Phase transformation; structural and optical properties of two dimensional MoO3
الموضوعات : Journal of Theoretical and Applied Physics
Mohammad Reza Khanlary
1
(Department of Physics, Imam Khomeini International University, Qazvin, Iran.)
Azita Keshavarz
2
(Department of Physics, Imam Khomeini International University, Qazvin, Iran.)
Reza Shakouri
3
(Department of Physics, Imam Khomeini International University, Qazvin, Iran.)
Milad Parhizkari
4
(Department of Physics, Imam Khomeini International University, Qazvin, Iran.)
الکلمات المفتاحية: Raman spectra, Thin Film, Heterostructure, Band gap,
ملخص المقالة :
Thin-layered materials from transition-metal dichalcogenides family and their preforms such as molybdenum three oxides (MoO3) exhibit great potential as active material in optoelectronics. Stable materials of few atoms thick have shown emerging capabilities in this area. In this article, we report the synthesis of MoO3 nanostructures by spray pyrolysis method. We will, perhaps for the first time, show that by varying the hot plate temperature from 350 to 4400C, a phase transformation reveal from h-MoO3 to α-MoO3. By annealing the prepared α-MoO3 sample a preferential orientation of (002) crystal plane is induced. A large band gap of 2.8 eV for the prepared MoO3 film was obtained from the transmission spectra. The Raman vibrational modes were investigated for excitation wavelength at 532 nm using MoO3 films and for a heterostructured MoO3/MoS2 film .Better crystallinity and decrease in defects intensity, as an advantage of MoO3 hybridization can be achieved by proper fabricating the heterostructure sample.
Phase transformation; structural and optical properties of Two-Dimensional MoO3
Mohammad.Reza Khanlary*, Azita.Keshavarz, Reza Shakoury ,Milad parhizkary
Physics Department of Imam Khomeini International University, Qazvin IRAN
Abstract
Thin-layered materials from transition-metal dichalcogenides family and their preforms such as molybdenum three oxides (MoO3) exhibit great potential as active material in optoelectronics. Stable materials of few atoms thick have shown emerging capabilities in this area. In this article, we report the synthesis of MoO3 nanostructures by spray pyrolysis method. We will, perhaps for the first time, show that by varying the hot plate temperature from 350 to 4400C, a phase transformation reveal from h-MoO3 to α-MoO3. By annealing the prepared α-MoO3 sample a preferential orientation of (002) crystal plane is induced. A large band gap of 2.8 eV for the prepared MoO3 film was obtained from the transmission spectra. The Raman vibrational modes were investigated for excitation wavelength at 532 nm using MoO3 films and for a heterostructured MoO3/MoS2 film .Better crystallinity and decrease in defects intensity, as an advantage of MoO3 hybridization can be achieved by proper bfaricating the heterostructure sample.
Keywords: band gap, thin film, Raman spectra, heterostructure.
1. Introduction
The top single, few or even all layers of multiphase materials might have the enough potential to be transformed into another phase by choosing appropriate physical or chemical reactions [1-2]. Molybdenum trioxide (MoO3) as semiconductor material with a wide band gap of ~ 3.2 eV [3] is one of the most interesting transition metal oxides with different forms of crystal structures [4-5]. Thermodynamically stable orthorhombic phase known as α –MoO3 is most attractive because of its diverse properties as in super capacitors [6]. Hexagonal h-MoO3 and monoclinic β – MoO3 are the metastable phases of n-type MoO3 semiconductors. MoO3 nanostructures have a wide application cases such as in lithium-ion batteries [7], catalysts [8], sensors [9], often in the form of a thin film.
It is known that, point defects; such as oxygen vacancies in metastable semiconductors can trap free charge carriers and local excitons. Then, the interaction between these defects and charge carriers is expected to influence physical properties of hosting materials. However, physics and behavior of point defects in 2D semiconductors is much more interesting, as they become stronger at reduced dimensionalities. A 2D MoO3 film is expected to have a variety of electronic and optical properties different from its bulk structure, originating from phase composition and oxygen stoichiometry, both of which are tunable to obtain desired properties [10]. In the case of α –MoO3, octahedral are distorted and form bilayers, which build up layered structure. Actually, MoO6 layers are separated by a Vander Waals gap of dispersed interactions, and oxygen sites on these symmetrically in equivalent sites, singly, doubly and triply coordinated to Mo ions. Generally, different geometries of MoO3 have been shown to enhance photoluminescence and sensing properties. Exciton bound to defects, if recombine radiatively, lead to light emission at energies lower than the interband optical energies [11]. Meanwhile, binding energy and recombination dynamics of Wannier and Frenkel excitons are expected to be very different for 2D MoO3.
Molybdenum trioxide can be deposited in a variety of methods; some of these methods are thermal evaporation [12], chemical vapor deposition [13], DC sputtering [14], RF sputtering [15], and atomic layer deposition [16]. The films prepared by each of these methods may not be the same in physical and structural properties due to the different heating regimes and etc. . Among the several approaches available for preparing MoO3 film, some techniques such as spray pyrolysis are involved to hot substrates.
Owning to the interface issues as well as the multilayer compositions, density of structural defects may increase in the heterogeneous layers [17-18]. The occurrence of this phenomenon can be justified. In this work, we have studied some structural defects as well as phase transformation of MoO3 thin films prepared on glass substrate, and compare it to its coating on a MoS2 substrate, synthesized by the same manner.
2-Experimental
For the deposition of MoO3 thin films 618 mg of ammonium heptamolybate tetra hydrate [AHM;(NH4)6Mo7O24.4H2O] in 50 ml of deionized distilled water and was stirred about an hour at room temperature. The solution was sprayed onto a clean glass substrate. The substrate temperature was maintained at 3500 C. Pyrolytic decomposition of the sprayed solution on the surface of the substrate results in the formation of MoO3 thin film. The films later on used for the characterization. In a second deposition process the above prepared solution was to another solution containing 0.19 gr of the thiourea [Sc (NH2)2] powder in25 mL of deionized water, and stirred for 60 min until reaching a clear colorless solution. Eventually, the solution was sprayed on preheated, the previously MoO3 thin film as a substrate, at 4400 C. In both the spray processes, the interval between nozzle and substrates was ~33cm, and the aqueous solution flow rate was 1 mL/ min. A numbers of the samples were annealed in atmospheric oven at 4400 C for 60 minutes. Structural properties of the prepared samples was carried out using X-ray diffraction with Cu (Cu Kα) line (λ= 1.54 A0). Raman spectra were collected using a Nicolet Almega Dispersive Raman Spectrometer (Thermo Fisher Scientific Inc.) equipped with a maximum power of 100 mW, 532 nm laser. All spectra were collected at 10 mW of power, 32 accumulations.
Optical absorption measurements in the range of 300-800 nm were carried out using a UV-Vis spectrometer (Rayleigh, model UV 2601 ).
3-Result and discussion
a) X-ray diffraction analysis
Fig.1 shows the XRD spectrum of the MoO3 film prepared on the heated glass substrate, by the hot plate of the spray unit, at 3500 C. the sample product crystalize in the hexagonal phase of MoO3 , and the diffraction peaks are indexed with reference to a standard (JCPDS-21-0569) data file. However, we note that fabricating in this substrate temperature has resulted a highly amorphous film. The highest intense peak observed at 2θ ~ 27 is broadened in comparison to the bulk compound. The observed broadening is a combination of the instrument error, the reduced crystallite size and the existence of micro strain in the synthesized sample product. As the h-MoO3 is thermodynamically metastable, the synthesis of its pure phase is usually more difficult, resulting in that the successful efforts to obtain h-MoO3 are very limited and thus limit its application. Here it seems that we were successfully able to synthesize, if not pure, but majorly h-MoO3 phase accompanied with orthorhombic α-MoO3 thin film. Actually, the prepared sample may be a composition of the hexagonal and orthorhombic MoO3 (ICCD 01-074-7909). H-MoO3 has been reported to have reduced crystallite size and grown as one-dimensional.
Fig. 1 XRD pattern of h-MoO3 thin films, with substrate temperature at 3500 C.
A phase transformation from h-MoO3 to α-MoO3 is reported to occur at 4000 C [19] which is in TGA/DTA studies. Therefore, we examined crystallinity of the as-prepared MoO3 films for the sample made in a higher substrate temperature in the spray unit. The XRD pattern of the sample prepared with a hot plate at 4400 C shows (Fig.2a) the polycrystalline nature with the mostly orthorhombic (α -MoO3) crystal structure. Here, we can see a new relatively high XRD peak value at 2θ ~ 39 related to (060) plane of α- MoO3.
Fig.2 XRD pattern of α-MoO3 thin films prepared by spray pyrolysis at 4400C substrate temperature a) as synthesized and b) after annealing at 4400C.
A comparison between the full width at half maximum (FWHM) for (020) of α- MoO3 and the similar peak for the (040) plane of h-MoO3 film (Fig.1), indicate a smaller crystallite size for h-MoO3 respect to α- MoO3 film with an increase in temperature, the crystallites aggregate and promote rapid growth of the particles. In some research the phase transformation is reported to be occurred in much lower temperature than in the case of the present work but with a longer heating process[20].No characteristic peaks of the other impurities are observed from this pattern indicating the high purity of prepared sample. However, the wide diffraction peaks suggest that the as-synthesized MoO3 film is not well crystallized. Amorphous MoO3 film like some metal oxide films such as TiO2 film or other nanostructure materials can be transferred to the crystal structure by annealing [16, 21-22].
We have to note that, temperature and annealing time are important parameters for the transition to the crystalline state and the density of the film changes during annealing. To investigate the effect of the further thermal treatment on the structure properties of the α - MoO3 sample, XRD pattern of the annealed sample at 4400 C was also taken and is shown in Fig.2 b). Thermal treatment of the α- MoO3 prepared film , shows the material to be highly crystalline, there were no peak shifts observed relative to the pattern of as-synthesized film. However, two smaller feature which can be related to the h-MoO3 are now reappeared at 2θ ~32 and 40.45. The peak at 2θ ≈ 49, attributed to (020) crystal plane of MoO3 is now much more intense. Actually, the peak intensities are significantly dependent on the treatment conditions. The origin of such a variation in the XRD peak intensity may be attributed to the change in the preferred crystal orientation, stoichiometry, and defects. An obvious difference in peak intensity at 2θ ~ 49 in the annealed sample can also be due to getting the larger crystallite size or obtaining a preferential crystalline orientation with (020) planes, which sometimes occurs in the nanostructures. Here, since the FWHM is not increased by sintering (37 nm before annealing and 36nm after annealing) it may induced prefer orientation.
It seems that the origin for phase transformation of the metastable phase h-MoO3 to thermodynamically stable phase α- MoO3 is due to high temperature heating effect on NH4+ Ions during the growth of the α- MoO3 crystal which occurs in this temperature away from annealing treatment. It has been reported [23] that slow annealing at a temperature of 4500 C in an air atmosphere transforms the metastable phase h-MoO3 to α- MoO3. This transformation is said to be coherent with the loss of NH4+ions in thermo gravimetric analysis (TGA). After dehydration and removal of ammonia compounds, a sharp exothermic peak at 4300 C is attributed to liberation of coordinated water and ammonia molecules from the internal structure of the MoO3 material, which promotes an irreversible phase transformation from the hexagonal to the orthorhombic structure [24].
b) UV-vis spectroscopy
Fir.3a) shows the transmittance spectra in the wavelength region 300-800 nm for the MoO3 film deposited at 4400 C. Generally, the optical transmission at the transmission edge corresponds to the transition from the valence to the conduction band, which in our sample has been occurred at energy related to ~ 450nm. According to this spectra, the MoO3 band gap (Eg) is calculated from the Tauc's plot (Fig.5b). The straight-line portion indicates the direct band gap in nature. The obtained band gap is 2.8eV that is in the range of reported Eg for α-MoO3 nanostructure [25]. Based on the kinds and their concentration of the structural defects created during the film preparation and their synthesize parameters, variation of Eg values from 2.5-3.2 eV is reported in the literature [26].
Fig.3 Optical transmission spectra (a) and Tauc’s plot of MoO3 film prepared at 4400 C (b)..
c) Raman spectroscopy
MoO3 offer a highly effective combination with a MoS2 layer. For MoO3 is a widely used p-type doping candidate due to the high work function, while MoS2 is a typical n-type semiconductor featured with low work function. Therefore, it is possible to convert n-type MoS2 to p-type MoO3 by partial oxidation reaction. Raman spectroscopy of a composite sample of MoO3/MoS2 was applied to monitor the coating treatment of MoO3 on MoS2 substrate. Raman spectra of the above mentioned α-MoO3 film and MoO3/MoS2 composite sample with the same manner preparation were collected using 532nm excitation and are presented in Fig.4a and Fig.4b respectively. The power of the laser was kept sufficiently low to avoid heating effect. Raman active modes identified in Fig 4a confirm the stable orthorhombic phase of MoO3 [27], with no indication of h-MoO3 peaks, which are reported to be in 252,690,899 and 976 cm-1 in the Raman spectrum. We have to note that the energy transfer by laser irradiation process in Raman scatting with this excitation wavelength may cause the omission of low consideration of h-MoO3 phase in the mixed phases prepared sample. However, there are some features in the Fig.4b with the probability of reappearance the mixture of the both phases in the hydride material. The 285 cm-1 (B2g, B3g) band in α-MoO3 is a doublet comprised of wagging modes of terminal oxygen atoms, the 339 (B1g) , 474 (Ag,vas O-M-O stretch and bend are closely resembles for the single crystal Raman spectrum [28]. The 668 cm-1 (B2g,B3g) is an asymmetric stretching of the MO-O-MO bridge along the C axis, the 822cm-1 (A1g,B1g) is a symmetric stretch of the terminal oxygen atoms, and the 996 cm-1(Ag,B1g) is the asymmetric stretch of the terminal oxygen atom [29]. Raman peak in the range830-1130 cm-1 are constructed from up to 3rd order combination of E12g (M2) + A1g(M) subtracted or added to TA(M) or LA(M) phonons. Type of the substrate is known to have noticeable effects on the amount and effectiveness of the defects created during fabrication hybrid nanostructures, causing significant variation on their Raman spectra. Another parameter for controlling structural defects detected by Raman spectra is a thermal treatment, sometimes is required during a thin film sanitization. In a spray pyrolysis process, substrate temperature may play a significant role in the physical properties of the products. One of the consequences of creation the defect in Raman analysis of our hybrid sample is slightly narrowing the main peak of MoO3 film in 822cm-1 and its redshift in MoO3/MoS2 hybrid film. Meanwhile, we believe the new 358cm-1,567 cm-1 and 1092cm-1 peaks are related to the defects of MoO3 film created during this new film fabrication. Narrowing, accompanied with a 2 cm-1 redshift of the 822cm-1 peak can be related to better crystallinity and decrease in defect density, respectively. These effects are not very clear for other peaks in the figure, indicating no noticeable effect on them by this extra processing on MoO3 film. MoS2 is known upon annealing in ~4500C by breaking the S-Mo-S bonds create S-vacancies, which facilitate the chemisorption of foreign molecules [30].
Fig.4 Raman spectra of the a) α- MoO3 and b) MoO3/MoS2 prepared samples.
4. Conclusion
Sintering effect and phase transformation studies on molybdenum trioxide thin films is done. Fitting the transmission plot with a theoretical curve drives MoO3 band gap of 2.8eV. Raman spectra of MoO3 and MoO3/MoS2 hetrostructured film show mixed phase existence of h-MoO3 and α-MoO3 in the prepared samples. The most intense Raman peak known as Ag mode located at 822 cm-1 and some other feature familiar Raman active mode of MoO3 are identified from the analyzed samples. Three additional modes peaked at 358,567 and 1092 cm-1 appeared in the MoO3/MoS2 film clearly shows the effect of MoS2 substrate in the prepared MoO3 film.
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