THE CONTROL OF ITO CONDUCTIVE COATING RELIEF VIA LASER-ORIENTED DEPOSITED CARBON NANOTUBES Текст научной статьи по специальности «Нанотехнологии»

Original papers Nanostructured, nanoscale materials and nanodevices

УДК 53.043, 532.6 DOI: 10.17277/jamt.2022.01.pp.058-067

The control of ITO conductive coating relief via laser-oriented deposited carbon nanotubes

Andrei S. Toikkaa,b,c4 Natalia V. Kamaninaa,b,c

a Vavilov State Optical Institute, 5/2, Kadetskaya St., St. Petersburg 199053, Russian Federation, b Saint Petersburg Electrotechnical University, 5, Professora Popova St., St. Petersburg 197376, Russian Federation, c Petersburg Nuclear Physics Institute named by B.P. Konstantinov of National Research Centre "Kurchatov Institute",

1, Orlova roscha St., Gatchina 188300, Russian Federation

И atoikka@obraz.pro

Abstract: The paper presents the analysis of the surface relief of ITO conductive layers structured with single-walled carbon nanotubes. The deposition of ITO films and carbon nanotubes was carried out by laser-oriented method using a CO2 laser. The optical scheme was matched to the control electric grid; the strength of the electric field was varied in the range of 100-600 V-cm-1. To analyze the surface, atomic force microscopy in the contact mode and measurement of the wetting angle by sessile drop method was used. Roughness of pure ITO was about 1.8 nm, while in laser-vacuum deposition of carbon nanotubes, roughness varied from 6.4 nm to 22.1 nm with an increase in the electric field during deposition. The relationship between roughness and surface area of coatings and the wetting angle was considered. An increase in the wetting angle from 101.8 degrees, in the case of pure ITO, to 115.0-128.6 degrees, with the deposition of carbon nanotubes, was shown. The results obtained indicate an improvement in the resistance of ITO coatings to moisture during the deposition of carbon nanotubes, which makes it possible to expand their operating conditions and scope in optical electronics, laser and display technology.

Keywords: laser-oriented deposition technique; indium and tin oxides; surface structurization; wetting angle; sessile drop method.

For citation: Toikka AS, Kamanina NV. The control of ITO conductive coating relief via laser-oriented deposited carbon nanotubes. Journal of Advanced Materials and Technologies. 2022;7(1):58-67. DOI: 10.17277/jamt.2022.01.pp.058-067

Управление рельефом проводящих ITO-покрытий при помощи лазерно-ориентированного осаждения углеродных нанотрубок

А. С. Тойкка^^, Н. В. Каманина^

а Государственный оптический институт им. С. И. Вавилова, Кадетская линия, 5/2, Санкт-Петербург 199053, Российская Федерация, ь Санкт-Петербургский государственный электротехнический университет «ЛЭТИ» им. В. И. Ульянова (Ленина),

ул. Профессора Попова, 5, Санкт-Петербург 197376, Российская Федерация, с Петербургский институт ядерной физики им. Б. П. Константинова Национального исследовательского центра «Курчатовский институт», Орлова Роща, 1, Гатчина 188300, Российская Федерация

И atoikka@obraz.pro

Аннотация: Представлен анализ рельефа поверхности проводящих контактов на основе 1ТО, структурированных одностенными углеродными нанотрубками. Осаждение пленок 1ТО и углеродных нанотрубок проводилось лазерно-ориентированным методом с использованием СО2-лазера. Оптическая схема была согласована с управляющей электрической сеткой, напряженность поля варьировалась в диапазоне 100...600 В/см. Для диагностики поверхности последовательно использовались атомно-силовая микроскопия в контактном режиме и измерение краевого угла смачивания методом висячей капли. Шероховатость чистого 1ТО составила порядка 1,8 нм, в то время как при лазерно-вакуумном осаждении углеродных нанотрубок шероховатость

варьировалась от 6,4 до 22,1 нм с ростом электрического поля в процессе напыления. Рассматривалась связь шероховатости и площади поверхности покрытий с краевым углом смачивания. Показан рост угла смачивания от 101,8° в случае чистого ITO до 115,0...128,6° при осаждении углеродных нанотрубок. Полученные результаты свидетельствуют об улучшении стойкости ITO-покрытий к влаге при осаждении углеродных нанотрубок, что позволяет расширить их условия эксплуатации и область применения в оптической электронике, лазерной и дисплейной технике.

Ключевые слова: метод лазерно-ориентированного осаждения; оксиды индия и олова; структурирование поверхности; краевой угол смачивания; метод висячей капли.

Для цитирования: Toikka AS, Kamanina NV. The control of ITO conductive coating relief via laser-oriented deposited carbon nanotubes. Journal of Advanced Materials and Technologies. 2022;7(1):58-67. DOI: 10.17277/jamt.2022.01. pp.058-067

1. Introduction

Along with functional photosensitive and modulating layers of optoelectronic devices, indium and tin oxides (ITO) are one of the key semiconductor materials in electronics, which, depending on the field of application of the developed system, can function both as transparent conductive contacts, antireflection coatings and buffer coatings [1-5]. ITO coatings are transparent in the visible and near infrared spectrum regions, flexible, non-toxic, and have a relatively high conductivity, which allows them to be used in photovoltaics [6-9], flexible electronics [10-13], and display technologies [14-17].

It should be noted that the content of indium in the Earth's crust is limited, and its cost is slowly but gradually increasing, therefore, in electronics, ITO is currently partially replaced by tin oxides with fluorine (FTO) [18-19], zinc oxides (ZnO), and zinc oxides with aluminum (AZO) [20-22], as well as using other materials [23-27]. Despite this, ITO is still in demand due to the possibility of optical refractive index matching with silicon, liquid crystals, and glass [28-33]. In photovoltaics, the ITO layer should reduce reflection losses and also accumulate charge carriers. In liquid-crystal technologies, contacts are made on the basis of this material at the interface: solid-liquid crystal. The ITO-glass tandem is used to protect against freezing, for example, car windshields.

Obtaining and controlled switching of the desired properties of ITO coatings often depends on the deposition method. In vacuum methods, for example, in magnetron sputtering and thermoresistive deposition, it is done by varying the power and oxygen concentration [34-38]. In chemical methods, for example, in sol-gel centrifugation, the rotation frequency and solvent composition are changed to control the ITO properties [39]. It is worth noting that thermal annealing is widely used in most methods [40-41].

In our study, we continue to improve the method of laser deposition of carbon nanotubes on the surface of various materials, including ITO [42-44]. Thus, in this work, the method of laser-oriented deposition of single-walled carbon nanotubes (CNTs) [45] is used to change the ITO parameters. The refractive index of CNTs (at the level of 1.05-1.10) is lower than that of ITO, which makes it possible to reduce reflection losses. CNTs with metallic chirality significantly reduce the electrical resistance of ITO [46]. Since they have a high Young's modulus, their deposition can also increase the microhardness of the coating, as was shown previously in the case of ZnSe and ZnS [47].

In the current work, we increased the ITO surface area due to the deposition of carbon nanotubes. A correlation of the control field during the deposition process with the relief parameters was obtained. The relief parameters were related to the value of the wetting angle.

2. Materials, methods and theoretical background

The K8 crown glasses were used as substrates. ITO films were sequentially deposited on them by laser-oriented method, and then single-walled carbon nanotubes (CNTs) were deposited in a varied average electric field from 0 to 600 V-cm-1. Powder (In2O3)0.9-(SnO2)0.1 with a purity of 99.99 % was used. CNTs had a diameter of 0.7-1.1 nm (No. 704121, Aldrich catalog). The deposition technique and ITO diagnostics is presented in more detail in the paper [48]. For deposition of carbon nanotubes, a CO2laser was used in a quasi-continuous mode (X = 10,6 ^m, W = 30 W, spot diameter was 5 mm).

The relief was measured by contact method of atomic force microscopy (Solver Next NT-MDT). Scanning area was 30 * 30 ^m, scanning frequency was 1 Hz. The results were processed in the Nova Px program. Roughness parameters were recalculated in

accordance with the standard [49] in the Image Analysis P9 image processing module [50]. The height of the AFM profile was measured on a two-dimensional grid of 256 * 256 points. The resulting sample was ranked in height, and statistical parameters were calculated. The normal distribution, which describes the distribution of the deposited film thickness in vacuum methods with sufficient accuracy, was taken as a basis.

To numerically characterize the homogeneity of the coating relief, the average roughness Sa was used:

m n

Sa =— HI f (x, y )|.

mn i=i j=i

(1)

Here, m = n = 256 is the number of split points along x and y axes, respectively, z = fx, y) is the

profile height sample measured with an atomic force

microscope.

The normal distribution is characterized by the

~ 2 mathematical expectation f and dispersion o . In the

case of atomic force microscopy, the absolute value of the mathematical expectation is not informative enough, since an arbitrary point within the profile can be taken as the reference point in height. The dispersion is associated with the root-mean-square roughness Sq, which is recalculated from the

profile height sample as follows:

Sq = - a -

—II (, y)-f )2 (2)

mn i=i j=i

Since deviations from the ideal case are observed in a real experiment, it is advisable to use higher-order statistical parameters: skewness Ssk and kurtosis Sku :

m n

Ssk -

13 1 a3

-II(/ (x, y )-f )

mn i-i j-i_

3/2

(||2)3/2 a3 ( i mn '

- II/ (x, y )-f)2

^ mn i-i j -i

S ku =

- I I ( (x, y )-f)4

14 14 _ mn i-i j=i

(l 2 ) a

4(

— I I ( (x, y )- f

^ mni-i j-i

(3)

. (4)

-\2

When ranking the sample z = fx, y) according to the frequency of repetitions, the left tail of the

distribution corresponds to depressions, and the right tail corresponds to ridges. With a negative skewness, the relief roughness is characterized by a large number of low ridges and a small number of deep depressions, and vice versa with positive values. At Ssk = 0, a symmetric case is observed. The kurtosis characterizes the sharpness of the peak in the distribution density. At a normal distribution Sku = 3. At Sku > 3 and above, the sharpness of the peak increases, and at Sku < 3 and below, the peak becomes flatter.

The relief diagnostics on a macro scale was carried out by measuring the wetting angle using hanging drop method. Drops of distilled water with a volume of 1 ^l were deposited on the surface of samples with structured ITO coatings for 1 second. The whole process was recorded by a CCD camera at a frequency of 60 fps. Using the SCA20 software, the contact boundary was determined, the droplet boundary was approximated, and the contact wetting angle was recalculated. The OSA 15 EC device for carrying out the above procedure is shown in Fig. 1.

It should be noted that the method of measuring the wetting angle 9 by sessile drop method is classically used for express surface diagnostics. When a drop spreads, the contact area of the liquid with the solid body and with the surrounding environment increases, and surface tension forces also work, which is illustrated in Fig. 2.

The change in free energy dF is equal to the work done by surface tension forces dW, and can also be expressed in terms of the change in the spreading area [51]:

dF - dW -adS,

(5)

where o is the surface tension coefficient at the interface of three phases, which is numerically equal to the superposition of surface tension forces in the projection onto the spreading plane reduced to unit length:

a - -

dFsl + dFlg + dFsg) eR

dr

-asl +algcos 0-asg. (6)

sg-

Here, a

sl'

a lg and asg are surface tension

coefficients at the solid-liquid, liquid-gas, and solidgas interfaces, respectively, and 9 is the wetting angle.

According to Fig. i (II), the change in area depends on the wetting angle and drop size:

dS - 2nR sin 9dR.

(7)

2

Fig. 1. System for optical registration of the wetting angle OSA 15 EC (DataPhysics)

(I)

Gas / Liquid

/0 R sin0 \

Solid body e r

(III)

dR

F

sg

F,

Fig. 2. Sessile drop method: (I) wetting angle 0, (II) change in the drop area during spreading, (III) direction of surface tension coefficients at the interface of three phases

In the equilibrium state, the free energy takes on a minimum value and does not change, therefore dF = dW = 0. In this case, by substituting (6) into (5), it is possible to express the wetting angle in terms of surface tensions and obtain the Young formula [52]:

cos 0 = ■

a, -a

sg

(8)

a

lg

This approach succinctly describes the case of a smooth homogeneous surface of a solid body.

In this paper, we propose to supplement the described approach for the case with a rough surface and consider the surface of ITO films with deposited carbon nanotubes in more detail.

In the real case, the surface of a solid body is rough, which means that the surface area is higher than in the ideal case. Graphically, this is shown in Fig. 3.

Analytically, the surface area is determined through the functional dependence of the height z = fx, y) [53]:

dSreal - ■

1 +

df (x, y ) 2 (df (x, y )

dx

dy

dxdy . (9)

dSreal

dx

Fig. 3. Demonstrated difference between the surface area of a real ITO film and a perfectly smooth surface

e

R

+

Since, in practice, the surface profile is measured by points, using a second-order difference relation, it is possible to do the point partition. Let us denote the ratio of the real surface area to the ideal one through the surface roughness coefficient:

Ks -

S real m "

Si

ZZV 1 + \Lx[f (x,y)] + \Ly[ (x,y)] ,

id i—1 j-1

(10)

where Lx [f (x, y)] and Ly [f (x, y)] are second-order difference relations in x and y coordinates:

Lx [f (x, y )] =

Ly [ (x, y )] —

f (x—, yi) - 2 f (x,, yi) + f (x,+1, yi) ;

(Ar)2 '

(11a)

f (xi, yi-1) - 2 f (xi, yi) + f (xi, yi+1)

(Ay )2

(11b)

It should be noted that the described roughness is a property of a solid body, therefore, in the expression for changing the free energy, this parameter will be used for osg and osl:

dF — (a

sl-asg)Ksds + algcos0ds . (12)

Thus, from the equilibrium condition, the following expression for the wetting angle can be obtained taking into account the surface roughness:

asi — as„

cos 0' = Ks-g = Ks cos0 . (13)

clg

According to expression (10), Ks = 1 in the case of an ideal smooth surface, and Ks > 1 in the case of a rough one.

3. Results and discussion

In laser-oriented deposition method, the material from the source substrate is sputtered under the action of a temperature gradient in a solid angle of 2n. Since the source substrate is heated unevenly during laser exposure, the number of particles deposited on the quartz substrate decreases as it deviates from its center. Directional deposition can be achieved by changing the distance from the grid to the upper carousel with samples, which also determines the magnitude of the control electric field.

During the ITO deposition, films with an average roughness of 2 nm or less are formed. The difference between the maximum height and maximum depth within the scanned sample is on the order of 10 nm. Carbon nanotubes under current conditions are deposited in the form of clusters with a height of about 60 nm. Depending on the magnitude of the electric field strength on the control contact grids, it is possible to influence the relief shape of the modified ITO surface (Fig. 4).

In this case, it is possible to rearrange the height distribution along the surface of samples. With an increase in the control field, an increase in roughness is observed (Table 1).

Fig. 4. Atomic force profiles of pure ITO surface (I) and ITO surface with deposited carbon nanotubes at different average electric field strengths: 100 V-cm-1 (II), 200 V-cm-1 (III), and 600 V-cm-1 (IV)

Table 1. Relief parameters of ITO films with deposited carbon nanotubes at different average electric field strengths

ITO + CNT with different electric field E, V-cm-

Parameter Pure ITO

i00 200 600

Average roughness Sa, nm i.8 6.4 i5.6 22.i

RMS roughness Sq, nm 2.2 7.9 i2.2 27.4

Skewness Ssk 0.02 -0.03 0.04 0.24

Kurtosis Sku 2.4 2.5 3.4 3.2

It should be noted that with increasing control field strength, the roughness Sa and Sq increase. At E = 200 V-cm-1 and E = 600 V-cm-1, the kurtosis is higher than 3, which means that the distribution peak in the vicinity of the mathematical expectation is sharper than in the case of a normal distribution, which can be associated with a more ordered deposition of carbon nanotubes on the ITO surface. The skewness at E = 600 V-cm-1 characterizes roughness associated with the influence of the number and magnitude of depressions in the relief.

The measured AFM profiles were imported into the MATLAB R2021a software and, in accordance

with expressions (i0)-(ii), the surface roughness coefficient was calculated, and a prediction was made for the wetting angle using formula (i3). A clean ITO coating with 9 = i02.8° was taken as a smooth surface (Table 2).

When measuring the wetting angle, the same trend was found (Fig. 5); its value increases with increasing surface in homogeneity, which can be controlled by the electric field during the deposition of carbon nanotubes (Table 3).

When the surface is structured with carbon nanotubes, in addition to the relief, the composition also changes; therefore, the surface tension coefficients differ.

Table 2. Calculation of the surface roughness coefficient and wetting angle

Parameter

Pure ITO

ITO + CNT with different electric field E, V-cm-i i00 200 600

Roughness coefficient, Ks Predicted wetting angle 9', deg

i.0i2 i02.8 (measured)

i.595 ii0.7

2.533 i24.i

3.004 i3i.7

C A left 102.8° CA right: 102.8°

CA right: 114.2"

1

CA left: 118.6" CA right: 118.7"

I

CA left: 128.7* CA right: 128.7'

I

Fig. 5. Wetting angle of pure ITO surface (I) and ITO surface with deposited carbon nanotubes at different average electric field strengths: 100 V-cm-1 (II), 200 V-cm-1 (III), and 600 V-cm-1 (IV)

Table 3. Statistics of the wetting angle on the ITO film surface with deposited CNTs

at different electric field strengths

Wetting angle parameter,

Pure ITO

ITO + CNT with different electric field E, V-cm-1 100 200 600

Mean value 101.8 115.0 119.8 128.6

Minimum value 99.3 111.5 115.6 128.0

Maximum value 102.9 117.6 125.4 129.0

4. Conclusion

Thus, based on the experimental data on atomic force microscopy and measurement of the wetting angle, taking into account the experiments and calculations performed, the following conclusions can be formulated:

1. Laser-oriented deposited carbon nanotubes are a scaffold that increases the surface roughness of the matrix material, in this case, ITO.

2. During deposition of carbon nanotubes, it is possible to control the relief parameters using an electric field on contact grids. As the tension increases, the roughness parameters Sa, Sq and Ks increase. At the same time, there is a tendency to an increase in the wetting angle with an increase in surface inhomogeneity. Thus, a clear correlation has been established between the surface roughness and the wetting angle.

3. The results obtained indicate a decrease in the adhesion energy to the surface of the structured ITO film, making it more resistant to moisture, which is important when working with optical equipment.

4. The results obtained and discussed can significantly expand the scope of ITO conductive contacts, not only for the field of general optoelectronics, but also for laser, biomedical and display technology.

5. Funding

This study is an initiative, however, it was partly funded by the Innovation Promotion Foundation, grant No. [72598/d-112174].

6. Acknowledgments

The authors are grateful to their colleagues at Vavilov State Optical Institute and Petersburg Nuclear Physics Institute for the fruitful discussion of the results.

7. Conflict of interest

The authors declare no conflict of interest.

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Информация об авторах / Information about the authors

Андрей Сергеевич Тойкка, лаборант, Государственный оптический институт им. С. И. Вавилова; студент магистратуры, Санкт-Петербургский государственный электротехнический университет «ЛЭТИ» им. В. И. Ульянова (Ленина), Санкт-Петербург, Российская Федерация; младший научный сотрудник, Петербургский институт ядерной физики им. Б. П. Константинова Национального исследовательского центра «Курчатовский институт», Гатчина, Российская Федерация; ORCID 0000-0002-86948497, e-mail: atoikka@obraz.pro

Наталия Владимировна Каманина, доктор физико-математических наук, заведующий отдела, Государственный оптический институт им. С. И. Вавилова; профессор, Санкт-Петербургский государственный электротехнический университет «ЛЭТИ» им. В. И. Ульянова (Ленина), Санкт-Петербург, Российская Федерация; ведущий научный сотрудник, Петербургский институт ядерной физики им. Б. П. Константинова Национального исследовательского центра «Курчатовский институт», Гатчина, Российская Федерация; ORCID 0000-0002-29032685, e-mail: nvkamanina@mail.ru

Andrei S. Toikka, Laboratory Assistant, Vavilov State Optical Institute; Master Student, Saint Petersburg Electrotechnical University, St. Petersburg, Russian Federation; Junior Researcher, Petersburg Nuclear Physics Institute named by B.P. Konstantinov of National Research Centre "Kurchatov Institute", Gatchina, Russian Federation; ORCID 0000-0002-86948497, e-mail: atoikka@obraz.pro

Natalia V. Kamanina, D. Sc. (Physics and Mathematics), Head of Department, Vavilov State Optical Institute; Professor, Saint Petersburg Electrotechnical University, St. Petersburg, Russian Federation; Principal Researcher, Petersburg Nuclear Physics Institute named by B.P. Konstantinov of National Research Centre "Kurchatov Institute", Gatchina, Russian Federation; ORCID 0000-0002-29032685, e-mail: nvkamanina@mail.ru

Received 14 January 2022; Accepted 01 March 2022; Published 14 April 2022

Copyright: © Toikka AS, Kamanina NV, 2022. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.Org/licenses/by/4.0/).

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