Topotactical Nitridation of α-MoO3 Fibers to γ-Mo2N Fibers and Its

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Topotactical Nitridation of r-MoO3 Fibers to γ-Mo2N Fibers and Its Field Emission Properties Khemchand Dewangan,† Sandip S. Patil,‡ Dilip S. Joag,‡ Mahendra A. More,*,‡ and N. S. Gajbhiye*,† Department of Chemistry, Indian Institute of Technology, Kanpur-208016, India, and Center for AdVanced Studies in Materials Science and Condensed Matter Physics, Department of Physics, UniVersity of Pune, Pune-41007, India ReceiVed: April 3, 2010; ReVised Manuscript ReceiVed: July 22, 2010

We demonstrate the synthesis of large-scale high-quality single-crystalline γ-Mo2N fibers using R-MoO3 fibers as precursors by temperature-programmed reactions with NH3. The formation mechanism is discussed, and the morphology, structure, composition, and chemical states of the prepared fibers were characterized by X-ray diffraction pattern, field emission scanning electron microscopy, high-resolution transmission electron microscopy, selected area electron diffraction pattern, and X-ray photoelectron spectroscopy studies. The normal field emission properties of the product were investigated, and the turn-on field of 0.6 V/µm is reported for the first time. The Fowler-Nordheim (F-N) plot obtained from the current density-applied field (J-E) characteristic is almost found to be linear, and the I-t plot shows that the emission current remains nearly constant over 2 h. The γ-Mo2N fibers exhibit an enhanced performance as compared to other nitrides. Therefore, our field emission results show that the γ-Mo2N fibers might be potential field emitters in the future devices. Introduction The unique physical and chemical properties of the transition metal nitrides prove them as an important class of materials for the fundamental and technological applications in diversified areas.1,2 Among these nitrides, molybdenum nitrides have been extensively studied as well-known transition-metal nitrides for applications of superconducting devices3-5 and active catalysts.6-8 Molybdenum forms several crystalline nitrides including Mo2N, MoN, and an R-phase, which is known as a Mo-N solid solution with low nitrogen contents.9 Mo2N crystallizes in two stable crystal forms: γ-Mo2N (cubic) and β-Mo2N (hexagonal). The theoretical energy band calculation predicted that MoN has a cubic rock-salt (NaCl) type structure (so-called B1-MoN structure).10 The B1-MoN phase, however, does not appear in the equilibrium phase diagram of the Mo-N system, as it is a metastable phase that is not stable in thermal equilibrium and prefers to crystallize the thermodynamically more stable hexagonal phase δ-MoN.11 Therefore, it is difficult to obtain crystalline and pure B1-MoN. Many researchers have adopted various thin film deposition techniques for the synthesis of this phase like reactive sputtering and N+ ions implantation.12,13 Similarly, other phases, such as γ-Mo2N, β-Mo2N, and δ-MoN, have been prepared through various physical methods using Mo-metal as the molybdenum source such as the nitridation by molecular N2/NH3 under high pressure and temperature9 or in NH3 atmosphere at room temperature by highenergy ball milling.14 The N+ ion implantation,15 reactive sputtering,12 and pulsed laser deposition under N2 radical irradiation3 have also been used to obtain these nitrides. On the other hand, temperature-programmed reactions of molybdate * To whom correspondence should be addressed. Tel: +91-512-2597080. Fax: +91-512-259-7436. E-mail: [email protected] (N.S.G.). Tel: +9120-2569-2678. Fax: +91-20-2569-1684. E-mail: [email protected] (M.A.M.). † Indian Institute of Technology. ‡ University of Pune.

with NH3 are the most common and widely used as the chemical approaches,16,17 including thermal reduction-nitridation in an autoclave,18 thermolysis of the Mo-N bond containing molybdenum complexes,19-22 ammonolysis of polysulfide precursors,23 a solid-state metathesis reaction between MoCl5 and Ca3N2,24 a low temperature solution route,25 and a simple inexpensive urea glass technique.4,26 In addition to the applications based on the catalytic and superconducting properties, the one-dimensional metal nitrides are thought to be potential candidates as field emitters in the various micro- and nanoelectronic devices. In this context, Ng et al. have reported enhanced field emission properties in patterned GaN nanowires.27 Ha et al. have studied the optical as well as the field emission properties of single-crystalline GaN nanowire.28 Various groups, Tang et al., Shi et al., and Liu et al., have reported field emission characteristics of AlN with different morphologies like nanorods, nanotips, and nanocones.29-31 The field emission properties of carbon nitride nanotubes were found to be familiar to the properties of carbon nanotubes.32 Tarntair et al. have observed that the field emission properties of quasi aligned silicon carbon nitride are superior to those of CNT and carbon-coated Si microtips.33 The exclusive field emission property of highly oriented boron carbonitride nanofibers was intended by Bai et al.34 In this article, we report topotactic transformation of R-MoO3 fibers into γ-Mo2N fibers by temperature-programmed reactions in NH3 atmosphere. Such a method was previously used for the preparation of molybdenum nitride catalysts with high surface areas. We now present its use, for the first time, to synthesize high-quality single-crystalline γ-Mo2N fibers in large scale using R-MoO3 fibers as a precursor. In addition, studies on the field emission behavior of the synthesized γ-Mo2N fibers are reported herein.

10.1021/jp103008f  2010 American Chemical Society Published on Web 08/12/2010

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Experimental Section Synthesis of r-MoO3 Fibers. The R-MoO3 fibers were produced as in our previously reported method.35 In a typical synthesis, a freshly prepared polymeric nitrosyl complex of molybdenum(II) (0.7 g) was dissolved in double-distilled water (35 mL) in the presence of a few drops of concentrated HNO3 under stirring. The resulting clear, pale yellow solution was transferred into a Teflon-lined autoclave with a stainless steel shell. The autoclave was filled up to 80% of the total volume and kept on a preheated laboratory furnace at 220 °C for 7-24 h. After it was cooled naturally to room temperature in ambient surroundings, the precipitate/product was collected after centrifugation, thoroughly rinsed with double-distilled water and absolute ethanol, and finally dried in an oven at 60 °C for 5 h. The proportion of the fibrous morphology was estimated to be 100% in the sample, based on microscopy results. Synthesis of γ-Mo2N Fibers. Temperature-programmed reductions of R-MoO3 fibers with NH3 gas (99.99% purity) were performed in a tubular quartz tube furnace. The NH3 gas was passed through the quartz tube and maintained at a flow rate of 94 cc min-1 during the entire synthesis. Approximately 250 mg of R-MoO3 fibers precursor was loaded into a quartz boat and placed inside a combustion quartz tube at the heating zone. The furnace was programmed to heat the material in three stages: (1) The temperature was increased from ambient to 357 °C at a rate of 5.6 °C min-1, (2) then to 447 at 0.5 °C min-1, and (3) then to 785 at 2.1 °C min-1. Finally, the furnace was held at a temperature of 785 °C for 5 h. The preparation temperature was varied in a furnace coupled with a PID controller and was monitored locally with a thermocouple. At the end of temperature program, the reactant gas was allowed to continue flowing, and the product was quenched. Once the system cooled down to room temperature, the product was passivated in the same tube by flushing a gas mixture of 0.1% O2/N2 to prevent bulk oxidation on exposure to air. After passivation, the product was taken out of the reactor and stored in a vacuum desiccator for subsequent evaluations. Characterizations of Material. The phase structure and crystallography of the samples were analyzed using a Thermo Electron Corporation, ARL X’TRA power X-ray diffractometer (XRD) (Cu KR radiation, λ ) 0.1542 nm, 40 kV, and 45 mA). A scanning rate of 1° min-1 and step size of 0.02° was applied to record the pattern in the range of 30-100°. A field emission scanning electron microscope (FESEM) Zeiss, SUPRA-40VP equipped with energy dispersive X-ray (EDX) spectroscopy, was used to analyze the sample for morphology, size, and elemental composition. Transmission electron microscope (TEM), selected area electron diffraction (SAED) pattern, and high-resolution transmission electron microscope (HRTEM) images were taken by using a Tecnai G2 instrument, operated at an accelerating voltage of 300 kV. X-ray photoelectron spectroscopy (XPS) measurements were carried out by SPECS photoelectron spectrometer (Phoibos 100 MCD Energy Analyzer) using monochromatic Mg KR radiation (1253.6 eV). Binding energies were calibrated with C (1s) at 285 eV with a precision of (0.2 eV. Field Emission of γ-Mo2N Fibers. The field emission studies of γ-Mo2N fibers were carried out in the planar “diode” configuration in an all metal ultra high vacuum (UHV) chamber at a base pressure of ∼1 × 10-8 Torr. The γ-Mo2N fibers were pasted on a precleaned tungsten substrate (6 mm × 3 mm) using silver paste. The tungsten substrate having γ-Mo2N fibers (cathode) was held parallel to the semitransparent phosphor

Figure 1. (a) Powder XRD pattern of γ-Mo2N fibers. (b) The bars represent data from the JCPDS file no. 25-1366 for γ-Mo2N.

screen (anode). The separation between the cathode and the anode was kept at ∼2 mm using linear drive motion. Results and Discussion Volpe and Boudart16 were the first to report the synthesis of high-surface area γ-Mo2N in temperature-programmed reactions of R-MoO3 in flowing NH3. They demonstrated that the transformation of R-MoO3 platelets is topotactic in the sense that {100} planes of γ-Mo2N are parallel to {010} planes of R-MoO3. The topotactic reaction is defined to occur when a solid product has well-defined crystallographically orientation relative to the starting (parent) crystal. To evaluate the nature of the crystal orientation, morphologies, crystal structure, and phase composition of the γ-Mo2N fibers, a number of experiments were carried out. First, single-crystalline γ-Mo2N fibers were prepared by NH3 reduction of single-crystalline R-MoO3 fibers. The detailed characterizations of the crystalline structure of this R-MoO3 sample can be found in ref 35. Briefly, each R-MoO3 fiber has grown along the [001] crystallographic direction because of the intrinsic structural anisotropy and growth rates (rhkl) variation of R-MoO3 in all three principal directions, that is, r001 . r100 . r010, and the fiber shape is represented by the planes {100} and {010}, which enclose the facets of the fiber along the longitudinal direction. Figure 1a shows an X-ray diffraction pattern (XRD) of the samples obtained by temperature-programmed reactions in the NH3 atmosphere of the R-MoO3 fibers. The sharp diffraction peaks suggest a well-crystallized product. The diffraction peaks in the pattern can be indexed to a face-centered cubic (fcc) phase with lattice parameter a ) 4.18 Å, which is close to that of γ-Mo2N, 4.163 Å (Joint Committee on Powder Diffraction Standard, JCPDS: 25-1366). Interestingly, no peaks of any other phases or impurities were detected on XRD, indicating high purity of the γ-Mo2N fibers. However, the (200) peak is more intense than the other peaks (Figure 1b) and indicates the anisotropic and preferred orientation on the fibers. This result agrees with the fact that the R-MoO3/NH3 reactions are topotactic, and crystallographically, {010} planes of R-MoO3 are transferred to the {100} planes of γ-Mo2N, or in other words, the crystallographic orientations of {100} planes of γ-Mo2N are parallel to {010} planes of R-MoO3 [{010}R-MoO3//{100}γ-Mo2N].16 The FESEM image in Figure 2a shows the fibrous morphology of the starting material R-MoO3, and Figure 2b shows its topotactic

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Figure 2. FESEM image of (a) R-MoO3 fibers and (b) γ-Mo2N fibers with EDX in the inset. TEM image of (c) γ-Mo2N fibers and (d) individual γ-Mo2N fibers. (e and f) SAED and HRTEM images of corresponding individual γ-Mo2N fibers, respectively.

nitrided transformation image of γ-Mo2N. EDX spectra were measured to determine the chemical composition of the γ-Mo2N fibers as shown in the inset of Figure 2b. The resulting γ-Mo2N sample has a fiberlike morphology with a diameter of about 300 nm and a length of several micrometers as depicted in the TEM image (Figure 2c,d). The SAED pattern (Figure 2e) of the γ-Mo2N fiber is similar to that of a single crystal, and the square pattern of the spots can be assigned to the [100] zone axis. In the HRTEM image in Figure 2f, the lattice fringes are clearly visible with a spacing of 0.21 nm, which corresponds to the (200) planes of γ-Mo2N. This result has good agreement with the XRD result that the {100}γ-Mo2N planes are predominantly orientated parallel to the {010}R-MoO3 planes.

A high-resolution XPS analysis was carried out on the passivated sample. Figure 3a shows signals corresponding to Mo (3d5/2) and Mo (3d3/2) electrons at 228.69 and 232.04 eV, respectively. The binding energy of this species Mo (3d5/2) is midway between those assigned to Mo(IV) (230.0 ( 0.2 eV) and Mo(0) (227.8 ( 0.2 eV). While it has been suggested that the binding energy near 228.9 eV is characteristic of Mo(II), we denoted this species as Mo(δ) where 0 < δ < IV.6 The binding energy at 235.49 eV is attributed to Mo(VI).23 Therefore, the existence of Mo(VI) in the samples is mainly due to the oxidation of the low valence state of Mo. However, no other peaks of impurities related to Mo(VI) state are observed from the XRD patterns. It can be explained from two aspects: First,

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Figure 3. High-resolution XPS spectra of γ-Mo2N fibers: (a) Mo (3d) and (b) Mo (3p) and N (1s).

the amount of Mo(VI) is below the resolution limit of XRD measurement; second, Mo(VI), which is in the form of oxynitride or amorphous oxide on the surface, formed during the passivation of sample. In Figure 3b, signals shown at 394.64, 397.57, and 411.99 eV correspond to Mo (3p3/2), N (1s), and Mo (3p1/2) electrons, respectively.25 It is noteworthy that the peaks of Mo (3p) and N (1s) overlap, and the N (1s) signal is characteristic for a metal nitride material. The signal of O (1s) electrons was observed at 530.43 eV. We know that R-MoO3 forms a crystalline solid with a layered structure and an orthorhombic crystal lattice (3.96, 13.86, and 3.7 Å). Each layered plane is composed of two sublayers, each of which is formed by corner-sharing [MoO6] octahedra along [001] and [100]; the two sublayers stack together by sharing the edges of the octahedra along [001]. An alternate stack of these layered sheets along [010] leads to the formation of R-MoO3, and the weak electrostatic van der Waals interaction is the major binding force between adjacent double layers.35 In γ-Mo2N, the crystal lattice has a fcc structure with a ) 4.18 Å, where each molybdenum atom is octahedrally surrounded by six nonmetallic nitrogen sublattice sites and vice versa. While R-MoO3 reacts with NH3-involved temperature program reactions, the solid morphology did not alter, viz., irrespective of the size and shape of the parent crystal, and the product (γMo2N) of the reactions turned out to be pseudomorphous with R-MoO3. Because of these topotactic reactions, the removal of oxygen and infusion of nitrogen must occur in a manner such that each intermediate reaction step is also topotactic so that the gross morphology of the parent material is typically maintained during transformations. In this way, structural symmetry between the molybdenum lattice points exists throughout the entire reactions. Volpe and Boudart16 and Jaggers et al.17 suggested that the solid state reactions of R-MoO3 with NH3 proceed through two parallel reaction pathways: R-MoO3 f γ-Mo2OxN1-x f γ-Mo2N and R-MoO3 f MoO2 f γ-Mo2N. The first reaction pathway involves an fcc oxynitride intermediate. This reaction is considered topotactic since the lattices of the γ-Mo2OxN1-x and γ-Mo2N phases have well-defined crystallographic orientations relative to R-MoO3 and lead to the formation of the high-surface area product. The second proposed reaction pathway involving the formation of MoO2 is not topotactic and is believed to produce lower surface area nitrides. Some of the authors reported a small amount of hydrogen bronze HxMoO3 generated as an intermediate at a slow heating rate at the early stage of reaction below 500 °C.6,7 All of the reaction processes can be summarized as follows: At the early stage of reaction, NH3 is adsorbed on the surface of R-MoO3 below a temperature of 347 °C and began to react with NH3 at a temperature of 357 °C.16 Thereafter, the reaction was carried out at temperatures from 357 to 447 °C with a slow heating rate that produced γ-Mo2OxN1-x, HxMoO3, and MoO2. The

slower heating rate was kept to avoid sintering and enhanced the selectivity to γ-Mo2OxN1-x and HxMoO3. The conversion of HxMoO3 to γ-Mo2OxN1-x occurred near the transition between the two heating segments (450 °C)7 and was favored by the use of higher rates during the second heating segment from 447 to 785 °C. Finally, at this temperature, nitrogen is incorporated onto the lattice of molybdenum, and hydrogen is combined with oxygen to form water while nitrogen stays in lattice interstices. During this process, only nonmetal atoms seem to move in and out of the lattice while molybdenum atoms remain almost motionless. The oxynitride can be further nitrided to γ-Mo2N by replacement of the lattice oxygen with nitrogen. When the nitridation temperature reached 500 °C, γ-Mo2OxN1-x started to produce γ-Mo2N; therefore, it can be considered that 500 °C is a critical temperature for the pseudomorphic transformation of R-MoO3. The appearance of MoO2 parallel with together the above process is negligible, and only above 657 °C, it reacts with NH3 to convert γ-Mo2N.16 Finally, the nitridation temperature was increased to 785 °C and maintained for 5 h, and then, γ-Mo2OxN1-x and MoO2 phases disappeared completely, and only well-defined and crystallized γ-Mo2N was observed.

The two heating segment program was employed to prepare γ-Mo2N via the temperature-programmed reactions of R-MoO3 with NH3. The rate of temperature increase becomes an important factor as follows: During the first heating segment (357-447 °C), a slow heating rate favored the formation of γ-Mo2OxN1-x and HxMoO3 with a negligible amount of MoO2. When the heating rate is sufficiently slow, hydrogen from dissociated NH3 has an opportunity to react with R-MoO3 before it decomposes to MoO2. Also, the slow heating rate during the first segment permits a longer diffusion time and deeper penetration of hydrogen into the R-MoO3 lattice. Hence, the HxMoO3 phase can be selectively formed via a topotactic reaction in which hydrogen is inserted between the loosely held double-thick layers of [MoO6] octahedra in R-MoO3. On the other hand, fast heating rate during the first segment, insufficient amounts of hydrogen may be available; therefore, competing reactions including the decomposition of R-MoO3 to MoO2 could occur. A fast heating rate during the second heating segment (447-785 °C) may favor the pseudomorphic nitridation of HxMoO3 to γ-Mo2OxN1-x by not permitting sufficient time for significant atom diffusion and morphological change. Volpe and Boudart16 have shown in Figure 4 the structural evolution of the topotatical nitridation of R-MoO3 through the restacking of layers.

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Figure 4. Scheme of layer structural evolution during the R-MoO3/ NH3 reaction: (a) (010) slabs spaced 0.6925 nm apart of R-MoO3, (b) during nitridation the oxynitride superlattice formation, (c) oxynitride supperlattice disappears, and (d) the layers become (200) γ-Mo2N planes as the fcc structure.

Figure 5. Current density electric field (J-E) plot of γ-Mo2N fibers with the F-N plot in the inset.

The field emission studies were carried out at a constant separation between anode and cathode of ∼2 mm. A typical emission current density-applied field (J-E) curve of γ-Mo2N fibers is shown in Figure 5. In the present studies, the turn on field defined as the applied filed required to draw an emission current of 1 nA was found to be ∼0.6 V/µm. Also, the threshold field required to extract the current density of 10 µA/cm2 was found to be ∼1.15 V/µm. The values of the turn on field and the threshold field observed in the present studies are superior as compared to the other nitrides (GaN, AlN, SiCN, CN, and BCN)27-34 as depicted in Table 1. The superior field emission characteristics observed in the present studies are due to the one-dimensional nature of the γ-Mo2N, offering a high aspect ratio. The emission current-applied field properties were characterized by Fowler-Nordheim (F-N) equation36

J ) A(β2E2 /φ) exp(-Bφ3/2 /βE)

(1)

where J is the emission current density, E is the applied field, A (1.54 × 10-6 A eV V-2) and B (6.83 × 103 V eV-3/2 µm-1)

Figure 6. Emission current-time (I-t) plot of γ-Mo2N fibers with the field emission micrographs in the inset.

are constants, φ is the work function, and β is the field enhancement factor. The corresponding F-N plots, that is, the plot of ln(J/E2) versus 1/E is shown as an inset in Figure 5. The nature of F-N plot was found identical to the F-N plots of other nitrides. In the context practical applications of the cold cathodes, the stability of the emission current is one of the decisive and important parameters. The emission current stability (I-t) measurements of the γ-Mo2N fibers was investigated at a base pressure of ∼1 × 10-8 Torr at the preset value of ∼1 µA over a duration of more than 2 h. A typical I-t plot is shown in Figure 6, with the field emission micrograph in the inset. The emission current shows excursions to low and high values with “spike” type current fluctuations superimposed on the base value. An initial increase in the emission current (up to 50 min) can be attributed to the cleaning effect of the emitter surface due to ion bombardment. Once the emitter surface gets clean, stabilization of the emission current could be expected. The “spike” type fluctuations are due to the adsorption-desorption and/or migration of the residual gas atoms/molecules on the emitter surface. In addition, creation of the new emission sites on the emitter surface due to the residual ion bombardment can be speculated. The inset of Figure 6 shows a field emission image, where the number spots correspond to the emission sites from the most protruding γ-Mo2N fibers. Conclusions In summary, a two segment heating-programmed reaction between R-MoO3 fibers and NH3 provides high-quality singlecrystalline γ-Mo2N fibers. The transformation of R-MoO3 is topotactic, and the heating rate of the two heating segment is a crucial factor for the same. As the R-MoO3 reacts with NH3, the gross morphology of the parent material is typically maintained during entire transformations, which passes through an oxynitride intermediate with changing crystal structure and

TABLE 1: Threshold Field for Various Nitrides materials

onset current/ current density

threshold field (V/µm)

GaN nanowires single-crystalline GaN nanowires AlN nanorods array quasi aligned AlN nanotips AlN nanocones aligned carbon nitride nanotubes (CN-NT) quasi aligned SiCN nanorods oriented boron cabonitride nanofibres

1 nA 10 µA/cm2 10 µA/cm2 1 µA/cm2 10 µA/cm2 10 µA/cm2 10 µA/cm2 2 nA 10 µA/cm2 0.26 nA

0.6 1.15 8.4 8.5 8.8 6 17.8 0.8 10 1.8

Mo2N fibers

references in the present studies 27 28 29 30 31 32 33 34

Nitridation of R-MoO3 Fibers to γ-Mo2N Fibers decreasing oxidation state of the metal. We put forth the outstanding metal nitride as a field emitter with its promising field emission properties. The values of the electric fields required to draw an emission current/current density of 1 nA and 10 µA/cm2 were found to ∼0.6 and 1.15 V/µm, respectively. The remarkable field emission current stability proves γ-Mo2N fibers as a good field emitter. The anisotropic morphology, robust nature, and overall promising field electron emission characteristics suggest prospective applications of γ-Mo2N fibers for vacuum cold-cathode flat-panel displays, microelectronics, etc. Acknowledgment. K.D. and N.S.G. thank the Department of Science and Technology (DST) and University Grants Commission (UGC), India, for financial support. S.S.P. is thankful to Bhabha Atomic Research Centre (BARC), Mumbai, India, for providing financial support. References and Notes (1) Toth, L. E. Transition Metal Carbides and Nitrides; Academic Press: New York, 1971. (2) Oyama, S. T. The Chemistry of Transition Metal Carbides and Nitrides; Blackie Academic and Professional: London, 1996. (3) Inumaru, K.; Baba, K.; Yamanaka, S. Synthesis and Characterization of Superconducting β-Mo2N Crystalline Phase on a Si Substrate: An Application of Pulsed Laser Deposition to Nitride Chemistry. Chem. Mater. 2005, 17, 5935–5940. (4) Gomathi, A.; Sundaresan, A.; Rao, C. N. R. Nanoparticles of Superconducting γ-Mo2N and δ-MoN. J. Solid State Chem. 2007, 180, 291– 295. (5) Inumaru, K.; Nishikawa, T.; Nakamura, K.; Yamanaka, S. HighPressure Synthesis of Superconducting Molybdenum Nitride δ-MoN by in Situ Nitridation. Chem. Mater. 2008, 20, 4756–4761. (6) Choi, J.-G.; Brenner, J. R.; Colling, C. W.; Demczyk, B. G.; Dunning, J. L.; Thompson, L. T. Synthesis and Characterization of Molybdenum Nitride Hydrodenitrogenation Catalysts. Catal. Today 1992, 15, 201–222. (7) Choi, J.-G.; Curl, R. L.; Thompson, L. T. Molybdenum Nitride Catalysts I. Influence of the Synthesis Factors on Structural Properties. J. Catal. 1994, 146, 218–227. (8) Volpe, L.; Boudart, M. Ammonia Synthesis on Molybdenum Nitride. J. Phys. Chem. 1986, 90, 4874–4877. (9) Jehn, H.; Ettmayer, P. The Molybdenum-Nitrogen Phase Diagram. J. Less-Common Met. 1978, 58, 85–98. (10) Papaconstantopoulos, D. A.; Pickett, W. E.; Klein, B. M.; Boyer, L. L. Electronic Properties of Transition-Metal Nitrides: The Group-V and Group-VI Nitrides VN, NbN, TaN, CrN, MoN and WN. Phys. ReV. B 1985, 31, 752–761. (11) Bull, C. L.; McMillan, P. F.; Soignard, E.; Leinenweber, K. Determination of the Crystal Structure of δ-MoN by Neutron Diffraction. J. Solid State Chem. 2004, 177, 1488–1492. (12) Ihara, H.; Kimura, Y.; Senzaki, K.; Kezuka, H.; Hirabayashi, M. Electronic Structure of B1 MoN, fcc Mo2N, and Hexagonal MoN. Phys. ReV. B 1985, 31, 3177–3178. (13) Savvides, N. High Tc Superconducting B1 Phase MoN films Prepared by Low-Energy Ion-Assisted Deposition. J. Appl. Phys. 1987, 62, 600–610. (14) An, G.; Liu, G. Ultramicro Molybdenum Nitride Powder Prepared Using High-Energy Mechanochemical Method. Rare Met. 2008, 27, 303– 307.

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