Corresponding author: Alexander P. Kuzmenko ( apk3527@mail.ru ) © 2020 Alexander P. Kuzmenko, Thet Phyo Naing, Andrey E. Kuzko, Alexey V. Kochura, Myo Min Than , Nay Win Aung.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Kuzmenko AP, Naing TP, Kuzko AE, Kochura AV, Than MM, Aung NW (2020) Formation of hierarchical structures from functionalized multiwalled carbon nanotubes in aerosil containing solutions. Modern Electronic Materials 6(1): 1723. https://doi.org/10.3897/j.moem.6.1.54811

Specific features and regularities of selfassembly and selforganization of multiwalled carbon nanotubes (MWCNT) have been studied for diffusionlimited conditions (method of drops) in waterbased (deionized water) colloidalal solutions with aerosil exposed to DC electric fields with magnitudes of 15 to 25 V. Studies of hierarchical structure formation during drop evaporation in uniform electric fields have revealed the formation of 40–120 nm sized linear piecewise formations, 25–45 nm sized fractal structures and 250 nm sized diffuse structures from MWCNT – COOH + aerosil + H_{2}O_{dw}. The structures have been studied using confocal microscopy, Xray diffraction, Raman scattering, atomic force microscopy, IR spectroscopy and scanning electron microscopy. The sizes of the observed micro and nanostructures decrease following the hyperbolic law d = 1/U in the approximation d → 2R, their growth rate increasing as U^{2}. We show that intense ultrasonication of functionalized MWCNT – COOH + aerosil + H_{2}O_{dw} in colloidal solutions causes the formation of the socalled “breathing" modes in axis centered singlewalled carbon nanotubes. This is confirmed by shortwave Raman scattering excitation and enables the existence of both combined sp^{2}hybridization types with π and σ carbon bonds and the metallic and semiconductor conductivity types in the material thus showing good promise of these structures for nanoelectronics.
selforganization and selfassembly, stabilized multiwalled carbon nanotubes, sp ^{2}hybridization, π and σcarbon bonds, controlled ordering of carbon nanotubes
New methods of synthesizing carbon nanotubes (CNT), both singlewalled (SWCNT) and multiwalled (MWCNT) ones, have been developed in recent years. CNT exhibit unique physicochemical properties:
The properties of CNT are largely determined by their extremely high aspect ratio l/d = 10^{6} [
Below we present data on the effect of electric fields on the selforganization of MWCNT – COOH + aerosil + H_{2}O_{dw} at different concentrations in colloidal solutions in deionized water (DW) with aerosil.
The test specimens were CCVDsynthesized (Catalytic Chemical Vapor Deposition) MWCNT by Nanocyl, Nelguim with CoO based nanocatalysts. Inset of Fig.
Fourier IR spectra of (1) initial and (2) functionalized MWCNT. Inset: SEM image of initial MWCNT (20 nm diameter).
The MWCNT functionalization degree in the initial and the astreated conditions was determined using Fourier IR spectroscopy (Fig.
Raman spectra were taken on an Omega Scope™ Raman microspectrometer (wavelength 532 nm, spectral resolution 0.8 cm^{1}). The Raman spectra contained the bands D = 1336÷1353 cm^{1} and G = 1567÷1600 cm^{1} the intensities of which were typical of MWCNT (I_{D} > I_{G}). The intensity I_{D} of the band D for MWCNT– COOH increased in comparison with its initial value for MWCNT whereas the intensity I_{G} of the band G did not change. After functionalization the defectiveness of the structures expressed as I_{D}/I_{G} was ~1.4 as compared with ~1.2 for the initial MWCNT.
The MWCNT– COOH complexes were exposed to sequential ultrasonic agitation and dispersion in order to obtain a colloidal solution containing MWCNT – COOH + aerosil + H_{2}O_{dw}. The agitation and dispersion operations were carried out on a Volna UZTA0,4/22OM ultrasonic device (22 kHz ultrasonic oscillation frequency, max. 20 W power, up to 55 min exposure time with intervals after 5 min of continuous exposure). The colloidal solution prepared using this method was stable during the experiment and remains stable until now (after almost 2 years) compared with several months stability periods for typical solutions. The colloidal solution was applied using the drop method into an electrode gap (100, 500, 1000 and 1500 µm) of a circuit board produced by photolithography on a sitall substrate capped with magnetronsputtered copper, chromium or gold films (Fig.
(a) IC, (b, c) confocal microscopy and (d–f) SEM images of colloidal solution residue (MWCNT – COOH + aerosil + H_{2}O_{dw}) on sitall: b, c: PLS, FS and DS formation at U = 19 and 19.5 V, respectively; d: FS and PLS formation at U = 19 V; e, f: PLS and DS formation at U = 19.5 V.
The micro and macroscopic reconstruction occurring in an electric field during evaporation of the colloidal solution drop was studied using confocal microscopy with a 0.46 digital aperture, an AISTNT atomic force microscope integrated into an OmegaScope Raman microscope and SEM. The particle dynamics was studied by video recording (frame rate 30 Hz, resolution 704 × 576 px). The particles moved in the colloidal solution from the negative electrode to the positive one by analogy with earlier data [
The structures forming in an electric field were systematized into fractal structures (FS), diffuse structures (DS) and piecewise linear structures (PLS). Their typical images are presented in Fig.
The variety of structural formations growing from MWCNT – COOH is illustrated in Fig.
AFM images of structures forming from MWCNT – COOH at U = 19 V: a–c: PLS, DS and FS from colloidal solution, respectively; d–f: at U = 19.5 V PLS, PLS and FS and DS, respectively. Inset: size distribution.
The reconstruction of the chemical structure in the FS, DS and PLS described herein was traced from changes in the Raman spectrum for colloidal solution residue using Raman microscope spectroscopy with a spatial resolution of 500 nm. The formation of a specific structure under equilibrium conditions between carbon atoms with sp^{2}hybridization is accompanied by growth of either πbound elongated carbon clusters (sized up to several decades of microns) or σbound graphite structures as demonstrated in earlier works [
The presence of the G band characterizing violation of symmetrical graphene sheet wrapping showing itself as degeneration of oscillations depending on orientation along (G^{+} – LO) or across (G – TO) the nanotube axis is only typical of SWCNT, and this was observed for the DSs and PLSs (Fig.
Defect concentration and residual surface stress in biased piecewise linear formations, diffuse structures and fractal structures as per Raman spectroscopy data.
Sample  I _{D}/I_{G}  I _{2D}/I_{G}  ΔI_{D 0.5}  ΔI_{G 0.5}  ΔI_{2D 0.5}  L, nm 

PLS  1.17  0.88  50  51  62  3.76 
DS  1.15  0.63  50  51  62  3.82 
FS  1.18  0.76  50  62  62  3.73 
Raman spectra of (1) PLS, (2) DS and (3) FS forming from MWCNT – COOH + aerosil + H_{2}O_{dw}. Inset: radial “breathing" mode region.
In the lowfrequency spectral region where SWCNT exhibit excitation of radial “breathing" mode at 100 to 600 cm^{1}, all the test MWCNT structures (FS, DS and PLS) exhibited untypical excitations (Fig.
Analysis of the experimental AFM images (Fig.
$\begin{array}{c}m\frac{\mathrm{d}{\mathbf{v}}_{i}}{\mathrm{d}t}=m\left(\frac{\partial \mathbf{V}}{\partial t}+\mathbf{v}(\nabla \mathbf{V})\right)\\ \sum _{i\ne j}^{N}\nabla U\left({R}_{ij}\right)+{\mathbf{F}}_{s}\left({R}_{s}\right)+{\mathbf{F}}_{L}\left({R}_{L}\right)\\ 6\pi a\eta \left({\mathbf{v}}_{i}\mathbf{V}\right)+{\mathbf{F}}_{B}+{\mathbf{F}}_{q}\end{array}$ (1)
Here m[(∂V/∂t) + v(∇V)] are the interaction forces allowing for the change in the volume of the drop (V); U (R_{ij}) is the particle interaction potential (q_{i} and q_{j} at the distance R_{ij}) in accordance with the Derjaguin–Landau–Verwey–Overbeek theoretical model; F_{L}(R_{L}) and F_{s} (R_{s}) are the interaction forces with the substrate and the liquid/air phase boundary, respectively; 6πRη(v_{i}  V)is the Stokes viscous friction force; F_{В} is the random BrownIan motion force of Gaussian type.
(a) size and (b) growth rate of nanostructures as a function of DC voltage amplitude as determined from AFM images and video monitoring at U = 19.5 V, respectively.
Comparison between the contributions of all the forces in Eq. (1) complicates the analytical solution of this equation which was only solved numerically [
F _{EF} = 2πε_{1}ReK (ω)R^{3}∇E^{2}, (2)
where ε_{1} and ε_{2} are the dielectric permeabilities of the medium (ε_{1} = ε_{Н2О} = 81) and the particles, respectively; σ_{1} and σ_{2} are the dielectric conductivities of the medium and the particles, respectively; ω is the AC field frequency; ReK (ω) = [(ε_{2}  ε_{1})/(ε_{2} + 2ε_{1})] + {3(ε_{1}σ_{2}  ε_{2}σ_{1}/[τ_{MW}(σ_{2} + 2σ_{1})^{2}(1 + ω^{2}τ_{MW}^{2})]} is the real part of the Clausius–Mossotti function; E is the electric field magnitude; R is the particle radius; τ_{MW} = (ε_{2}+2ε_{1})/(σ_{2}+2σ_{1}) is the particle recharging time (Maxwell–Wagner charge relaxation time) in AC electric fields. In DC electric fields (ω = 0) K (ω) = 1 and Eq. (1) simplifies to:
F _{EF} = 2πε_{1}R^{3}∇E^{2}. (3)
In order for nanostructures in the form of PLS or FS to be equilibrium, their growth from colloidal solutions in an electric field should probably satisfy the following equation:
F _{EF} = F_{q} = q^{2}/(4πε_{1}ε_{0}R^{2}) = qЕ.
In order for N complexes of a colloidal solution with the radius R to fill the electrode gap Z the equality Z = 2NR should be met. The change in the size of the MWCNT – COOH complexes as a function of applied bias d = f (U) can be derived from Eq. (3) taking into account that Е = U/Z. The change max(∇Е^{2}) at d → 2R will obey the hyperbolic law d = 1/U, in agreement with experimental data (Fig.
Basing on the fact that the particles move mainly due to the electrophoretic force (Eq. (2)), one can convert the motion equation F_{EF} = mdv/dt at a constant field magnitude to the following empirical function:
$V\left(U\right)=\frac{3}{2}(\rho {)}^{1}{\epsilon}_{1}{E}^{2}\int dt={v}_{0}+B{E}^{2}$ (4)
where v_{0} is the initial velocity determined by diffusion and convection of particles in the test colloidal solution; ρ is the density of carbon particles; B is the constant that accounts for ρ; ε_{1} is the dielectric permeability of the liquid phase of the colloidal solution; Z is the electrode gap which was 100, 500, 1000 or 1500 µm in the experiment; t is the evaporation time of a solution drop (3.5 min).
A Raman spectrum taken in perpendicular radiation incidence and scattering geometry contained bands in the radial “breathing" mode region that are only typical of SWCNT on PLS and DS (276 cm^{1}) and FS (216 cm^{1}), see Fig.
At an excitation energy of 2.33 eV (532 nm), taking into account the Kataura Table [
Selfassembly and/or selforganization in purified and functionalized MWCNT – COOH + aerosil + H_{2}O_{dw} produces diffuse structures, fractal structures or local piecewise formations the sizes of which obey the 1/U law and the growth rate of which obeys the U^{2} law. MWCNT – COOH + aerosil + H_{2}O_{dw} complexes are oriented in a controlled manner in electric fields. We show that intense ultrasonication of MWCNT – COOH + aerosil + H_{2}O_{dw} complexes produces axis centered SWCNT inside MWCNT as confirmed by Raman scattering excitation in the short wave region (radial “breathing" modes). The variety of structures forming in DC electric fields is caused by the existence of combined sp^{2}hybridization types with π and σ carbon bonds and the metallic and semiconductor conductivity types in the material which alone shows good promise of these structures for nanoelectronics.
The work was performed with financial support of the Ministry of Education and Science of the Russian Federation within the basic part of State Assignment No. 16.2814.2017/PCh (Project No. 39.13) and Agreement No. 14.577.21.0181 (unique ID RFMEFI57715X0181).