Morphological, structural and optical properties of ... - CyberLeninka

Cent. Eur. J. Chem. • 10(4) • 2012 • 1106-1118 DOI: 10.2478/s11532-012-0012-7

Central European Journal of Chemistry

Morphological, structural and optical properties of MoO2 films electrodeposited on SnO2|glass plate Research Article

Nijole Dukstiene1*, Dovile Sinkeviciute1, Asta Guobiene2 Department of Physical Chemistry, Faculty of Chemical Technology, Kaunas University of Technology, LT–50254 Kaunas, Lithuania

1

Institute of Materials Science, Kaunas University of Technology, LT-50131 Kaunas, Lithuania

2

Received 24 October 2011; Accepted 17 January 2012

Abstract: MoO2 films were prepared by electrodeposition under potential controlled conditions from an aqueous alkaline solution of sodium molybdate. Optical microscopy showed that films of different morphology were deposited. The surface roughness and grain size were determined by atomic force microscopy. The characterization of as-deposited films by X-ray diffraction analysis revealed their amorphous nature. The optical constants of films were derived from transmittance spectra recorded in the 310-1100 nm wavelength range. All films were highly absorptive and showed a direct band to band transition. From the absorption edge data, the values of the optical band gap Eg and the Urbach energy EU were determined based on Tauc’s model. The influence of film thickness on the extinction coefficient k, refractive index n, absorption coefficient a and the band gap energy Eg was studied. Keywords: MoO2 film • Morphology • Transmittance spectra • Tauc model • Optical constants. © Versita Sp. z o.o.

1. Introduction Nanometric molybdenum oxides have attracted a great technological interest because of their interesting physico-chemical properties. The most extensively studied molybdenum oxides are MoO3 and MoO2. The MoO3 has the unique two dimensional layered structure. Due to this specific structure MoO3 films possess good electrochromic [1,2] and photochromic [3,4] properties and have shown significant promise for applications as smart windows [5] and display devices [6]. Among the other transition metal oxides, MoO2, has the unusual metal-like electrical conductivity [7,8] and exhibits excellent field emission properties [9,10]. MoO2 is a highly important material in the selective oxidation catalysis of various organic compounds [11-13]. Besides catalytic applications, recently there has been an upsurge of interest in MoO2 films for energy

storing [14,15], and as a soft magnetic [16] and optical [17-20] material. In the past decades, MoO2 in nano-scale size has been synthesized by different methods. The most widely used methods are hydrothermal synthesis [15,21] and thermal evaporation [16,22]. The MoO2 preparation methods also include laser beam deposition [23] and the Pechini method [24]. MoO2 can also be obtained by chemical and electrochemical techniques. The former method includes chemical vapor deposition [17] and solgel [25], while the latter comprise of electrodeposition from solutions. The electrosynthesis of nanometric MoO2 materials with unique properties and their applications in a wide variety of technologies are just beginning to generate tremendous interest. Recently, the number of MoO2 nanostructures, such as nanowires [26-28] and nanofibers [29], have been synthesized by electrodeposition * E-mail: [email protected]

1106

N. Dukstiene, D. Sinkeviciute, A. Guobiene

techniques. As reported by the Gao group [26], composite electrodes based on electrochemically synthesized MoO2 nanowires possess long-term cycle stability and specific capacitance as high as 597 Fg-1. Electrolytic MoO2 nanowires offer an alternative substitute for manufacturing Mo-based emitters [27] and can be used as a precursor [29] for the production of Mo nanowires. The electrodeposition of hydrous molybdenum(IV) oxide as a thin film has been also reported by several groups [30,31]. In some cases, the deposition leads to the formation of a stable MoO2•xH2O [30], while others have reported depositing thin films with a MoO(2-n)•(OH)2n composition [31]. The post-deposition argon-annealing treatment of these films resulted in polycrystalline MoO2 with crystallites aligned perpendicularly to the substrate. An optical absorption study indicated a direct band gap of 2.83 eV. Recently, we have reported our success in depositing thin molybdenum(IV) oxide films onto a Se|SnO2|glass plate [32]. We have suggested a two step route for the MoO2 film electrodeposition. The main advantage of this method is the depositing of a non hydrous MoO2 film. In the first step, the selenium layer is electrodeposited onto a SnO2|glass plate. In the second step, the Se|SnO2|glass plate is transferred in to a sodium molybdate solution. Under the potential controlled conditions the MoO42ions discharge to form hydrous MoO2•xH2O, which in the interaction with pre-deposited selenium layer losses molecular water: (1) (2) In this paper, we continue to report our findings and describe the optical properties of these MoO2 films. The optical properties of films strongly depend on the preparation method and are very sensitive to the morphology [33,34]. Numerous experimental results [17,34-37] are available for the optical characterization of molybdenum oxides prepared by different techniques, while the electrolytic molybdenum oxide films are less studied and need more clarification regarding morphological and optical properties. Notably, only a few papers have appeared in the literature on the optical behaviour of electrolytic MoO2 films [31]. To the best of our knowledge, the relation between the electrolytic MoO2 film morphology and optical properties has not yet been discussed. In the present work the MoO2 films were obtained under different electrodeposition conditions. The morphology of obtained films has been mainly determined by the electrolysis temperature, while the film thickness has been influenced by both the deposition

potential and the electrolysis temperature. The present paper explores the film morphology and has been carried out with the following well focused aims: first, to investigate the morphology and structure of MoO2 films; secondly, to determine the optical characteristics of the films and, finally, to discuss a correlation between their optical properties and morphology.

2. Experimental procedure 2.1. Materials

All solutions were prepared using doubly distilled water and analytical grade reagents. Only freshly prepared solutions were used for measurements, and they were not de-aerated during the experiments. SeO2 (>99%, Reachim, Russia), Na2MoO4•2H2O (>99%, SigmaAldrich, Germany), sodium citrate (C6H5Na3O7•2H2O) (>99%, Lachema, Czech Republic), acetone (Standard, Poland), HNO3 (65%, Penta, Czech Republic), H2SO4 (96%, Barta a Cihlar, Czech Republic) Na2SO4 (>99%, Lach-Ner, Czech Republic) and NaOH (Standard, Poland) were used as received.

2.2. Film deposition

A commercially available ISE-2 three-compartment cell and a PI-I-50 potentiostat coupled to PR programmer (ZIP, Russia) were employed to electrodeposit the molybdenum oxide films onto initially selenium predeposited tin oxide coated glass plates. The thickness of the SnO2 (Nippon Sheet Glass Co, Japan) layer was ~1 µm. The presence and crystallinity of the SnO2 films were confirmed by the XRD analysis. The results confirmed the tetragonal structure and the polycrystalline nature of the SnO2 layers. Before experiments, the SnO2|glass plates had been cleaned with acetone, followed by HNO3 (0.1 M) solution, and finally rinsed with distilled water. The SnO2|glass plate was fixed in a stainless steel cylindrical holder, and the electrical contact was realized using a copper stick. The geometrical area of the SnO2|glass plate was 2.25 cm2.

2.2.1. Se|SnO2|glass plate preparation

A thin selenium film was deposited on the SnO2|glass plate using a H2SeO3 (0.01 M) solution in sodium citrate (0.22 M) as a supporting electrolyte (pH 5.76). Electrodeposition was performed by applying a cathodic current density of 0.222 mA cm-2 for 10 min. The electrolysis temperature was 20oC. The resulting Se films were rinsed with double distilled water. XRD analysis revealed the amorphous structure of the selenium film. The selenium surface morphology was compared with that of the pure SnO2|glass plate surface. 1107

Morphological, structural and optical properties of MoO2 films electrodeposited on SnO2|glass plate

Figure 1. AFM image of selenium layer on SnO2|glass surface: (a) 2D topography; (b) 3D topography.

Figure 2. AFM image of SnO2|glass surface: (a) 2D topography; (b) 3D topography. The AFM image of the Se layer on the SnO2|glass plate surface presented in Fig. 1, indicates a compact film composed of grain-type particles. Several measurements in different spots on the surface showed a homogeneous distribution. The maximum grain height was estimated to be about 75 nm, while the average grain width was about 728 nm. A root mean square roughness (Rq) equaled to 14.8 nm. Polycrystalline SnO2 film consists of cones like structures (Figs. 2a and 2b) with an average diameter of 400 nm. The height of cones varied from 30 to 115 nm. A root means square roughness equaled to 20.8 nm.

2.2.2. MoO2 film deposition

The electrodeposition of MoO2 films was carried out as previously described in our paper [32]. Briefly, thin films were electrodeposited by applying the definite cathodic potentials for 30 min in an ISE-2 cell where a Se|SnO2|glass electrode, a platinum spiral with an active area of 12.5 cm2 and an Ag|AgCl, KCl(sat), electrode were used as a working, counter and reference electrodes, respectively. Throughout the paper, all potential values are referred to this reference electrode. The electrolyte solution was Na2MoO4 (0.2 M) in sodium citrate (0.22 M), and the pH value of 8.3 was adjusted 1108

by adding H2SO4 (1 M) or NaOH (6 M) solutions. MoO2 electrodeposition conditions and sample labelling are summarized in Table 1. Subsequent to deposition, films were washed with ethanol and then subjected to further characterization. As-deposited thin films were well-adherent to the substrate, and no visible defects such as cracks, voids, porosity or loose particles were found. The prevailing molybdenum species was Mo(IV) at all deposition potentials. A more detailed information on the film composition obtained by the XPS depth profiling analysis can be found in our previous paper [32]. The body colour of MoO2 films varied from brown to black.

2.3. Characterization of MoO2 film

X-ray diffraction was carried out under a Bragg-Brentano geometry on a diffractometer (Dron-3.0 Bourevestnic Inc., Russia) using the CuKα (λ = 0.154178 nm) radiation, 30 kV voltage and the 20 mA current. The 2θ scanning range varied from 10 to 70 degrees. The scanning speed was 1omin-1. The results were registered with a computer and then processed by the Searchmath program. ATR−FTIR spectra were recorded using the compensation method with a Perkin Elmer FTIR Spectrum GX spectrophotometer (USA) by averaging


Table 1. MoO2

film electrodeposition labelling.

No.

Sample

conditions

and

sample

Electrodeposition conditions Potential, V

Temperature, oC

1

S1

-1.0

20

2

S2

-1.0

40

3

S3

-1.0

60

4

S4

-1.1

20

5

S5

-1.1

40

6

S6

-1.1

60

7

S7

-1.2

20

8

S8

-1.2

40

9

S9

-1.2

60

of scanning probe microscopy data were performed using a Windows-based program (Surface View version 2.0). Optical microscopy was carried out with an Olympus CX31 optical microscope (Olympus, Philippines) and an Olympus C-5050 photo camera (Olympus, Japan), with a magnification of 400x. The optical parameters of MoO2 films were deduced from the UV–VIS spectroscopy measurements. The transmittance and absorbance UV–VIS spectra were recorded using a UV–VISIBLE Spectronic Genesis spectrophotometer (Perkin Elmer Spectrum GX, USA) in the range 300–1100 nm, and the sampling interval was 1 nm. The spectra of films were taken using an identical transparent tin oxide-coated glass plate as a reference. Measurements were performed at room temperature. The reproducibility of the measurements was good, ranging within approximately ±0.5%. The optical constants were computed by fitting the obtained data to the exact expressions. The thickness and refractive index of an electrodeposited film were measured with a GAERTHER L115 laser ellipsometer (USA, Gaertner Scientific Corporation). Ellipsometer operated in the polarizercompensator sample-analyzer (PCSA) mode at an angle of incidence of 50o. The polarizer was set to a fixed angle of 45o. The light source was a He-Ne laser (5 mV), with the incident light wavelength of 632.8 nm. An accuracy of the refractive index measurements was ±0.01. The refractive index of pure SnO2 film on glass was evaluated for the determination of MoO2 film thickness and the refractive index.

3. Results and discussion 3.1. Film surface morphology

Figure 3.

Molybdenum oxide film surface macroscopic images: (a) S1; (b) S7; (c) S2; (d) S8; (e) S3; (f) S6.

64 scans with 0.3 cm–1 resolution in the range 4000–600 cm–1 for each sample. The surface morphology of the films was investigated using an atomic force microscope (NANOTOP-206, Microtestmachines, Belarus). An ULTRASHARP Si cantilever (force constant 5.0 N m-1, curvature radius less than 10 nm) was used. The measurements were performed in the contact mode. The AFM characteristics: maximum scan field area up to 30×30 microns, measurement matrix up to 512×512 points. The image processing and the analysis

The surface morphology of the MoO2 films was observed with an optical microscope and the micrographs are presented in Figs. 3a-3f. The top view image analysis of these films provides the direct evidence that the electrolysis temperature plays the key role in film texture. Molybdenum oxide films electrodeposited at a temperature of 20oC (Figs. 3a and 3b) are relatively smooth and densely packed, which is a characteristic feature of an amorphous film. An increase in electrolysis temperature leads to an increased discontinuity. Films electrodeposited at 40oC appear to be composed of at least two components, one being randomly orientated needles, and the other appears to be amorphous and formed by nanoparticles (Figs. 3c and 3d). A further increase in temperature up to 60oC leads to the formation of hill-like blocks with a random orientation (Figs. 3e and 3f). 1109


Figure 4.

AFM images of sample S1: (a) 2D topography; (b) 3D topography.

Figure 5. AFM images of sample S8: (a) 2D topography; (b) 3D topography. More detailed information about MoO2 film surface morphology was obtained from AFM measurements. The typical AFM image of a molybdenum oxide film, obtained at a bath temperature of 20oC (sample S1), is shown in Fig. 4. Electrolytic molybdenum oxide film exhibits a grain-type surface morphology; however, from the 3D topography images in Figs. 1b, 2b and 4b it is evident that the film surface does not follow the pattern of the selenium and SnO2|glass layers. Tightly packed grains and randomly located clusters are clearly observed on the film surface. The same surface morphology as that shown in Fig. 4 was observed also for S4 and S7 samples. An AFM top view image of a MoO2 film deposited at 40oC (sample S8) is shown in Fig. 5. An increase in the electrolysis temperature has a distinct influence on the film morphology. Films consist of large agglomerates and clusters. Agglomerates whose size varied from 1 to 4 µm were formed of tightly bounded grains and the clusters are randomly distributed on the surface. The same morphological features as those shown in Fig. 5 were also observed for other S2 and S5 samples deposited at a 40oC temperature. 1110

The surface morphology of the films electrodeposited at 60oC was more complex in character. As mentioned above, surface macroscopic examination (Figs. 3e-3f) revealed hill- like blocks structures with an average width of 100 µm. AFM images (Figs. 6 and 7) show the development of morphology on the crest of these structural elements. A close inspection of these images (Figs. 6 and 7) revealed surface fragmentation. Molybdenum oxide films were damaged, showing a rippled morphology consisting of cavities, separate grains and channels. At the deposition potential of E = -1.0 V (Fig. 6), cavities approximately 2 µm width and 30 nm deep, and single particles 20 nm high and 200 nm width were observed on film surface. As the deposition potential decreased so too did the size of cavities and the agglomeration of particles (Figs. 6 and 7). The morphology, surface roughness and grain size parameters of molybdenum oxide films, determined from AFM measurements, are presented in Table 2. A careful adjustment of the deposition conditions allowed for the depositing of MoO2 films of a different morphology. No exact correlation between the deposition conditions and the particle size were found.


Table 2. Sample

MoO2 film surface morphology, roughness parameters and grain size. Morphology

S1 S4

Continuous

S7 S2

Thickness, nm

Ra, nm

Rq, nm

Grains Maximum height, nm

Average width, nm

100

16.5

26.0

200

560

460

8.2

14.2

65.5

543

600

6.0

8.9

93.2

351

190

7.0

10.3

98.5

883

480

10.0

13.9

80.0

800

S8

740

5.9

9.0

103.1

699

S3

380

8.6

17.4

41.6

280

S5

S6

Compact with needles

Hill-like

S9

960

3.1

5.2

25.8

216

1150

6.0

10.8

163.3

558

Figure 6. AFM images of sample S3: (a) 2D topography; (b) 3D topography. Film morphology depended primarily on the electrolysis temperature and changed from continuous to compact with needles embedded in the film matrix and finally to hill- like blocks of a random orientation. Throughout the remainder of the paper, these three types of morphologies would be refereed to as “continuous”, “compact with needles” and “hill-like”.

3.2. Structural studies

Fig. 8 shows the recorded XRD pattern of a SnO2|glass plate (Fig. 8, curve a) and the MoO2 films of a different morphology (Fig. 8 curves b-e). Irrespective of their morphology, all thin films do not show any well-defined peaks corresponding to MoO2 (Fig. 8 curves b-e), meanwhile the pattern at 2θ 26.4 as compared with that of SnO2|glass (Fig. 8 curve a) decreases. This fact indicates that MoO2 films are amorphous or low-crystalline, which means that if there are some crystallites, their size and/ or amount are rather small, and therefore beyond the measurement sensitivity. In order to describe the vibrational modes of the obtained MoO2 films, their ATR−FTIR spectra were analysed. The spectra of films with “continuous” (sample

S1), “compact with needles” (sample S2) and “hill- like” (sample S3) morphology are shown in Fig. 9. Only one spectrum of each film of a characteristic morphology is given, because no major differences were observed. Over the recent years, there have been reported numerous data on the vibrational properties of MoO2 obtained by different methods [15,31,38−40]. Hydrous molybdenum(IV) oxide films are characterized by two sharp absorption peaks at 1400 and 1600 cm-1 and a broad band at about 3400 cm-1. The peak at 1600 cm-1 is caused by H−O−H deformation vibrations, while the peak at 1400 cm-1 is due to the vibration of Mo−OH bond [31,38]. A broad band at about 3400 cm-1 results from the stretching mode of adsorbed water and the OH groups linked to the films. The absence of these characteristic peaks (Fig. 9) can be regarded as an indication of the non-hydrous nature of the MoO2 films. On the other hand, MoO2 is composed of distorted MoO2 octahedra joined by edges along the [100] direction and connected by corner-sharing into a threedimensional structure. In all spectra, three characteristic vibration modes at about 1014, 836, and 648 cm-1 were observed. The symmetric stretching modes of the double terminal 1111


Figure 7. AFM images of sample S6: (a) 2D topography; (b) 3D topography.

Figure 8. XRD

spectra of (a) SnO2|glass plate and samples of deposited MoO2 films: (b) S1; (c) S2; (d) S7; (e) S9.

Mo=O bond in MoO2 IR spectra appears at 985 cm-1 [39]. The terminal double bond shifts to higher frequencies by 29 cm-1. This shift in the spectra of the MoO2 films can be related to their amorphous nature. The peak at about 836 cm-1 results from asymmetric stretching vibrations of the O−Mo−O short-length bonds [40]. The features at 648 cm-1, have been assigned to the vibration mode of Mo−O−Mo bonds and are in a good agreement with those reported for electrolytic molybdenum(IV) oxide films deposited on a ITO surface [31]. A peak at about 694 cm-1 is characteristic of the asymmetric stretching vibrations of the O−Mo−O medium-length bonds [40]. This vibration was observed in the spectra of the films of “continuous” and “hill- like” morphologies (Fig. 9 curves a and c). The spectrum of the film of the “compact with needles” morphology shows a peak at 739 cm-1, which corresponds to the stretching vibration mode of the Mo−O bonds and is typical of the monoclinic rutile structure of MoO2. This suggests the possible presence of a very small fraction of crystalline MoO2 in these films.

3.3. Determination properties

of

optical

3.3.1. Optical properties of films of “continuous” morphology

Figure 9. ATR−FTIR

spectra of deposited MoO2 films of: (a) “continuous“; (b) “compact with needles” and (c) “hilllike” morphologies.

1112

As presented in Table 2, MoO2 films of “continuous” morphology are labelled as samples S1, S4 and S7. The film thickness was 100, 460 and 600 nm, respectively. Representative transmittance spectra of these films are presented in Fig. 10. The transmission spectra do not exhibit any sharp absorption edge characteristics of amorphous material, and confirm the results of the XRD analysis. As one can see from Fig. 10, the film thickness influences the shape of the spectrum. The spectrum of sample S1 shows an intensive transmittance minimum in the UV–VIS region and a shoulder in the range


films were taken from Tables 2 and 3, respectively. The refractive index of air (n0) equals 1. According to Eq. 3, the calculated SSL at λ = 500 nm was 3.06%, 0.97% and 0.57% for samples S1, S4 and S7, respectively. It is obvious that the thicker films scatter less light relative to the thinner ones. The optical absorption coefficient α was calculated by the following equation [44]: (4)

Figure 10. Transmittance spectra of samples: (a) S1; (b) S4; (c) S7. 480−630 nm. The position of the minimum depends slightly on the film thickness and shifts from 310 to 320 nm, while the shoulder for a thicker film disappears (Fig. 10 curves b and c). The transmittance of the thicker samples (S4 and S7) significantly decreases in the ultraviolet and visible region, whereas the spectra shift to longer wavelengths. In all spectra, transmittance approaches the maximum value at around λ = 800 nm and then slightly decreases. The refractive index n is an important parameter for optical material and their applications. The refractive indices of MoO2 films were measured at λ = 632.8 nm and are given in Table 3. The increase of n with MoO2 film thickness can be related to the higher packing density of the films. The values of the refractive indices (at λ = 633 nm) reported in literature for molybdenum oxide films prepared by different methods are 1.95–2.02 [41], 1.91−2.07 [42] and 2.01−2.18 [35]. The refractive indices (i.e., 1.85−2.00) for the MoO2 films of “continuous” morphology lie well within range of reported values. The optical transmittance is determined by the film quality and the surface roughness. The transmittance surface scattering loss (SSL) can be expressed as follows [43]: (3) where T0 is the transmittance of an ideally smooth surface; and n0 and n are the refractive indices for air and molybdenum oxide, respectively; and Rq, is the root mean square for roughness. In our calculations, the transmittance of an ideal smooth surface (T0) was assigned to be of 100%, the representative measured value of Rq and refractive index determined by ellipsometry, for each of the MoO2

where d is the sample thickness, cm; and T is transmittance. For all samples, the absorption coefficients were in the order of 104 cm-1 throughout the wavelength region studied. The extinction coefficient was calculated by the equation [33]: (5) where λ is wavelength, cm; and α is absorption coefficient, cm-1. In general, the coefficient k increases with increasing the wavelength of the incident light and approaches a nearly constant value in the near-infrared region (Fig. 11). The values of the extinction coefficient significantly decrease as the film thickness increases from 100 to 460 nm (samples S1 and S4). In addition, the extinction coefficients of the films are a very low in the spectral range between 300-600 nm. Some representative values of the extinction coefficient at λ = 500 nm are given in Table 3. Thus, based on the above analysis, we can conclude that a red shift and a decrease of transmittance result from the absorption and scattering of a rough surface. The spectral dependence α vs. f(hv) shows the high and the exponential absorption regions. In the high absorption region (α equals to 104 cm-1), absorption is characterized by the Tauc equation [33]: (6) where B is an inverse proportion to the width of the conduction band and the valence band tail; hv is the photon energy; and Eg is the optical band gap; r characterizes the transition type. The values of r are ½ and 2 for direct and indirect allowed transitions, respectively [45,46]. In order to check whether the optical data was a better fit to the direct or the indirect formula, the dependences (αhv)2 vs. hv and (αhv)1/2 vs. hv were plotted. In these 1113


plots, any direct or indirect transition should manifest itself as a straight line, close to fundamental absorption edge. As is shown in (Figs. 12a-12c), the experimental data fit better the straight line plot in the (αhv)2 vs. hv dependences, while the (αhv)1/2 vs. hv plots show no linear behaviour near the optical absorption edge. The value of the direct band gap was determined by extrapolating the straight-line portion to the zero absorption (α = 0). By fitting the experimental data Eg values to Eq. 6, the band edge width parameter (B) was calculated. The cutoff wavelength (λc) is the fundamental absorption edge which corresponds to electron

transitions from valence to conduction band and is related to the optical band gap by equation [47]: .

(7)

Table 3 lists the corresponding values of band gap energy; parameter B and the fundamental absorption edge for films of different thickness. The optical absorption coefficient just below the absorption edge shows an exponential variation with photon energy, indicating the presence of Urbach’s tail. In the exponential edge, absorption follows the Urbach rule [46]: (8)

Figure 11.

Spectral dependence of the extinction coefficient for samples: (a) S1; (b) S4; (c) S7.

where B is a constant; and EU is the Urbach energy. The Urbach energy is interpreted as a broadening of the intrinsic absorption edge due to disorder and is related to transitions from the extended valence band states to the localized states at the conduction band tail. According to Eq. 8, the value of Urbach’s energy can be calculated from the inverse slope of plots lnα vs. hv (Fig. 13). A graph was plotted for each film sample, and the calculated values of Urbach energy are listed in Table 3. A decrease of the band gap correlates to the change of the Urbach energy (Table 3). The change of EU does not directly correlate with film thickness; however,

Figure 12. Plots of (αhv)2 vs. hv for samples: (a) S1; (b) S4; (c) S7. Table 3. Sample

Optical characteristics of thin molybdenum oxide films of “continuous” morphology. Film thickness, nm

nλ

= 632.8

kλ=500

Eg, eV

B×105, cm1eV-1

Fundamental absorption edge, nm

*αλc×104, cm-1

Eu, eV

S1

100

1.85

0.68

2.53

5.94

490

4.21

0.29

S4

460

1.96

0.11

2.42

5.78

512

2.50

0.45

S7

600

2.00

0.11

2.38

4.35

518

1.39

0.37

*Absorption coefficient corresponding to fundamental absorption edge

1114


Figure 13. Plots of lnα vs. hν for: (a) S1; (b) S4; (c) S7.

respectively. For each of the samples the value of Rq and n were taken from Tables 2 and 4, respectively. The optical absorption and extinction coefficients, band gap energy, the fundamental absorption edge and the Urbach energy were calculated by fitting the spectral data to the equations discussed above. The spectral dependences of the extinction coefficient k vs. hv do not show any shape-dependant changes; however, its values are lower (Tables 3 and 4). As shown in Fig. 15, the MoO2 films of “compact with needles” morphology exhibit a direct band gap allowed transition. An increase of film thickness results in a systematic band gap shift from the green region and extending down to the yellow and then to the red spectral region. The values of EU are higher than those determined for films of “continuous” morphology and increase with film thickness. Optical constants characterizing these films are summarized in Table 4.

3.3.3. Optical properties morphology

Figure 14.

Transmittance spectra of samples: (a) S2; (b) S5; (c) S8.

it indicates structural disorder in thicker (samples S4 and S7) MoO2 films’ network as compared with thinner films. This disorder leads to an increase of point defect concentration and consequently to a greater number of possible bands to tail and tail to tail transitions [48].

3.3.2. Optical properties of films of “compact with needles” morphology

Samples of “compact with needles” morphology are labelled as S2, S5 and S8 (Table 2). The transmittance spectra of these samples are shown in Fig. 14. The MoO2 films of a “compact with needles” morphology have a lower transmittance in the ultraviolet and visible regions and approach a transmittance maxima in a longer wavelengths region as compared with films of “continuous” morphology (Figs. 10 and 14). The measured refractive indices of these films are in the range of 1.98−2.06 (Table 4). The surface scattering loss at λ = 500 nm was calculated according to Eq. 3 and was found to be 1.59%, 2.88% and 1.43% for samples S2, S5 and S8,

of films

of

“hill-like”

Fig. 16 shows the transmittance spectra of molybdenum oxide films of “hill- like” morphology. The optical behaviour of films of “hill- like” morphology significantly differs from that of the other films studied. Various structural defects, such as cavities and channels on film surface, are the sources of scattering sites which contribute to a loss in transmittance. In addition, the extinction coefficients of thicker films (samples S6 and S9) are very low and close to zero between 300 and 700 nm. The extinction coefficient includes contribution from the absorption and the light scattering of grains. In the case of films of “hill-like” morphology, the low extinction coefficients do not arise from the intrinsic absorption properties of film, but rather from the light scattering of complex film surface. The observed behaviour is interesting but difficult to interpret. Some representative values of the extinction coefficient at λ = 600 nm for each film are given in Table 5. The MoO2 films of “hill- like” morphology possess the lowest refractive indices among all investigated films (Tables 3, 4 and 5). The low refractive index supports a loose packing structure and low packing density of film. Based on the transmittance spectra, the optical constants of films were determined and are presented in Table 5. The Urbach energy changes inversely with the band gap energy. The high values of the Urbach energy correlate with complex film surface morphology and can be ascribed to two factors. The first comes from morphological defects and the second results from 1115


Table 4. Optical characteristics of thin molybdenum oxide films of “compact with needles” morphology. Sample

Film thickness, nm

nλ

= 632.8

kλ = 500

Eg, eV

B×105, cm1eV-1


*αλc×104, cm-1

Eu, eV

S2

190

1.98

0.23

2.35

4.56

527

3.70

0.45

S5

480

1.98

0.12

2.08

4.42

596

1.50

0.50

S8

740

2.06

0.05

1.99

4.13

623

1.39

0.59


Table 5. Optical characteristics of thin molybdenum oxide films of “hill- like” morphology. Sample

Film thickness, nm

nλ

= 632.8

kλ = 600

Eg, eV

B×105, cm1eV-1


*αλc×104, cm-1

Eu, eV

S3

380

1.77

0.17

2.39

4.44

518

2.04

0.66

S6

960

1.92

0.05

1.99

3.25

623

1.54

0.77

S9

1150

1.98

0.03

1.98

3.76

626

1.81

0.79


Figure 15. Plots of (αhv)2 vs. hv for samples: (a) S2; (b) S5; (c) S8. a tendency to convert into defects, which produce localized states. These, in turn, increase the density of localized states in the band gap structure. As a result, both a decrease in the band gap and a broadening of the Urbach tail takes place.

4. Conclusions

Figure 16. Transmittance spectra of samples: (a) S3; (b) S6; (c) S9. a structural disorder in the amorphous film matrix as proposed by the Mott and Davies [49] model. The insufficient number of atoms deposited in amorphous films leads to the formation of both saturated and unsaturated bonds [50]. The unsaturated bonds have 1116

The amorphous MoO2 films were electrodeposited on selenium pre-deposited SnO2|glass plates. The film morphology depended on the electrolysis temperature and changed from “continuous” to “compact with needles” and then to “hill- like” blocks of random orientation. In the ATR-FTIR spectra the basic characteristic peaks corresponding to different vibrational modes of molybdenum(IV) oxide were observed at about 1014, 836, and 648 cm-1. Optical studies showed that MoO2 films had a high optical absorption coefficient and the absorption mechanism was due to direct band to band transitions. The extinction coefficient k, refractive index n, absorption


coefficient α and the band gap energy Eg depended both on the film thickness and morphology. Irrespectively of MoO2 film morphology, the red shift of the fundamental absorption edge with an increase in the film thickness was observed for all films studied. A decrease of band gap correlated with an increase of the Urbach energy EU. The observed absorption behaviour was related with the structural disorder defects in the

MoO2 film network. The degree of disorder increased with an increase of film thickness, which was confirmed by both the electrolysis temperature and the deposition potential. The MoO2 films of “hill- like” morphology had the lowest optical band gap energy Eg values as compared with the other films studied, and they lay in the red spectral region.

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