresponds to one unit cell; MnB cations are situated in the yellow octahedra, MnA’ cations are – at the centres of the blue tetragonal pyramids and octahedra, MnA cations where ni is the intrinsic concentration and e is the electron charge) as ni ≈ 6 × 1014 and are shown as brown spheres, and oxygen atoms are shown as small red spheres. The lines in 2 × 1015 cm−3 at 309.1 K and 349.2 K, respec(b) are the unit cell borders. tively. The electrical resistivity of γ-Mn3O4 was too high to be measured by our setup. character of electrical conduction with activation energy about We investigated the electronic band structure of the both semEa ≈ 0.27 ± 0.02 eV (Figure 2b). The Hall constant curves for iconductors by near-IR and optical absorption spectroscopy and distinctly detected the absorption edges (Figure 3a). The edge these samples (Figure 2c) show that at temperatures someprofiles suggest a parabolicity of the energy bands. Hence, using what below room ζ-Mn2O3 turns to p-type, while at temperan the standard “parabolic” model (α = α 0 ⎡⎣(E − E g ) / E g ⎤⎦ + C , tures above ambient the hole and electron contributions to
Figure 2. Temperature dependencies of: a) the electrical resistivity ρ, b) the logarithm of conductivity and of c) the Hall constant RH. d) The magnetic field dependence of MR effect of the perovskite-type Mn2O3 at ambient pressure. Three sets of the symbols in a), b) and c) correspond to three different samples investigated. In (a), the resistivity was measured in a magnetic field of 13.6 T. In (b), the activation energies Ea are given near each curves. In (c), the inset shows enlarged part of the plot in a region near RH = 0. In (d), the curves were measured at two temperatures.
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Adv. Mater. 2014, DOI: 10.1002/adma.201403304
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COMMUNICATION Figure 3. Near-infrared and optical absorption α spectra of the perovskite-type Mn2O3 and orthorhombic Pbcm-Mn3O4. a) The original spectra in [cm−1] units. b,c) The same spectra replotted as α vs E and α2 vs E′ (1 eV ≈ 8065 cm−1) for determination of indirect and direct energy bandgaps, respectively.
where α is the absorption coefficient, α0 is a constant, E is the energy, C is an instrumental shift, and n ≈ 1/2 and ≈ 2 for direct and indirect gaps, respectively), we established that ζ-Mn2O3 has a direct fundamental bandgap, Eg ≈ 0.45 eV (Figure 3b). Likewise, γ-Mn3O4 has an indirect fundamental bandgap, Eg ≈ 0.95 eV and a minimal direct one about 1.3 eV (Figure 3b). These Eg values are determined for the first time and they are lower than those in the ambient-pressure polymorphs, e.g., Eg ≈ 1.3 eV in α-Mn2O3,[12] and Eg ≈ 2.5 eV in α-Mn3O4.[12–15] The optical gap (Figure 3b) and thermal bandgap (Eg = 2Ea ≈ 0.54 eV, Figure 2b) in ζ-Mn2O3 show a sizable difference of ca. 90 meV, thereby suggesting a large exciton binding energy. This is characteristic for ionic semiconductors with strongly localized states at the anion sites, like ZnO.[18] Raman spectra of the ambient- and high-pressure polymorphs of Mn2O3 and Mn3O4 are shown at Figure 4. The spectra of the high-pressure phases have more Raman peaks that correlates with the higher symmetry of the conventional phases. The strongest peaks in the denser high-pressure polymorphs have noticeably lower frequencies (Figure 4). This fact hints that a simple analysis of average Mn–O chemical bonds[19] cannot explain the spectra, and further investigations are needed. Our findings for Mn3O4 agree with earlier Raman studies that observed the α–γ transition under pressure.[20,21]
Adv. Mater. 2014, DOI: 10.1002/adma.201403304
Figure 4. Raman spectra of the perovskite-type Mn2O3 and the marokitetype Mn3O4 collected at ambient conditions. The spectra of conventional polymorphs, Ia3-Mn2O3 and I41/amd-Mn3O4 are also shown for comparison.
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The powder XRD studies of ζ-Mn2O3 to 16 GPa show the persistence of the perovskite-type structure (Figure 5a). Previous work has found a direct transformation of α-Mn2O3 to a postperovskite CaIrO3-type δ-Mn2O3 above 28 GPa and 1000 K,[22] and hence, no phase transitions in the range to 16 GPa were anticipated. As the triclinic perovskite structure of ζ-Mn2O3 is very complex (20 crystallographic sites for the Mn ions and 24 for O),[6] we consider here only the volume compressibility. Using the third-order Birch-Murnaghan equation of state we find the bulk modulus value B0 = 156.5(8) GPa and B0′ = 9.4(4) at the fixed unit cell volume, established earlier at ambient conditions, V0 = 3157.73(9) Å3 (inset in Figure 5a). In direct examination of microhardness of the ζ-Mn2O3 at different loads ranging between 2 and 5 N we determine the Vickers hardness as HV ≈ 15(2) GPa (one example of the indentation imprint at load to 3 N is shown as inset in Figure 5e). This value is typical for nitride ceramics (e.g., for Si3N4HV ≈15.5 GPa). The known industrial semiconductors are rather brittle and their HV values are about several GPa.[23,24] For instance, direct-bandgap semiconductors, InAs with Eg ≈ 0.36 eV, PbS with Eg ≈ 0.4 eV, and Hg0.7Cd0.3Te with Eg ≈ 0.3 eV have HV ≈ 2.6 GPa, [23,24] HV ≈ 0.8 GPa,[25] and HV ≈ 0.7 GPa,[26] respectively. The electrical resistance of ζ-Mn2O3 measured on bulk samples up to ca. 8 GPa (Figure 5b) shows moderate and reversible changes suggesting a minor decrease of Eg with pressure. The electrical resistance measurements on thin-film samples up to higher pressures of ca. 23 GPa give the consistent results but, in addition, register the noticeable variations above 15 GPa (Figure 5c). The thermoelectric power studies establish that at ambient conditions the conduction in ζ-Mn2O3 is a bit shifted to p-type (Figure 5d,e), in agreement with the Hall effect data (Figure 2c). Under applied pressure, the Seebeck effect gradually diminishes and inverts its sign near 10 GPa, and then shows a crossover near 15 GPa (Figure 5d,e). One can figure out two major effects of the high-pressure treatment on the Seebeck effect, namely: i) a reversible p–n switching of the dominant conduction type at moderate pressures of ca. 11–13 GPa (cycle 1 in Figure 5d) (the ‘hysteresis’ on the decompression cycle is likely related to formation of defects), and ii) an irreversible p–n switching if applied pressure exceeds 15–16 GPa (Figure 5e). A possibility of simple switching of the conduction type in semiconductors opens new technological opportunities. It is worth mentioning here a case of Ag10Te4Br3 with a bandgap Eg ≈ 1.1 eV that shows a reversible and abrupt p–n inversion near room temperature, at 375–400 K.[27] However, such examples are very rare. In the well-established technology of manufacture of high-quality single crystalline silicon by the Czochralski technique, the electrical conduction type may be controlled by a high-temperature annealing that redistributes the residual interstitial oxygen into electrically active clusters and electrically-passive precipitates related to SiO2.[28,29] In oxide semiconductors, e.g., in hematite (α-Fe2O3), a controlled oxygen absorption can induce an ‘n–p’ inversion in a surface conduction type.[30] We can propose that the gradual p–n inversion in the conductiontype of ζ-Mn2O3 around 10 GPa (Figure 5d,e) may be caused by pressure-driven shifts of the valence and conduction bands. Such an evolution of the electronic band structure is normally reversible. The crossover near 15–16 GPa in ζ-Mn2O3 may be 4
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related to charge disproportionation between different cation sites in this mixed-valence perovskite having at ambient conditions a valence configuration, like Mn2+(Mn3+)3(Mn3.25+)4O12.[6] Previous low-temperature investigations detected three magnetic transitions in ζ-Mn2O3 at 99, 47, and 12 K, that were accompanied by subtle structural changes.[6] Thus, applied high pressure could also induce variations in magnetic and electronic properties of ζ-Mn2O3. Charge disproportionation normally leads to structural distortions and can result in formation a number of electrically-active defects. This process can modify the native defect structure of the samples and thereby shift the charge balance. This sort of changes may be irreversible, and can induce a switching of the dominant type of electrical conduction in intrinsic semiconductors, like ζ-Mn2O3 with ni ≈ 6 × 1014 cm−3 at ambient conditions (Figure 2). Notice, that stress-related switching or control the properties have enormous potential for nanoelectronics, e.g., mechanical loads are applied for data recording in IBM memory devices of ultrahigh storage density.[31,32] Likewise, stresses above ca. 12 GPa generated by nanoindentation can be used for “writing electrically conductive zones” on silicon substrates;[33] this process corresponds to the known phase transitions sequence, – above 11 GPa, the semiconducting Si-I transforms to the metallic Si-II that upon pressure releasing eventually turns to the metastable semimetal Si-III with a high electrical conduction.[34] One should recall that the double-perovskite structure of ζ-Mn2O3 has a slight triclinic distortion (α ≈β ≈ γ ≈ 89.2°) and two-fold enlargement of the lattice parameters (a = 14.6985(2) Å, b = 14.6482(2) Å, c = 14.6705(2) Å) (Figure 1a).[6] The pronounced superlattice features of ζ-Mn2O3 together with the charge disproportionation (2Mn3+ → Mn2+ + Mn4+) between the Mn cations suggest that the bandgap in this perovskite might be related to the charge density wave.[35,36] Usually, such charge density waves and related Peierls transitions can show up at sufficiently low temperatures, but high applied pressures increasing the charge density at atoms could also lead to them. Materials with negative effective correlation energy U are believed to generally tend to form the charge density waves, superconductivity, or spin density waves.[35,36] Theoretical investigations of possibilities for realization of the negative effective correlation energy U found two feasible mechanisms for that, namely, exchange correlation and charge excitation.[35,36] The former might be expected in 3d transition atoms having either d4 or d6 electrons, of which charge disproportionation (2dn → dn−1 + dn+1) can result in negative values of U.[35,36] Likewise, the latter is potentially possible in systems with either s1 or d9 electrons. Considering the existing oxidation states, this leaves a few options only, e.g., Mn3+(3d4), Fe 4+(3d4),Fe2+(3d6), Co3+(3d6), or Cu2+(3d9).[36] Thus, the realization of charge density wave and related Peierls transition in the high-pressure polymorph ζ-Mn2O3 at ambient conditions is apparently an exceptional case, hardly having any analogues among simple oxide systems. It seems that at a Peierls transition the bandgap in ζ-Mn2O3 is opened at one point of the Brillouin zone, and the following tiny triclinic distortion cannot change the direct-type of this gap. In contrast, the case of γ-Mn3O4 adopting the known marokite-type structure and demonstrating the typical indirect bandgap looks rather regular.
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COMMUNICATION Figure 5. Pressure evolution of: a) X-ray diffraction patterns, b,c) electrical resistance, and d,e) thermoelectric power of the perovskite-type Mn2O3 at ambient temperature. a) All the XRD peaks correspond to the perovskite structure. The dashes denote the expected reflection positions for the perovskite structure of ζ-Mn2O3. The inset shows a relative variation of the cell volume under pressure. b-e) The resistance and thermopower curves were measured on different samples for several pressure cycles in high-pressure cells with hemispherically concave (plot b)) and conventional flat anvils (plots c,d,e)). e) The inset shows a scanning electron microscopy image of an indentation imprint at the surface of Mn2O3 after loading to 3 N.
Adv. Mater. 2014, DOI: 10.1002/adma.201403304
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Table 1. The crystal structure parameters of γ-Mn3O4. Pbcm space group (#57), a = 3.0276(1) Å, b = 9.8176(3) Å, c = 9.5799(3) Å, V = 284.751(16) Å3, and Z = 4
Unit cell:
Atomic coordinates (x, y, z) and isotropic displacements (Ueqa): Atom
Wykoff site
Mn(1) Mn(2)
y
4d
0.1864(2)
0.85445(6)
1/4
0.0103(1)
8e
0.29434(13)
0.11463(4)
0.06957(4)
0.0069(1)
O(1)
4c
0.1203(10)
3/4
0
0.0109(5)
O(2)
4d
0.3115(9)
0.1980(3)
1/4
0.0085(4)
O(3)
8e
0.2936(6)
0.47054(19)
0.1113(2)
0.0094(3)
The ζ-Mn2O3 with the direct bandgap Eg ≈ 0.45 eV remarkably complements the limited group of narrow- and direct-bandgap semiconductors that are of exceptional technological importance; just to mention, such as: InSb (Eg ≈ 0.17 eV),[17,37] PbTe and PbSe (Eg ≈ 0.27 eV),[37,38] InAs (Eg ≈ 0.36 eV),[17,37] PbS (Eg ≈ 0.4 eV),[37,38] InN (Eg ≈ 0.7 eV),[39,40] and GaSb (Eg ≈ 0.73 eV),[17,37] and Hg1−xCdxTe solid solutions with tunable Eg ≈ 0–1.5 eV.[41] Mn2O3 comprises two nontoxic, earth-abundant and inexpensive elements, and thus, has an economical advantage over the above semiconductors consisting of scarce or/and toxic In, Pb, Cd, Hg, As, Te, Se, and other elements. Furthermore, ζ-Mn2O3 is much more incompressible and stronger than the usual non-oxide semiconductors. The ‘softness’ of the above semiconductors is a serious obstacle hindering their industrial use. For instance, handling of ‘soft’ HdCdTe elements with the excellent electronic transport characteristics is known to be particularly difficult during a technological device process,[26] and hence, ‘hard’ semiconductors with essentially similar electronic band structure will be in high demand. The unique properties of ζ-Mn2O3 suggest that this material is promising for new industrial applications. Usually, high-pressure application is well simulated in thin strained films,[42] and hence, the perovskite-type ζ-Mn2O3 might be potentially prepared just at ambient pressure conditions. In addition, a possibility of reversible and irreversible p–n switching of the dominant conduction type by applied pressure at room temperature (Figure 5d,e) indicates that ζ-Mn2O3 has a potential as ‘smart’ material with remarkable opto-electro-mechanical characteristics. In summary, we found that the ζ-Mn2O3 and γ-Mn3O4 polymorphs are narrow-bandgap semiconductors, unlike the wide-bandgap cubic-bixbyite α-Mn2O3,[12] and hausmannite α-Mn3O4.[12–15] We established that ζ-Mn2O3 has a direct fundamental bandgap of Eg ≈ 0.45 eV, likewise, γ-Mn3O4 has an indirect fundamental bandgap of Eg ≈ 0.95 eV and small direct gap of 1.3 eV. In examination of the elastic and plastic properties of ζ-Mn2O3, we determined the high values of bulk modulus, B0 = 156.5(8) GPa (at B0′ = 9.4(4)) and of Vickers hardness, HV = 15.2 GPa. A combination of the direct and narrow bandgap with a high Vickers hardness strongly suggests that ζ-Mn2O3 may have important technological applications beyond those of the known ‘soft’ narrow-direct-bandgap semiconductors, like InSb, PbTe, PbSe, PbS, InAs, InN, and HgCdTe.
Sample Preparation and Characterization: The polycrystalline samples of ζ-Mn2O3 and single crystals of γ-Mn3O4 were synthesized at HP-HT
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z
conditions near 20 GPa and 1400–1700 °C. The chemical composition and morphology of the samples were examined by scanning electron microscopy (SEM) at a LEO-1530 instrument, and by microprobe analysis at a JEOL JXA-8200 electron microprobe. X-ray Diffraction: Using single-crystal X-ray diffraction (XRD) data collected on a four-circle Oxford Diffraction Xcalibur diffractometer (λ = 0.7107Å) equipped with an Xcalibur Sapphire2 CCD detector (see Supporting information for details), we confirmed the CaMn2O3-type structure of γ-Mn3O4 (Figure 1b, Table 1).[9–11,13] Pure bulk samples of ζ-Mn2O3 with the double-perovskite-type structure were selected for this work. The detailed analysis of the double-perovskite structure of ζ-Mn2O3 was reported earlier.[6] Electronic Transport Experiments: Electrical and galvanomagnetic properties of the samples were measured by a conventional Montgomery method (a modification of the Van der Pauw method) using an Oxford Instruments setup.[43] The measurements were performed in magnetic fields up to 13.6 T. Optical Experiments: Near-Infrared (IR) and optical absorption spectra were collected on double-side polished samples of thickness ca. 15 µm at Bruker IFS 120 Fourier transform spectrometer.[16] Raman spectra were excited with the 632.8 nm line of a He–Ne laser and recorded by a LabRam spectrometer in a back-scattering geometry. High-Pressure Structural and Electronic Transport Experiments: In addition, we probed the structural and electronic stability of ζ-Mn2O3 under high pressure. The structural behavior of ζ-Mn2O3 was studied at the ID09a line of ESRF (λ = 0.41481 Å) in diamond anvil cell with Re gasket with a hole filled with He pressure-transmitting medium.[44] The electrical and thermoelectric properties of ζ-Mn2O3 were investigated at room temperature in two anvil-type cells with flat and hemispherically concave anvils.[45] The cells were loaded in an automated mini-press setup allowing to tune applied stress continuously with simultaneous measurements of properties of a sample under multiple pressure cycling.[46] The electrical resistance was measured by quasi-four probe method, and the thermopower - by the same method as in previous works.[46] Hardness: The mechanical hardness of the perovskite-type Mn2O3 has been examined by microindentation under loads ranging between 2 and 5 N. We performed a series of tests at different points of well-polished samples and then averaged the results.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements
Experimental Section
6
Ueqa, Å2
x
The authors thank H. Keppler, V. Dmitriev and M. Hanfland for assistance in the optical and structural experiments. S.V.O. thanks the Deutsche Forschungsgemeinschaft (DFG, project OV-110/1–2) for the
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Adv. Mater. 2014, DOI: 10.1002/adma.201403304
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Received: July 22, 2014 Revised: September 17, 2014 Published online:
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[45]
[46]
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financial support. A part of this work was performed as a part of the State Program «Potok» No. 01201463334 (Project No. 12-P-2-1004) and under financial support of the Russian Foundation for Basic Research, RFBR (Project No. 14–02–00622a).
I. Yonenaga, Mater. Transact. 2005, 46, 1979. A. Sher, A.-B. Chen, W. E. Spicer, Appl. Phys. Lett. 1985, 46, 54. R. L. Stanton, Mineral. Mag. 1970, 37, 852. M. Martyniuk, R. H. Sewell, C. A. Musca, J. M. Dell, L. Faraone, Appl. Phys. Lett. 2005, 87, 251905. T. Nilges, S. Lange, M. Bawohl, J. M. Deckwart, M. Janssen, H.-D. Wiemhöfer, R. Decourt, B. Chevalier, J. Vannahme, H. Eckert, R. Weihrich, Nat. Mater. 2009, 8, 101. K. Sueoka, N. Ikeda, T. Yamamoto, S. Kobayashi, J. Appl. Phys. 1993, 74, 5437. K. Sueoka, N. Ikeda, T. Yamamoto, Appl. Phys. Lett. 1994, 65, 1686. A. Gurlo, N. Bârsan, A. Oprea, M. Sahm, T. Sahm, U. Weimar, Appl. Phys. Lett. 2004, 85, 2280. P. Vettiger, G. Cross, M. Despont, U. Drechsler, U. Durig, B. Gotsmann, W. Haberle, M. A. Lantz, H. E. Rothuizen, R. Stutz, G. K. Binnig, IEEE Trans. Nanotechnol. 2002, 1, 39; DOI: 10.1109/ TNANO.2002.1005425. M. I. Lutwyche, M. Despont, U. Drechsler, U. Durig, W. Haberle, H. Rothuizen, R. Stutz, R. Widmer, G. K. Binnig, P. Vettiger, Appl. Phys. Lett. 2000, 77, 3299. S. Ruffell, K. Sears, J. E. Bradby, J. S. Williams, Appl. Phys. Lett. 2011, 98, 052105. J. M. Besson, E. H. Mokhtari, J. Gonzalez, G. Weill, Phys. Rev. Lett. 1987, 59, 473. H. Katayama-Yoshida, A. Zunger, Phys. Rev. Lett. 1985, 55, 1618. H. Katayama-Yoshida, K. Kusakabe, H. Kizaki, A. Nakanishi, Appl. Phys. Express 2008, 1, 081703. List of semiconductors, http://en.wikipedia.org/wiki/List_of_semiconductor_materials, accessed October 2014. Y. I. Ravich, B. A. Efimova, I. A. Smirnov, Semiconducting Lead Chalcogenides, (Ed: L. S. Stil’bans), Springer Science and Business Media, New York 1970, pp 51–56. J. Wu, W. Walukiewicz, K. M. Yu, J. W. AgerIII, E. E. Haller, H Lu, W. J. Schaff, Y. Saito, Y. Nanishi, Appl. Phys. Lett. 2002, 80, 3967. V. Y. Davydov, A. A. Klochikhin, R. P. Seisyan, V. V. Emtsev, S. V. Ivanov, F. Bechstedt, J. Furthmuller, H. Harima, A. V. Mudryi, J. Aderhold, O. Semchinova, J. Graul, Phys. Status Solidi B 2002, 229, R1. A. Rogalski, Rep. Prog. Phys. 2005, 68, 2267. M. Baleva, E. Mateeva, Phys. Rev. B 1994, 50, 8893. S. V. Ovsyannikov, N. V. Morozova, A. E. Karkin, V. V. Shchennikov, Phys. Rev. B 2012, 86, 205131. S. V. Ovsyannikov, X. Wu, G. Garbarino, M. Núñez-Regueiro, V. V. Shchennikov, J. A. Khmeleva, A. E. Karkin, N. Dubrovinskaia, L. Dubrovinsky, Phys. Rev. B 2013, 88, 184106. S. V. Ovsyannikov, I. V. Korobeinikov, N. V. Morozova, A. Misiuk, N. V. Abrosimov, V. V. Shchennikov, Appl. Phys. Lett. 2012, 101, 062107. V. V. Shchennikov, N. V. Morozova, I. Tyagur, Y. Tyagur, S. V. Ovsyannikov, Appl. Phys. Lett. 2011, 99, 212104.
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A Hard Oxide Semiconductor with A Direct and Narrow Bandgap and Switchable p–n Electrical Conduction Sergey V. Ovsyannikov,* Alexander E. Karkin, Natalia V. Morozova, Vladimir V. Shchennikov, Elena Bykova, Artem M. Abakumov, Alexander A. Tsirlin, Konstantin V. Glazyrin, and Leonid Dubrovinsky Chemical bonding in inorganic solids partly predetermines and interconnects their elastic and electronic properties; metals are usually plastic, semiconductors are brittle, and insulating ceramics may be rather hard. Overplaying the combination of these properties could greatly extend the applicability of materials for industrial use. Here we report an oxide semiconductor (perovskite-type Mn2O3) having a narrow and direct bandgap of 0.45 eV and a high Vickers hardness of 15 GPa. All the known materials with resembling electronic band structures (e.g., InSb, PbTe, PbSe, PbS, and InAs) play crucial roles in the semiconductor industry, e.g., in IR detectors, thermal image detectors, photodiodes or photo-electromagnetic detectors, and in other devices. The perovskite-type Mn2O3 comprising nontoxic, earth-abundant and inexpensive elements, is much more incompressible and stronger than the above semiconductors, and unique combination of its electronic and mechanical properties makes it particularly promising. Furthermore, we show that applied pressure (stress) can reversibly or irreversibly (in dependence on pressure value) ‘switch’ the electrical conduction type of perovskite-type Mn2O3. Development of semiconductor technologies is closely related to advances in materials science and engineering. Modern technological challenges require designing and creating novel unique and “smart” materials. Newly fabricated semiconductor Dr. S. V. Ovsyannikov, E. Bykova, Dr. K. V. Glazyrin, Prof. L. S. Dubrovinsky Bayerisches Geoinstitut Universität Bayreuth Universitätsstrasse 30, D-95447 Bayreuth, Germany E-mail: [email protected] Dr. A. E. Karkin, N. V. Morozova, Prof. V. V. Shchennikov Institute of Metal Physics Russian Academy of Sciences Urals Division GSP-170, 18 S. Kovalevskaya Str., Yekaterinburg 620219, Russia E. Bykova Laboratory of Crystallography Universität Bayreuth D-95440, Bayreuth, Germany Prof. A. M. Abakumov Electron Microscopy for Materials Research (EMAT) University of Antwerp Groenenborgerlaan 171, B-2020 Antwerp, Belgium Dr. A. A. Tsirlin National Institute of Chemical Physics and Biophysics Akadeemia tee 23, 12618 Tallinn, Estonia Dr. K. V. Glazyrin Petra III, P02, Deutsches Elektronen Synchrotron Notkestr. 85, 22607 Hamburg, Germany
DOI: 10.1002/adma.201403304
Adv. Mater. 2014, DOI: 10.1002/adma.201403304
systems with advanced properties or unusual electron band structure features may, in turn, lead to emergent industrial applications. The overwhelming majority of existing industrial semiconductor devices is based on solid solutions, doped modifications, and heterostructures of a number of basic semiconductors that are distinguished by electronic, chemical, mechanical and other properties. Extending the list of the basic semiconductors remains one of primary goals for semiconductor science and technology. For instance, it is worth mentioning recent advances in fabrication of multifunctional hard wide-bandgap semiconductors, like A 3IV N 4 (AIV – Si, Sn, Ge, Zr, Hf) with tunable optoelectronic properties,[1–4] comparable with those of In1−xGaxN.[5] High-pressure high-temperature (HP-HT) synthesis is known to be a powerful tool for preparation of new unique materials and phases of which properties can be very different from those of their ambient pressure analogues. In this work we examined the properties of two high-pressure polymorphs of manganese oxides, perovskite-type ζ-Mn2O3,[6] and marokitetype Mn3O4[7] (may be labeled as γ-Mn3O4 after hausmannite, α-Mn3O4 and cubic spinel, β-Mn3O4). The γ-Mn3O4 polymorph was discovered several decades ago;[8] it was found to crystallize in the orthorhombic CaMn2O3 (marokite)-type structure,[9,10] and to show anti-ferromagnetic ordering below 210 K.[11] Very recently a recoverable at ambient conditions ζ-Mn2O3 with a double-perovskite-type structure with intricate electronic configuration, Mn2+(Mn3+)3(Mn3.25+)4O12 has been revealed.[6] The ζ-Mn2O3 polymorph has a triclinic distortion (space group F1, # 2, P in the standard settings) (Figure 1a), and fine details of its 1 crystal structure are yet to be studied. Both polymorphs, perovskite-type ζ-Mn2O3 and marokite-type γ-Mn3O4 are considerably denser than the ambient-pressure α-Mn2O3 and α-Mn3O4 phases,[12–15] and therefore, they could demonstrate new physical characteristics for these oxides. Since the magnetic properties of these polymorphs were recently reported,[6,11] in the present work we investigate their optical and electronic transport properties by means of near-infrared and optical absorption spectroscopy, by Raman spectroscopy and by measurements of the electrical resistivity. The case of ζ-Mn2O3 appears to be more spectacular, and, in addition, we examine its mechanical properties by means of compressibility study and Vickers hardness testing. Furthermore, we probe the evolution of the electronic transport properties of ζ-Mn2O3 upon compression. The temperature dependencies of the electrical resistivity were investigated on three bulk samples of pure ζ-Mn2O3 (Figure 2a) synthesized at slightly different HP-HT conditions in the stability region of this phase.[6] These curves unambiguously suggest an intrinsic semiconducting
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conductivity are nearly equal. For intrinsic semiconductor this fact means the equivalence of hole and electron mobilities, and hence, from these data we cannot determine the carrier concentration and mobility. The magnetoresistance (MR) effect in ζ-Mn2O3 looks as a conventional parabolic function of the magnetic field, B (Figure 2d).[16] Using a standard expression, MR = A1(µB)2 (where, µ is the carrier mobility) and assuming the coefficient A1 to be about 1,[17] we find the carrier mobility µn,p ≈ 123 and 106 cm2/(V s) at 309.1 K and 349.2 K, respectively. Then, from the resistivity data we can estimate the Figure 1. The crystal structures of: a) the perovskite-type Mn2O3 and b) orthorhombic Pbcmintrinsic carrier concentration (1/ρ ≈ niµ円e円, Mn3O4. (a) corresponds to one unit cell; MnB cations are situated in the yellow octahedra, MnA’ cations are – at the centres of the blue tetragonal pyramids and octahedra, MnA cations where ni is the intrinsic concentration and e is the electron charge) as ni ≈ 6 × 1014 and are shown as brown spheres, and oxygen atoms are shown as small red spheres. The lines in 2 × 1015 cm−3 at 309.1 K and 349.2 K, respec(b) are the unit cell borders. tively. The electrical resistivity of γ-Mn3O4 was too high to be measured by our setup. character of electrical conduction with activation energy about We investigated the electronic band structure of the both semEa ≈ 0.27 ± 0.02 eV (Figure 2b). The Hall constant curves for iconductors by near-IR and optical absorption spectroscopy and distinctly detected the absorption edges (Figure 3a). The edge these samples (Figure 2c) show that at temperatures someprofiles suggest a parabolicity of the energy bands. Hence, using what below room ζ-Mn2O3 turns to p-type, while at temperan the standard “parabolic” model (α = α 0 ⎡⎣(E − E g ) / E g ⎤⎦ + C , tures above ambient the hole and electron contributions to
Figure 2. Temperature dependencies of: a) the electrical resistivity ρ, b) the logarithm of conductivity and of c) the Hall constant RH. d) The magnetic field dependence of MR effect of the perovskite-type Mn2O3 at ambient pressure. Three sets of the symbols in a), b) and c) correspond to three different samples investigated. In (a), the resistivity was measured in a magnetic field of 13.6 T. In (b), the activation energies Ea are given near each curves. In (c), the inset shows enlarged part of the plot in a region near RH = 0. In (d), the curves were measured at two temperatures.
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Adv. Mater. 2014, DOI: 10.1002/adma.201403304
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COMMUNICATION Figure 3. Near-infrared and optical absorption α spectra of the perovskite-type Mn2O3 and orthorhombic Pbcm-Mn3O4. a) The original spectra in [cm−1] units. b,c) The same spectra replotted as α vs E and α2 vs E′ (1 eV ≈ 8065 cm−1) for determination of indirect and direct energy bandgaps, respectively.
where α is the absorption coefficient, α0 is a constant, E is the energy, C is an instrumental shift, and n ≈ 1/2 and ≈ 2 for direct and indirect gaps, respectively), we established that ζ-Mn2O3 has a direct fundamental bandgap, Eg ≈ 0.45 eV (Figure 3b). Likewise, γ-Mn3O4 has an indirect fundamental bandgap, Eg ≈ 0.95 eV and a minimal direct one about 1.3 eV (Figure 3b). These Eg values are determined for the first time and they are lower than those in the ambient-pressure polymorphs, e.g., Eg ≈ 1.3 eV in α-Mn2O3,[12] and Eg ≈ 2.5 eV in α-Mn3O4.[12–15] The optical gap (Figure 3b) and thermal bandgap (Eg = 2Ea ≈ 0.54 eV, Figure 2b) in ζ-Mn2O3 show a sizable difference of ca. 90 meV, thereby suggesting a large exciton binding energy. This is characteristic for ionic semiconductors with strongly localized states at the anion sites, like ZnO.[18] Raman spectra of the ambient- and high-pressure polymorphs of Mn2O3 and Mn3O4 are shown at Figure 4. The spectra of the high-pressure phases have more Raman peaks that correlates with the higher symmetry of the conventional phases. The strongest peaks in the denser high-pressure polymorphs have noticeably lower frequencies (Figure 4). This fact hints that a simple analysis of average Mn–O chemical bonds[19] cannot explain the spectra, and further investigations are needed. Our findings for Mn3O4 agree with earlier Raman studies that observed the α–γ transition under pressure.[20,21]
Adv. Mater. 2014, DOI: 10.1002/adma.201403304
Figure 4. Raman spectra of the perovskite-type Mn2O3 and the marokitetype Mn3O4 collected at ambient conditions. The spectra of conventional polymorphs, Ia3-Mn2O3 and I41/amd-Mn3O4 are also shown for comparison.
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The powder XRD studies of ζ-Mn2O3 to 16 GPa show the persistence of the perovskite-type structure (Figure 5a). Previous work has found a direct transformation of α-Mn2O3 to a postperovskite CaIrO3-type δ-Mn2O3 above 28 GPa and 1000 K,[22] and hence, no phase transitions in the range to 16 GPa were anticipated. As the triclinic perovskite structure of ζ-Mn2O3 is very complex (20 crystallographic sites for the Mn ions and 24 for O),[6] we consider here only the volume compressibility. Using the third-order Birch-Murnaghan equation of state we find the bulk modulus value B0 = 156.5(8) GPa and B0′ = 9.4(4) at the fixed unit cell volume, established earlier at ambient conditions, V0 = 3157.73(9) Å3 (inset in Figure 5a). In direct examination of microhardness of the ζ-Mn2O3 at different loads ranging between 2 and 5 N we determine the Vickers hardness as HV ≈ 15(2) GPa (one example of the indentation imprint at load to 3 N is shown as inset in Figure 5e). This value is typical for nitride ceramics (e.g., for Si3N4HV ≈15.5 GPa). The known industrial semiconductors are rather brittle and their HV values are about several GPa.[23,24] For instance, direct-bandgap semiconductors, InAs with Eg ≈ 0.36 eV, PbS with Eg ≈ 0.4 eV, and Hg0.7Cd0.3Te with Eg ≈ 0.3 eV have HV ≈ 2.6 GPa, [23,24] HV ≈ 0.8 GPa,[25] and HV ≈ 0.7 GPa,[26] respectively. The electrical resistance of ζ-Mn2O3 measured on bulk samples up to ca. 8 GPa (Figure 5b) shows moderate and reversible changes suggesting a minor decrease of Eg with pressure. The electrical resistance measurements on thin-film samples up to higher pressures of ca. 23 GPa give the consistent results but, in addition, register the noticeable variations above 15 GPa (Figure 5c). The thermoelectric power studies establish that at ambient conditions the conduction in ζ-Mn2O3 is a bit shifted to p-type (Figure 5d,e), in agreement with the Hall effect data (Figure 2c). Under applied pressure, the Seebeck effect gradually diminishes and inverts its sign near 10 GPa, and then shows a crossover near 15 GPa (Figure 5d,e). One can figure out two major effects of the high-pressure treatment on the Seebeck effect, namely: i) a reversible p–n switching of the dominant conduction type at moderate pressures of ca. 11–13 GPa (cycle 1 in Figure 5d) (the ‘hysteresis’ on the decompression cycle is likely related to formation of defects), and ii) an irreversible p–n switching if applied pressure exceeds 15–16 GPa (Figure 5e). A possibility of simple switching of the conduction type in semiconductors opens new technological opportunities. It is worth mentioning here a case of Ag10Te4Br3 with a bandgap Eg ≈ 1.1 eV that shows a reversible and abrupt p–n inversion near room temperature, at 375–400 K.[27] However, such examples are very rare. In the well-established technology of manufacture of high-quality single crystalline silicon by the Czochralski technique, the electrical conduction type may be controlled by a high-temperature annealing that redistributes the residual interstitial oxygen into electrically active clusters and electrically-passive precipitates related to SiO2.[28,29] In oxide semiconductors, e.g., in hematite (α-Fe2O3), a controlled oxygen absorption can induce an ‘n–p’ inversion in a surface conduction type.[30] We can propose that the gradual p–n inversion in the conductiontype of ζ-Mn2O3 around 10 GPa (Figure 5d,e) may be caused by pressure-driven shifts of the valence and conduction bands. Such an evolution of the electronic band structure is normally reversible. The crossover near 15–16 GPa in ζ-Mn2O3 may be 4
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related to charge disproportionation between different cation sites in this mixed-valence perovskite having at ambient conditions a valence configuration, like Mn2+(Mn3+)3(Mn3.25+)4O12.[6] Previous low-temperature investigations detected three magnetic transitions in ζ-Mn2O3 at 99, 47, and 12 K, that were accompanied by subtle structural changes.[6] Thus, applied high pressure could also induce variations in magnetic and electronic properties of ζ-Mn2O3. Charge disproportionation normally leads to structural distortions and can result in formation a number of electrically-active defects. This process can modify the native defect structure of the samples and thereby shift the charge balance. This sort of changes may be irreversible, and can induce a switching of the dominant type of electrical conduction in intrinsic semiconductors, like ζ-Mn2O3 with ni ≈ 6 × 1014 cm−3 at ambient conditions (Figure 2). Notice, that stress-related switching or control the properties have enormous potential for nanoelectronics, e.g., mechanical loads are applied for data recording in IBM memory devices of ultrahigh storage density.[31,32] Likewise, stresses above ca. 12 GPa generated by nanoindentation can be used for “writing electrically conductive zones” on silicon substrates;[33] this process corresponds to the known phase transitions sequence, – above 11 GPa, the semiconducting Si-I transforms to the metallic Si-II that upon pressure releasing eventually turns to the metastable semimetal Si-III with a high electrical conduction.[34] One should recall that the double-perovskite structure of ζ-Mn2O3 has a slight triclinic distortion (α ≈β ≈ γ ≈ 89.2°) and two-fold enlargement of the lattice parameters (a = 14.6985(2) Å, b = 14.6482(2) Å, c = 14.6705(2) Å) (Figure 1a).[6] The pronounced superlattice features of ζ-Mn2O3 together with the charge disproportionation (2Mn3+ → Mn2+ + Mn4+) between the Mn cations suggest that the bandgap in this perovskite might be related to the charge density wave.[35,36] Usually, such charge density waves and related Peierls transitions can show up at sufficiently low temperatures, but high applied pressures increasing the charge density at atoms could also lead to them. Materials with negative effective correlation energy U are believed to generally tend to form the charge density waves, superconductivity, or spin density waves.[35,36] Theoretical investigations of possibilities for realization of the negative effective correlation energy U found two feasible mechanisms for that, namely, exchange correlation and charge excitation.[35,36] The former might be expected in 3d transition atoms having either d4 or d6 electrons, of which charge disproportionation (2dn → dn−1 + dn+1) can result in negative values of U.[35,36] Likewise, the latter is potentially possible in systems with either s1 or d9 electrons. Considering the existing oxidation states, this leaves a few options only, e.g., Mn3+(3d4), Fe 4+(3d4),Fe2+(3d6), Co3+(3d6), or Cu2+(3d9).[36] Thus, the realization of charge density wave and related Peierls transition in the high-pressure polymorph ζ-Mn2O3 at ambient conditions is apparently an exceptional case, hardly having any analogues among simple oxide systems. It seems that at a Peierls transition the bandgap in ζ-Mn2O3 is opened at one point of the Brillouin zone, and the following tiny triclinic distortion cannot change the direct-type of this gap. In contrast, the case of γ-Mn3O4 adopting the known marokite-type structure and demonstrating the typical indirect bandgap looks rather regular.
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COMMUNICATION Figure 5. Pressure evolution of: a) X-ray diffraction patterns, b,c) electrical resistance, and d,e) thermoelectric power of the perovskite-type Mn2O3 at ambient temperature. a) All the XRD peaks correspond to the perovskite structure. The dashes denote the expected reflection positions for the perovskite structure of ζ-Mn2O3. The inset shows a relative variation of the cell volume under pressure. b-e) The resistance and thermopower curves were measured on different samples for several pressure cycles in high-pressure cells with hemispherically concave (plot b)) and conventional flat anvils (plots c,d,e)). e) The inset shows a scanning electron microscopy image of an indentation imprint at the surface of Mn2O3 after loading to 3 N.
Adv. Mater. 2014, DOI: 10.1002/adma.201403304
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Table 1. The crystal structure parameters of γ-Mn3O4. Pbcm space group (#57), a = 3.0276(1) Å, b = 9.8176(3) Å, c = 9.5799(3) Å, V = 284.751(16) Å3, and Z = 4
Unit cell:
Atomic coordinates (x, y, z) and isotropic displacements (Ueqa): Atom
Wykoff site
Mn(1) Mn(2)
y
4d
0.1864(2)
0.85445(6)
1/4
0.0103(1)
8e
0.29434(13)
0.11463(4)
0.06957(4)
0.0069(1)
O(1)
4c
0.1203(10)
3/4
0
0.0109(5)
O(2)
4d
0.3115(9)
0.1980(3)
1/4
0.0085(4)
O(3)
8e
0.2936(6)
0.47054(19)
0.1113(2)
0.0094(3)
The ζ-Mn2O3 with the direct bandgap Eg ≈ 0.45 eV remarkably complements the limited group of narrow- and direct-bandgap semiconductors that are of exceptional technological importance; just to mention, such as: InSb (Eg ≈ 0.17 eV),[17,37] PbTe and PbSe (Eg ≈ 0.27 eV),[37,38] InAs (Eg ≈ 0.36 eV),[17,37] PbS (Eg ≈ 0.4 eV),[37,38] InN (Eg ≈ 0.7 eV),[39,40] and GaSb (Eg ≈ 0.73 eV),[17,37] and Hg1−xCdxTe solid solutions with tunable Eg ≈ 0–1.5 eV.[41] Mn2O3 comprises two nontoxic, earth-abundant and inexpensive elements, and thus, has an economical advantage over the above semiconductors consisting of scarce or/and toxic In, Pb, Cd, Hg, As, Te, Se, and other elements. Furthermore, ζ-Mn2O3 is much more incompressible and stronger than the usual non-oxide semiconductors. The ‘softness’ of the above semiconductors is a serious obstacle hindering their industrial use. For instance, handling of ‘soft’ HdCdTe elements with the excellent electronic transport characteristics is known to be particularly difficult during a technological device process,[26] and hence, ‘hard’ semiconductors with essentially similar electronic band structure will be in high demand. The unique properties of ζ-Mn2O3 suggest that this material is promising for new industrial applications. Usually, high-pressure application is well simulated in thin strained films,[42] and hence, the perovskite-type ζ-Mn2O3 might be potentially prepared just at ambient pressure conditions. In addition, a possibility of reversible and irreversible p–n switching of the dominant conduction type by applied pressure at room temperature (Figure 5d,e) indicates that ζ-Mn2O3 has a potential as ‘smart’ material with remarkable opto-electro-mechanical characteristics. In summary, we found that the ζ-Mn2O3 and γ-Mn3O4 polymorphs are narrow-bandgap semiconductors, unlike the wide-bandgap cubic-bixbyite α-Mn2O3,[12] and hausmannite α-Mn3O4.[12–15] We established that ζ-Mn2O3 has a direct fundamental bandgap of Eg ≈ 0.45 eV, likewise, γ-Mn3O4 has an indirect fundamental bandgap of Eg ≈ 0.95 eV and small direct gap of 1.3 eV. In examination of the elastic and plastic properties of ζ-Mn2O3, we determined the high values of bulk modulus, B0 = 156.5(8) GPa (at B0′ = 9.4(4)) and of Vickers hardness, HV = 15.2 GPa. A combination of the direct and narrow bandgap with a high Vickers hardness strongly suggests that ζ-Mn2O3 may have important technological applications beyond those of the known ‘soft’ narrow-direct-bandgap semiconductors, like InSb, PbTe, PbSe, PbS, InAs, InN, and HgCdTe.
Sample Preparation and Characterization: The polycrystalline samples of ζ-Mn2O3 and single crystals of γ-Mn3O4 were synthesized at HP-HT
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z
conditions near 20 GPa and 1400–1700 °C. The chemical composition and morphology of the samples were examined by scanning electron microscopy (SEM) at a LEO-1530 instrument, and by microprobe analysis at a JEOL JXA-8200 electron microprobe. X-ray Diffraction: Using single-crystal X-ray diffraction (XRD) data collected on a four-circle Oxford Diffraction Xcalibur diffractometer (λ = 0.7107Å) equipped with an Xcalibur Sapphire2 CCD detector (see Supporting information for details), we confirmed the CaMn2O3-type structure of γ-Mn3O4 (Figure 1b, Table 1).[9–11,13] Pure bulk samples of ζ-Mn2O3 with the double-perovskite-type structure were selected for this work. The detailed analysis of the double-perovskite structure of ζ-Mn2O3 was reported earlier.[6] Electronic Transport Experiments: Electrical and galvanomagnetic properties of the samples were measured by a conventional Montgomery method (a modification of the Van der Pauw method) using an Oxford Instruments setup.[43] The measurements were performed in magnetic fields up to 13.6 T. Optical Experiments: Near-Infrared (IR) and optical absorption spectra were collected on double-side polished samples of thickness ca. 15 µm at Bruker IFS 120 Fourier transform spectrometer.[16] Raman spectra were excited with the 632.8 nm line of a He–Ne laser and recorded by a LabRam spectrometer in a back-scattering geometry. High-Pressure Structural and Electronic Transport Experiments: In addition, we probed the structural and electronic stability of ζ-Mn2O3 under high pressure. The structural behavior of ζ-Mn2O3 was studied at the ID09a line of ESRF (λ = 0.41481 Å) in diamond anvil cell with Re gasket with a hole filled with He pressure-transmitting medium.[44] The electrical and thermoelectric properties of ζ-Mn2O3 were investigated at room temperature in two anvil-type cells with flat and hemispherically concave anvils.[45] The cells were loaded in an automated mini-press setup allowing to tune applied stress continuously with simultaneous measurements of properties of a sample under multiple pressure cycling.[46] The electrical resistance was measured by quasi-four probe method, and the thermopower - by the same method as in previous works.[46] Hardness: The mechanical hardness of the perovskite-type Mn2O3 has been examined by microindentation under loads ranging between 2 and 5 N. We performed a series of tests at different points of well-polished samples and then averaged the results.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements
Experimental Section
6
Ueqa, Å2
x
The authors thank H. Keppler, V. Dmitriev and M. Hanfland for assistance in the optical and structural experiments. S.V.O. thanks the Deutsche Forschungsgemeinschaft (DFG, project OV-110/1–2) for the
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Adv. Mater. 2014, DOI: 10.1002/adma.201403304
www.advmat.de www.MaterialsViews.com [23] [24] [25] [26]
Received: July 22, 2014 Revised: September 17, 2014 Published online:
[27]
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financial support. A part of this work was performed as a part of the State Program «Potok» No. 01201463334 (Project No. 12-P-2-1004) and under financial support of the Russian Foundation for Basic Research, RFBR (Project No. 14–02–00622a).
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