Low Temperature Nanoscale Oxygen-Ion Intercalation Into Epitaxial

Subscriber access provided by University of Colorado Boulder

Article

Low Temperature Nanoscale Oxygen-Ion Intercalation Into Epitaxial MoO Thin Films 2

EunYoung Ahn, Joonhyuk Lee, Yoon Young Koh, Jaekwang Lee, ByeongGyu Park, Jae-Young Kim, Inwon Lee, Chan-Woo Lee, and Hyoungjeen Jeen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11959 • Publication Date (Web): 24 Jan 2017 Downloaded from http://pubs.acs.org on January 27, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 27

1 2 3 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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Low Temperature Nanoscale Oxygen-ion Intercalation into Epitaxial MoO2 Thin Films EunYoung Ahn†, Joonhyuk Lee†, Yoon Young Koh‡, Jaekwang Lee†, Byeong-Gyu Park‡, Jae-Young Kim‡, Inwon Leeǁ, Chan-Woo Lee§, Hyoungjeen Jeen†,* †

Department of Physics, Pusan National University, Busan, 46241, Korea Pohang Accelerator Laboratory, Pohang University of Science and Technology, Pohang, 37673, Korea ║ Department of Naval Architecture and Ocean Engineering, Pusan National University, Busan, 46241, Korea § Conversion Materials Laboratory, Energy Materials and Process Research Division, Korea Institute of Energy Research, Daejeon, 34129, Korea ‡

ABSTRACT In transition metal oxides (TMOs), lots of physical phenomena such as metalinsulator transitions (MIT), magnetism, and ferroelectricity are closely related to the amounts of oxygen contents. Thus, understanding surface oxidation process in TMOs and its effect are important for enhancing performances of modern electronic and electrochemical devices due to miniaturization of those devices. In this regard, MoO2+x (0 ≤ x ≤ 1) is an interesting TMO, which shows MIT driven by the change of its oxygen content, i.e. metallic MoO2 and insulating MoO3. Hence, understanding thermally-driven oxygen intercalation into MoO2 is very important. In this work, we conducted in-situ post-annealing of as-grown epitaxial MoO2 thin films at different temperatures in oxidative condition to investigate the thermal effect on oxygen ion intercalation and resultant MIT in MoO2+x. Through the spectroscopic techniques such as spectroscopic ellipsometry and x-ray absorption spectroscopy, we observed that oxygen ion can intercalate into MoO2 and trigger a phase transition in nanoscale at surprisingly low-temperature as low as 250oC. In addition, after oxygen annealing at 350oC, we find that both hybridization and interband transition energy between O 2p and Mo 4d t2g are significantly shifted to low energy nearly 0.2 eV, which clearly 1 ACS Paragon Plus Environment


1 2 3 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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 27

supports that the electronic transition of MoO2+x is predominantly driven by change of oxygen contents.

INTRODUCTION A wide variety of phenomena such as magnetic transition, metal-insulator transition, and topotactic transformation found in transition metal oxides (TMOs) are often due to the oxygen contents of materials, since oxygen has high electron affinity resulting in easy modulation of electronic structure of the neighboring atoms.1-7 Recently, such oxygen-driven tuning of physical properties has attracted great attentions for the potential of applications such as fuel cells, catalysts, smart windows and etc.8-16 Among many oxides, molybdenum oxide is especially interesting, since molybdenum belonging to 4d transition metals has multivalent state from +2 to +6. Thus, molybdenum oxide has a variety of oxygen compounds such as MoO2, Mo4O11, MoO3 and etc.17-19 In molybdenum oxides, oxygen contents are critical parameter to determine the physical properties of materials. MoO2 and MoO3 are especially well-known to show drastic differences in electronic ground state and crystallographic structure. Dark blue-colored MoO2 with monoclinic structure has a metallic ground state.20 Contrary to MoO2, transparent MoO3 with orthorhombic structure is a bandgap insulator which has a wide bandgap nearly 3 eV.21-22 Thus, the oxidation process in MoO2 to MoO3 leads compositionally-driven metal to insulator transition (MIT) accompanying the color change of materials. Interestingly, both MoO2 and MoO3 have MoO6 octahedra as a framework with a different arrangement of octahedra. Understanding of the oxygen ion intercalation processes gives an opportunity of MoO2+x materials in the applications mentioned above. Despite these points of interest, this structural phase transition between the monoclinic and orthorhombic structures is a formidable task, since it has only been reported when thermal oxidation at 400oC or higher of powdered or bulk MoO2 took 2 ACS Paragon Plus Environment

Page 3 of 27

1 2 3 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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


place.23-26 In this report, we induced the oxygen ion intercalation into MoO2 thin films and observed resultant changes in several intrinsic physical properties such as optical transitions and hybridization energy between oxygen and molybdenum. Interestingly, we also clearly observed the spectroscopic evidence of nanoscale oxidation as low as 250oC by spectroscopic methods.

METHODS 100-nm-thick epitaxial (100) MoO2 thin films are prepared by RF magnetron sputtering on (0001) Al2O3 substrates. Al2O3 substrates were treated at 1400oC in air for 6 hours to create atomically-flat step terraces. Film growth was performed at 500oC of substrate temperature and 7 mTorr of Ar partial pressure. Target to substrate distance is kept about 13 cm. More growth details can be found elsewhere.27 After the growth, films were cooled down to 50oC. We confirmed the epitaxial growth of (100) MoO2 films on (0001) Al2O3 substrates using high-resolution x-ray diffraction. We tried to intercalate oxygen ions into MoO2 by five minutes annealing at 100oC, 250oC, 350oC, and 400 oC with 400 Torr of oxygen partial pressure without exposure to the air after each growth.

To confirm the structural phase transition of MoO2, we performed θ - 2θ measurements and ω scans using a high resolution x-ray diffractometer (HRXRD, SmartLab, Rigaku) with Ge (220) double-crystal monochromator. We collected the optical spectra of MoO2+x thin films prepared in different annealing temperatures by a spectroscopic ellipsometer (VASE Ellipsometer, J.A. Woollam Co., PNU-ell) at room temperature. Optical conductivities are deduced from analysis of the spectra. The photon energy ranges from 0.75 eV to 6 eV, incidence angles are 65o, 70o, and 75o. Multi-beam interference of MoO2+x/Al2O3 is considered. To obtain optical constants of the MoO2+x the collected spectra were fitted by 3 ACS Paragon Plus Environment


1 2 3 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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 27

Newton Rapson method.28 To elucidate the origin of optical transitions in MoO3, firstprinciples calculations based on density functional theory were performed for the optical properties of MoO3 using a plane-wave basis with projector-augmented-wave method and generalized-gradient approximation to the exchange correlation potential as implemented in the Vienna ab initio simulation package (VASP) code. The imaginary part of the dielectric function is first estimated, within the independent particle approximation in the longwavelength limit q → 0, by a summation over empty states using the following equation: =

4 1 2 , − , − 〈",#$%& |", 〉 〈",#$)& |", 〉∗ . ⟶

,,

, where the indices v and c refer to valence and conduction band states respectively, and ",, is the periodic part of the Bloch wave at the k-point (all-electron Kohn-Sham wave

function with band index n at the k-point is written as Ψ, = ",, ./ 0∙2 ), are the kpoint weights, which are defined such that they sum to 1; the factor 2 accounts for spin degeneracy. The vectors q are unit vectors for the three Cartesian directions and Ω is the volume of the primitive cell. Then the real part of the optical conductivity directly related to optical transition is obtained by; Re6 =

Ιm . 4

Note that standard DFT commonly underestimates the band gap due to the self-interaction error. Accordingly, in order to achieve overall agreement with the experimental absorption spectrum of MoO3, the DFT-calculated absorption spectrum was shifted towards 0.45 eV higher energy. From the x-ray absorption spectroscopy (XAS, 2A beamline, Pohang Accelerator Laboratory) measurements at room temperature with normal incidence, we observed O K-edge spectra of MoO2+x thin films to study hybridization energy between O 2p and Mo 4d. 4 ACS Paragon Plus Environment

Page 5 of 27

1 2 3 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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


RESULTS AND DISCUSSION Figure 1 is a schematic diagram to explain phase transition from MoO2 to MoO3 via oxygen-ion intercalation. The valence state of monoclinic MoO2 is Mo (IV); it has an electron configuration as 4d2. Lattice constants and an angle are a = 5.61, b = 4.86, c = 5.63 Å, and β = 120.94o.29 The structure has MoO6 octahedra as a framework sharing the corners with a rutile arrangement.30 The bonding is made through hybridization between O 2p and Mo 4d.31 Note that degenerated d orbitals are split to t2g and eg according to crystal field theory and predicted by density functional theory.32-33 As a result of the d orbital splitting, and electron configuration of Mo 4d2, the density of state is separated to partially filled t2g near the Fermi level and empty eg band.32-33 Therefore, optical transitions between the d orbitals are possible. Due to this band structure, MoO2 shows metallic behavior.20 In contrast, lattice constants of orthorhombic MoO3 are a = 3.92, b = 13.94, and c = 3.66 Å.34 The MoO6 octahedral framework shares the corners and edges.34 The valence state of MoO3 is Mo (VI); the electron configuration is 4d0. It is well known as an insulator with a wide bandgap nearly 3 eV.21-22 To induce structural and electronic phase transitions, oxygen-ion intercalation into asgrown MoO2 thin films was attempted by an in-situ post annealing process at different temperatures with 400 Torr of oxygen partial pressure (PO2) after the growth of each film. As-grown MoO2 thin film shown a dark blue color, and this fact can be supported by the large absorption coefficient of metallic MoO2 thin films at visible range from the previous work.27 The structural analysis was conducted by x-ray diffraction with θ - 2θ scans and ω scans. Generally, θ - 2θ diffraction patterns give the information on epitaxy (See Figure S1). Note that only (h00) peaks of MoO2 are shown in diffraction patterns of thin films annealed up to 350oC. This means no global structural phase transition. Note that when we annealed 5 ACS Paragon Plus Environment


1 2 3 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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

MoO2 thin films at the slightly higher temperature (400oC), we clearly observed the transition to (010) MoO3 (See Figure S1). Figure 2 (a) shows the photography of 100-nm-thick MoO2+x thin films after oxygen ion intercalation at the different temperature. The color of MoO2 is dark blue, due to their metallic properties and large absorption coefficient in a visible range.27 However, The color of the sample after annealing at 350oC became clearly brighter and more transparent than the other samples. This may be due to high transmittance (80 %) of MoO3 at visible ranges.21 With these facts, we could conjecture that the color change of the sample by oxygen annealing at 350oC is possibly due to microscopic oxidation on MoO2 surface. To study possible structural change by local oxidation, ω - scans around (200) peak of MoO2+x were performed. Interestingly, in Figure 2 (b) by increasing the annealing temperature, we observed a gradual increase of ∆ω in (200) peak of the films up to 350oC. We characterized ∆ω values from ω - scan with 0.012o as a step size of the measurement. From the ∆ω ≈ 0.12o of the 100-nm-thick as-grown MoO2 thin films, it increases up to ∆ω ≈ 0.60o of the MoO2+x thin films annealed at 350oC gradually. We believe these variations are closely linked to local oxidation of MoO2 thin films. Thus, the increase of the Δω implies the reduction of crystalline quality and degradation of unit cell as an effect of local oxygen-ion intercalation of MoO2+x annealed up to 350oC. It is interesting to note that the ∆ω of MoO2 thin film annealed at 400oC is drastically reduced to ∆ω ≈ 0.10o. This indicates complete conversion to MoO3 taken place as the case of other oxides.35-36

In order to confirm the ionic intercalation at nanoscale of MoO2 upon the in-situ postannealing at the oxidative condition, we performed spectroscopic ellipsometry of the annealed films at room temperature. From the optical technique, we could derive the optical conductivity of MoO2+x, and clearly saw the evolutions of the spectral shapes upon annealing temperatures. The Lorentz oscillation model which strongly related with dielectric properties 6 ACS Paragon Plus Environment

Page 6 of 27

Page 7 of 27

1 2 3 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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of materials accounts for the spectrum of optical conductivity (σ), and the real part of σ shows the allowed direct electronic transitions.37 Figure 3 shows the optical conductivity spectrum of MoO2+x at the different annealing temperature. In order to understand our theoretical result for the spectrum of MoO2+x annealed at 350oC, we added the optical conductivity spectrum of MoO3 calculated by DFT in Figure 3. Note that the similar optical conductivity of MoO3 thin films from annealing of MoO2 at 400oC can be gotten experimentally shown in Figure S2. To investigate the optical electron transition of MoO2+x, we separated each hidden peak by peak deconvolution. Each peak was assigned based on previously reported DFT results.32-33 Note that the electron configuration of MoO2 is 4d2. α and β peaks below 3 eV correspond to the transitions originated from partially filled t2g orbitals split by crystal field theory. The lower peak (α) near 1 eV denotes the optical transition to empty space of t2g band above the Fermi level, while β peak near the 3 eV denotes the transition to completely empty eg band. We assigned a double peak feature of γ between 4 ~ 5 eV explained by the interband transition from O 2p to Mo 4d t2g. This double peak feature of γ is explained by the separation between t2g orbital caused by a distortion of MoO6 octahedra. Moreover, the signature of another peak estimated as the interband transition from O 2p to Mo 4d eg was shown above 5 eV as a result of peak deconvolution.38-39 The variations of the spectra depending on the oxygen ionic intercalation process were clearly seen by the decrease of the peak intensity of α and β peaks (d – d transition) and energy shift of the γ1 peak (p - d transition). Each peak area of α (Mo 4d t2g to t2g) and β (Mo 4d t2g to eg), which represents the allowed transitions from t2g orbitals, decreased due to a reduction of the number of electrons in Mo 4d. Moreover, the photon energy of each transition underwent red-shift upon an increase of annealing temperature. Table 1 shows the position and area of each peak with their annealing temperature. At higher temperature above the 350oC, we labeled spit peak of γ1 as γ11 and γ12. Red shift of γ1 and β 7 ACS Paragon Plus Environment


1 2 3 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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 27

depending on the annealing temperature also shown in this table. In Figure 3, the optical conductivity of MoO2+x annealed at 350oC shows the clearly different feature, comparing to that of the as-grown MoO2. These new features cannot be explained by the DFT result of MoO2. The spectrum of 350oC is composed of very complex features of optical transition. The sign of peak splitting and new peak feature near 5 eV are shown. An abrupt increase in optical conductivity near 5 eV estimated as the optical transition from O 2p to Mo 4d t2g is caused by an increase of the number of empty t2g orbitals. Also, the peak splitting of p – d transitions near the 3 eV is similar to the result of our DFT calculation of MoO3 given in Figure 3. This result is from the sum of σxx + σzz, and we considered experimental geometry in the ellipsometry measurement. Due to the difference in crystal structures and electron configurations of Mo ion, the electronic structure of MoO3 is clearly different from that of MoO2, including the intrinsic optical transitions. Each peak position of optical conductivity, corresponding to the required photon energy for optical transitions, is in approximate agreement with our theoretical calculation, which indicates that MoO2+x annealed at 350oC indeed verges on MoO3. However, optical transitions α and β still can be found while the areas of two peaks are reduced. It is interesting in that both optical properties of MoO2 and MoO3 are observed in the spectrum of MoO2+x annealed at 350oC. These changes of spectra begin to appear from 250oC. Importantly, the peak area of α and β already reduces, and γ1 is under red-shift. Based on these results, we could conclude the decreases of electron occupancies in t2g orbitals at 250oC accommodated with changes of band structure. Note that the reduction of α and β depends on the number of electron in Mo 4d orbitals. Thus, this behavior is concrete evidence of oxidation in MoO2. In other word, MoO2 thin films are oxidized at 250oC, although it could not be observed by XRD measurement. In order to confirm the low temperature

8 ACS Paragon Plus Environment

Page 9 of 27

1 2 3 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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


oxidation on MoO2 surface and the variations of t2g orbitals, we used another surface sensitive probe.

To confirm the evolution of t2g band upon oxidation, we collected the O K-edge spectra of x-ray absorption spectroscopy shown in Figure 4 (a), since we could observe hybridization between Mo and O. The spectrum of O K-edge at the x-ray energy range 530 ~ 540 eV describes the hybridization between Mo 4d and O 2p. In these XAS measurements, a circular polarized beam was used at room temperature. To compare the change of each peak of all the spectra, we normalized the absorption intensity in the end of spectra (550 eV) based on hybridization energy less than 530 eV. Assignments of the hybridization peaks are based on DFT results32-33 like the case of optical conductivity spectra. There are four distinct hybridization peaks (A ~ D) in the spectra; A and B are related to the hybridization of O 2p – Mo 4d t2g and C and D are that of O 2p – Mo 4d eg.40-42 Note that we used two detection modes such as surface-sensitive total electron yield (TEY) shown in Figure 4 (a) and bulksensitive total fluorescent yield (TFY) shown in Figure S3. Each mode has a different penetration depth: several nm for TEY and nearly 100 nm for TFY. Both the TEY and TFY of MoO2+x annealed at 350oC are similar. This means the degree of hybridization for both the surface and bulk is the same in each sample. On the other hand, the TFY spectrum of MoO2+x annealed at 250oC is very different from that of TEY. The TEY spectrum is similar with the XAS of MoO3, whereas the TFY spectrum is similar to the TEY spectrum of as-grown MoO2. The difference in the spectra between TEY and TFY of MoO2+x at 250oC clearly proved the unexpectedly low temperature nanoscale oxygen-ion intercalation of MoO2 at 250oC. Interestingly, the TFY spectrum at 250oC can be drawn by combining the two TFY spectra, i.e. 75 % from that of the as-grown MoO2 and 25 % from that of the MoO3 (See Figure S4). This implies the maximal depth of MoO2+x would be as deep as 25 nm based on the typical 9 ACS Paragon Plus Environment


1 2 3 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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 27

penetration depth of the TFY mode.43 We observed the color of 25-nm-thick MoO2+x annealed at 250oC with 400 Torr of oxygen pressure became transparent (Figure S5). In addition, we observed that the increase of pre-peak intensity near the 530 eV is prominent and it resembles the O K-edge of MoO3 previously reported.40, 42, 44-45 The change of pre-peak intensity at MoO2+x is similar to that of vanadium oxide during the oxidation of VO2 to V2O5.46 It is considered as the level of t2g orbital occupancy varied by the variations of Mo valence states. In other words, spectral changes of optical transitions and hybridizations caused by the ionic intercalation into MoO2+x indicate that the electronic structures (band structures) change. Figure 4 (b) shows the variations of peak B in the XAS spectra (diamond symbol), and γ1 in optical conductivity (triangular symbol). Note that step size of each spectrum is 0.01 eV. Each B and γ1 is related to the hybridization and interband transition between O 2p to Mo 4d t2g. Peak positions of both B and γ1 shift to low energy nearly 0.2 eV after annealing at 350oC. The positions of B of MoO2+x annealed at 250oC have different values of TEY and TFY. Like a preceding result of XAS spectral shape peak B of TFY is located near the MoO2 and that of TEY is located near the MoO2+x annealed at 350oC. Due to the longer penetration depth of spectroscopic ellipsometry, the decrement of peak γ1 is located between TEY and TFY of B at 250oC. It is interesting that the decrease of peak positions shown in the B is analogous to the decrement of γ1. Note that both B and γ1 shift to low energy nearly 0.2 eV after annealing at 350oC. Therefore, these oxygen-ion intercalation processes lead to the electronic structure changes of the Mo 4d t2g band.

CONCLUSIONS In conclusion, we intercalate oxygen ion into epitaxial MoO2 using an in-situ postannealing process at 100oC, 250oC, and 350oC with 400 Torr of oxygen partial pressure. There is no global structural phase transition of MoO2 but the broadening of x-ray diffraction 10 ACS Paragon Plus Environment

Page 11 of 27

1 2 3 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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


peaks during the ionic intercalation up to 350oC. To obtain precise evidence of surface oxidation, we performed surface sensitive spectroscopic methods. We observed the unexpected low temperature surface oxidation of MoO2 as low as 250oC. In addition, this ionic intercalation leads to a shift of hybridization and optical transition between the O 2p and Mo t2g to lower energy by nearly 0.2 eV.

ASSOCIATED CONTENT SECTION Supporting Information X-ray diffraction patterns, Optical conductivity of MoO3, O K-edge TFY spectra, Reconstruction of TFY spectrum of the MoO2+x film annealed at 250oC, and Evidence of full oxidation in the 25-nm-thick MoO2 thin films.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest

ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) and grant funded by the Korea government (MSIP) through GCRC-SOP (No. 2011-0030013) and through a Pusan National University Grant 2014. Also, this research was supported by the Basic Science Research Program through the NRF funded by the Ministry of Education (NRF-2015R1D1A1A02062175). 11 ACS Paragon Plus Environment


1 2 3 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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 27

REFERENCES (1)

Cheong, S.-W., Transition metal oxides: The exciting world of orbitals. Nat. Mater.

2007, 6, 927-928. (2)

Ohtomo, A.; Hwang, H. Y., A high-mobility electron gas at the LaAlO3/SrTiO3

heterointerface. Nature 2004, 427, 423-426. (3)

Ohtomo, A.; Muller, D. A.; Grazul, J. L.; Hwang, H. Y., Artificial charge-

modulationin atomic-scale perovskite titanate superlattices. Nature 2002, 419, 378-380. (4)

Jeen, H.; Choi, W. S.; Biegalski, M. D.; Folkman, C. M.; Tung, I. C.; Fong, D. D.;

Freeland, J. W.; Shin, D.; Ohta, H.; Chisholm, M. F., et al., Reversible redox reactions in an epitaxially stabilized SrCoOx oxygen sponge. Nat. Mater. 2013, 12, 1057-1063. (5)

Jeong, J.; Aetukuri, N.; Graf, T.; Schladt, T. D.; Samant, M. G.; Parkin, S. S.,

Suppression of metal-insulator transition in VO2 by electric field–induced oxygen vacancy formation. Science 2013, 339, 1402-1405. (6)

Kanki, T.; Tanaka, H.; Kawai, T., Electric control of room temperature

ferromagnetism in a Pb(Zr0.2Ti0.8)O3⁄La0.85Ba0.15MnO3 field-effect transistor. Appl. Phys. Lett. 2006, 89, 242506. (7)

Hayward, M. A.; Cussen, E. J.; Claridge, J. B.; Bieringer, M.; Rosseinsky, M. J.;

Kiely, C. J.; Blundell, S. J.; Marshall, I. M.; Pratt, F. L., The hydride anion in an extended transition metal oxide array: LaSrCoO3H0.7. Science 2002, 295, 1882-1884.


Page 13 of 27

1 2 3 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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(8)

Norskov, J. K.; Bligaard, T.; Rossmeisl, J.; Christensen, C. H., Towards the

computational design of solid catalysts. Nat. Chem. 2009, 1, 37-46. (9)

Dombrovskis, J. K.; Jeong, H. Y.; Fossum, K.; Terasaki, O.; Palmqvist, A. E. C.,

Transition metal ion-chelating ordered mesoporous carbons as noble metal-free fuel cell catalysts. Chem. Mater. 2013, 25, 856-861. (10) Dang, H.; Singh, V. P., Nanowire CdS-CdTe solar cells with molybdenum oxide as contact. Sci. Rep. 2015, 5, 14859. (11) Chroneos, A.; Yildiz, B.; Tarancon, A.; Parfitt, D.; Kilner, J. A., Oxygen diffusion in solid oxide fuel cell cathode and electrolyte materials: mechanistic insights from atomistic simulations. Energy Environ. Sci. 2011, 4, 2774-2789. (12) Maier, J., Nanoionics: ion transport and electrochemical storage in confined systems. Nat. Mater. 2005, 4, 805-815. (13) Xu, Y.; Yi, R.; Yuan, B.; Wu, X.; Dunwell, M.; Lin, Q.; Fei, L.; Deng, S.; Andersen, P.; Wang, D., et al., High capacity MoO2/graphite oxide composite anode for lithium-ion batteries. J. Phys. Chem. Lett 2012, 3, 309-314. (14) Xu, Z.; Wang, H.; Li, Z.; Kohandehghan, A.; Ding, J.; Chen, J.; Cui, K.; Mitlin, D., Sulfur refines MoO2 distribution enabling improved lithium ion battery performance. J. Phys. Chem. C 2014, 118, 18387-18396. (15) Yao, D. D.; Rani, R. A.; O’Mullane, A. P.; Kalantar-zadeh, K.; Ou, J. Z., Enhanced coloration efficiency for electrochromic devices based on anodized Nb2O5/electrodeposited MoO3 binary systems. J. Phys. Chem. C 2014, 118, 10867-10873.



1 2 3 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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 27

(16) Sun, D.; Gu, X.-K.; Ouyang, R.; Su, H.-Y.; Fu, Q.; Bao, X.; Li, W.-X., Theoretical study of the role of a metal–cation ensemble at the oxide–metal boundary on CO oxidation. J. Phys. Chem. C 2012, 116, 7491-7498. (17) Brewer, L.; Lamoreaux, R. H., The Mo-O system (molybdenum-oxygen). Bull. APD. 1, 85-89. (18) Clentsmith, G. K.; Cloke, F. G.; Green, J. C.; Hanks, J.; Hitchcock, P. B.; Nixon, J. F., Stabilization of low-oxidation-state early transition-metal complexes bearing 1,2,4triphosphacyclopentadienyl ligands: structure of [{Sc(P3C2tBu2)2}2]; ScII or mixed oxidation state? Angew. Chem. Int. Ed. 2003, 42, 1038-1041. (19) Walia, S.; Balendhran, S.; Nili, H.; Zhuiykov, S.; Rosengarten, G.; Wang, Q. H.; Bhaskaran, M.; Sriram, S.; Strano, M. S.; Kalantar-zadeh, K., Transition metal oxides – thermoelectric properties. Prog. Mater. Sci. 2013, 58, 1443-1489. (20) Shi, Y.; Guo, B.; Corr, S. A.; Shi, Q.; Hu, Y.-S.; Heier, K. R.; Chen, L.; Seshadri, R.; Stucky, G. D., Ordered mesoporous metallic MoO2 materials with highly reversible lithium storage capacity. Nano Lett. 2009, 9, 4215-4220. (21) Boukhachem, A.; Bouzidi, C.; Boughalmi, R.; Ouerteni, R.; Kahlaoui, M.; Ouni, B.; Elhouichet, H.; Amlouk, M., Physical investigations on MoO3 sprayed thin film for selective sensitivity applications. Ceram. Int. 2014, 40, 13427-13435. (22) Scanlon, D. O.; Watson, G. W.; Payne, D. J.; Atkinson, G. R.; Egdell, R. G.; Law, D. S. L., Theoretical and experimental study of the electronic structures of MoO3 and MoO2. J. Phys. Chem. C 2010, 114, 4636-4645.


Page 15 of 27

1 2 3 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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(23) Ressler, T.; Wienold, J.; Jentoft, R. E.; Neisius, T., Bulk structural investigation of the reduction of MoO3 with propene and the oxidation of MoO2 with oxygen. J. Catal. 2002, 210, 67-83. (24) Wang, L.; Zhang, G.-H.; Chou, K.-C., Study on oxidation mechanism and kinetics of MoO2 to MoO3 in air atmosphere. Int. J. Refract. Met. H. 2016, 57, 115-124. (25) Brox, B.; Olefjord, I., ESCA studies of MoO2 and MoO3. Surf. Interface Anal. 1988, 13, 3-6. (26) Ressler, T.; Jentoft, R. E.; Wienold, J.; Timpe, O., Solid-state kinetics from timeresolved in situ XAFS investigations: reduction and oxidation of molybdenum oxides. J. Synchrotron Radiat. 2001, 8, 683-685. (27) Ahn, E.; Seo, Y.-S.; Cho, J.; Lee, I.; Hwang, J.; Jeen, H., Epitaxial growth and metallicity of rutile MoO2 thin film. RSC Adv. 2016, 6, 60704-60708. (28) Ryaben'kii, V. S.; Tsynkov, S. V., A theoretical introduction to numerical analysis. Chapman and Hall//CRC: Boca Raton, FL, USA, 2006. (29) Ghedira, M.; Do-Dinh, C.; Marezio, M.; Mercier, J., The crystal structure of Mo0.975Ti0.025O2 between 24 and 900°C. J. Solid State Chem. 1985, 59, 159-167. (30) Magnéli, A.; Andersson, G., On the MoO2 structure type. Acta Chem. Scand. 1955, 9, 1378-1381. (31) Rogers, D. B.; Shannon, R. D.; Sleight, A. W.; Gillson, J. L., Crystal chemistry of metal dioxides with rutile-related structures. Inorg. Chem. 1969, 8, 841-849. (32) Eyert, V.; Horny, R.; Höck, K. H.; Horn, S., Embedded peierls instability and the electronic structure of MoO2. J. Phys.: Condens. Matter 2000, 12, 4923-4946.

ACS Paragon Plus Environment

15


1 2 3 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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(33) Moosburger-Will, J.; Kündel, J.; Klemm, M.; Horn, S.; Hofmann,

Page 16 of 27

P.;

Schwingenschlögl, U.; Eyert, V., Fermi surface of MoO2 studied by angle-resolved photoemission spectroscopy, de haas-van alphen measurements, and electronic structure calculations. Phys. Rev. B 2009, 79, 115113. (34) Wooster, N., The crystal structure of molybdenum trioxide, MoO3. Z. Krist. 1931, 80, 504-512. (35) Jeen, H.; Lee, H. N., Structural evolution of epitaxial SrCoOx films near topotactic phase transition. AIP Advances 2015, 5, 127123. (36) Lee, S.; Meyer, T. L.; Sohn, C.; Lee, D.; Nichols, J.; Lee, D.; Seo, S. S. A.; Freeland, J. W.; Noh, T. W.; Lee, H. N., Electronic structure and insulating gap in epitaxial VO2 polymorphs. APL Mater. 2015, 3, 126109. (37) Johnson, P. B.; Christy, R. W., Optical constants of the noble metals. Phys. Rev. B 1972, 6, 4370-4379. (38) Chase, L. L., Optical properties of CrO2 and MoO2 from 0.1 to 6 eV. Phys. Rev. B 1974, 10, 2226-2231. (39) Dissanayake, M. A. K. L.; Chase, L. L., Optical properties of CrO2, MoO2, and WO2 in the range 0.2-6 eV. Phys. Rev. B 1978, 18, 6872-6879. (40) Lajaunie, L.; Boucher, F.; Dessapt, R.; Moreau, P., Quantitative use of electron energy-loss spectroscopy Mo-M2,3 edges for the study of molybdenum oxides. Ultramicroscopy 2015, 149, 1-8. (41) Thakur, P.; Cezar, J. C.; Brookes, N. B.; Choudhary, R. J.; Prakash, R.; Phase, D. M.; Chae, K. H.; Kumar, R., Direct observation of oxygen induced room temperature 16 ACS Paragon Plus Environment

Page 17 of 27

1 2 3 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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ferromagnetism in MoO2 thin films by x-ray magnetic circular dichroism characterizations. Appl. Phys. Lett. 2009, 94, 062501. (42) Thakur, P.; Cezar, J.; Brookes, N.; Choudhary, R.; Phase, D.; Chae, K.; Kumar, R., X-ray absorption and magnetic circular dichroism characterization of Mo1–xFexO2 ( x = 0– 0.05) thin films grown by pulsed laser ablation. Hyperfine Interact. 2010, 197, 95-100. (43) Schnohr, C.; Ridgway, M., X-ray absorption spectroscopy of semiconductors. Springer: Heidelberg, Berlin, 2015; p 78-80. (44) Sing, M.; Neudert, R.; Von Lips, H.; Golden, M.; Knupfer, M.; Fink, J.; Claessen, R.; Mücke, J.; Schmitt, H.; Hüfner, S., Electronic structure of metallic K0.3MoO3 and insulating MoO3 from high-energy spectroscopy. Phys. Rev. B 1999, 60, 8559-8568. (45) Chen, J. G.; Eng Jr, J.; Kelty, S. P., NEXAFS determination of electronic and catalytic properties of transition metal carbides and nitrides: from single crystal surfaces to powder catalysts. Catal. Today 1998, 43, 147-158. (46) Hébert, C.; Willinger, M.; Su, D. S.; Pongratz, P.; Schattschneider, P.; Schlögl, R., Oxygen K-edge in vanadium oxides: simulations and experiments. Eur. Phys. J. B 2002, 28, 407-414.



1 2 3 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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 27

Figure 1. Schematic diagram of oxidative phase transition from monoclinic MoO2 to orthorhombic MoO3. MoO2 has metallic ground state, while MoO3 is an insulator with Eg ≈ 3 eV. 18 ACS Paragon Plus Environment

Page 19 of 27

(a)

M o O2+x 250oC

M oO2

M oO2+x 100 oC

(b) 0.7

M oO3

M oO2+x 350oC

MoO2 MoO2 +x 100oC

0.6

MoO2 +x 250oC

0.60

o

MoO2 +x 350 C MoO3

0.5

o

FWHM ( )

1 2 3 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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.4

0.43

0.3 0.2

0.24

0.1 0.12

0.10

50 100 150 200 250 300 350 400 o

T ( C)

Figure 2. (a) Picture of 100-nm-thick MoO2 thin films oxidized at different temperatures. (b) The ∆ω of rocking curves from (200) MoO2+x. The ∆ω values increase with the increase of oxidation temperature. After oxidizing at 350oC, ∆ω is nearly 0.60o and the sample color is changed. 19 ACS Paragon Plus Environment


1 2 3 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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 27

Figure 3. Optical conductivities measured by spectroscopic ellipsometry at RT. Spectral shapes abruptly changed after oxidizing at 250oC. Unfilled triangles, labeled as α and β are dd transitions, while filled triangles, labeled as γ1, are p-d transitions of the films annealed at different temperature. 20 ACS Paragon Plus Environment

Page 21 of 27

1 2 3 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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Summary of Peak Deconvolution of Optical Conductivity Spectra α

β

γ11

γ12

Position (eV)

Area (arb. units)

Position (eV)

Area (arb. units)

Position (eV)

Area (arb. units)

MoO2

0.72

4013

2.74

1256

3.75

845

MoO2+x 100oC annealed

0.72

2810

2.74

1166

3.73

1104


0.71

3055

2.68

1009

3.71

1177


0.71

1836

2.43

86

3.57 2.91

MoO3

Position (eV)

Area (arb. units)

717

3.63

392

670

3.32

1800



B (eV) 12

γ1 (eV) 3.80

(a)

533.00 ( b ) MoO3 TFY

10

3.75 532.95

o

Intenstiy (arb. units)

1 2 3 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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 27

MoO2+x 350 C

8

3.70 532.90

MoO2+x 250oC TEY

6

3.65 532.85

o

MoO2+x 250 C TFY

4

C

A B

2

3.60 532.80

MoO2 MoO2+x 250oC TFY

MoO2

D

MoO2+x 250oC TEY

532.75 525

530

535

540

545

550

3.55

MoO2+x 350oC

50

100 150 200 250 300 350

3.50

T (oC)

E (eV)

Figure 4. (a) O K-edge spectra measured by x-ray absorption spectroscopy. It is clearly seen that bulk-sensitive TFY O K-edge spectrum of MoO2+x oxidized at 250oC is still similar to the TEY spectrum of as-grown MoO2. However, surface-sensitive TEY O K-edge spectrum of the MoO2+x is similar to that of MoO3. (b) Plot of peak positions of B (filled diamonds) and γ1 (open triangles) from XAS of O K-edge and spectroscopic ellipsometry spectra. The positions depend on the annealing temperatures. Each peak undergoes red-shift about 0.2 eV.


Page 23 of 27

1 2 3 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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC Graphic



1 2 3 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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Page 24 of 27

Page 25 of 27

(a)

M oO2

M o O2+x 250oC

M oO2+x 100oC

(b) 0.7

M o O3

M oO2+x 350oC

MoO2 MoO2+x 100oC

0.6

0.60

MoO2+x 250oC o

MoO2+x 350 C

0.5

FWHM (o)

1 2 3 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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


MoO3

0.4

0.43

0.3 0.2 0.1

0.24

0.12

0.10

50 100 150 200 250 300 350 400

T (oC)



4000

MoO2

 2000











4000

MoO2+x 100oC



2000 



4000

 (-1 cm-1)

1 2 3 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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 27

MoO2+x 250oC 

2000  

4000 2000

MoO2+x 350oC



 

MoO3 DFT

6000 4000 2000 1

2

3

4

5

 (eV)


6

Page 27 of 27

 (eV)

B (eV) 12

(a)

3.80

533.00 ( b ) MoO3 TFY

10

3.75 532.95

Intenstiy (arb. units)

1 2 3 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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


o

MoO2+x 350 C

8

3.70 532.90

o

MoO2+x 250 C TEY

6

3.65 532.85

o

MoO2+x 250 C TFY

4

A 2

B

3.60 532.80

C

MoO2

MoO2

D

MoO2+x 250oC TFY MoO2+x 250oC TEY

532.75 525

530

535

540

545

550

3.55

MoO2+x 350oC

50

100 150 200 250 300 350

T (oC)

E (eV)


3.50

Low Temperature Nanoscale Oxygen-Ion Intercalation Into Epitaxial

Recommend Documents