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Semiconductor Physics. Physics of nanostructures
UDC 538.9 + 539.8 EDN RRDAEF
https://www.doi.org/10.33910/2687-153X-2023-4-4-195-202
Peculiarities of the structure and optical properties of amorphous semiconductor plasmon nanocomposites TiO2<Ag>, prepared by ion plasma co-sputtering
^ Dauithan1, O. Yu. Prikhodko1, S. L. Peshaya™, Y. S. Mukhametkarimov1, S. A. Kozyukhin2,
S. Ya. Maksimova1, G. A. Ismaylova1
1 NAO al-Farabi Kazakh National University, 71 al-Farabi Ave., Almaty 050040, Kazakhstan 2 N.S. Kurnakov Institute of General and Inorganic Chemistry of RAS, 31 Leninsky Ave., Moscow 119991, Russia
Authors
Kuanyshbek Dauithan, e-mail: [email protected]
Oleg Yu. Prikhodko, ORCID: 0000-0002-7506-2762, e-mail: prikhodko [email protected] Svetlana L. Peshaya, ORCID: 0000-0002-1814-7567, e-mail: [email protected] Yerzhan S. Mukhametkarimov, ORCID: 0000-0003-1381-4532, e-mail: [email protected] Sergey A. Kozyukhin, ORCID: 0000-0002-7405-551X, e-mail: [email protected] Suyumbika Ya. Maksimova, ORCID: 0000-0001-7759-0326, e-mail: [email protected] Guzal A. Ismaylova, ORCID: 0000-0002-1813-2031, e-mail: guzal [email protected]
For citation: Dauithan, K., Prikhodko, O. Yu., Peshaya, S. L., Mukhametkarimov, Y. S., Kozyukhin, S. A., Maksimova, S. Ya., Ismaylova, G. A. (2023) Peculiarities of the structure and optical properties of amorphous semiconductor plasmon nanocomposites TiO2<Ag>, prepared by ion plasma co-sputtering. Physics of Complex Systems, 4 (4), 195-202. https://www.doi.org/10.33910/2687-153X-2023-4-4-195-202 EDN RRDAEF
Received 28 June 2023; reviewed 20 September 2023; accepted 20 September 2023.
Funding: This study was supported by the Science Committee of the Ministry of Science and Higher Education of the Republic of Kazakhstan (Grant No. AP09258922).
Copyright: © K. Dauithan, O. Yu. Prikhodko, S. L. Peshaya, Y. S. Mukhametkarimov, S. A. Kozyukhin, S. Ya. Maksimova, G. A. Ismaylova (2023) Published by Herzen State Pedagogical University of Russia. Open access under CC BY-NC License 4.0.
Abstract. Structure and optical properties of amorphous semiconductor nanocomposite TiO2<Ag> films prepared by ion-plasma RF magnetron co-sputtering of a TiO2 and Ag target only in the argon atmosphere are studied. Matrix of TiO2<Ag> films is amorphous with inclusions of isolated 3-4 nm sized silver nanoparticles. The optical transmission spectra have a sharp edge of fundamental absorption band at ~ 300 nm, which is formed by direct and indirect allowed optical transitions. The band gap of the TiO2<Ag> films increases with the Ag concentration due to some ordering of the matrix. The absorption band is observed in the visible region of the spectrum due to the effect of local surface plasmon resonance absorption (LSPR) on silver nanoparticles. The LSPR intensity increases with the silver concentration, and the maximum of the absorption band, depending on the Ag concentration, is located in the region from 455 to 488 nm.
Keywords: ion-plasma sputtering, amorphous plasmon semiconductor nanocomposites TiO2<Ag>, silver nanoparticles, structure, optical properties, surface plasmon resonance absorption
Introduction
Titanium dioxide films (TiO2), a polymorphic wide-gap semiconductor material (anatase, rutile, brookite), are widely used in various fields: from biomedicine to solar energy (Banerjee et al. 2015; Cao et al. 2016; Ghann et al. 2016; Kulkarni et al. 2015; Pakdel et al. 2013). This is due to the generation of high-energy electrons in TiO2 under the action of near-UV radiation. In recent years, many studies have been aimed to expand of the functional properties of these films by embedding elements of different chemical nature and creating composites on their basis. Special attention is paid to composite films based
on TiO2 with inclusions of isolated nanoparticles of noble metals such as Ag and Au (Prakash et al. 2016; Wodka et al. 2010; Yu et al. 2017; Zhao et al. 1996). In such wide-gap semiconductor nanocomposite TiO2<Ag> and TiO2<Au> films, an absorption band is observed in the visible region of the spectrum due to local surface plasmon resonance (LSPR) on metal nanoparticles. Such films are of significant interest for the physics of plasmon phenomena and have important applied significance, in particular, for improving the photocatalytic and optical properties of TiO2 films (Bueno-Alejo et al. 2017).
The semiconductor composite TiO2 films with inclusions of Ag and Au nanoparticles are obtained by various chemical and physical methods. For example, the sol-gel method, PLD, photoreduction treatment, ion doping and magnetron ion-plasma DC/RF co-sputtering (Ghidelli et al. 2020; Navabpour et al. 2014; Ozimek et al. 2016; Pan, Heagy 2019; Usha et al. 2016; Yuan et al. 2017; Zada et al. 2017). Ion-plasma RF magnetron co-sputtering has a number of advantages over other methods. This is due to the possibility of obtaining continuous and homogeneous composite films and the possibility to be well integrated into the planar technology of electronic device manufacturing (Prikhodko et al. 2014). However, in the technology of obtaining TiO2 films with isolated Ag and Au nanoparticles and with the LSPR effect, the sputtering is carried out in the atmosphere of a mixture of inert gas argon with oxygen and with the obligatory subsequent thermal treatment of the films in air or in the argon and hydrogen mixture (Rodrigues et al. 2018; Torrell et al. 2011), which ultimately complicates the technology of their production.
In this work, we show the possibility of obtaining amorphous semiconductor nanocomposite films TiO2<Ag> with a brightly pronounced LSPR effect, i. e. plasmon nanocomposites, synthesized by ionplasma radio-frequency (RF) magnetron co-sputtering only in argon atmosphere, and the peculiarities of their structure and optical properties are studied.
Experimental Technique
Thin nanocomposite TiO2<Ag> films were obtained by ion-plasma RF (13.56 MHz) magnetron sputtering of a combined TiO2 and Ag target in an argon atmosphere (99.998%) at a pressure ~ 0.5 Pa. A preliminary high vacuum (~ 10-4 Pa) was created by oil-free forevacuum (BocEdwards XDS10) and turbomolecular (BB-150) pumps. The films were deposited on KU-1 fused silica substrates, fresh cleavages of NaCl and KBr crystals, and single-crystal silicon (c-Si) plates that were kept at room temperature. The technological parameters of the sputtering were selected so as to obtain an amorphous TiO2 matrix with inclusions of isolated silver nanoparticles.
The elemental composition and morphology of the films were monitored by energy dispersive analysis (EDX) using a Quanta 3D 200i SEM with an EDAX. The thickness of the films was measured by scanning a cleavage of the c-Si/TiO2 and c-Si/TiO2<Ag> sandwich structures on SEM and ranged from 40 to 50 nm. For these measurements, the films were deposited on c-Si substrates cleaned from the SiO2 oxide layer. It was found that the composition of TiO2 films is close to stoichiometric, and the Ag concentration in TiO2<Ag> films reaches 9.0 at. %.
The structure of TiO2 and TiO2<Ag> films was studied by transmission electron microscopy (TEM JEM 2100 JEOL). The optical properties (transmission T = f(X) and reflection spectra R = f(X)) of films deposited on quartz substrates were studied on a Shimadzu UV2000 spectrophotometer in the spectral range from 250 to 1100 nm with the recording step—1 nm.
Results and discussion
The structure of TiO2 and TiO2<Ag> films
Fig. 1 a-e shows typical TEM images and selected area electron diffraction (SAED) patterns of TiO2 and TiO2<Ag> films with silver concentration 9.0 at. %. Figs. 1 b and d show typical SAED patterns of TiO2 films and the matrix of the TiO2<Ag> film. The SAED of TiO2 film (Fig. 1 b) and matrix of the film TiO2<Ag> (Fig. 1 d) are characterized by a diffuse halo, which is indicative of their amorphous structure. However, in the matrix of TiO2<Ag> films, the diffuse halo is less blurred, so its structure is more ordered. The decoding of SAED ring reflections (Figs. 1 d and e) is performed using the CrysTBox program (Klinger, Jäger 2015).
Fig. 1. TEM images (a, c) and SAED patterns (d, e) of TiO2 (a, b) and Ti02<9.0 at. % Ag> (c, d, e) films, as well as the size distribution of Ag nanoparticles in Ti02<9.0 at.% Ag> film (f)
The Ti02<Ag> film matrix contains isolated nanoparticles (Fig. 1 c). SAED of the nanoparticle (Fig. 1 e) shows clearly pronounced point reflections characteristic of a single-crystal structure. It follows from the interpretation of the SAED that this nanoparticle is silver.
Fig. 1 f shows a histogram of the size distribution of Ag nanoparticles in Ti02<9.0 at.% Ag> films, obtained as a result of the TEM image processing. We can see that most silver nanoparticles have a diameter ~ 3-4 nm, and the distribution itself is lognormal. The same sizes for silver nanoparticles were obtained for Ti02<Ag> films with other Ag concentrations. Similar results were obtained for wide-gap semiconductor nanocomposite a-C:H<Ag> films with the LSPR effect, also prepared by ion-plasma co-sputtering (Prikhodko et al. 2016; 2017).
Thus, the nanocomposite Ti02<Ag> films, obtained by ion-plasma RF magnetron co-sputtering of Ti02 and Ag in only argon atmosphere, consist of amorphous Ti02 matrix and isolated crystalline silver nanoparticles.
Optical properties of TiO2 and TiO2<Ag> films
Fig. 2 shows the optical transmission spectra of the Ti02 and Ti02<Ag> films with different silver concentrations. It can be seen that the spectra of the films are characterized by the sharp transmission band edge (fundamental absorption band edge) in the near UV range from 280 to 320 nm. The slope of the passband edge changes with an increase in the silver concentration in the Ti02<Ag> films: it increases significantly with the Ag concentration from 0 to 4.9 at. %, and then decreases slightly. The Ti02 films in the visible and near IR regions (from 400 to 1100 nm) have a sufficiently high transmittance. In contrast, a pronounced absorption band with a maximum in the visible region of the spectrum is observed in Ti02<Ag> films.
Fig. 2. Transmission spectra of TiO2 and TiO2<Ag> films with different concentration of silver
Fig. 2 shows that an increase in the Ag concentration (the volume fraction of Ag nanoparticles) leads to a significant increase in the intensity of this absorption band. The maximum of the absorption band, depending on the Ag concentration, is located in the range from 455 to 488 nm. As the concentration increases, the resonance peak shifts towards longer wavelengths. The main shift of the maximum is observed in the region up to 4.9 at. % Ag.
According to (Manikandan et al. 2003), the observed absorption band in TiO2<Ag> films is due to the effect of surface plasmon resonance (LSPR) in consequence of resonant absorption of electromagnetic radiation by free surface electrons of isolated silver nanoparticles.
In (Ievlev et al. 2014), it was shown for polycrystalline semiconductor nanosized anatase films that the edge of the fundamental absorption band is formed by both indirect and direct allowed transitions. In this connection, we analyze the features of the fundamental absorption of the studied amorphous semiconductor nanosized TiO2 and TiO2<Ag> films.
The optical band gap (Eg) of the films was estimated using the Tauc method (Tauc et al. 1966). According the method, in the fundamental absorption region when the absorption coefficient a > 104 cm1, the relationship between the light photon energies hv, a, and Eg is described by the relation
ahv ~ (hv-Eg)1/n, (1)
where n = 1/2 corresponds to allowed direct-gap optical transitions, n = 2—to allowed indirect-gap transitions (quadratic absorption law) (Tauc et al. 1966).
The optical band gap of TiO2 and TiO2<Ag> films was determined for both allowed direct-gap and indirect-gap optical transitions. The error in Eg did not exceed ± 0.01 eV. Fig. 3 shows the spectral dependences (ahv)1/2 and (ahv)2 for nanosized TiO2 and TiO2<Ag> films with an Ag concentration of 4.9, 5.3, and 9.0 at. %.
Fig. 3. Spectral dependences (ahv)1'2 and (ahv)2 for TiO2 (a) and TiO2<Ag> (b-d) films
It can be seen that in the TiO2 and TiO2<Ag> films spectra there are rather extended characteristic rectilinear sections, and extrapolation of them to the energy axis hv gives the values of E. However, the detailed analysis shows that the most extended characteristic rectilinear sections are that when n = 2 (quadratic absorption law). It follows that the edge of the fundamental absorption band in the TiO2 and TiO2<Ag> films is mainly formed by indirect allowed optical transitions.
Fig. 4 (curves 1 and 2) shows the concentration dependences of TiO2<Ag> films optical band gap. The Eg values of the films correspond to semiconductor wide-gap materials. Optical band gap for TiO2 and TiO2<Ag> films in the case of n = 2 (the quadratic absorption law) are significantly lower than that of n = 1'2. When n = the insignificant rise of Eg with Ag concentration occurs, but when n = 2, the significant increase of Eg takes place in the region of Ag concentration from 0 to 4.9 %, and later on Eg practically is invariable. Note, that the Eg for TiO2<Ag> films considering both absorption laws with Ag concentration exceeding 3.8 at.% practically coincide with the data (Zhang et al. 2006) for polycrys-talline anatase films.
Fig. 4. Concentration dependences of TiO2<Ag> films E' for different laws of absorption (curves 1 and 2)
and Urbach energy Eu (curve 3)
To explain the increase in the optical band gap in nanocomposite Ti02<Ag> films, let us consider the variation with the Ag concentration of the Urbach energy (Eu) (Fig. 4, curve 3). According to (Akshay et al. 2019; Urbach et al. 1953), Eu characterizes the degree of semiconductor structure disorder, which determines the tail extension of the density of states at the boundaries of the allowed energy bands Ec and Ev. The relationship between the absorption coefficient a, the energy of the incident photon hv, and the Urbach energy Eu is given by the following relation (the empirical Urbach's rule)
a ~ exp(hv/E). (2)
The Urbach energy was estimated from the low-energy region of the absorption spectrum. It is seen from Fig. 4 that the Urbach energy significantly decreases in the region from 0 to 4.9 at. % with Ag concentration increase, but the optical band gap of Ti02<Ag> films significantly rises, in the case of a quadratic absorption mechanism. A further increase in the silver concentration to 9.0 at. % leads to an insignificant increase in the Urbach energy and an insignificant decrease in E^ i. e., there is a correlation between changes in Eu and Eg when a quadratic absorption law take place. It follows that the increase in the Ag concentration in the Ti02<Ag> films leads to the decrease of the density of localized states tails extension in the band gap, which is indicative of the ordering of the amorphous Ti02<Ag> films structure. This is in good agreement with the change in the steepness of the transmission (absorption) band edge founded in this work.
Considering our data on TEM (Peshaya et al. 2022) the ordering of the amorphous structure of Ti02<Ag> films with an increase in the concentration of Ag from 0 to 4.9 at. %, apparently, is due to a decrease in the porosity of the films because of the filling of pores in them with silver nanoparticles. This, in turn, leads to an increase in the refractive index of the matrix and, as a consequence, to a shift of the resonant peak towards longer wavelengths.
A further increase in the concentration of silver in the films to 9.0 at. % leads to some disordering of the structure as a result of an increase in the amount of silver nanoparticles in the matrix of the films.
It should be noted that an increase in silver volume fraction in the Ti02<Ag> nanocomposite can also lead, as the theory (Gibson et al. 1982) shows, to a shift of the resonance peak maximum towards longer wavelengths. Apparently, this is the reason for the further shift of the maximum to the red region.
Thus, the optical properties of amorphous semiconductor nanocomposite Ti02<Ag> films are characterized by a sharp edge of the fundamental absorption band in the near UV region, which is formed mainly by indirect allowed optical transitions. The slope of the absorption band edge and the optical band gap depend on the silver concentration in the films. There is a pronounced LSPR effect in the visible region of the spectrum, the intensity of which increases significantly with the rise of Ag concentration.
Conclusions
Using the ion-plasma RF magnetron sputtering of a combined Ti02 and Ag target in an atmosphere of only argon without subsequent heat treatment, amorphous semiconductor nanocomposite Ti02<Ag> films with a thickness from 40 to 50 nm and with a silver concentration of up to 9.0 at. % were obtained.
It was found by TEM that the structure of the films consists of practically amorphous Ti02 matrix and isolated silver nanoparticles with the diameter ~ 3-4 nm.
The resulting Ti02<Ag> films are wide-gap semiconductor materials. The optical band gap of the films increases significantly with the silver concentration. The optical properties of Ti02 and Ti02<Ag> films are characterized by a sharp fundamental absorption edge at ~ 300 nm, which is due to direct and indirect allowed optical transitions. In the visible region of the spectra of Ti02<Ag> films, a pronounced surface plasmon resonance absorption is manifested. The LSPR intensity significantly increases with an increase in the silver concentration in the films, and the absorption maximum, depending on the Ag concentration, is located in the region from 455 to 488 nm. The presence of isolated silver nanoparticles and their concentration influences on the degree of the structure order of the amorphous matrix of the Ti02<Ag> film and, thus, the optical band gap of the films. Amorphous Ti02<Ag> films, in comparison with Ti02 films, have a more ordered structure of the amorphous Ti02 matrix and, as a consequence, higher Eg.
The shown possibility of obtaining amorphous plasmon nanocomposite Ti02<Ag> films using simpler technology is of significant interest for their application in structures fabricated in a continuous cycle of planar technology.
Conflict of Interest
The authors declare that there is no conflict of interest, either existing or potential.
Author Contributions
The authors have made an equal contribution to the paper.
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