Controlled Synthesis of Single-Walled Carbon ... - ACS Publications

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Controlled Synthesis of Single-Walled Carbon Nanotubes in an Aerosol Reactor Ying Tian,† Albert G. Nasibulin,*,† Brad Aitchison,‡ Timur Nikitin,§ Jan v. Pfaler,^ Hua Jiang,† Zhen Zhu,† Leonid Khriachtchev,§ David P. Brown,‡ and Esko I. Kauppinen*,† †

NanoMaterials Group, Department of Applied Physics and Center for New Materials, Aalto University, Puumiehenkuja 2, 00076 AALTO, Finland ‡ Canatu Ltd., 02150, Espoo, Finland § Department of Chemistry, University of Helsinki, P.O. Box 55, 00014 Helsinki, Finland ^ Department of Mathematics and Systems Analysis, Aalto University, Espoo, Finland ABSTRACT: The growth mechanism and influence of synthesis parameters on the properties of single-walled carbon nanotubes (SWNTs) produced by ferrocene vapor decomposition in a carbon monoxide atmosphere have been investigated in detail by a combined study of Raman and UV-vis-NIR absorption spectroscopy and transmission electron microscopy (TEM). CO2 plays an essential role in selective etching of small diameter nanotubes and the purification of SWNTs. This etching effect is beneficial to narrow the diameter distribution and to control the average diameter of SWNTs. Increasing the synthesis temperature results in the formation of larger catalyst particles due to a higher agglomeration rate, thereby forming larger diameter nanotubes. Decreasing the CO flow rate, and thus lengthening the agglomeration time, also provides the possibility to enlarge the diameter of SWNTs. Therefore, by varying the growth parameters, the mean diameter of SWNTs can be effectively changed from 1.2 to 1.8 nm to satisfy the needs of various applications.

1. INTRODUCTION Carbon nanotubes (CNTs), and in particular SWNTs, have attracted great interest due to their remarkable properties, which have been utilized in many different applications such as sensors, transistors, optoelectronic devices, and composites and for energy storage.1-4 It is well-known that the properties of SWNTs can vary significantly depending upon their diameters and chiral angles. Therefore, it is crucial to produce SWNTs with selective properties to fit the requirements for specific applications. However, a bottleneck for the application of SWNTs is the limited diameter and chirality control of produced nanotubes and low production rates. The CVD method has been found to be efficient and selective for the growth of either SWNTs or multiwalled carbon nanotubes. The aerosol (floating catalyst CVD) synthesis method,5-9 in particular, has potential for large scale SWNT production since it is a continuous process involving both catalyst particle formation and subsequent SWNT growth occurring in the same reactor. In addition, direct sampling of SWNTs from the gaseous phase by filtering and subsequent transfer onto practically any substrate at room temperature gives another practical advantage over other methods.10 Recently, our group has reported a technique for the production of rather long (up to a few micrometers) highpurity SWNTs based on ferrocene vapor decomposition in an atmosphere of ambient pressure CO.11 Previously, we revealed an essential role of CO2 in the process of SWNT formation.12 However r 2011 American Chemical Society

the effect of CO2 as well as other experimental parameters on the characteristics of produced SWNTs remained unclear. In this work, we carried out the investigations of the influence of various parameters (temperature, flow rate, and CO2 concentration) on properties of SWNTs and studied the details of CNT formation. The SWNTs were synthesized by a pilot aerosol reactor which is scaled up by a factor of 25 as compared to our previous reactor based on the ferrocene decomposition in CO atmosphere.7 This allowed us to produce longer SWNTs with higher yield because of longer residence time and higher flow rates in the reactor. This is the first detailed parametric analysis of SWNTs as a function of growth conditions using optical methods for this type of reactor. The sample morphology, mean diameter, and diameter distribution are analyzed by a combined study of TEM, Raman, and UV-vis-NIR absorption spectroscopy.

2. EXPERIMENTAL SECTION The continuous synthesis of SWNTs using this method was described in detail elsewhere.7 Briefly, CO was used as the carbon source and ferrocene as the catalyst precursor was vaporized by passing ambient temperature CO through a cartridge filled with the ferrocene powder. The flow containing ferrocene vapor and Received: December 27, 2010 Revised: February 7, 2011 Published: March 24, 2011 7309

dx.doi.org/10.1021/jp112291f | J. Phys. Chem. C 2011, 115, 7309–7318

The Journal of Physical Chemistry C

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Figure 1. Low-magnification TEM images of the as-deposited SWNTs synthesized at 920 C with CO2 concentration of (a) 0%, (b) 0.4%, (c) 0.7%, (d) 1.0%, and (e) 1.2%.

CO was then introduced into the heated reactor. The samples were collected from the scaled-up aerosol reactor operated under various conditions. The reactor temperature was set at 840, 880, and 920 C. The concentration of added CO2 was changed from 0 to 1.2% at 920 C. In order to examine the effects of residence time on the CNT production, the CO flow rate was varied from 2.0 to 8.0 L/ min. It is worth noting that, in order to study the effects of a particular parameter, all other variables were kept constant while varying the given studied parameter. The SWNT samples were collected downstream of the reactor on a TEM grid for TEM (Philips CM200 FEG) observation or by filtering the flow through 2.45 cm diameter nitrocellulose disk filters (Millipore Corp., Billerica, MA) for optical measurements. The asdeposited samples on filters were analyzed by Raman spectroscopy by using four different laser excitations in three Raman spectrometers equipped with cooled CCD detectors, namely a Wintech alpha300 Raman spectrometer using a Nd:YAG green laser at 532 nm (2.33 eV), an Acton SpectraPro 500I spectrometer with a He-Ne laser at 633 nm (1.96 eV), and a JY-Horiba LabRAM HR 800 spectrometer using excitations at 488 nm (2.54 eV) and 785 nm (1.58 eV). To measure the absorption spectra of the SWNT thin films, the sample was transferred by the dry transfer technique10 from a filter to an optically transparent, 1 mm thickness quartz window substrate (material: HQS300, Heraeus). The absorption spectra were recorded by a double-beam Perkin-Elmer Lambda 950 UV-vis-NIR spectrometer equipped with two excitation sources of a deuterium lamp and a halogen lamp, which cover the working wavelength range from 175 to 3300 nm. An uncoated substrate was used in the reference beam to exclude the effect of the substrate.

3. RESULTS AND DISCUSSION 3.1. Effects of CO2 Concentration. It has been found that CO2 acting as an etching agent plays an essential role in the SWNT formation process.13,14 In order to study the influence of CO2 on the properties of SWNTs, a set of samples was collected at CO2 concentration of 0, 0.4, 0.7, 1.0, and 1.2%, while keeping other parameters constant: temperature of 920 C and CO flow rate of 4.0 L/min. Figure 1 shows the TEM images of the as-deposited samples collected at different concentrations of CO2 at 920 C. One can observe that the SWNT bundles are quite long and support a number of inactive catalyst particles, and the amount of impurities can be controlled by varying the concentration of CO2. The highest purity of SWNTs is obtained at a CO2 concentration of 0.7%. Beyond this CO2 level, the quality of SWNTs drops, and finally SWNTs growth is terminated. As shown in Figure 1e, the products collected at 1.2% CO2 mainly consist of inactive catalyst particles. To further characterize the SWNT samples, Raman spectroscopy was employed using excitation energies at 1.58, 1.96, 2.33, and 2.54 eV as shown in Figure 2. As expected, the Raman spectra show the strong G and RBM features of SWNT samples made by this method. Very low intensity D bands are observed in the figure. The G band is related to the vibration of sp2-bonded carbon atoms in a two-dimensional hexagonal lattice of graphite layer; on the other hand, the D band corresponds to the presence of defects on the nanotube walls or amorphous carbon material in the sample.15 Therefore, the ratio of G and D band intensities (IG/ID) has been commonly used as a reflection of the quality of SWNT samples. Nevertheless, the sample collected at 1.2% CO2 7310

dx.doi.org/10.1021/jp112291f |J. Phys. Chem. C 2011, 115, 7309–7318


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Figure 2. Raman spectra of SWNT samples synthesized at 920 C showing (a) G, D bands excited by 2.33 eV and RBM bands excited (b) 2.33 eV, (c) 1.58 eV, (d) 1.96 eV and (e) 2.54 eV energies as a function of CO2 concentration. The inset to (a) is the calculated IG/ID ratio as a function of CO2 concentration.

does not show any D band and presents a very low intensity of G band. This shows, as expected, that there are many catalyst particles along with little SWNT material which is in good agreement with the TEM image (Figure 1e). The ratio of IG/ID for the samples collected at CO2 concentration from 0 to 1.0% is plotted in the inset to Figure 2a. It shows that the optimum CO2 concentration corresponds to about 0.7%, at which the highest quality of SWNTs is obtained. Parts b, c, d, and e of Figure 2 show the RBM region of SWNT samples using the excitation energies of 2.33, 1.58, 1.96, and 2.54 eV, respectively. For each excitation energy, the spectra

show similar RBM positions for the SWNT samples collected at different conditions and similar dependence on CO2 concentration. The relative intensity of the RBMs located at higher frequency (indicated in the dashed frame) decreases with increasing CO2 concentration. The inverse relationship between RBM frequency and nanotube diameter indicates that the relative abundance of smaller diameter SWNTs decreases with the increase of CO2. Although Raman spectroscopy is a very sensitive method to characterize the diameters of SWNTs, it is still challenging to obtain a full diameter distribution from the resonant Raman 7311



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Figure 3. Absorption spectra of SWNT samples synthesized at 920 C (a) before and (b) after background subtraction (in black dots) along with the fitted curves (in solid lines) as well as (c) fitted diameter distributions of SWNT samples from the corresponding absorption spectra as a function of CO2 concentration. The mean diameters dt are 1.4, 1.4, 1.6, and 1.7 nm for the SWNT samples collected at 0, 0.4, 0.7 and 1.0% of CO2, respectively.

data. Thus, UV-vis-NIR absorption spectrometry was further applied to characterize the mean diameter and diameter distribution of the SWNT samples. Figure 3a shows absorption spectra of the SWNT thin film samples. The absorption peaks from the interband electronic transitions strongly overlap for the SWNT samples collected with 0 and 0.4% CO2. The absorption peaks become more resolved and narrower for the samples collected at higher CO2 concentrations, which implies a narrower diameter distribution in these samples. The rather flat absorption profile from the sample synthesized at 1.2% CO2 confirms the low SWNT fraction in this sample. This result is consistent with the Raman analysis (Figure 2) and TEM imaging (Figure 1e). A universal method for evaluating the mean diameter and diameter distribution of SWNTs has been developed recently based on optical absorption spectra.16 Without making a strict assumption for the form of diameter distribution, the optical absorption from the transition energies of each nanotube was modeled by summing contributions over the entire absorption spectrum. Using this method, it is feasible to quantify the diameter distributions of SWNT samples with any shape of UVvis-NIR absorption profile. Figure 3b shows the original absorption spectra after background subtraction (dotted black lines) along with the fitted spectra (solid lines) on the top. The associated diameter histograms of the SWNT samples are presented in Figure 3c. In order to confirm the data obtained from absorption spectra, we carried out statistical measurements from high-resolution TEM images for the sample collected at 0.4% of CO2 (with the statistical sample of 90 SWNTs). As can be seen, the comparison of the data obtained by these two different methods shows very similar results (Figure 4).

For the SWNT sample collected without CO 2 , the diameter histogram shows a bimodal distribution with a Gaussian mean diameter of 1.4 nm. Upon adding a small amount of CO 2 , the bimodal diameter distribution merges to a single modal distribution. The SWNT sample collected at 0.4% CO 2 shows a mean diameter of 1.4 nm. When the CO 2 concentration increases to 0.7%, the mean diameter shifts to 1.6 nm with significantly fewer small-diameter nanotubes. For the SWNT sample synthesized at 1% CO 2 , the mean diameter continuously increases to 1.7 nm. The calculated results of absorption spectra are in a good agreement with the previous Raman studies; i.e., the relative abundance of smaller diameter nanotubes decreases with the increase of CO 2 concentration. This results in a narrower diameter distribution and increase of the mean diameter. Further high-resolution TEM (HRTEM) studies were conducted on the same SWNT samples collected at 0, 0.4%, and 0.7% CO 2 . 16 The diameter diagrams obtained from the direct observation of HRTEM images support the results that the bimodal diameter distribution of SWNTs collected without CO 2 and the variation of mean diameter as a function of CO2 concentration. Let us discuss the possible role of CO2 during the synthesis of SWNTs. As was found on the basis of TEM and Raman spectroscopy analyses, CO2 plays a role of an etching agent which can remove amorphous carbon coated on the surfaces of catalyst particles and nanotubes, thereby enhancing the activity and lifetime of the catalysts for the nanotube growth and improving the quality of carbon nanotube.13,14 Similar etching behavior against amorphous carbon coating was also observed in the H2O-assisted growth.17-19 7312



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Figure 4. Comparison of SWNT diameter distributions obtained on the basis of (a) absorption spectrum and (b) measurements from high-resolution TEM images for the sample collected at 0.4% of CO2. (c) An example of the HRTEM image used for statistical diameter measurements.

Figure 5. Low- and high-magnification TEM images of the as-produced SWNTs synthesized without adding CO2 at (a, d) 840 C, (b, e) 880 C, and (c, f) 920 C.

In addition, another possibility which cannot be ruled out is that CO2 can prevent the formation of cementite, Fe3C, though the reaction Fe3 C þ CO2ðgÞ S 3Fe þ 2COðgÞ ,

ΔH ¼ 146 kJ=mol ð1Þ

since cementite is believed to be an inactive phase for fiber and nanobues growth.17,20 Meanwhile, the selective etching of small diameter nanotubes by CO2 can be understood since the relatively more reactive

carbon atoms in the carbon network of high curvature are easily etched due to the inverse Bouduard reaction, consequently suppressing the growth of small diameter nanotubes as observed in the experimental results.21 This provides the possibility to narrow the diameter distribution and enlarge the mean diameter of SWNT samples, which can be very important for some applications, e.g., optical laser absorbers. However, the excessive amount of CO2 reduces the quality of CNTs and finally terminates the nanotube growth, which might be due to the further 7313



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Figure 6. Raman spectra of SWNT samples showing (a) G, D bands excited by 2.33 eV and RBM bands excited by (b) 2.33 eV, (c) 1.58 eV, (d) 1.96 eV, and (e) 2.54 eV laser energies as a function of synthesis temperature without adding CO2. The inset to (a) shows the IG/ID ratio as a function of temperature.

oxidation of carbon nanotube or preventing the CO disproportionation by shifting the equilibrium reaction to the left: COðgÞ þ COðgÞ S CðsÞ þ CO2ðgÞ , ¼ - 169 kJ=mol

ΔH ð2Þ

3.2. Effects of Temperature. Another important parameter affecting the properties of carbon nanotubes is the synthesis temperature. The SWNT samples were collected at 840, 880, and 920 C. The CO flow rate was fixed at 4.0 L/min and no CO2

was added in the reactor. The morphology of the SWNT samples was characterized by TEM measurements. The overview TEM images (Figure 5) show that the as-deposited nanotube bundles are quite long with only a small amount of inactive catalyst particles. The higher magnification TEM images in Figure 5 reveal that the sample synthesized at 920 C presents cleaner SWNT surfaces than the other two samples. Raman spectra, using laser excitation energies of 1.58, 1.96, 2.33, and 2.54 eV, for the samples collected at 840, 880, and 920 C are shown in Figure 6. The G and D bands of the SWNT samples 7314



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Figure 7. Absorption spectra of SWNT samples (a) before and (b) after background subtraction (in black dots) along with the fitted curves (in color solid lines) as well as (c) fitted diameter distributions of SWNT samples from the correspondence absorption spectra as a function of the synthesis temperature without adding CO2. The mean diameters dt of the samples are 1.2, 1.3, and 1.4 nm at temperatures of 840, 880, and 920 C, respectively.

excited by 2.33 eV are plotted in Figure 6a. The inset to Figure 6a shows the ratio of IG/ID as a function of growth temperature. The results indicate that the highest quality SWNTs are obtained at 920 C. This is quite reasonable since defects are annealed away at higher temperature, which results in the more crystalline graphitic nanotube walls.3 Another explanation could be that the same small amount of CO2 generated inside the reactor plays a more important role in the equilibrium reaction (reaction 2) at higher temperature based on the thermodynamics of CO disproportionation.,22 Thus, the etching role of CO2 against amorphous carbon favorably occurs at higher temperature by shifting the equilibrium of reaction 2 to the left, consequently improving the quality of the SWNT products. The RBM spectra excited by 2.33 eV in Figure 6b show similar peak positions for the three samples. However, the relative intensity of the RBM modes in the scale of 220-270 cm-1 becomes more prominent at lower temperature (840 and 880 C). This suggests the relative abundance of smaller diameter nanotubes in these two samples. On the basis of the corrected Kataura plot which is in good agreement with the experimental results from our SWNT thin film samples,16 the RBM modes at 228 and 269 cm-1 correspond to the 24 and 21 families of metallic nanotubes, respectively. From the observation of the G band in Figure 6a, the two samples collected at temperatures of 840 and 880 C indeed show a distinct Breit-Wigner-Fano (BWF) line shape (indicated by an arrow), a signature of metallic tubes.23 This supports the analysis of RBM spectra. Using the excitation energies of 1.58, 1.96, and 2.54 eV to probe different band-gap regions, the same trend is observed, namely, that the relative intensity of RBM peaks at higher frequency increases with the decrease of the growth temperature. This corresponds to the increase of relative concentration of smaller diameter tubes.

The absorption spectrometer was utilized to further characterize the full diameter distribution of the SWNT samples synthesized at different temperatures. The original absorption spectra are shown in Figure 7a. The SWNT samples collected at 840 and 880 C present similar fine structures but different from the sample collected at 920 C. Using the same method, the fitted spectra are plotted in Figure 7b along with the absorption spectra after background subtraction. Very little difference between the calculated and experimental spectra is observed in the figure. The corresponding calculated diameter distributions are shown in Figure 7c. As previously discussed the SWNT sample synthesized at 920 C without adding CO2 presents a bimodal diameter distribution with peaks at 1.3 and 1.7 nm and a mean diameter of 1.4 nm. When decreasing temperature to 880 C, the diameter distribution still shows a bimodal structure which is implied by the shape of absorption peak in Figure 7b. However, the relative abundance of the two fractions changed. There is larger fraction of smaller diameter nanotubes. The calculated mean diameter is 1.3 nm. For the 840 C sample, the relative abundance of smaller diameter nanotubes further increases in the range of 0.8-1.4 nm with a mean diameter of 1.2 nm. Therefore, the results obtained from the absorption spectra are in good agreement with the Raman analyses; i.e., the relative concentration of smaller diameter nanotubes increases with the decrease of temperature, which results in the decrease of the mean diameter of the SWNT samples. It has been shown that the diameter of nanotubes was correlated to the size of the metal catalyst particles from which they grow;24,25 i.e., larger catalyst particles yield larger diameter nanotubes. It has been previously observed that an increase in temperature results in an increase in catalyst particles size due to the higher rate of agglomeration of nucleated particles, thereby forming larger diameter nanotubes.26,27 Our results, 7315



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Figure 8. Raman spectra of SWNT samples showing (a) G, D bands excited by 2.33 eV and RBM bands excited by (b) 2.33, (c) 1.58, (d) 1.96, and (e) 2.54 eV energies as a function of CO flow rate with a constant CO2 concentration of 1.2%. The inset to (a) shows the calculated IG/ID ratio as a function of CO flow rate.

as analyzed by Raman and absorption spectra, support this observation. In addition to the above, a bimodal diameter distribution of SWNTs is observed in the present study, which we believe is correlated to the bimodal size distribution of catalyst particles. 3.3. Effects of Flow Rate. The formation of SWNTs is studied at different residence time in the reactor by varying the flow rate of CO. The samples were collected at CO flow rates of 2.0, 4.0, and 8.0 L/min at 880 C. In order to exclude the effects of CO2, the CO2 concentration was kept constant at 1.2%. The TEM

images show SWNT bundles. No obvious difference is observed among these samples. The Raman spectra using the excitation energies of 1.58, 1.96, 2.33, and 2.54 eV are plotted in Figure 8. The inset to Figure 8a shows the ratio of IG/ID as a function of CO flow rate using the laser energy of 2.33 eV. The intensity ratios of G and D bands are all greater than 50, which indicates the high quality of these SWNT samples. The highest value is obtained for the sample synthesized at CO flow rate of 2.0 L/min. Similar RBM spectra are observed in Figure 8b when excited by 2.33 eV. While using 7316



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Figure 9. Absorption spectra of SWNT samples (a) before and (b) after background subtraction (in black dots) along with the fitted curves (in color solid lines) as well as (c) fitted diameter distributions of SWNT samples from the correspondence absorption spectra as a function of CO flow rate with a constant CO2 concentration of 1.2%. The mean diameters dt of the samples are 1.8, 1.7, and 1.6 nm at CO flow rate of 2.0, 4.0, and 8.0 L/min, respectively.

the excitation energy of 1.58 eV, the RBMs show an increase of intensity in the range of 200-250 cm-1 as a function of the CO flow rate. Similarly, the data from excitation energies of 1.96 and 2.54 eV show the relative intensity of RBMs located at higher frequency increases when the CO flow rate is 8 L/min. This implies an increase in the relative concentration of smaller diameter nanotubes with higher CO flow rate. The absorption spectroscopy measurements were performed to complete the picture of the diameter distribution of this set of SWNT samples. As shown in Figure 9a, the lowest energy absorption peaks of the three samples exhibit bimodal structure and the relative intensity of the peak at about 0.7 eV increases with the CO flow rate. Parts b and c of Figure 9 display the fitted absorption spectra along with the spectra after background subtraction and the calculated diameter distributions, respectively. For the SWNT samples collected at CO flow rate of 2.0 and 4.0 L/min, the bimodal distributions both consist of most abundant diameter fractions in the scale of 1.2-1.6 and 1.62.2 nm; however, the relative concentration of the two fractions changes with the flow rate. The percentage of nanotubes with a diameter in the range of 1.2-1.6 nm increases from 21% to 32% when the CO flow rate increases from 2.0 to 4.0 L/min. For the SWNT sample synthesized at a CO flow rate of 8.0 L/min, the concentration of the nanotubes in the smaller diameter scale (1.0-1.6 nm) increases dramatically up to 50%. Further, the mean diameters are 1.8, 1.7, and 1.6 nm for the SWNT samples synthesized at CO flow rate of 2.0, 4.0, and 8.0 L/min, respectively. Therefore, it is clear that the fraction of smaller diameter nanotubes increases with the CO flow rate. These results obtained from absorption spectra are in good agreement with the analysis of Raman spectra. Namely, with an increase in CO flow rate, the relative fraction of smaller diameter nanotubes

increases which results in a decrease of the mean diameter. The CO flow rate influences both the ferrocene heating rate and the residence time in the reactor. Lower flow rates lead to lower heating rate and longer residence time which provides a greater opportunity for Fe catalyst particles to collide and to aggregate forming larger particles. As discussed in the previous section, larger catalyst particles result in the formation of SWNTs with larger diameters. Therefore, lower flow rates result in larger mean diameter of SWNTs, as observed in Raman and absorption spectra. Additionally, the highest purity of the SWNT sample is obtained at the CO flow rate of 2.0 L/min. This may be also due to the longer residence time in the high-temperature zone of the furnace. The impurities and amorphous carbon can be annealed and etched away by greater exposure time to the high temperature, since CO disproportination reaction is significantly shifted to the direction of initial components.

4. CONCLUSIONS A detailed parametric analysis of SWNTs synthesized in a scaled-up aerosol reactor is performed utilizing a combined study of Raman and UV-vis-NIR absorption spectroscopy with electron microscopy. Depending on different growth conditions such as CO2 concentration, temperature, and CO flow rate, the mean diameter of a SWNT sample can be varied from 1.2 to 1.8 nm. The addition of a small amount of CO2 improves the quality of SWNT samples. Meanwhile, selective etching of small diameter nanotubes by CO2 provides a way to narrow the diameter distribution and to vary the mean diameter of a SWNT sample. In addition, increasing temperature results in larger catalyst particles due to the higher possibility of agglomeration after nucleation and hence forms larger diameter nanotubes. The 7317


The Journal of Physical Chemistry C quality of SWNTs is improved by the increase of growth temperature. The flow rate of CO is also a parameter to influence the nature of SWNT products. The mean diameter of SWNTs decreases with increasing the CO flow rate. To conclude, by varying the growth parameters, the properties of SWNT samples (mean diameter, diameter distribution, and purity) can be tuned to meet the requirements for specific applications.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: albert.nasibulin@tkk.fi (A.G.N.), esko.kauppinen@tkk.fi (E.I.K.). Fax: þ358 94513517. Tel: þ358 405098064.

’ ACKNOWLEDGMENT This work was financially supported by the Academy of Finland (Project Nos. 128495 and 128445), the TEKES GROCO project (1298/31/08), Canatu Ltd., and the CNB-E project in the Aalto University Multidisciplinary Institute of Digitalization and Energy (MIDE) programme. T.N. and L.K. thank the Finnish Centre of Excellence in Computational Molecular Science, the FinNano Program (OPNA Consortium), and the University of Helsinki Research Funds (HENAKOTO).

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