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Effect of lithium nanosandwiching on the structural, optical and dielectric performance of MoO3
Physica E: Low-dimensional Systems and Nanostructures 114 (2019) 113569
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Effect of lithium nanosandwiching on the structural, optical and dielectric performance of MoO3
T
S.E. Al Garnia,b, A.F. Qasrawic,d,∗ a
Physics Department, Faculty of Science - Al Faisaliah, King Abdulaziz University, Jeddah, Saudi Arabia Department of Physics, Faculty of Science, University of Jeddah, Jeddah, Saudi Arabia c Department of Physics, Arab American University, Jenin, Palestine d Group of Physics, Faculty of Engineering, Atilim University, 06836, Ankara, Turkey b
A R T I C LE I N FO
A B S T R A C T
Keywords: Li/MoO3 X-ray diffraction Nanosandwiching Optical conduction Dielectric
In this article, we discuss the effects of lithium nanosheets on the structural, optical, dielectric and optical conductivity parameters of the MoO3 films. The nanosandwiching of Li layers between two layers of MoO3 of thicknesses larger than 20 nm induced the crystallization process of the amorphous MoO3. Namely, MoO3 thin films that are nanosandwiched with Li sheets of thicknesses larger than 50 nm, exhibit structural phase transitions from hexagonal to monoclinic and reveals larger crystallite sizes. The possible formation of Li2O at the MoO3/Li/MoO3 interfaces is simulated and discussed. Optically, the Li nanosandwiching is observed to enhance the light absorbability by 11.0 times at 2.0 eV and successfully engineered the energy bands gap in the range of 3.05–0.45 eV. It also enhances the dielectric performance. In addition, relatively thick layers of lithium (200 nm) succeeds in converting the conductivity type from n-to p-type. The modeling of the dielectric spectra in accordance with the Drude- Lorentz approach have shown that the presence of Li in the structure of MoO3 significantly increases the drift mobility values of electrons from 5.86 to 11.40 cm2/V. The plasmon frequency range for this system varies in the frequency domain of 0.32–5.94 GHz. The features of MoO3/Li/MoO3 interfaces make them attractive for thin film transistor technology as optical receivers being promising for use in optical communications.
1. Introduction Molybdenum Trioxide (MoO3) is an attractive marital for it wide applications. It can be used in the fabrication of gas sensor [1–3], as supercapacitors [4,5] and as transparent electrodes [11–16]. Systematic gas sensing measurements has shown that the flower-like MoO3 shows excellent gas sensing performances toward ethanol. Liquid-exfoliated MoO3 is used to prepare supercapacitor electrodes that have relatively low capacitance (∼2 F g−1 at 10 mV/s) because of the low electrical conductivity of the MoO3. However, addition of carbon nanotubes beyond the percolation threshold yielded a 100-fold increase in capacitance values [4,5]. MoO3 which was synthesized by the thermal decomposition of ammonium molybdate displayed specific capacitance of value of 148.9 F g-1 at 0.7 A g-1 [6]. In the same context, MoO3 which is used as material in lithium batteries exhibits exceptional high specific capacity that nominates it as high performance anode materials [7]. Different methods are used to prepare MoO3. Polycrystalline MoO3 thin films have been prepared by oxidation at high temperature of
∗
molybdenum compound layers deposited by chemical vapor deposition (CVD) from molybdenum hexacarbonyl Mo(CO)6 [8]. In addition, the MoO3 are also prepared by the physical vapor deposition technique [9]. The as-deposited amorphous MoO3 films were reported to crystallize into a stable orthorhombic phase on annealing in the air at 350 °C. For these films, the feasibility of using Mg ions in polycrystalline MoO3 thin films as intercalants, was explored [9]. These Mg ions which were intercalated Galvano statically at a constant current density of 166 μA/ cm2 revealed lattice expansion along b axes by 2.35% for x = 0.1 and 3.17% for x = 0.3 [9]. Moreover, the physically prepared films by the PVD technique are shown as an efficient reverse-electron recombination barrier layer (RBL) at the fluorine doped tin oxide (FTO)/titanium dioxide (TiO2) interfaces in dye sensitized solar cells (DSSCs). These films displayed an average optical transmittance of ∼77% in a spectral range of 350–800 nm with bandgap value of ∼3.1 eV [10]. MoO3/Metal/MoO3 interfaces are also used as transparent layer electrodes [11–16]. Mainly, MoO3/Ag/MoO3 stacks are investigated for utilization of transparent cathodes in organic light-emitting devices
Corresponding author. Department of Physics, Arab American University, Jenin, Palestine. E-mail addresses: [email protected], [email protected] (A.F. Qasrawi).
https://doi.org/10.1016/j.physe.2019.113569 Received 29 March 2019; Received in revised form 26 April 2019; Accepted 17 May 2019 Available online 28 May 2019 1386-9477/ © 2019 Elsevier B.V. All rights reserved.
Physica E: Low-dimensional Systems and Nanostructures 114 (2019) 113569
S.E. Al Garni and A.F. Qasrawi
exclusion of the glassy background. As appears in Fig. 1 (b), increasing the thickness of the lithium nanosheet further to 100 nm (MLM-100) and to 200 nm (MLM-200), forces the appearance of sharp peaks. While the most intensive peak of the MLM-100 samples appears at diffraction angle of 2θ = 21.40o , the most intensive peak of MLM-200 sample appears at diffraction angles of 2θ = 25.20o. The peaks which appeared at 2θ = 21.40o in the XRD patterns of MLM-100 split into two peaks (Fig. 1 (b)) when the Li thickness is increased to 200 nm. The split peaks are centered at 21.40 and 21.80°. To understand the physics beyond this interesting behavior of XRD of the samples under investigation, complete software analysis using “TREOR 92”, “Crystdiff” and “Crystal Maker” software packages are employed. The structural analysis on the MLM samples have shown that the resulting films comprise two phases in its structure. Although the MLM-0 samples show amorphous nature of structure, the MLM-20 displayed weak XRD patterns that are hardly assigned to the hexagonal structural phase of MoO3. Increasing the thickness of lithium to 50 nm, causes a disappearance of the main peak which was observed at 2θ = 39.35o for the MLM-20 and forces the appearance of new diffraction patterns. The diffraction angles at 2θ = 39.35o are consistent with the published literature data [17] and is assigned to the hexagonal phase of MoO3. The peaks which appeared in the XRD of MLM-50 samples are assigned to both hexagonal and monoclinic structural phases of molybdenum oxide. The picture of the lithium induced crystallography of molybdenum oxide become brighter as the lithium nanosheet increases to 100 and 200 nm. Particularly, the amorphous sea which is observed for the MLM-0, MLM-20 and MLM-50 disappears and clearly noticeable crystal structure is observable (Fig. 1 (b)). The indexing of the XRD patterns for the samples which contain Li nanosheets of thickness of 100 and 200 nm reveal that the material is mostly composed of monoclinic MoO3 as major phase and hexagonal MoO3 as minor phase. The calculated lattice parameters for the monoclinic unit cell of MoO3 are found to be a = 13.885, b = 3.696, c = 3.963Å and θ = 90.450 . Those which are calculated for hexagonal MoO3 are a = b = 10.522 and c = 14.888Å . The lattice parameters values are consistent with literature data [17,18]. The observed and the analyzed XRD patterns for the stacked layers of MoO3 indicate that the presence of Li in the structure of MoO3 forces the crystallization process of the material. The more thick the Li layer, the more dominant the monoclinic phase over that of hexagonal. Calculations of the crystallite size (D ) with the help of modified Scherrer equation [17] from the maximum peak broadening (β ) revealed D values of 52 nm for the MLM-20 samples and D value of 56 nm for MLM-100 and MLM 200. The strain which exhibits value of 2.2× 10−3 for the hexagonal phase that is dominant in MLM-20, became 3.5× 10−3 when the monoclinic phase dominates in the MLM-100 samples and reduces to 3.0× 10−3 as the lithium layer thickness increases to 200 nm. These values strongly affect the dislocation density (δ ). The dislocation density which exhibits value of 6.0× 1010 cm-2 in the MLM20 displays values of 6.75× 1010 cm-2 and 5.8× 1010 cm-2 in the MLM100 and MLM-200 films, respectively. Previous studies on the lithium insertion/extraction reactions on MoO3 samples with different crystallinity have shown that the insertion of Li into pristine MoO3 Leads to large structural distortions without altering the electrochemical performance of MoO3 [19]. Particularly, the insertion of solvated Li ions increase the interlayer spacing between the Mo-O octahedron layers by 5.5 Å . This increase is followed by a shrunk in the interlayer from 12.40 to 11.15. Å. In an attempt to explain the strongly induced crystallization which is achieved by the Li nanosandwiching, we turn the attention to the bonding mechanism and ionic radiuses. The ionic radius of Mo+6 being 62 p.m. [20] is very close to the value being 60 p.m. which is reported for Li+1 [21]. These values indicate that the lithium ion can replace sites of Mo+6. The replacement of Mo with Li leads to the formation of Li2O bonds. The possibility of formation of this phase at the interface region is simulated by the “Crystalmaker” software packages. The results of the simulator are shown in Fig. 2 (a) and (b). For the cubic unit
(OLED) [10]. The interface performed as the transparent anode in the organic solar cells [11,12]. In addition, MoO3/Au/MoO3 is designed for use as transparent electrodes in green OLEDs [13]. They achieved highly transparent electrode features with low resistivity [14], and semi-transparent contact. This structure yielded a power conversion efficiency of 5.5% with an average transparency of 26% [15]. Furthermore, MoO3/Al/MoO3 displayed high optical transmittance (approximately 90%) in visible light region [16]. In the light of these interesting features of MoO3, here in this article, we aim to improve the optical and electrical performance of the MOO3 prior to optoelectronic technology applications. Particularly, The MoO3 films are deposited onto glass substrates and are used as substrates to deposited Li films of thicknesses in the range of 20–200 nm. The resulting multilayers will be coated with another MoO3 film to encapsulate the Li thin layer. The structure will then by subjected to optical analysis to reveal its optical and dielectric performances. These parameters are the keys to use this interface in visible light communication technology. Particularly, based on the optical conductivity which is estimated from the imaginary part of the dielectric constant, the optical conduction parameters presented by the drift mobility, plasmon frequency, and free carrier concentration will be investigated at frequencies that suits visible light communications. 2. Experimental details MoO3 thin films are grown onto cleaned glass substrates by the thermal deposition technique under vacuum pressure of 10-5 mbar using VCM-600 evaporator. The source material is high purity (99.999%) MoO3 nano-powders (Alfa-Aeser). The film thicknesses are monitored by an in situ thickness monitor (Inficon STM-2). The first layer is 500 nm thick. This layer is coated with Li+1 nanosheets of thicknesses of 20–200 nm. The prepared films are given the codes MLM0, MLM-20, MLM-50, MLM-100 and MLM-200 for MoO3 films sandwiched with 0, 20, 50, 100 and 200 nm Li layer, respectively. The produced MoO3/Li films are recoated with another MoO3 layer of thickness of 500 nm. The fabricated thin films are studied by means of X-ray diffraction (XRD) technique using Miniflex 600 XRD unit, and by the ultraviolet–visible light spectrophotometry technique using Thermo-scientific Evolution 300 spectrophotometer. The structural analysis and computer simulations are carried out with the help of “TREOR 92”, “Crystdiff” and “Crystalmaker” software packages. 3. Results and discussion Owing to its importance as nonlinear optical material, the two nanosandwiched stacked layers of molybdenum oxide are subjected to Xray diffraction (XRD) analysis. The resulting diffraction patterns are shown in Fig. 1 (a) and (b). As seen from Fig. 1 (a) for two stacked layers of MoO3/MoO3 (MLM-0) no intensive peaks are observable. For MoO3/Li/MoO3 films sandwiched with lithium nanosheet of thickness of 20 (MLM-20) and 50 nm (MLM-50), no remarkable intensive peak is observable but some minor peaks which are hardly detectable after
Fig. 1. The X-ray diffraction patterns for MoO3 nanosandwiched with lithium layer of thicknesses of (a) 00 and 20 and 50 nm and (b) 100 and 200 nm. 2
Physica E: Low-dimensional Systems and Nanostructures 114 (2019) 113569
S.E. Al Garni and A.F. Qasrawi
Fig. 1. (continued)
to 200 nm. The secondary peak which appears at 39.04° in the simulated XRD also appears at 38.61° in the experimentally measured one. If one assumes crystallites of size of 56 nm and microstrain of 0.34% like the experimentally determined values, new peaks appear at 19.24o and 27.33o . The simulation with the crystallite size value of 56 nm and microstrain value of 0.34% is excluded as no evidence of the existence of such patterns was observed in the experimental XRD patterns. In the light of these analyses it is possible to assign the induced crystallization process to the exchange in the bonding mechanism between MoO3 and Li2O. The effects of the sandwiching of lithium nanosheets on the optical performance of MoO3 thin films are readable from the measured transmittance (T) and reflectance (R) spectra which are employed to calculate the absorption coefficient (α ) following the previously mentioned procedures [22]. The transmittance, reflectance and absorption coefficient spectra for the studied samples are displayed in Fig. 3 (a), (b) and (c), respectively. As seen from Fig. 3 (a), for the samples which are nanosandwiched with lithium sheets of thicknesses of 20, 50 and 100 nm, the effect of Li layer appears in the incident wavelength range
cell which is shown in Fig. 2 (a), the lattice parameter is 4.61 Å (crystallography open database code: 1514086). The average bond length which is executed over 32 visible bonds by the simulator is found to be 199.6 p.m. This value is larger than that of Mo-O being 169 p.m. [22] indicating the weaker van der Waals force of attraction between Li-O compared to Mo-O. The weaker forces lead to an enlargement between the interlayer spacing. Similar behavior was also mentioned for MoS2/Bi19Cl3S27 interfaces [23]. The smaller bond length of Mo-O compared to Mo-S and the weaker van der Waals attraction forces is believed to cause an enlargement in the interlayer spacing and a shift in the XRD patterns. In accordance with the simulator results, the formation of Li2O should lead to X-ray diffraction patterns which are shown in Fig. 2 (b). The simulator was run assuming a microstrain of 0.1% (1.0× 10−3 ) and grain size of ∼20 nm for Li2O. The position of the peaks is also demonstrated in the same figure. The strongest reflection of Li2O appears at 2θ=33.64o in the (111) direction. This peak appears at 2θ=33.45o in the experimentally determined XRD patterns which appear in Fig. 1 (b) for samples nanosandwiched with Li nanosheets of thickness of 100 nm and at 2θ=33.35o when the Li thickness is increased
Fig. 2. (a) the schematic of the Li2O unit cells and (b) the simulated XRD patterns for Li2O films with grain sizes of 20 nm and strain of 0.1%. 3
Physica E: Low-dimensional Systems and Nanostructures 114 (2019) 113569
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increasing with increasing incident light wavelengths reaching a maxima at 2.0 eV. The energy where the maxima are observed does not depend on the Li layer thickness. However, the light absorbability in the studied range of light increases with increasing Li thickness. Namely, Rλ increases from 3.1 to 6.3 and reaches 11.0 as the lithium nanosheet thickness increases from 50 to 100 and reaches 200 nm, respectively. No significant difference between the light absorbability of MLM-20 and MLM-50 was observed. On the other hand, the plotting of Tauc's equation [22], ((αE )1/2 ∝ (E − Eg ) ), which is best fitted for indirect allowed electronic transitions as demonstrated in Fig. 4 (b) reveal energy band gap values of 3.05, 1.88, 1.78, 1.1 and 0.47 eV for the molybdenum oxide films nanosandwiched with Li layers of thicknesses of 0, 20, 50, 100 and 200 nm, respectively. Wide range of band gap engineering is obtained via Li nanosandwiching. The addition of relatively thin layers of Li is sufficient to alter the band gap from the blue light limit to the IR light limit. Engineering of the energy band gap via nanosandwiching technique was previously observed for Ga2S3/In/Ga2S3 thin films. The variation of the indium layer thickness in the range of 20–200 nm succeeds in lowering the energy band gap from 3.70 to 1.40 eV. It also enhances the light absorbability by 54.6 times [24]. Similarly, simulation studies on the MoO3/Ag/MoO3 (MAM) nanostructure as transparent anode [14] has shown that the light absorbance of thin organic solar cells is enhanced significantly and that the MAM structure could replace ITO in solar cells. The reason for the efficient performance of the MAM as light absorber is the propagating surface plasmon polaritons. In addition, it is observed that the MoOx (40 nm)/ Au (10 nm)/MoOx (40 nm) structures [16] reveal much higher transmission in the green-red region and leads to lower sheet resistance than indium tin oxide (ITO). This nanostructure is experimentally tested and verified to be more suitable as transparent electrodes for organic light emitting diodes. The energy band gap of Li2O is ∼7.99 eV [25] which is much higher than our measuring ability range. Since the observed values are much less than this value, the redshift in the energy band gap as a result of Li participation could be assigned to reasons other than the formation of Li2O like the introduction of unoccupied surfaces states in the gap or due to the introduction of occupied surface states near the conductionband minimum [26]. Band gap narrowing in Mg doped ZnO was attributed to the formation of impurity bands in the vicinity of valence band from oxygen vacancies [27]. It could also be assigned to the valence/conduction energy bands bending and image force lowering [28].
Fig. 3. The optical (a) transmittance, (b) reflectance and (c) absorption coefficient spectra for the MoO3/Li/MoO3 samples.
of 366–670 nm only. Li has no effect on the optical transmittance of MoO3 above 670 nm. In the effective range of spectra, the transmittance decreases with increasing Li layer thickness and also redshifts. The MLM-200 sample exhibit lower T values in the range of 366–1100 nm. On the other hand, the reflectance spectra which are illustrated in Fig. 3 (b) decreases with increasing Li layer thickness and the number of the observed reflection peaks also decreases from three peaks to one peak as the thickness of the Li layer reaches 100 nm and no peak appears when the Li slab thickness is 200 nm. The absorption coefficient spectra for the MoO3/Li/MoO3 film are shown in Fig. 3 (c). As seen from the figure, while the α values of MLM0 sharply decrease with decreasing incident light wavelengths reaching near zero value at 2.80 eV, the absorption coefficient values for the Li nanosandwiched samples never reach zero at that energy. It approaches zero value in the IR region (E < 1.60 eV). Increasing the lithium nanosheets thickness decreases the absorption coefficient value in the visible region of light and increases it in the IR range. The effect of Li on the light absorbability is pictured in Fig. 4 (a). The figure represents the light absorbability (Rλ = αMLM − x / αMLM − 0 ) in the incident light region of 2.45–1.33 eV. This region is the region where the absorption coefficient shifts up with increasing Li layer thickness. The light absorbability start
Fig. 4. (a) the light absorbability and (b) the Tauc's equation fitting for the MoO3/Li/MoO3 samples. 4
Physica E: Low-dimensional Systems and Nanostructures 114 (2019) 113569
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at 1.60 eV was also attributed to the electronic transitions between the d yz and d xy energy levels that are located at 1.60 eV [32]. We believe that the redshift in the position of the peaks of the dielectric spectra that resulted from the participation of the Li nanosheets in the structure of MoO3 is ascribed to the same reasons that caused the redshifts in the energy band gap as we mentioned in the preceding paragraph. The imaginary part of the dielectric spectra for MLM-0 and for MLM-20, MLM-50, MLM-100 and MLM-200 samples are illustrated in Fig. 5 (b) and (c), respectively. The values of εim increase with increasing incident light energy following a sharp slope above 3.0 eV. The slope of variation of the εim − E dependence becomes smoother as the thickness of the Li layer increases. To give significance to the observed behavior of the imaginary part of the dielectric constant we turn the attention to the relation between this parameter and the optical conductivity (σ (w ) ). The optical conductivity is related the imaginary part through the relation, σ (w ) = εim w /(4π ) with w being the angular frequency [22,24]. Obtaining information about the optical conductivity parameters like the free charge carrier density (n ), drift mobility ( μ ), electron-plasmon reduced frequency (wei ), scattering time (τi ) and plasmon frequency (wpi ) with the help of Drude-Lorentz approach for frequency dependent conductivity allow guessing the performance of the studied samples as optical signal receivers. The imaginary part of the dielectric constant and the plasmon frequency is given by the equations,
Fig. 5. (a) The real part of the dielectric constant and (b) the optical conductivity for the MoO3/Li/MoO3 samples. The blue circles of (b) shows the computational results of the conductivity spectra obtained by Eqn. (1).
However, as the MLM-0, MLM-20, MLM-50 and MLM-100 are n-type semiconductor (tested by hot probe technique) and MLM-200 is p-type semiconductor whose work function is larger than 6.7 eV (electron affinity (qχ =6.7 eV [22]) and as the work function of lithium metal is 2.6 eV [29], the Li/MLM-x (0 ≤ x ≤ 100nm ) MoO3 forms an ohmic contact (band bending is not expected) and Li/MLM-200 creates a Schottky type of interface. The barrier height for the Li/MLM-200 (qφ = Eg + qχ − ϕm ) is larger than 4.55 eV. This is extremely deep barriers that causes large band bending which explains the abnormal decrease in the value of the band gap from 3.05 to 0.45 eV. Fig. 5 (a) and (b) displays the real (εr ) and imaginary (εim ) parts of the dielectric constant being calculated by the previously described methods [22,24] from the measured transmittance and reflectance spectra. As seen from Fig. 5 (a), the real part of the dielectric constant of the two stacked layers of MoO3 without participation of lithium nanosheets display four maxima at 3.79, 3.27, 2.54 and 1.60 eV. While the first peak (3.79 eV) disappears upon insertion of Li nanosheet of thicknesses of 20 nm and 50 nm, the other remaining three peaks redshifts to 3.17, 2.33 and 1.49 eV and to 2.76, 2.01 and 1.28 eV, respectively. Further increase in the thickness of Li to 100 nm causes the disappearance of one more peaks and shifts the peaks to 2.79 and 1.28 eV. Consistently, increasing the Li layers thickness to 200 nm shifts the peaks to 2.15 and 1.20 eV, respectively. In addition to the observed redshifts in the positions of the critical points of the dielectric spectra, the value of the dielectric constant also decreases with increasing Li layer thickness. In our previous investigations we have observed that the dielectric spectra of one layer of MoO3 reveal peaks in the dielectric spectra at 3.62 and at 2.72 eV [22]. These two peaks are comparable with those we found here for two stacked layers of MoO3 as 3.79 and 2.54 eV. The high energy peak was previously assigned to the localization (quantization) of charge carriers in the individual nanocrystals [30]. The peak which appeared near 2.54 eV was previously ascribed to the Mo+5 penta-coordinated [MoO5]5+ [31]. The peak which appeared
k
εim =
∑
((wei2 i=1
wpi2 w − w 2)2 + w 2τi−2)
(1)
and,
wpi =
4πne 2 , respectively m∗
(2)
In Eqn. (2) m∗ is the effective mass of electrons or holes in MoO3 ∗ ∗ = 1.28mo [33])). The reduced effec= 0.6mo [22]) and in Li (mLi (mMoO 3 tive mass for samples which are nanosandwiched with Li layers is cal2 1 ∗ + m∗ )−1=0.409mo . culated from the equation, mMLM − x ≠ 0 = ( m∗ Li
MoO3
Running the series for k = 4 was sufficient to reproduce the experimental data. The results of the computational fittings are illustrated in Fig. 5 (b) and (c) by the blue colored circles. The computed optical conductivity parameters for the studied samples are also shown in Table 1. As the table suggests for the linearly connected four oscillators of one particular sample, while the scattering time (τi ) decreases with increasing numbers of oscillators (i ), the reduced frequency (wei ) increases. This increase is accompanied with increase in the free carrier density and increase in the value of plasmon frequency and decrease in the values of drift mobility. The lower the incident photon energy, the more mobile the free charge carriers. As the highest drift mobility corresponds to the first oscillator (i = 1) which exhibit μ values of 5.86 cm2 / Vs at reduced frequency of 2.25 × 1015 Hz and free electron concentration of 7.0 × 1017 cm−3, it will play the main roles when performing as light resonator. The insertion of lithium nanosheets of thicknesses of 20, 50 and 100 nm increased the drift mobility to 10.7, 10.7 and 11.4 cm2 / Vs , respectively. Accordingly, the free electron
Table 1 Optical conductivity parameters for the MoO3 stacked layers nanosandwiched with Li. MLM-0 n-type
τi (fs ) wei(x1015Hz) n (x 1017 cm-3) μ(
cm2 ) Vs
wpei (GHz)
MLM-20 n-type
MLM-50 n-type
MLM-100 n-type
MLM-200 p-type
1
2
3
4
1
2
3
4
1234
1
2
3
4
1
2
3
4
2.00 2.25 7.00 5.86
1.50 5.00 1.00 4.39
1.00 5.50 1.00 2.93
0.85 6.30 600 2.49
1.50 2.50 0.70 10.7
1.50 5.00 1.00 10.7
0.60 5.00 50.0 4.27
0.60 6.50 120 4.27
1.60 2.25 0.90 10.7
1.50 0.90 0.45 4.5 4.5 6.50 1.00 20.0 200 10.7 6.4 3.2
1.60 2.25 3.00 11.4
0.70 3.30 3.00 4.98
0.60 4.50 35.0 4.27
0.52 6.75 130 3.70
0.60 1.70 9.00 4.27
0.51 3.20 15.0 3.63
0.47 4.55 25.0 3.34
0.40 6.80 120 2.84
0.64
0.24
0.24
5.94
0.32
0.38
2.67
4.14
0.36 0.38 1.69 5.35
0.66
0.66
2.24
4.31
1.13
1.46
1.89
4.14
5
Physica E: Low-dimensional Systems and Nanostructures 114 (2019) 113569
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concentration decreased to 7.0 × 1016 cm−3 , 9.0 × 1016cm−3 and 3.0 × 1017 cm−3 , respectively. The later parameter alters the values of plasmon frequency. The decrease in the free electron concentration upon Li nanosandwiching is ascribed to the electron hole recombination at the MoO3/Li interface to reach the equilibrium Fermi levels in the ohmic type interfaces [22,33]. Increasing the thickness of Li layers to 200 nm, increased the number of free charge carriers to 9.0 × 1017cm−3 and lowers the drift mobility values from 11.4 to 4.27 cm2 / Vs , respectively. In accordance with the hot probe technique test, the values being 9.0 × 1017cm−3 is accounted for holes rather than electrons. Thus, the mobility value being 4.27 cm2 / Vs is also regarded as holes drift mobility which is usually less than electrons drift mobility. In general, except for the sample which is nanosandwiched with lithium of thickness of 200 nm, the range of plasmon frequency is not remarkably altered. The MLM-200 samples displayed a larger plasmon frequency value for the first oscillator compared to other MLM samples.
[2] H.M.M. Arachchige, E. Comini, D. Zappa, G. Sberveglieri, Gas Sensing Properties of MoO3 vol. 1, Multidisciplinary Digital Publishing Institute Proceedings, 2017, p. 449 H.M. Arachchige, E. Comini, D. Zappa, G. Sberveglieri, Gas Sensing Properties of MoO3, p. 449. [3] Y. Liu, W. Zeng, J. Mater. Sci. Mater. Electron. 27 (2016) 12996–13001. [4] D. Hanlon, C. Backes, T.M. Higgins, M. Hughes, A. O'Neill, P. King, N. McEvoy, G.S. Duesberg, B. Mendoza Sanchez, H. Pettersson, Chem. Mater. 26 (2014) 1751–1763. [5] S. Wang, K. Dou, Y. Dong, Y. Zou, H. Zeng, Int. J. Nanomanufacturing 12 (2016) 404–414. [6] X. Zhou, L. Ma, X. Xu, Z. Feng, Adv. Mater. Res. 1008 (2014) 361–364. [7] Q. Xia, H. Zhao, Z. Du, C. Gao, Z. Zeng, Z. Zhang, T. Zhang, Facile synthesis of MoO3 nanobelts with carbon dispersed structure and its application as anode of lithium ion batteries, ECS Meeting Abstracts, 2015 580-580. [8] M.R. Velazco, W. McDonnell, A.W. Smith, Johnson Matthey 61 (2017) 5. [9] T.S. Sian, G.B. Reddy, S.M. Shivaprasad, Jpn. J. Appl. Phys. 43 (2004) 6248. [10] A. Ashok, S. Vijayaraghavan, S.V. Nair, M. Shanmugam, RSC Adv. 7 (2017) 48853–48860. [11] Y.C. Kim, S.J. Lee, H. Jung, B.E. Park, H. Kim, W. Lee, J.M. Myoung, RSC Adv. 5 (2015) 65094–65099. [12] L. Hrostea, M. Boclinca, M. Socol, L. Leontie, A. Stanculescu, M. Girtan, Sol. Energy 146 (2017) 464–469. [13] B. Tian, G. Williams, D. Ban, H. Aziz, J. Appl. Phys. 110 (2011) 104507. [14] Y. Zhang, Y. Cui, T. Ji, Y. Hao, F. Zhu, IEEE Photonics J 9 (2017) 1–7. [15] X. Tian, Y. Zhang, Y. Hao, Y. Cui, W. Wang, F. Shi, H. Wang, B. Wei, W. Huang, J. Nanophotonics 9 (2015) 093043-093043. [16] M. Kim, C. Lim, D. Jeong, H.S. Nam, J. Kim, J. Lee, Org. Electron. 36 (2016) 61–67. [17] A. Chithambararaj, A.C. Bose, Beilstein J. Nanotechnol. 2 (2011) 585. [18] S. Sood, Polymorphism Control in Nanostructured Metal Oxides, PhD Diss, State University of New York at Stony Brook, 2014. [19] T. Tsumura, M. Inagaki, Solid State Ionics 104 (1997) 183–189. [20] E. Sindhuja, K. Ravichandran, Mater. Res. Bull. 103 (2018) 299–308. [21] M. Salah, S. Azizi, A. Boukhachem, C. Khaldi, M. Amlouk, J. Lamloumi, J. Mater. Sci. 52 (2017) 10439–10454. [22] S.E. Al Garni, A.F. Qasrawi, Physica E 105 (2019) 162–167. [23] Z. Wu, Y. Liu, S. Zhang, Z. Huang, Q. Jiang, T. Zhou, J. Hu, J. Mater. Chem. A 6 (2018) 21404–21409. [24] E.O. Nazzal, A.F. Qasrawi, S.R. Alharbi, Plasmonics 13 (2018) 1049–1056. [25] M.M. Islam, T. Bredow, C. Minot, J. Phys. Chem. B 110 (2006) 9413–9420. [26] A.S. Barnard, S.P. Russo, I.K. Snook, Phys. Rev. B 68 (2003) 235407. [27] Z. Quan, X. Liu, Y. Qi, Z. Song, S. Qi, G. Zhou, X. Xu, Appl. Surf. Sci. 399 (2017) 751–757. [28] S.N. Mohammad, J. Appl. Phys. 95 (2004) 4856–4865. [29] S. Schuld, R. Hausbrand, M. Fingerle, W. Jaegermann, K.M. Weitzel, Adv. Energy Mater. 8 (2018) 1703411. [30] I. Navas, R. Vinodkumar, V.M. Pillai, Appl. Phys. A 103 (2011) 373. [31] S.P. Ramachandran, B. Saravanakumar, V. Ganesh, G. Ravi, A. Sakunthala, R. Yuvakkumar, J. Mater. Sci. Mater. Electron. 28 (2017) 13780–13786. [32] G. Zhang, T. Xiong, M. Yan, L. He, X. Liao, C. He, C. Yin, H. Zhang, L. Mai, Nano Energy 49 (2018) 555–563. [33] S.O. Kasap, Principles of Electronic Materials and Devices, second ed., McGraw-Hill, New York, 2002http://Materials.Usask.Ca.
4. Conclusions In the current work, we have studied the effects of lithium nanosandwiching on the structural, dielectric and optical properties of molybdenum oxide thin films. The insertion of Li nanolayers between two layers of molybdenum oxide of thicknesses 500 nm successfully induced the crystallization process in the material, engineered the energy band gap, increased the light absorbability, decreased the magnitude of the real part of the dielectric constant and improved the optical conductivity parameters. The nanosandwiching of thin metal slabs between two transparent dielectric layers appears to be effective in altering the physical properties of the dielectric molybdenum oxide so that it becomes more appropriate for the production of thin film transistors owing to better drift mobility values and make it attractive for use as receivers of optical signals. Acknowledgments This work was supported by the Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah, under grant No. (D-004-3631440). The authors, therefore, gratefully acknowledge the DSR technical and financial support. References [1] P. Dwivedi, S. Dhanekar, S. Das, Semicond. Sci. Technol. 31 (2016) 115010.
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Physica E: Low-dimensional Systems and Nanostructures journal homepage: www.elsevier.com/locate/physe
Effect of lithium nanosandwiching on the structural, optical and dielectric performance of MoO3
T
S.E. Al Garnia,b, A.F. Qasrawic,d,∗ a
Physics Department, Faculty of Science - Al Faisaliah, King Abdulaziz University, Jeddah, Saudi Arabia Department of Physics, Faculty of Science, University of Jeddah, Jeddah, Saudi Arabia c Department of Physics, Arab American University, Jenin, Palestine d Group of Physics, Faculty of Engineering, Atilim University, 06836, Ankara, Turkey b
A R T I C LE I N FO
A B S T R A C T
Keywords: Li/MoO3 X-ray diffraction Nanosandwiching Optical conduction Dielectric
In this article, we discuss the effects of lithium nanosheets on the structural, optical, dielectric and optical conductivity parameters of the MoO3 films. The nanosandwiching of Li layers between two layers of MoO3 of thicknesses larger than 20 nm induced the crystallization process of the amorphous MoO3. Namely, MoO3 thin films that are nanosandwiched with Li sheets of thicknesses larger than 50 nm, exhibit structural phase transitions from hexagonal to monoclinic and reveals larger crystallite sizes. The possible formation of Li2O at the MoO3/Li/MoO3 interfaces is simulated and discussed. Optically, the Li nanosandwiching is observed to enhance the light absorbability by 11.0 times at 2.0 eV and successfully engineered the energy bands gap in the range of 3.05–0.45 eV. It also enhances the dielectric performance. In addition, relatively thick layers of lithium (200 nm) succeeds in converting the conductivity type from n-to p-type. The modeling of the dielectric spectra in accordance with the Drude- Lorentz approach have shown that the presence of Li in the structure of MoO3 significantly increases the drift mobility values of electrons from 5.86 to 11.40 cm2/V. The plasmon frequency range for this system varies in the frequency domain of 0.32–5.94 GHz. The features of MoO3/Li/MoO3 interfaces make them attractive for thin film transistor technology as optical receivers being promising for use in optical communications.
1. Introduction Molybdenum Trioxide (MoO3) is an attractive marital for it wide applications. It can be used in the fabrication of gas sensor [1–3], as supercapacitors [4,5] and as transparent electrodes [11–16]. Systematic gas sensing measurements has shown that the flower-like MoO3 shows excellent gas sensing performances toward ethanol. Liquid-exfoliated MoO3 is used to prepare supercapacitor electrodes that have relatively low capacitance (∼2 F g−1 at 10 mV/s) because of the low electrical conductivity of the MoO3. However, addition of carbon nanotubes beyond the percolation threshold yielded a 100-fold increase in capacitance values [4,5]. MoO3 which was synthesized by the thermal decomposition of ammonium molybdate displayed specific capacitance of value of 148.9 F g-1 at 0.7 A g-1 [6]. In the same context, MoO3 which is used as material in lithium batteries exhibits exceptional high specific capacity that nominates it as high performance anode materials [7]. Different methods are used to prepare MoO3. Polycrystalline MoO3 thin films have been prepared by oxidation at high temperature of
∗
molybdenum compound layers deposited by chemical vapor deposition (CVD) from molybdenum hexacarbonyl Mo(CO)6 [8]. In addition, the MoO3 are also prepared by the physical vapor deposition technique [9]. The as-deposited amorphous MoO3 films were reported to crystallize into a stable orthorhombic phase on annealing in the air at 350 °C. For these films, the feasibility of using Mg ions in polycrystalline MoO3 thin films as intercalants, was explored [9]. These Mg ions which were intercalated Galvano statically at a constant current density of 166 μA/ cm2 revealed lattice expansion along b axes by 2.35% for x = 0.1 and 3.17% for x = 0.3 [9]. Moreover, the physically prepared films by the PVD technique are shown as an efficient reverse-electron recombination barrier layer (RBL) at the fluorine doped tin oxide (FTO)/titanium dioxide (TiO2) interfaces in dye sensitized solar cells (DSSCs). These films displayed an average optical transmittance of ∼77% in a spectral range of 350–800 nm with bandgap value of ∼3.1 eV [10]. MoO3/Metal/MoO3 interfaces are also used as transparent layer electrodes [11–16]. Mainly, MoO3/Ag/MoO3 stacks are investigated for utilization of transparent cathodes in organic light-emitting devices
Corresponding author. Department of Physics, Arab American University, Jenin, Palestine. E-mail addresses: [email protected], [email protected] (A.F. Qasrawi).
https://doi.org/10.1016/j.physe.2019.113569 Received 29 March 2019; Received in revised form 26 April 2019; Accepted 17 May 2019 Available online 28 May 2019 1386-9477/ © 2019 Elsevier B.V. All rights reserved.
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exclusion of the glassy background. As appears in Fig. 1 (b), increasing the thickness of the lithium nanosheet further to 100 nm (MLM-100) and to 200 nm (MLM-200), forces the appearance of sharp peaks. While the most intensive peak of the MLM-100 samples appears at diffraction angle of 2θ = 21.40o , the most intensive peak of MLM-200 sample appears at diffraction angles of 2θ = 25.20o. The peaks which appeared at 2θ = 21.40o in the XRD patterns of MLM-100 split into two peaks (Fig. 1 (b)) when the Li thickness is increased to 200 nm. The split peaks are centered at 21.40 and 21.80°. To understand the physics beyond this interesting behavior of XRD of the samples under investigation, complete software analysis using “TREOR 92”, “Crystdiff” and “Crystal Maker” software packages are employed. The structural analysis on the MLM samples have shown that the resulting films comprise two phases in its structure. Although the MLM-0 samples show amorphous nature of structure, the MLM-20 displayed weak XRD patterns that are hardly assigned to the hexagonal structural phase of MoO3. Increasing the thickness of lithium to 50 nm, causes a disappearance of the main peak which was observed at 2θ = 39.35o for the MLM-20 and forces the appearance of new diffraction patterns. The diffraction angles at 2θ = 39.35o are consistent with the published literature data [17] and is assigned to the hexagonal phase of MoO3. The peaks which appeared in the XRD of MLM-50 samples are assigned to both hexagonal and monoclinic structural phases of molybdenum oxide. The picture of the lithium induced crystallography of molybdenum oxide become brighter as the lithium nanosheet increases to 100 and 200 nm. Particularly, the amorphous sea which is observed for the MLM-0, MLM-20 and MLM-50 disappears and clearly noticeable crystal structure is observable (Fig. 1 (b)). The indexing of the XRD patterns for the samples which contain Li nanosheets of thickness of 100 and 200 nm reveal that the material is mostly composed of monoclinic MoO3 as major phase and hexagonal MoO3 as minor phase. The calculated lattice parameters for the monoclinic unit cell of MoO3 are found to be a = 13.885, b = 3.696, c = 3.963Å and θ = 90.450 . Those which are calculated for hexagonal MoO3 are a = b = 10.522 and c = 14.888Å . The lattice parameters values are consistent with literature data [17,18]. The observed and the analyzed XRD patterns for the stacked layers of MoO3 indicate that the presence of Li in the structure of MoO3 forces the crystallization process of the material. The more thick the Li layer, the more dominant the monoclinic phase over that of hexagonal. Calculations of the crystallite size (D ) with the help of modified Scherrer equation [17] from the maximum peak broadening (β ) revealed D values of 52 nm for the MLM-20 samples and D value of 56 nm for MLM-100 and MLM 200. The strain which exhibits value of 2.2× 10−3 for the hexagonal phase that is dominant in MLM-20, became 3.5× 10−3 when the monoclinic phase dominates in the MLM-100 samples and reduces to 3.0× 10−3 as the lithium layer thickness increases to 200 nm. These values strongly affect the dislocation density (δ ). The dislocation density which exhibits value of 6.0× 1010 cm-2 in the MLM20 displays values of 6.75× 1010 cm-2 and 5.8× 1010 cm-2 in the MLM100 and MLM-200 films, respectively. Previous studies on the lithium insertion/extraction reactions on MoO3 samples with different crystallinity have shown that the insertion of Li into pristine MoO3 Leads to large structural distortions without altering the electrochemical performance of MoO3 [19]. Particularly, the insertion of solvated Li ions increase the interlayer spacing between the Mo-O octahedron layers by 5.5 Å . This increase is followed by a shrunk in the interlayer from 12.40 to 11.15. Å. In an attempt to explain the strongly induced crystallization which is achieved by the Li nanosandwiching, we turn the attention to the bonding mechanism and ionic radiuses. The ionic radius of Mo+6 being 62 p.m. [20] is very close to the value being 60 p.m. which is reported for Li+1 [21]. These values indicate that the lithium ion can replace sites of Mo+6. The replacement of Mo with Li leads to the formation of Li2O bonds. The possibility of formation of this phase at the interface region is simulated by the “Crystalmaker” software packages. The results of the simulator are shown in Fig. 2 (a) and (b). For the cubic unit
(OLED) [10]. The interface performed as the transparent anode in the organic solar cells [11,12]. In addition, MoO3/Au/MoO3 is designed for use as transparent electrodes in green OLEDs [13]. They achieved highly transparent electrode features with low resistivity [14], and semi-transparent contact. This structure yielded a power conversion efficiency of 5.5% with an average transparency of 26% [15]. Furthermore, MoO3/Al/MoO3 displayed high optical transmittance (approximately 90%) in visible light region [16]. In the light of these interesting features of MoO3, here in this article, we aim to improve the optical and electrical performance of the MOO3 prior to optoelectronic technology applications. Particularly, The MoO3 films are deposited onto glass substrates and are used as substrates to deposited Li films of thicknesses in the range of 20–200 nm. The resulting multilayers will be coated with another MoO3 film to encapsulate the Li thin layer. The structure will then by subjected to optical analysis to reveal its optical and dielectric performances. These parameters are the keys to use this interface in visible light communication technology. Particularly, based on the optical conductivity which is estimated from the imaginary part of the dielectric constant, the optical conduction parameters presented by the drift mobility, plasmon frequency, and free carrier concentration will be investigated at frequencies that suits visible light communications. 2. Experimental details MoO3 thin films are grown onto cleaned glass substrates by the thermal deposition technique under vacuum pressure of 10-5 mbar using VCM-600 evaporator. The source material is high purity (99.999%) MoO3 nano-powders (Alfa-Aeser). The film thicknesses are monitored by an in situ thickness monitor (Inficon STM-2). The first layer is 500 nm thick. This layer is coated with Li+1 nanosheets of thicknesses of 20–200 nm. The prepared films are given the codes MLM0, MLM-20, MLM-50, MLM-100 and MLM-200 for MoO3 films sandwiched with 0, 20, 50, 100 and 200 nm Li layer, respectively. The produced MoO3/Li films are recoated with another MoO3 layer of thickness of 500 nm. The fabricated thin films are studied by means of X-ray diffraction (XRD) technique using Miniflex 600 XRD unit, and by the ultraviolet–visible light spectrophotometry technique using Thermo-scientific Evolution 300 spectrophotometer. The structural analysis and computer simulations are carried out with the help of “TREOR 92”, “Crystdiff” and “Crystalmaker” software packages. 3. Results and discussion Owing to its importance as nonlinear optical material, the two nanosandwiched stacked layers of molybdenum oxide are subjected to Xray diffraction (XRD) analysis. The resulting diffraction patterns are shown in Fig. 1 (a) and (b). As seen from Fig. 1 (a) for two stacked layers of MoO3/MoO3 (MLM-0) no intensive peaks are observable. For MoO3/Li/MoO3 films sandwiched with lithium nanosheet of thickness of 20 (MLM-20) and 50 nm (MLM-50), no remarkable intensive peak is observable but some minor peaks which are hardly detectable after
Fig. 1. The X-ray diffraction patterns for MoO3 nanosandwiched with lithium layer of thicknesses of (a) 00 and 20 and 50 nm and (b) 100 and 200 nm. 2
Physica E: Low-dimensional Systems and Nanostructures 114 (2019) 113569
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Fig. 1. (continued)
to 200 nm. The secondary peak which appears at 39.04° in the simulated XRD also appears at 38.61° in the experimentally measured one. If one assumes crystallites of size of 56 nm and microstrain of 0.34% like the experimentally determined values, new peaks appear at 19.24o and 27.33o . The simulation with the crystallite size value of 56 nm and microstrain value of 0.34% is excluded as no evidence of the existence of such patterns was observed in the experimental XRD patterns. In the light of these analyses it is possible to assign the induced crystallization process to the exchange in the bonding mechanism between MoO3 and Li2O. The effects of the sandwiching of lithium nanosheets on the optical performance of MoO3 thin films are readable from the measured transmittance (T) and reflectance (R) spectra which are employed to calculate the absorption coefficient (α ) following the previously mentioned procedures [22]. The transmittance, reflectance and absorption coefficient spectra for the studied samples are displayed in Fig. 3 (a), (b) and (c), respectively. As seen from Fig. 3 (a), for the samples which are nanosandwiched with lithium sheets of thicknesses of 20, 50 and 100 nm, the effect of Li layer appears in the incident wavelength range
cell which is shown in Fig. 2 (a), the lattice parameter is 4.61 Å (crystallography open database code: 1514086). The average bond length which is executed over 32 visible bonds by the simulator is found to be 199.6 p.m. This value is larger than that of Mo-O being 169 p.m. [22] indicating the weaker van der Waals force of attraction between Li-O compared to Mo-O. The weaker forces lead to an enlargement between the interlayer spacing. Similar behavior was also mentioned for MoS2/Bi19Cl3S27 interfaces [23]. The smaller bond length of Mo-O compared to Mo-S and the weaker van der Waals attraction forces is believed to cause an enlargement in the interlayer spacing and a shift in the XRD patterns. In accordance with the simulator results, the formation of Li2O should lead to X-ray diffraction patterns which are shown in Fig. 2 (b). The simulator was run assuming a microstrain of 0.1% (1.0× 10−3 ) and grain size of ∼20 nm for Li2O. The position of the peaks is also demonstrated in the same figure. The strongest reflection of Li2O appears at 2θ=33.64o in the (111) direction. This peak appears at 2θ=33.45o in the experimentally determined XRD patterns which appear in Fig. 1 (b) for samples nanosandwiched with Li nanosheets of thickness of 100 nm and at 2θ=33.35o when the Li thickness is increased
Fig. 2. (a) the schematic of the Li2O unit cells and (b) the simulated XRD patterns for Li2O films with grain sizes of 20 nm and strain of 0.1%. 3
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increasing with increasing incident light wavelengths reaching a maxima at 2.0 eV. The energy where the maxima are observed does not depend on the Li layer thickness. However, the light absorbability in the studied range of light increases with increasing Li thickness. Namely, Rλ increases from 3.1 to 6.3 and reaches 11.0 as the lithium nanosheet thickness increases from 50 to 100 and reaches 200 nm, respectively. No significant difference between the light absorbability of MLM-20 and MLM-50 was observed. On the other hand, the plotting of Tauc's equation [22], ((αE )1/2 ∝ (E − Eg ) ), which is best fitted for indirect allowed electronic transitions as demonstrated in Fig. 4 (b) reveal energy band gap values of 3.05, 1.88, 1.78, 1.1 and 0.47 eV for the molybdenum oxide films nanosandwiched with Li layers of thicknesses of 0, 20, 50, 100 and 200 nm, respectively. Wide range of band gap engineering is obtained via Li nanosandwiching. The addition of relatively thin layers of Li is sufficient to alter the band gap from the blue light limit to the IR light limit. Engineering of the energy band gap via nanosandwiching technique was previously observed for Ga2S3/In/Ga2S3 thin films. The variation of the indium layer thickness in the range of 20–200 nm succeeds in lowering the energy band gap from 3.70 to 1.40 eV. It also enhances the light absorbability by 54.6 times [24]. Similarly, simulation studies on the MoO3/Ag/MoO3 (MAM) nanostructure as transparent anode [14] has shown that the light absorbance of thin organic solar cells is enhanced significantly and that the MAM structure could replace ITO in solar cells. The reason for the efficient performance of the MAM as light absorber is the propagating surface plasmon polaritons. In addition, it is observed that the MoOx (40 nm)/ Au (10 nm)/MoOx (40 nm) structures [16] reveal much higher transmission in the green-red region and leads to lower sheet resistance than indium tin oxide (ITO). This nanostructure is experimentally tested and verified to be more suitable as transparent electrodes for organic light emitting diodes. The energy band gap of Li2O is ∼7.99 eV [25] which is much higher than our measuring ability range. Since the observed values are much less than this value, the redshift in the energy band gap as a result of Li participation could be assigned to reasons other than the formation of Li2O like the introduction of unoccupied surfaces states in the gap or due to the introduction of occupied surface states near the conductionband minimum [26]. Band gap narrowing in Mg doped ZnO was attributed to the formation of impurity bands in the vicinity of valence band from oxygen vacancies [27]. It could also be assigned to the valence/conduction energy bands bending and image force lowering [28].
Fig. 3. The optical (a) transmittance, (b) reflectance and (c) absorption coefficient spectra for the MoO3/Li/MoO3 samples.
of 366–670 nm only. Li has no effect on the optical transmittance of MoO3 above 670 nm. In the effective range of spectra, the transmittance decreases with increasing Li layer thickness and also redshifts. The MLM-200 sample exhibit lower T values in the range of 366–1100 nm. On the other hand, the reflectance spectra which are illustrated in Fig. 3 (b) decreases with increasing Li layer thickness and the number of the observed reflection peaks also decreases from three peaks to one peak as the thickness of the Li layer reaches 100 nm and no peak appears when the Li slab thickness is 200 nm. The absorption coefficient spectra for the MoO3/Li/MoO3 film are shown in Fig. 3 (c). As seen from the figure, while the α values of MLM0 sharply decrease with decreasing incident light wavelengths reaching near zero value at 2.80 eV, the absorption coefficient values for the Li nanosandwiched samples never reach zero at that energy. It approaches zero value in the IR region (E < 1.60 eV). Increasing the lithium nanosheets thickness decreases the absorption coefficient value in the visible region of light and increases it in the IR range. The effect of Li on the light absorbability is pictured in Fig. 4 (a). The figure represents the light absorbability (Rλ = αMLM − x / αMLM − 0 ) in the incident light region of 2.45–1.33 eV. This region is the region where the absorption coefficient shifts up with increasing Li layer thickness. The light absorbability start
Fig. 4. (a) the light absorbability and (b) the Tauc's equation fitting for the MoO3/Li/MoO3 samples. 4
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at 1.60 eV was also attributed to the electronic transitions between the d yz and d xy energy levels that are located at 1.60 eV [32]. We believe that the redshift in the position of the peaks of the dielectric spectra that resulted from the participation of the Li nanosheets in the structure of MoO3 is ascribed to the same reasons that caused the redshifts in the energy band gap as we mentioned in the preceding paragraph. The imaginary part of the dielectric spectra for MLM-0 and for MLM-20, MLM-50, MLM-100 and MLM-200 samples are illustrated in Fig. 5 (b) and (c), respectively. The values of εim increase with increasing incident light energy following a sharp slope above 3.0 eV. The slope of variation of the εim − E dependence becomes smoother as the thickness of the Li layer increases. To give significance to the observed behavior of the imaginary part of the dielectric constant we turn the attention to the relation between this parameter and the optical conductivity (σ (w ) ). The optical conductivity is related the imaginary part through the relation, σ (w ) = εim w /(4π ) with w being the angular frequency [22,24]. Obtaining information about the optical conductivity parameters like the free charge carrier density (n ), drift mobility ( μ ), electron-plasmon reduced frequency (wei ), scattering time (τi ) and plasmon frequency (wpi ) with the help of Drude-Lorentz approach for frequency dependent conductivity allow guessing the performance of the studied samples as optical signal receivers. The imaginary part of the dielectric constant and the plasmon frequency is given by the equations,
Fig. 5. (a) The real part of the dielectric constant and (b) the optical conductivity for the MoO3/Li/MoO3 samples. The blue circles of (b) shows the computational results of the conductivity spectra obtained by Eqn. (1).
However, as the MLM-0, MLM-20, MLM-50 and MLM-100 are n-type semiconductor (tested by hot probe technique) and MLM-200 is p-type semiconductor whose work function is larger than 6.7 eV (electron affinity (qχ =6.7 eV [22]) and as the work function of lithium metal is 2.6 eV [29], the Li/MLM-x (0 ≤ x ≤ 100nm ) MoO3 forms an ohmic contact (band bending is not expected) and Li/MLM-200 creates a Schottky type of interface. The barrier height for the Li/MLM-200 (qφ = Eg + qχ − ϕm ) is larger than 4.55 eV. This is extremely deep barriers that causes large band bending which explains the abnormal decrease in the value of the band gap from 3.05 to 0.45 eV. Fig. 5 (a) and (b) displays the real (εr ) and imaginary (εim ) parts of the dielectric constant being calculated by the previously described methods [22,24] from the measured transmittance and reflectance spectra. As seen from Fig. 5 (a), the real part of the dielectric constant of the two stacked layers of MoO3 without participation of lithium nanosheets display four maxima at 3.79, 3.27, 2.54 and 1.60 eV. While the first peak (3.79 eV) disappears upon insertion of Li nanosheet of thicknesses of 20 nm and 50 nm, the other remaining three peaks redshifts to 3.17, 2.33 and 1.49 eV and to 2.76, 2.01 and 1.28 eV, respectively. Further increase in the thickness of Li to 100 nm causes the disappearance of one more peaks and shifts the peaks to 2.79 and 1.28 eV. Consistently, increasing the Li layers thickness to 200 nm shifts the peaks to 2.15 and 1.20 eV, respectively. In addition to the observed redshifts in the positions of the critical points of the dielectric spectra, the value of the dielectric constant also decreases with increasing Li layer thickness. In our previous investigations we have observed that the dielectric spectra of one layer of MoO3 reveal peaks in the dielectric spectra at 3.62 and at 2.72 eV [22]. These two peaks are comparable with those we found here for two stacked layers of MoO3 as 3.79 and 2.54 eV. The high energy peak was previously assigned to the localization (quantization) of charge carriers in the individual nanocrystals [30]. The peak which appeared near 2.54 eV was previously ascribed to the Mo+5 penta-coordinated [MoO5]5+ [31]. The peak which appeared
k
εim =
∑
((wei2 i=1
wpi2 w − w 2)2 + w 2τi−2)
(1)
and,
wpi =
4πne 2 , respectively m∗
(2)
In Eqn. (2) m∗ is the effective mass of electrons or holes in MoO3 ∗ ∗ = 1.28mo [33])). The reduced effec= 0.6mo [22]) and in Li (mLi (mMoO 3 tive mass for samples which are nanosandwiched with Li layers is cal2 1 ∗ + m∗ )−1=0.409mo . culated from the equation, mMLM − x ≠ 0 = ( m∗ Li
MoO3
Running the series for k = 4 was sufficient to reproduce the experimental data. The results of the computational fittings are illustrated in Fig. 5 (b) and (c) by the blue colored circles. The computed optical conductivity parameters for the studied samples are also shown in Table 1. As the table suggests for the linearly connected four oscillators of one particular sample, while the scattering time (τi ) decreases with increasing numbers of oscillators (i ), the reduced frequency (wei ) increases. This increase is accompanied with increase in the free carrier density and increase in the value of plasmon frequency and decrease in the values of drift mobility. The lower the incident photon energy, the more mobile the free charge carriers. As the highest drift mobility corresponds to the first oscillator (i = 1) which exhibit μ values of 5.86 cm2 / Vs at reduced frequency of 2.25 × 1015 Hz and free electron concentration of 7.0 × 1017 cm−3, it will play the main roles when performing as light resonator. The insertion of lithium nanosheets of thicknesses of 20, 50 and 100 nm increased the drift mobility to 10.7, 10.7 and 11.4 cm2 / Vs , respectively. Accordingly, the free electron
Table 1 Optical conductivity parameters for the MoO3 stacked layers nanosandwiched with Li. MLM-0 n-type
τi (fs ) wei(x1015Hz) n (x 1017 cm-3) μ(
cm2 ) Vs
wpei (GHz)
MLM-20 n-type
MLM-50 n-type
MLM-100 n-type
MLM-200 p-type
1
2
3
4
1
2
3
4
1234
1
2
3
4
1
2
3
4
2.00 2.25 7.00 5.86
1.50 5.00 1.00 4.39
1.00 5.50 1.00 2.93
0.85 6.30 600 2.49
1.50 2.50 0.70 10.7
1.50 5.00 1.00 10.7
0.60 5.00 50.0 4.27
0.60 6.50 120 4.27
1.60 2.25 0.90 10.7
1.50 0.90 0.45 4.5 4.5 6.50 1.00 20.0 200 10.7 6.4 3.2
1.60 2.25 3.00 11.4
0.70 3.30 3.00 4.98
0.60 4.50 35.0 4.27
0.52 6.75 130 3.70
0.60 1.70 9.00 4.27
0.51 3.20 15.0 3.63
0.47 4.55 25.0 3.34
0.40 6.80 120 2.84
0.64
0.24
0.24
5.94
0.32
0.38
2.67
4.14
0.36 0.38 1.69 5.35
0.66
0.66
2.24
4.31
1.13
1.46
1.89
4.14
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Physica E: Low-dimensional Systems and Nanostructures 114 (2019) 113569
S.E. Al Garni and A.F. Qasrawi
concentration decreased to 7.0 × 1016 cm−3 , 9.0 × 1016cm−3 and 3.0 × 1017 cm−3 , respectively. The later parameter alters the values of plasmon frequency. The decrease in the free electron concentration upon Li nanosandwiching is ascribed to the electron hole recombination at the MoO3/Li interface to reach the equilibrium Fermi levels in the ohmic type interfaces [22,33]. Increasing the thickness of Li layers to 200 nm, increased the number of free charge carriers to 9.0 × 1017cm−3 and lowers the drift mobility values from 11.4 to 4.27 cm2 / Vs , respectively. In accordance with the hot probe technique test, the values being 9.0 × 1017cm−3 is accounted for holes rather than electrons. Thus, the mobility value being 4.27 cm2 / Vs is also regarded as holes drift mobility which is usually less than electrons drift mobility. In general, except for the sample which is nanosandwiched with lithium of thickness of 200 nm, the range of plasmon frequency is not remarkably altered. The MLM-200 samples displayed a larger plasmon frequency value for the first oscillator compared to other MLM samples.
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4. Conclusions In the current work, we have studied the effects of lithium nanosandwiching on the structural, dielectric and optical properties of molybdenum oxide thin films. The insertion of Li nanolayers between two layers of molybdenum oxide of thicknesses 500 nm successfully induced the crystallization process in the material, engineered the energy band gap, increased the light absorbability, decreased the magnitude of the real part of the dielectric constant and improved the optical conductivity parameters. The nanosandwiching of thin metal slabs between two transparent dielectric layers appears to be effective in altering the physical properties of the dielectric molybdenum oxide so that it becomes more appropriate for the production of thin film transistors owing to better drift mobility values and make it attractive for use as receivers of optical signals. Acknowledgments This work was supported by the Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah, under grant No. (D-004-3631440). The authors, therefore, gratefully acknowledge the DSR technical and financial support. References [1] P. Dwivedi, S. Dhanekar, S. Das, Semicond. Sci. Technol. 31 (2016) 115010.
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