New Insights into Planar Defects in Layered Î±-MoO3 Crystals

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Interface-Rich Materials and Assemblies

New Insights into Planar Defects in Layered #-MoO3 Crystals Hongfei Liu, Coryl Jing Jun Lee, Shifeng Guo, and Dongzhi Chi Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03102 • Publication Date (Web): 26 Oct 2018 Downloaded from http://pubs.acs.org on October 28, 2018

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New Insights into Planar Defects in Layered α-MoO3 Crystals Hongfei Liu*, Coryl J. J. Lee, Shifeng Guo, and Dongzhi Chi Institute of Materials Research and Engineering (IMRE), A*STAR (Agency for Science, Technology and Research), 2 Fusionopolis Way, Singapore 138634, Singapore Abstract: The observation of regular (h0l) planar defects in α-MoO3 crystals can be traced back to over 60 years ago. Two mechanisms have been proposed to interpret the formation of the planar defects. One is related to the diffusion of oxygen vacancies due to thermal-driven release of oxygen atoms in vacuum and the consequent crystallographic shear of α-MoO3. The other is associated with redox reactions of moisture and/or hydrocarbons that give rise to HxMoO3 precipitates. Here, we report that regularly spaced (302) planar defects can be introduced into -MoO3 belt crystals by heating in liquid sulfur at 300 C. These defects are undetectable neither by atomic-force microscopy nor by scanning-electron microscopy at the crystal surface. Raman scattering enhancement and weakening have been observed for different phonon modes of -MoO3 at the (302) planar defects as probed from the (010) surface. Their comparisons with the Raman scattering enhancements at the edges, as well as the argon-plasma-induced Raman spectral evolutions, of the as-grown -MoO3 belt crystals, provide new insights into the planar defects with regarding to their formation and characteristics.

Keywords: α-MoO3 crystals, planar defects, oxygen vacancies, crystallographic shears, Raman scattering

*Author to whom correspondence should be addressed; electronic mail: [email protected]

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1. Introduction Molybdenum trioxides, -MoO3, have long been extensively investigated as i) the catalysts and/or primary components of catalysts for important applications in chemical industry,1-3 ii) the components of electrical and optoelectronic devices,4-8 and iii) most recently, the van der Waals crystals towards their two-dimensional (2D) nanosheets with novel and unique properties for next generation ubiquitous nanoelectronics.9-15 As a typical transition metal oxide, -MoO3 has the common issue of oxygen deficiencies, i.e., MoO3-x.16-23 To create and/or engineer the surficial oxygen deficiencies of -MoO3 could have important consequences when enhancing their oxygen reduction reactions for catalytic applications. In a recent study carried out at Peetes’ group, thermal chemical reactions between poly-(diallyldimethylammonium chloride) and stoichiometric -MoO3 have been employed to create oxygen vacancy (VO) defects on the surface of MoO3.24-25 It is generally believed that slight oxygen deficiencies (x < 0.002) are dominated by randomly distributed oxygen vacancies while increased oxygen deficiencies give rise to crystallographic shears (CS) in the crystal matrix of -MoO3.26 The oxygen vacancies are intrinsic point defects and invisible by most of the traditional microscopic techniques. In comparison, the crystallographic shears are planar defects, they usually arrange themselves in regular patterns that can be visualized and/or analyzed by using microscopy, X-ray diffraction (XRD), spectroscopy, etc., techniques.1, 27-35 However, the coexistence of VO and CS in -MoO3 complicates the analysis, especially in reducing atmosphere and/or at elevated temperatures where reducing reactions and release of oxygen atoms at the surface of -MoO3 might occur. In fact, although the observation and assessment of heat-treatment-induced CS in -MoO3 has been started over 60 years 2


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ago,35-36 detailed role/behavior of VO, either intrinsic or recreated on the surface via releasing oxygen atoms, in the formation of CS is still a subject of debate due partly to the presence of undesired hydrogen from moisture absorption in air and/or residual hydrocarbon contaminations in vacuum.26, 37-45

The presence of atomic hydrogen, even at room temperature,37 could induce defects into -MoO3 crystals via forming acicular precipitates of HxMoO3 that topotactically aligned along the [203] directions of the MoO3 (010) atomic planes.26,

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From the

visualization point of view, both the shapes and orientations of the HxMoO3 acicular precipitates observed by Smith et al.28 are nearly the same as those of heat-treatmentinduced CS (i.e., via releasing oxygen atoms and generating VO) in -MoO3 observed by Bursill et al.33 The latter were also confirmed by heating -MoO3 crystals in sealed glass at pressure of 2  10-6 Torr.33 These results indicate that both HxMoO3 and MoO3-x could exhibit in similar planar defects in the crystal matrix of -MoO3. In this light, the detailed formation mechanism of the planar defects (i.e., their initialization) might be a complex problem in transmission electron microscopy (TEM) observations where both the electron-beam-induced heating and the residual hydrocarbon contaminations are most likely inevitable.33, 36

Both XRD and Raman scattering are mature nondestructive techniques that have been used, besides TEM, in studying the structural evolutions of -MoO3 from its stoichiometric phase to HxMoO3 and/or MoO3-x.28,

38, 40, 42-43, 46-52

However, XRD

typically probes the average information of the material structures, which is inappropriate for localized analyzing the microscale planar defects in -MoO3; so does the traditional

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spectroscopic Raman scattering, especially when studying powder samples, where the analysis is mainly based on dynamic sampling and statistic processing.38, 42-43, 51 Recent developments in confocal micro-Raman technology make it feasible for lattice dynamic studies of thin film crystals as well as their defective structures,53-55 which has been greatly helping advancing the latest researches on 2D materials.56-63 Unfortunately, Raman mapping studies of the planar defects in -MoO3 are missing in the literature although they have already provided direct evidence for the enhanced Raman scattering at the edge areas of -MoO3 belt crystals.54, 64

In this work, we have introduced (302) planar defects into high-quality α-MoO3 belt crystals. These microscale defects, visible under optical microscope, are undetectable by using either atomic-force microscopy (AFM), scanning-electron microscopy (SEM), or energy-dispersive X-ray spectroscopy (EDX). Confocal micro-Raman spectroscopy and mappings, together with the AFM, SEM, and EDX studies, revealed that the effects of the planar defects on the Raman features of -MoO3 are apparently different from those of VO at the crystal edges.38, 54, 64 These observations provide new insights into the formation and properties of such planar defects, which could have important roles in understanding the morphological/structural evolutions of α-MoO3 crystals upon heat treatments as well as their conversions towards MoO2 and/or MoS2 for not only novel electronics and optoelectronics but also catalysis and energy conversion applications.24-25, 44, 47, 65-67

2. Materials and Methods

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High-quality single-crystalline α-MoO3 belts were synthesized by thermal vapor transport (TVT) at the temperature of  600 C and verified by single-crystal XRD (SCXRD).12, 14 These individual α-MoO3 belt crystals are generally grown with their length- and widthdirection along their [001]- and [100]-axis, respectively; so that their surface normal directions are along their [010]-axis. To introduce planar defects, disassembled α-MoO3 belts were sandwiched between sulfur flakes (Aldrich, 99.98%), followed by putting on sapphire wafers (double-side polished and epiready) and loading in an alumina crucible. The crucible was then covered and heated on a hot plate in a fume hood at 300 C for about eight hours. At the end of the heat treatment, the liquid sulfur was just about consumed and the remains were then evaporated within a few minutes with the cover of the crucible removed. In this way, regularly spaced planar defects were introduced into the α-MoO3 belt crystals with the effect of the moisture and/or oxygen from the air environment minimized because of their limited solubility in liquid sulfur.68-70

The post-growth introduced planar defects of α-MoO3 were then studied at room temperature by employing Raman spectroscopy mappings in a confocal micro-Raman system (Witec alpha-300). A 532-nm wavelength argon ion laser was used as the excitation light source. The beam spot of the laser on the sample surface is about ~900 nm in diameter, which provides a spatial resolution high enough for probing the microscale defective structures when doing the feature mappings. Optical microscopy, AFM (tapping mode), SEM, and EDX were also employed to assist the data analysis and interpretation.

3. Results and Discussion

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3. 1 Raman Spectroscopy and Mapping of As-Grown Crystal Belts. Figure 1 presents the microphotograph [Fig. 1(a)], mappings of Raman features [Figs. 1(b)-1(h)], and Raman spectra [Fig. 1(i)] collected from the as-grown α-MoO3 (~1.0 m in thickness, see Supporting Information, Figure S1). One sees in Fig. 1(a) that the two layered crystals, partly overlapped, exhibit well shaped belt structures. Figures 1(b) and 1(c) present the intensity and frequency distributions, respectively, of the B3g mode at about 127 cm-1, showing that this mode is only detectable at the edges rather than the center areas of the belt crystals. Likewise, Figure 1(d) shows the intensity distribution of the Ag mode at about 470 cm-1; Figures 1(e)-1(g) show the intensity, frequency, and linewidth distributions, respectively, of the Ag mode at about 818 cm-1; and Figure 1(h) presents the intensity distribution of the B3g mode at about 290 cm-1. The eight Raman spectra in Fig. 1(i) were collected with four each from the edge and center areas of the belts. They were normalized to the most strengthened Mo-O-Mo stretching mode (Ag) at about 818 cm-1 and collectively shifted in the vertical axis for easier comparisons. It is seen that all the Raman modes observed at the center areas have been enhanced in their intensities at the edges. Some modes, hardly detectable at the center areas, are clearly seen at the edges as indicated by the arrows in Fig. 1(i); a typical example from the Ag mode at about 470 cm1

is shown in the inset of Fig. 1(i), which is consistent with the intensity distributions

shown in Fig. 1(d).

The edge-enhanced Raman scattering has been attributed to increased oxygen deficiencies at the edge areas.54, 64 This attribution can be traced back to the systematic studies carried out by Mestl et al. for mechanically activated MoO3 powders.38-39,

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Oxygen deficiencies, typically in the form of VO, tend to reduce the valance states of Mo

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in the lattice matrix of MoO3, leading to gap states that, in turn, resulted in resonant and enhanced Raman scattering at the wavelength around 532 nm.47,

52, 64

However, a

comparison of the microscopic images in Figs. 1(a), 1(b), 1(d), 1(e), and 1(h) shows that light wave-guiding and/or extraction, beside VO, could also contribute the Raman intensity enhancements. This is manifested by the similar distributions of the contrast fluctuations throughout the belt crystals in Figs. 1(a), 1(d), 1(e), and 1(h). The arrows in Fig. 1(a) indicate two typical areas where weak (arrow 1) and strong (arrow 2) light outputs occurred, respectively. Likewise, these two areas exhibit weak and strong Raman scattering enhancements, respectively, in Figs. 1(d), 1(e), and 1(h). Because -MoO3 is a high-k crystal, its waveguide effect (~1.0 m in thickness) might lead to enhanced light extraction at the edges and/or surface curved areas [e.g., the area indicated by arrow 2 in Fig. 1(a)] of the crystal.71 In principle, the light extraction effect is a kind of magnifier for the resonance enhanced Raman scattering. However, one sees that the area indicated by arrow 2 is much weaker than that indicated by arrow 1 in Fig. 1(b), which is in reverse to those displayed in Figs. 1(a), 1(d), 1(e) and 1(h), where the former is stronger than the latter. This comparison provides direct evidence that the B3g mode at about 127 cm-1 is much more sensitive than the other modes to the oxygen deficiency-induced Raman scattering enhancement of -MoO3 via the VO-related resonance mechanism. Also seen is that the extra enhancement over the resonant effect of the B3g (~127 cm-1) mode is only dominant at the belt edges [see Figs. 1(b), 1(c), and 1(f)] of the as-grown -MoO3 crystal, which will be further discussed in a later section.

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3.2 Structural Defects Introduced onto/into -MoO3 by Plasma Treatments at Room Temperature. Figures 2(a)-2(d) present the morphological evolutions of a -MoO3 crystal belt [Figure 2(a)] upon an argon-plasma-treatment (APT) for 60 min [Figure 2(b)], followed by oxygen-plasma-treatment (OPT) for 30 min [Figure 2(c)] and then another OPT for 30 min [Figure 2(d)]. The setup and processing procedures for the plasma treatments have been reported in an earlier publication.14 It is seen that darkening of the -MoO3 has been introduced by APT at its surficial defective areas where present the cracks and/or atomic layer steps as those seen in Fig. 2(a) before the plasma treatments. Because of the APT-induced darkening, the surficial defects of -MoO3 became dim and hardly seen in Fig. 2(b). However, they are a bit clearer in Fig. 2(c) and much clearer in Fig. 2(d) due to the OPT processes. This observation, together with the comparisons between the image contrasts in Fig. 2(b) and those in Figs. 2(c) and 2(d), reveals that the APT-induced darkening of the -MoO3 crystals tends to be recovered by the OPT process. A careful look at the image in Fig. 2(d) revealed that the remaining darkening mainly locate at the defects, either the initial ones or those generated during the APT and/or OPT processes [see Figs. 2(a) and 2(d)].

A higher magnification of the APT-induced -MoO3 darkening is presented in Fig. 2(e), which shows that the dark structures exhibit regular orientations besides those of the initial defects on the (010) surfacetheir fronts are around 33  2 away from the [001]axis of the -MoO3 crystal. The typical orientation of the APT-induced dark structures, their locating at the vicinity of surficial defects, as well as their tendency of recovery by the OPT process, provide evidence that they are HxMoO3 precipitates, similar to those

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observed by Smith et al. when heating MoO3 in MeOH.26 This is also, more or less, supported by XRD comparisons of the -MoO3 belt crystals before and after APT (Supporting Information, Figure S2), where the APT-induced broadening of the (040) diffraction peak to the lower angular side is consistent with that of HxMoO3 recently reported by Zhang et al.72 In our study, the hydrogen is most likely from the residual air due to the low background vacuum of the plasma treatment chamber (i.e., ~350 mtorr).14 The topotactic hydrogen reduction of -MoO3 could be significantly enhanced by the APT process, especially at the vicinity of those inherent surface defects where oxygen deficiencies are most likely enriched,26,

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and reverted by the OPT process due to

enriched reactive oxygen species.42

Figure 2(f) compares the Raman spectra collected from the APT processed -MoO3 crystal at its bright- and dark-area (hereafter, referred as to BA and DA, respectively) with that from the as-grown sample. One sees that all the distinguishable Raman features of APT-DA have been softened (i.e., red shifted) as compared with those of APT-BA while the latter exhibit minor changes from those of the as-grown sample. A typical softening of the B2g mode is highlighted in the inset of Fig. 2(f), which is at ~282.0 cm-1 in the spectrum of APT-BA but red-shifted to ~280.0 cm-1, i.e., softened by ~2.0 cm-1 in the spectrum of APT-DA. Also seen in Fig. 2(f) is that a broad band emerged in the APTDA spectrum at ~444 cm-1 but it is not distinguishable at all for the as-grown -MoO3. The feature intensity comparisons in Fig. 2(f) reveal that the oxygen deficiency induced Raman scattering enhancements, which have been observed at the belt edges of the MoO3 crystal due to VO-caused resonance, are not detectable at all, although VO might be

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yielded in -MoO3 when interacting with hydrogen during the APT process.42 This observation, in turn, provides evidence that the darkened -MoO3 is dominated by HxMoO3 rather than MoO3-x. This is further supported by the earlier reports that the former have slightly softened Raman features with respect to stoichiometric -MoO3 while VO have minor effects on the mode frequencies of -MoO3 [see Figs. 1(f) and 1(i)].40, 42, 52

3.3 Regularly Spaced Planar Defects Introduced into -MoO3 by Low-Temperature Heat Treatment in Liquid Sulfur. Although thermal vapor sulfurization (TVS) of MoO3 have been extensively employed for producing low-dimensional MoS2 nanostructures (i.e., 1D nanotubes and 2D nanosheets),73-74 chemical reactions between sulfur molecular and stoichiometric -MoO3 are unlikely to occur at low temperatures on the (010) surface due to the activation barrier.75 In this light, we have carried out lowtemperature heat treatment of -MoO3 belt crystals in liquid-sulfur at 300 C. The morphological and microstructural changes of the treated belt have been measured from both the top- and bottom-surface and the results are presented in Fig. 3. A typical microphotograph taken from the top surface of the belt is shown in Fig. 3(a). One sees that dark needle-like defective structures emerged in the belt crystal; they regularly spaced and collectively aligned themselves in the two orientations of about 34  1 away from the [001]-axis, which correspond to the intersections of the (302) atomic planes on the (010) surface of the -MoO3 crystal belt. Typical AFM and SEM images recorded from the locations around the cycle indicated in Fig. 3(a) are presented in Figs. 3(b) and 3(g), respectively; detailed comparisons are provided as Supporting Information (Figure

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S1). One sees that neither AFM nor SEM could reveal the regularly spaced defective structures on the surface of the heat treated -MoO3 belt. Neither do the element distributions of Mo [Fig. 3(h)] and O [Fig. 3(i)] recorded by EDX mapping along with the SEM imaging [Fig. 3(g)]; they are uniform and featureless. Figure 3(c) presents the intensity mapping of the Raman mode at about 818 cm-1, which shows exactly the same distribution of the defects as that observed by the photomicroscope in Fig. 3(a). This result, together with the AFM and SEM observations, as well as the featureless EDX mappings, provides evidence that the defect structures are intrinsic and they are most likely lying in the (302) atomic planes rather than on the surface of -MoO3. This conclusion is further supported by the microphotograph [Fig. 3(d)], AFM [Fig. 3(e)], and Raman mapping [Fig. 3(f)] recorded from the bottom surface of the belt after it’s flipped over by a scotch-tape, where the mirror-symmetry of the defects viewed from the topand bottom-surface of the belts indicates that they are indeed perpendicular to the (010) surface. Such heat-treatment-induced intrinsic planar defects are apparently different formation from those of HxMoO3 precipitates induced by APT (Fig. 2) although they have quite similar orientations with respect to the [001]-axis of -MoO3.

A comparison of the Raman spectra collected from the top- and bottom-surface of the low-temperature treated -MoO3 crystal belt at its defect-free- and defect-area is shown in Fig. 4; the spectra were normalized to the Ag (~818 cm-1) mode [see Fig. 4(b)]. It is seen that the defects did not introduce apparent frequency shifts but enhanced the intensity of some of the Raman modes [e.g., see the B2g (~665 cm-1) mode in Fig. 4(b)]. This behavior is, more or less, similar to the VO-induced Raman scattering enhancement at the belt edges (see Fig. 1) but different from the effect of the HxMoO3 precipitates [see 11


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Fig. 2(f)] where the Raman modes of -MoO3 were shifted without apparent intensity enhancements. A common observation for the planar defects and the HxMoO3 precipitates is that both structures induced a broad Raman feature at ~444 cm-1 [see the arrows in Figs. 4(a) and 2(f)]. This common observation suggests that the broad feature might be related to localized lattice disorders of -MoO3, which were induced either by the inward diffusion of H atoms through reactions with O to form VO or by the (302) planar defects due to the VO-induced CS.26, 42 This is also supported by the fact that the (302)-oriented CS has already been observed by Bursill et al. in their TEM studies of MoO3.33 In addition, our X-ray photoelectron spectroscopy (XPS) studies of the -MoO3 crystals before and after the low-temperature heat treatment in liquid sulfur (Supporting Information, Figure S3) reveal that reductions in the valance states of Mo from Mo6+ to Mo5+ with [Mo5+]/[Mo6+] = ~6% are indeed occurred during the treatment, most likely at the surface of the crystal belts.

More details about the typical Raman modes of B3g at ~127 cm-1, Ag/B2g at ~156 cm-1, B2g at ~282 cm-1, the broad feature at ~444 cm-1, and Ag at ~818 cm-1 have been mapped around the planar defects of the heat treated -MoO3 belt. Their intensity and frequency mappings are presented in Figs. 5(a)-5(e) and Figs. 5(f)-5(j), respectively. It is seen that the B3g (~127 cm-1) mode [Fig. 5(a)] and the broad feature (~444 cm-1) [Fig. 5(d)] have been enhanced, while the others [Figs. 5(b), 5(c), and 5(e)] have been weakened, in their intensities by the planar defects. In contrast, all the modes have been, more or less, softened by the planar defects [see Figs. 5(f)-5(j)]. A careful look at the mode frequency maps reveals that the mode softening is minor (< 0.2 cm-1) for the B2g (~282 cm-1) mode [Fig. 5(h)] and the Ag (~818 cm-1) mode [Fig. 5(j)]. Fragmental 12


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comparisons for 10 spectra, 5 each from the defect-free- and defect-area, are highlighted in Figs. 5(k)-5(m) with the peak intensity normalized. These comparisons, together with the intensity mappings in Figs. 5(a)-5(e), show that the effect of the planar defects on the mode intensities is essentially different from that of VO at the edge areas of the as-grown crystal belts (see Fig. 1). First, all the Raman modes are enhanced in intensity by VO at the crystal edges while most of the Raman modes are weakened [e.g., the modes at ~156 cm-1, ~282 cm-1, and ~818 cm-1, see Figs. 5(b), 5(c), and 5(e), respectively] and only a few modes [i.e., the B3g at ~127 cm-1 and the broad feature at ~444 cm-1, see Figs. 5(a) and 5(d), respectively] are enhanced by the planar defects. Next, the B3g mode at ~127 cm-1, the B3g mode at ~244 cm-1, the B3g mode at ~290 cm-1, the B1g mode at ~378 cm-1, and the Ag mode at ~470 cm-1 emerged at the crystal edges but undistinguishable at all at the center areas of the as-grown crystal belt [see the arrows in Fig. 1(i)]. In comparison, after the heat treatment i) the B3g (~127 cm-1) mode emerged throughout the vicinities of the planar defects [Fig. 5(k)] with its intensity enhanced by the planar defects [Fig. 5(a)]; ii) the B2g (~282 cm-1) mode is weakened and minor softened at the planar defects [Figs. 5(c) and 5(h)] while its shoulder (i.e., the B3g mode at ~290 cm-1) is not detectable at all [see Fig. 5(i)]; iii) the Ag mode [~470 cm-1, see Fig. 5(m) and Supporting Information, Figure S4] emerged throughout the vicinities of the planar defects [Fig. 5(k)] with its intensity weakened by the planar defects; and iv) the broad feature (~444 cm-1) emerged only at the locations of the planar defects [see Figs. 5(d) and 5(m)].

It has been widely observed that lattice disorders of metal sulfides and oxides can induce activations of their intrinsic ‘silent’ Raman modes due physically to the infraredactive to Raman-active (IA-to-RA) transitions via partly breaking/relaxing the selection

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rules.76-78 For example, additional Raman modes have been observed in crystalline ZnO individually doped with Fe, Sb, Al, Ga, Mn, Co, N, or P atoms, and the vibration frequencies of these additional Raman modes are constant and independent of the doping elements and their concentrations (see, e.g., ref.

78

and references therein). In this light,

the broad feature at ~444 cm-1, which is absent from both the edge and the center areas of the as-grown -MoO3 belt crystal [see the comparisons in the inset of Fig. 1(i)] but appeared after either the APT [Fig. 2(f)] or the heat treatment [Figs. 5(d) and 5(m)] at the defective areas, is most likely due to the disorder-induced activation of a ‘silent’ mode, i.e., via an IA-to-RA transition; the IA-to-RA transition also happened to the B3g mode at ~127 cm-1. This conclusion is further supported by the absence of the B3g (~290 cm-1) at the high-energy shoulder of the B2g (~282 cm-1) mode, since the intensity ratio of I290/I282 has been proposed to be propositional to the concentration of VO in -MoO3.64 It is also supported by the intensity weakening of the Ag/B3g (~156 cm-1) mode, the B2g (~282 cm-1) mode, and the Ag (~818 cm-1) mode at the planar defects, which indicates the absence of the VO-induced resonance effect wherein. These results, in turn, provide evidence that the resonance Raman enhancement is dominated by VO-induced changes in the absorption wavelength at the edges of the as-grown -MoO3 belt crystal. Moreover, both the VO and the oxygen-deficiency-induced lattice disorders (in the states of VO, Mo interstitials, and/or CS) of -MoO3 could active its IA-to-RA transitions and, meanwhile, weaken the intrinsic Raman active modes via partly breaking/relaxing the selection rules. It is thus clear that the weakening of most of the Raman modes and the enhancement of a few ones at the planar defects are due to the lattice disorders, typically CS, since VO are absent wherein. On the other hand, the Vo-induced resonance enhanced all the Raman active

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phonon modes while the lattice-disorder-induced IA-to-RA transitions resulted in the extra enhancement of the B3g (~127 cm-1) mode at the edges of the belt crystal (see the discussion in Section 3.1).

These Raman mapping comparisons between the as-grown and the low-temperature treated -MoO3 crystal belts provide further insights that the heat treatment under certain conditions tends to convert the dominant intrinsic oxygen deficiencies of -MoO3 from VO to CS, giving rise to the regularly spaced (302) planar defects. Figure 6(a) presents a distribution of the planar defects at the edge areas of the heat treated crystal belt and Fig. 6(b) presents the number of defects accounted as the distance away from the belt edge is increased. One sees that the density of the planar defects at the crystal edge is the highest and it monotonically decreases when moving away from the edge towards the center areas. This defect density distribution, strongly correlated with the enriched VO at the crystal edges, provides further evidence for the VO-to-CS conversion under certain conditions. This distribution also suggests that these VO are dominated by those created intrinsically during the crystal growth of -MoO3 rather than recreated on the (010) surface during the low-temperature heat treatment in liquid sulfur.

4. Conclusion In conclusion, we have introduced regularly spaced (302) planar defects into high-quality α-MoO3 single-crystal belts by carrying out a heat treatment at low-temperature in liquid sulfur. Raman spectroscopy and the spectral mappings of the planar defects and their vicinity areas, together with their comparisons with those of the as-grown belts at the crystal edges and those of the mode evolutions upon argon-plasma treatment, confirmed 15


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that these planar defects were formed due to thermally driven conversions of intrinsic oxygen deficiencies from oxygen vacancies to crystallographic shears under certain conditions. They also confirmed a higher mobility of the oxygen vacancies in the in-plane directions, i.e., along the [100]- and [001]-axis, than that along the out-of-plane direction, i.e., the [010]-axis, of the layered -MoO3 crystal. The recreation of oxygen vacancies via releasing oxygen atoms at the (010) surface played a minor role in the vacancy-tocrystallographic shear conversions, which is different from earlier reported observations upon electron-beam heating in the TEM chamber and the heat treatment under low pressures in the sealed glass.

Supporting Information Available Detailed comparisons among the optical microscopy, SEM, and AFM images, targeting at the regularly spaced defects; AFM height profile and absorption spectra with thickness-related fringes; XRD comparisons of the -MoO3 crystal belts before and after the APT or low-temperature heat treatment in liquid sulfur; XPS comparisons of the valance states of Mo of the -MoO3 crystal belts before and after the low-temperature heat treatment in liquid sulfur; and Raman mappings of the Ag (~470 cm-1) mode of the -MoO3 belt crystal after the low-temperature heat treatment in liquid sulfur.

Acknowledgement This research is supported by A*STAR Science and Engineering Research Council Pharos 2D Program (SERC Grant No. 152-70-00012).

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Figure Captions Figure 1: Raman spectroscopy and mapping of as-grown -MoO3 belt crystals. (a) Microphotograph, (b) Intensity mapping of the B3g mode at ~127 cm-1, (c) Mode frequency mapping of the B3g mode at ~127 cm-1, (d) Intensity mapping of the B2g/Ag mode at ~156 cm-1, (e) Intensity mapping of the Ag mode at ~818 cm-1, (f) Mode frequency mapping of the Ag mode at ~818 cm-1, (g) Linewidth mapping of the Ag mode at ~818 cm-1, (h) Intensity mapping of the mode at ~290 cm-1, and (i) Spectral comparisons with four each collected from the edge and center areas, respectively. The spectra were normalized to the strongest mode at ~818 cm-1 and the inset highlights the comparisons of the Ag mode at ~470 cm-1. Please note the Raman scattering enhancements at the belt edges, the comparisons between the areas indicated by the arrows in the microphotograph, and the minor mode shifts in the spectra. Figure 2: Morphological evolutions of an -MoO3 belt crystal upon argon-plasmatreatment (APT) and consequent oxygen-plasma-treatment (OPT) at room temperature. (a) As-grown belt with surface defects, (b) After APT for 60 min, (c) Consequent OPT for 30 min, (d) Another OPT for 30 min, (e) Enlarged microphotograph of the belt after APT for 60 min, and (f) Comparisons of the Raman spectra collected from the as-grown -MoO3 crystal belt and the brightand dark-area of the belt after APT for 60 min. Figure 3: Visualization of the regularly spaced (302) planar defects introduced into the MoO3 belt crystal by low-temperature heat treatment in liquid sulfur. (a)

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Microphotograph from the top surface, (b) AFM image from the top surface, (c) Raman intensity mapping of the Ag (~818 cm-1) mode from the top surface, (d) Microphotograph from the bottom surface, (e) AFM image from the bottom surface, (f) Raman intensity mapping of the Ag (~818 cm-1) mode from the bottom surface, (g) SEM image from the top surface, (h) EDX mapping of Mo from the top surface, and (i) EDX mapping of O from the top surface of the MoO3 belt after the heat treatment. Figure 4: Raman spectral comparisons of the low-temperature heat treated -MoO3 belt crystal collected from the top- and bottom-surface at the defect and defect-free areas. (a) Raman features with mode frequencies smaller than 550 cm-1 and (b) Raman features with mode frequencies larger than 550 cm-1. Figure 5: Raman mappings and spectral comparisons made for the planar defects and their vicinities of the -MoO3 belt crystal after the low-temperature heat treatment in liquid sulfur. (a)-(e) Intensity mappings of the modes at ~127, ~156, ~282, ~444, and ~818 cm-1, respectively; (f)-(j) Mode frequency mappings of the modes at ~127, ~156, ~282, ~444, and ~818 cm-1, respectively; (k)-(m) Raman spectral comparisons between the defect and defect-free areas of the -MoO3 crystal belt. Figure 6: Density distribution of the planar defects formed in -MoO3 crystal belt. (a) Microphotograph of the planar defects and (b) Number of defects as a function of distance away from the crystal edge, showing a monotonically decrease. The

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defect numbers are simply counted in the individual areas separated by adjacent dashed lines.

TOC Graphic

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Figure 1: Raman spectroscopy and mapping of as-grown α-MoO3 belt crystals. (a) Microphotograph, (b) Intensity mapping of the B3g mode at ~127 cm-1, (c) Mode frequency mapping of the B3g mode at ~127 cm-1, (d) Intensity mapping of the B2g/Ag mode at ~156 cm-1, (e) Intensity mapping of the Ag mode at ~818 cm-1, (f) Mode frequency mapping of the Ag mode at ~818 cm-1, (g) Linewidth mapping of the Ag mode at ~818 cm-1, (h) Intensity mapping of the mode at ~290 cm-1, and (i) Spectral comparisons with four each collected from the edge and center areas, respectively. The spectra were normalized to the strongest mode at ~818 cm-1 and the inset highlights the comparisons of the Ag mode at ~470 cm-1. Please note the Raman scattering enhancements at the belt edges, the comparisons between the areas indicated by the arrows in the microphotograph, and the minor mode shifts in the spectra. 115x94mm (300 x 300 DPI)


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Figure 2: Morphological evolutions of an α-MoO3 belt crystal upon argon-plasma-treatment (APT) and consequent oxygen-plasma-treatment (OPT) at room temperature. (a) As-grown belt with surface defects, (b) After APT for 60 min, (c) Consequent OPT for 30 min, (d) Another OPT for 30 min, (e) Enlarged microphotograph of the belt after APT for 60 min, and (f) Comparisons of the Raman spectra collected from the as-grown α-MoO3 crystal belt and the bright- and dark-area of the belt after APT for 60 min. 168x202mm (300 x 300 DPI)


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Figure 3: Visualization of the regularly spaced (302) planar defects introduced into the α-MoO3 belt crystal by low-temperature heat treatment in liquid sulfur. (a) Microphotograph from the top surface, (b) AFM image from the top surface, (c) Raman intensity mapping of the Ag (~818 cm-1) mode from the top surface, (d) Microphotograph from the bottom surface, (e) AFM image from the bottom surface, (f) Raman intensity mapping of the Ag (~818 cm-1) mode from the bottom surface, (g) SEM image from the top surface, (h) EDX mapping of Mo from the top surface, and (i) EDX mapping of O from the top surface of the α-MoO3 belt after the heat treatment. 82x82mm (300 x 300 DPI)


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Langmuir

Figure 4: Raman spectral comparisons of the low-temperature heat treated α-MoO3 belt crystal collected from the top- and bottom-surface at the defect and defect-free areas. (a) Raman features with mode frequencies smaller than 550 cm-1 and (b) Raman features with mode frequencies larger than 550 cm-1. 78x73mm (300 x 300 DPI)


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Figure 5: Raman mappings and spectral comparisons made for the planar defects and their vicinities of the α-MoO3 belt crystal after the low-temperature heat treatment in liquid sulfur. (a)-(e) Intensity mappings of the modes at ~127, ~156, ~282, ~444, and ~818 cm-1, respectively; (f)-(j) Mode frequency mappings of the modes at ~127, ~156, ~282, ~444, and ~818 cm-1, respectively; (k)-(m) Raman spectral comparisons between the defect and defect-free areas of the α-MoO3 crystal belt. 114x93mm (300 x 300 DPI)


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Langmuir

Figure 6: Density distribution of the planar defects formed in α-MoO3 crystal belt. (a) Microphotograph of the planar defects and (b) Number of defects as a function of distance away from the crystal edge, showing a monotonically decrease. The defect numbers are simply counted in the individual areas separated by adjacent dashed lines. 102x127mm (300 x 300 DPI)


New Insights into Planar Defects in Layered Î±-MoO3 Crystals

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