Seed-Mediated Growth of Conductive and Transparent Anatase Nb

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Seed-mediated growth of conductive and transparent anatase Nb-TiO2 films for CdS based devices Liangliang Liu, Shu Xiao, Zhengyong Ma, Cuiqing Jiang, Zhongcan Wu, Suihan Cui, Ricky K Y. Fu, Hai Lin, Zhongzhen Wu, Feng Pan, and Paul K Chu ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00334 • Publication Date (Web): 16 Apr 2019 Downloaded from http://pubs.acs.org on April 17, 2019

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ACS Applied Nano Materials

Seed-mediated growth of conductive and transparent anatase Nb-TiO2 films for CdS based devices

Liangliang Liu a,b,1, Shu Xiao a,1, Zhengyong Ma a, Cuiqing Jiang a, Zhongcan Wu a, Suihan Cui a, Ricky K.Y. Fu b, Hai Lin a, Zhongzhen Wua,b,, Feng Pan a, Paul K. Chu b*

a School

of Advanced Materials, Peking University Shenzhen Graduate School, Shenzhen 518055, China

b

Department of Physics and Department of Materials Science and Engineering, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China

1 The

authors made equal contributions

Corresponding authors. E-mail [email protected] (P.K. Chu) 

address:

[email protected]

1

ACS Paragon Plus Environment

(Z.Z.

Wu);

ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract: Owing to the small natural abundance of indium, high-purity anatase Nb-TiO2 (NTO) is proposed as an alternative of indium tin oxide but materials fabrication is quite difficult. Herein, a simple and scalable seed-mediated method is designed to produce high-purity anatase NTO. Owing to the smaller nucleation free energy of anatase than rutile when the grain size is smaller than 14 nm, nano-scale anatase seeds nucleate preferentially by precise energy control. The mechanism of the seed-mediated NTO is studied by TEM. The conductivity depends on the purity of anatase and high transparency with the optimal antireflection layers is demonstrated by simulation.

Keywords: Seed-mediated growth; NTO films; Energy controlling; Conductivity; Transparency;

2


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Inexpensive transparent conducting materials with good etching properties and durability highly are desirable as alternatives to indium tin oxide (ITO) in applications such as solar cells, organic light emission diodes (OLED), and photonic devices.1–3 Owing to the small natural abundance of In, Al-ZnO (AZO), F-SnO2 (FTO), and Cu/Ag films have been proposed to replace ITO but none of these materials have the desirable transparency, conductivity, thermal/chemical stability, and mechanical strength.4,5 Theoretical and experimental investigations have been conducted on oxygen-deficient TiO2 by Nb doping as transparent conducting oxide (TCO).6 The structure is crucial to the conductivity and transparency and generally the anatase phase has higher conductivity than the rutile phase for the same amount of oxygen vacancies which can destroy the order of TiO2 and decrease the transparency.7,8 Although anatase and rutile have different crystallization temperature, crystallization is generally uncontrollable resulting in growth of both structures.9,10 Epitaxial growth on SrTiO3 or sapphire has been reported as the only means to obtain high purity anatase NTO films with conductivity of 10-4 Ω·cm and transparency larger than 80%,11,12 but the materials are costly and the process is difficult for large-scale commercial production. Seed-mediated growth is a common strategy to produce a particular phase,13 and it should be feasible to achieve orientated film growth using this concept. According to the film growth theory, the crystal faces or phase structures with the smallest Gibbs free energy are preferred for nucleation and crystallization based on the minimum energy principle.14,15 The Gibbs free energy includes not only the kinetic energy of the 3



depositing particles, but also the surface energy produced at the hetero-interface, which may be dominant especially in the beginning of deposition.16,17 As the film thickens, the interface effect weakens and the Gibbs free energy is dominated by the kinetic energy of the depositing particles again. Therefore, exploiting this energy evolution with film thickness and precise energy control of the incident particles, special crystalline seeds can be created at the hetero-interface. In this work, the average free energy of the anatase and rutile structures depending on the grain size are calculated by the Gibbs free energy equation using the Vienna Ab initio Simulation Package (VASP) and Matlab18-20 and the details are described in the methods section of the supporting information. As shown in Fig. 1(a), Fig.S1 and Table. S1 (in the supporting information), both anatase NTO and rutile NTO belong to the tetragonal system and the degree of distortion and asymmetry of anatase NTO are larger than those of rutile NTO leading to instability of anatase NTO. 21 The anatase NTO has a smaller free energy than rutile NTO when the grain size is smaller than 14 nm suggesting the possibility of seed-mediated growth of anatase TiO2. According to the calculation results in Fig.1(a), we implant the anatase seeds with the (004) preferred orientation into the hetero-interface between the NTO film and soda-lime glass substrate by precise energy controlling of the incident particles in low-energy magnetron sputtering. For comparison, the amorphous and multi-phase NTO films are deposited by decreasing and increasing the particle energy during magnetron sputtering as shown in Fig. 1(b). Post-annealing is performed in the transmission electron microscope (TEM) by in situ heating by which the pure anatase 4


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NTO is obtained. The detailed deposition parameters are shown in the supporting information. The cut-samples are observed by high-resolution TEM, as shown in Fig. 2. Because of superposition of the surface energy and kinetic energy of the incoming particles, the nanoscale anatase grains can form at the hetero-interface when deposition starts. With increasing film thickness, the surface energy diminishes and the kinetic energy of the incident particles is not enough for anatase to crystallize and so amorphous NTO is formed subsequently. Therefore, the seed-implanted NTO film has a graded structure. There are three typical points at the bottom, middle and top of the NTO film designated as 1, 2 and 3, respectively. They are characterized by electron diffraction and the crystal lattice is fitted with the fast Fourier transform (FFT) algorithm in the corresponding insets. As expected, many anatase nano-grains with the (004) preferred orientation are observed from the bottom layer of the cut-NTO film near the substrate.

The grain size is about 5 nm and anatase NTO has

a smaller free energy than rutile NTO as shown in Fig. 1(a). With increasing film thickness, NTO cannot nucleate further without the surface energy at the hetero-interface and an amorphous structure is formed as shown in Fig. 2(a). If the sputtering power is decreased, nucleation cannot take place because of the small growth energy even at the hetero-interface and therefore, amorphous NTO is present from the bottom to the top surface (Fig. 2(c)).

If the sputtering power is large, many

other crystalline phases such as rutile and larger grains (>14 nm) are produced because the high kinetic energy of the incident particles induces multi-phase growth. 5



As shown in Fig. 2(d), both anatase and rutile can be observed at the beginning of NTO deposition and a polycrystalline and multi-phase structure is observed.

The phase transition from room temperature to 400 ℃ is shown in Fig. 3. The anatase (004) seed crystals with a small size of about 5 nm are observed from the hetero-interface near the glass substrate. When the temperature is increased to 200℃, more anatase (004) NTO seed grains are created at the bottom of the film and the grain size increases. When the cut-sample is heated to 400 ℃ and kept for 10 min, anatase (004), anatase (101), and anatase (105) NTO crystals are present in the whole film without nucleation of rutile crystals and the grain size increases with annealing time. The anatase (004) NTO seeds decrease the nucleation threshold producing high crystallinity without the rutile phase,13 as shown in Fig. 3(a). When the annealing temperature is 500 ℃ , rutile begins to appear as shown in Fig. S2(in the supporting information). However, no crystal can be found from the amorphous cut-sample (Fig. 3(b)) and the film does not exhibit a crystallization trend when the temperature is increased to 200℃ due to the high energy threshold of nucleation. After heating to 400℃ which is larger than the normal nucleation temperature of NTO22 and keeping for 10 min, many anatase and rutile grains appear and start to grow into a mixed phase structure. Both the grain size and number are less than those of the seed-meditated sample indicating poor crystallization. The multi-phases sample in Fig. 3(c) has many anatase and rutile grains due to the higher deposition energy. When the annealing temperature is 200℃, all the grains start to grow and the rutile grains also begin to 6


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coalesce to form bigger ones. Further increasing the temperature to 400℃ causes the anatase and rutile grains to grow and the grain size is larger revealing better crystallization.

The XRD and Raman patterns of the three samples before and after annealing in Fig. S3 (in the supporting information) show similar results. No obvious diffraction peaks are detected from the as-deposited seed-mediated sample due to the low percentage and small size of the seed crystals and only anatase phase can be observed after 400℃ annealing from XRD and Raman. The XRD patterns of the amorphous sample before and after annealing are similar with the seed-mediated samples. However, rutile phase (marked by arrows in Fig.S3(b)) could be identified by Raman after annealing, which was consisted with the TEM results. The multi-phases sample shows competitive growth of anatase (004) and rutile (210) and multi-phases intergrowth during annealing.

The conductivity and transparency properties are determined and shown in Fig. 4(a) and Fig. S4 (in the supporting information). The resistivity of NTO film is related to the crystal phase and orientation. Generally, rutile suppresses the conductivity compared to anatase7,23 and the (004) orientation shows higher electron mobility than the other orientations due to the static eﬀective mass along the a axis (m(100)*) is smaller than that along the c axis (m(001)*)

24,25.

Thus, a high Hall mobility of 6.85

cm2/Vs is obtained in the seed-mediated NTO samples which is higher than those in 7



literatures (mostly between 2-5 cm2/Vs).4,15 The carrier concentrations have a direct relationship with the oxygen vacancies. Thus, XPS was performed on those annealed samples and shown in Fig.S5 in the supporting information. No obvious difference can be observed in the elements concentration of Ti, O and Nb. The Nb3d peaks presented at 204.1eV and 207.4 eV are referred to Nb2+ and Nb5+, respectively. The rations between Nb2+ and Nb5+ are ranged from 0.173 to 0.177 without any difference among the three samples. Besides, the Ti2P peaks at 458.6 eV and 464.1 eV are referred to Ti4+ with 0.4 eV peak shift due to the Nb doping.4 The carrier concentrations in the three samples are similar because of the similar concentration and valence state of Nb, as shown in XPS results. Based on the Hall mobility and carrier concentration results, the best electrical properties of 7.1×10-4 Ω·cm could be attributed to the pure anatase phase and (004) preferred orientation of the seed-mediated sample. The photos of different samples before and after annealing at 400℃ are shown in Fig.S6. The transmittance of the NTO films in the visible range (320-780 nm) depends on the crystallinity when the carrier concentration is similar and hence, the multi-phases NTO sample shows the best transmittance and seed-mediated sample is second. It is noted that the average transmittance values of the three samples in the visible region (400-760 nm) are only 70.78%, 67.72%, and 71.48% because of significant reflection arising from the large difference in the refractive indexes (n) between the NTO films (n = 2.55 @ 550 nm), soda-lime glass (n = 1.53 @ 550 nm), and air (n = 1.00 @ 550 nm)26, as shown in Fig. S4 (in the supporting information). 8


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To match the refractive indexes and decrease reflection at the interfaces, CdS, a common n-layer material in solar cells and OLED 27,28 with a refractive index of 2.53 at 550 nm24 is applied to contact the NTO directly and two anti-reflection layers are designed on both sides of the glass (FL-AR at the front and BL-AR at the back). The process is simulated by the Essential Macleod 8.0 as shown in Fig. 4(b) and Table. S2

(in

the

supporting

information).

In

the

multilayer

system

(air/FL-AR/glass/BL-AR/NTO/CdS/…), light dispersion is relatively low and there is large transmittance from air to the CdS layer. Figs. 4(c) and (d) show the relationship between the transmittance curves of the multilayer system and CdS thickness and NTO thickness. Owing to intrinsic absorption by the CdS layer at short wavelengths (350-600 nm), the transmittance curves of the multilayer system after deducting absorption by the CdS layer are also displayed. By using the anti-reflection layers (FL-AR layer and BL-AR layer) on the two sides of glass (Detailed information about the anti-reflection layers can be found in the supporting information), all the samples exhibit an average transmittance larger than 80%. In particular, when the NTO thickness is below 300 nm, the average transmittance is 85% and nearly no change in the average transmittance can be observed when the thickness of the CdS layer is changed from 50 nm to 200 nm.

In summary, a seed-mediated technique is developed to prepare high-purity anatase NTO films based on the energy difference of anatase and rutile NTO on the 9



grain size. By precisely controlling the energetics including the surface energy at the hetero-interface, kinetic energy of the incoming particles, and annealing temperature, nano-grain seeds are produced resulting in seed-mediated growth of pure anatase NTO.

The seed-mediated grown NTO films have large electron mobility and small

resistivity after annealing. To address reflection due to the refractive index difference between glass and NTO, an anti-reflection layer and CdS layer (similar n with NTO) are introduced to the multilayer system. As a result, the NTO film shows transmittance larger than 80% which is similar to that of ITO films and thus has large potential in photovoltaic and photonic devices.

ASSOCIATED CONTENT Supporting Information Details of the method and simulation are shown in the supporting information.

AUTHOR INFORMATION Corresponding Author Corresponding authors. [email protected] (Z. Z. Wu); [email protected] (P.K. Chu) ORCID: Zhongzhen Wu: 0000-0002-3728-7928 Paul K. Chu: 0000-0002-5581-4883 Feng Pan: 0000-0002-8216-1339 10


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Liangliang Liu: 0000-0003-3405-355X Shu Xiao: 0000-0002-7211-7211 Author Contributions Z. Wu designed the experiments. L. Liu. S. Xiao and C. Jiang carried out the experiments. Z. Ma, R.K.Y. Fu, F. Pan, and P. K Chu contributed to data analysis. Z. Wu, S. Xiao, L. Liu, and P.K. Chu co-wrote the paper. All authors provided valuable feedbacks.

Notes The authors declare no competing financial interest.

Acknowledgements This work was financially supported by Soft Science Research Project of Guangdong Province (No. 2017B030301013), New Energy Materials Genome Preparation

&

Test

Key-Laboratory

Project

of

Shenzhen

(No.

ZDSYS201707281026184), Shenzhen Science and Technology Research Grants (JCYJ20170306165240649), as well as City University of Hong Kong Applied Research Grant (ARG) No. 9667122 and Strategic Research Grant (SRG) No. 7004644.

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transition and textural changes upon calcination of two commercial titania samples: A pure anatase and a mixed anatase-rutile [J]. Journal of Solid State Chemistry, 2015, 232: 42-49. [24] Yamada N, Shibata T, Taira K, et al. Enhanced Carrier Transport in Uniaxially (001)-Oriented Anatase Ti0.94Nb0.06O2 Films Grown on Nanosheet Seed Layers [J]. Applied Physics Express, 2011, 4(4): 045801. [25] Hirose Y, Yamada N, Nakao S, et al. Large electron mass anisotropy in a d -electron-based transparent conducting oxide: Nb-doped anatase TiO2 epitaxial films [J]. Physical Review B Condensed Matter, 2009, 79(16): 897-899. [26] Rubin M. Optical properties of soda lime silica glasses[J]. Solar Energy Materials, 1985, 12(4):275-288. [27] Britt J, Ferekides C. Thin‐film CdS/CdTe solar cell with 15.8% efficiency[J]. Applied Physics Letters, 1993, 62(22):2851-2852. [28] Chirilă A, Reinhard P, Pianezzi F, et al. Potassium-induced surface modification of Cu(In,Ga)Se2 thin films for high-efficiency solar cells.[J]. Nature Materials, 2013, 12(12):1107.

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Figure captions: Fig.1: (a)The Gibbs free energy of anatase and rutile NTO crystal and their variations with different grain sizes; (b)The schematic diagram of different samples at different depositing energy while annealing. Fig.2: The TEM results of as-deposited of NTO thin films. (a) The seed-mediated sample;(b) The high magnification of Zone1 in (a); (c) The completely amorphous sample; (d) The multi-phases sample. Fig. 3: Processes of crystallization in the different samples at different temperature: (a) The seed-mediated sample; (b) The completely amorphous sample; (c) The multi-phases sample. Fig. 4: The performances of the NTO films. (a) Carrier concentration, mobility and resistivity of different NTO samples; (b) Simulation model of the NTO thin films used in the CdS/CdTe thin film solar cell; (c) The transmittance and the average transmittance of the CdS/CdTe thin film solar cell with various thicknesses of CdS layers (320 nm NTO layer). (d) The transmittance and the average transmittance of the CdS/CdTe thin film solar cell with various thicknesses of NTO layers (150 nm CdS layer).

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Figures:

Fig.1: (a)The Gibbs free energy of anatase and rutile NTO crystal and their variations with different grain sizes; (b)The schematic diagram of different samples at different depositing energy while annealing.

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Fig.2: The TEM results of as-deposited of NTO thin films. (a) The seed-mediated sample;(b) The high magnification of Zone1 in (a); (c) The completely amorphous sample; (d) The multi-phases sample.

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Fig. 3: Processes of crystallization in the different samples at different temperature: (a) The seed-mediated sample; (b) The completely amorphous sample; (c) The multi-phases sample.

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Fig. 4: The performances of the NTO films. (a) Carrier concentration, mobility and resistivity of different NTO samples; (b) Simulation model of the NTO thin films used in the CdS/CdTe thin film solar cell; (c) The transmittance and the average transmittance of the CdS/CdTe thin film solar cell with various thicknesses of CdS layers (320 nm NTO layer). (d) The transmittance and the average transmittance of the CdS/CdTe thin film solar cell with various thicknesses of NTO layers (150 nm CdS layer).

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Seed-Mediated Growth of Conductive and Transparent Anatase Nb

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