Summary

In this chapter, the study started with the selection of dopants. In order to clarify the influence of properties of dopants on thermoelectric characteristic, we chose three tetravalent metal ions in the same group(ⅣB group) as dopants, i.e. Ti⁴⁺, Zr⁴⁺ and Hf⁴⁺; moreover, we also used the precursors with the same functional ligand to control the reactivity. After systematical investigation, we concluded that Hf⁴⁺ showd the greatest benefit on enhancing thermoelectric performance of ZnO (6 times improvement), since the incorporation of Hf didn’t decrease the electrical mobility severely. This advantage ensured the enhancement of the electrical conductivity without increasing carrier concentration greatly. Thus, the high absolute value of Seebeck coefficient was maintained. Besides, the great mass difference between Hf and Zn suppressed the thermal conductivity effectively.

However, the electrical conductivity was only enhanced by the factor of 2.5 due to the low doping efficiency of conventional ALD doping process. Thus, we proposed a new doping technique, i.e. mixed ALD doping process, to further increase the electrical conductivity. At the optimal doping concentration, the highest conductivity was almost 5-fold more compared with undoped ZnO. Unfortunately, an abundance of free carriers lowered the absolute value of Seebeck coefficient and the loosely distributed dopants no longer impeded the transportation of phonons effectively. For this reason, the degree of

enhancement on ZT value, which was only about 4.5 times, was unsatisfactory.

Afterward, we combined the advantages of conventional process and mixed ALD doping process to deposit HZO superlattices, i.e. heat blocking for conventional process and excellent electrical performance for mixed ALD doping process. After striking a balance between all effects, we conclude that 7-cycle HfO2 was the optimal heat and electrical barrier to incorporate with super ZnO, because it could increase the absolute value of Seebeck coefficient by energy filtering effects and impeded heat transport by forming distinct HfO2/ZnO interface. Hence, the overall enhancement on ZT value was over an order of magnitude (~13 times).

Finally, we investigated the effect of isotope incorporation on thermoelectricity in compound materials for the first time. After modifying the deposition parameters, the electrical performance of ¹⁸O-containing films was about the same with the normal films (¹⁸O-excluding). After that, we controlled the period length of superlattice to probe the interface-distance dependent thermal conductivity in our systems. It turned out that the optimal periodicity was determined to be 4.5/4.5 nm, which best matches the majority phonon wavelengths of ZnO. Therefore, regarding the enhancement on ZT, the further 20% increment was achieved and the ZT value of optimal ZnO was about 0.023 at 300K, which was about 16 times higher than that of undoped ZnO.

Chapter 4

Metal oxide/polymer superlattice

As we stated in section 1.8.3, the deposition of conducting polymers by MLD technique was developed to prepare the insertion material in metal oxide/polymer superlattice. In the research, three common conducting polymers were tested, i.e.

polythiophene, polyaniline and PEDOT. Among the deposition processes, PEDOT-process possessed the following merits: reproducibility, robustness and the high electrical conductivity of the deposited films. Although the research started with the deposition of polythiophene and polyaniline, the focus of this chapter was on the characteristic and the deposition process of PEDOT and its metal oxide/polymer superlattice. As for the deposition of polythiophene and polyaniline, they are stated in the latter part of this section.

4.1 Poly(3,4-ethylenedioxythiophene) (PEDOT) 4.1.1 The deposition of PEDOT

In the development of depositing polyaniline by MLD technique, we noticed that the vapor pressure of oxidant, i.e. SbCl5, decayed with time if the temperature was elevated. It might be due to the high oxidizing power nature which made itself unstable.

For addressing the stability issue, prolonging the pulse time was utilized to increase the

amount of vapor phase SbCl5 molecules instead of elevating the temperature of SbCl5. Thus, we set the temperature of SbCl5 at 75°C, at which the degradation was less obvious. For the detailed experimental settings, they can be found in Table 2.6. The characteristics of deposited PEDOT films are summarized in Table 4.1.

First, the blue appearance of deposited films shown in Figure 4.1 and the

characteristic absorption peaks in FTIR spectrum (Figure 4.2) confirmed the formation of PEDOT by MLD process. In FTIR spectrum, 10 typical absorption bands of PEDOT are listed as follow: 1519, 1312, 1206, 1133, 1079, 1032, 969, 912, 829, and 690 cm^-1. The bands at 1519 and 1312 cm^-1 are originated from the stretching mode of C = C and C-C in thiophene ring. The bands around 1206, 1133, 1079, and 1032 cm^-1 are

attributed to the bending mode of C-O-C in ethylenedioxy groups; the bands at 969, 912, 829, and 690 cm^-1 are identified as the stretching mode of the C-S-C bond in thiophene ring. All bands mentioned above and the absence of the peak at 754 cm^-1 which is attributed to C-H of the aromatic structure confirmed the successful formation of the PEDOT in MLD process.

As for thermoelectric properties, MLD-deposited PEDOT showed a high

electrical conductivity without post-treatments, which were common ways to promote the electrical conductivity of PEDOT; however, it showed a poor absolute value of Seebeck coefficient. This characteristic was attributed to high doping level of deposited

PEDOT. In Figure 4.3, which is the UV-Vis spectrum of our PEDOT, there are two characteristic peaks at about 400-700 nm and 1000-1060 nm with a free tail extending into the near-infrared region. These two peaks are ascribed to the π-π* transitions of thiophene ring and polaron and/or bipolaron bands for oxidized PEDOT with long conjugation length, respectively. A high absorbance in near-infrared region relative to visible light region indicated the high doping level of PEDOT deposited by MLD process; hence, a high carrier concentration in PEDOT was expectable, which yielded a high electrical conductivity but low absolute value of Seebeck coefficient, thus a low power factor. Regarding thermal conductivity, our PEDOT showed a typical behavior of polymers that is almost thermally insulated and the thermal conductivity is lower than unity. Considering all effects, the ZT value of our PEDOT was only 0.051 at 300 K, which made it improper to serve as a particle thermoelectric material by itself.

Compared to the MLD polyaniline process we developed, there were two merits in PEDOT deposition process. First, it is obvious that the deposited films were highly conductive unlike MLD deposited polyaniline and performed a typical p-type

conducting. Without post-treatment which was commonly conducted to promote electrical conductivity, such as acid rinse, the conductivity of our PEDOT films were over 400 S cm^-1 that could compete with those synthesized by traditional solution process. Second, it is noteworthy that the growth rate of PEDOT was almost ten times

higher than that of polyaniline we deposited, i.e. 0.3~0.4 nm/cycle and 0.03 nm/cycle, respectively; hence, high growth rate ensured the practicability of MLD deposited PEDOT in various applications. Thanks to these advantages, MLD deposited PEDOT was promising and then chosen as the insertion material in developing metal

oxide/polymer superlattice thermoelectric materials.

Table 4.1: The characteristic of deposited PEDOT films.

Thickness

(nm)

Growth rate

( nm/cycle)

σ

(S cm^-1)

Seebeck

(µV K^-1)

PF

(10^-4 W m^-1 K^-2)

κ

(W m^-1 K^-1)

ZT (300k)

60.6 0.303 456.6 8.39 0.032 0.19 0.0051

Figure 4.1: The photographs of deposited PEDOT films and blank glass substrates.

Figure 4.2: The FITR spectrum of deposited PEDOT films.

Figure 4.3: The UV-Vis spectrum of deposited PEDOT films.

4.1.2 Interface-engineering of metal oxide/PEDOT superlattice

As we stated in previous section, PEDOT was selected as an insertion material and utilized to develop conducting polymer incorporated HZO superlattice

thermoelectric materials due to its high growth rate and conductivity. However, the integration of two processes pressed for an investigation before depositing

HZO/PEDOT superlattices, since PEDOT and HZO didn’t offer proper reaction sites for each other to nucleate during switching.

To address this issue, we first monitored the growth of PEDOT on the surface of ZnO using the pristine deposition settings (same as those in previous section) by QCM.

In Figure 4.4, it shows the mass gain of the one-cycle PEDOT on the surface of ZnO using different pulse sequence of precursors. It can be seen clearly that mass gain of EDOT dose approached to zero in the pristine pulse sequence (EDOT first) even incoming SbCl5 indeed chemisorbed onto the surface. This phenomena revealed two facts: EDOT monomers couldn’t adsorb onto the surface on ZnO; SbCl5 molecules could react with surface functional groups of ZnO. These two facts concluded that no PEDOT formed after one cycle of PEDOT-deposition. However, as we changed the pulse sequence (SbCl5 first), there were positive mass gains after the pulse of both precursors, which meant PEDOT indeed formed by this pulse sequence. Besides QCM analysis, the growth mechanism could be also discerned by cross sectional TEM images

presented in Figure 4.5: the inserted PEDOT layers can be seen in Figure 4.5 (b), but no layered structures in Figure 4.5 (a). Hence, the modified pulse sequence, i.e. SbCl5 was introduced first, was utilized to deposit inserted PEDOT layers in superlattice;

moreover, the multiple pulse process of EDOT, i.e. SbCl5EDOTEDOTEDOT, was also utilized to ensure the growth of PEDOT under few cycles of deposition.

After dealing with the modification of PEDOT deposition process, we further investigated the growth of incoming ZnO on the surface of inserted PEDOT layers to optimize the deposition process of HZO. In Figure 4.6, it shows the MGPC of ZnO at the steady-growth region and the mass gain of the first cycle using the normal

depositing parameters (i.e. without exposure) of ZnO on surface of inserted PEODT with various modifications. The low mass gain of ZnO depositing without any modification interpreted the poor nucleation and growth of ZnO on the surface of inserted PEDOT as we expected. However, after conducting each modification, the mass gain truly increased which validates the profit of interface-engineering. In type A, the exposure of SbCl5 during depositing PEDOT was abrogated to eliminate redundant physisorbed molecules since unreacted SbCl5 molecules were nearly inert to the incoming H2O which was analyzed by QCM shown in Figure 4.7, and thus hinder the nucleation of ZnO; in type B, long exposure time (25 seconds) of each precursor was utilized to increase the physisorbed precursors and/or reaction probability (by increasing

collision between gas-phase molecules and surface), and then improved the nucleation of ZnO on the surface of PEDOT; in type C, the dose of H2O2 was believe to increase the density of hydroxyl groups on the organic surface which offered chemical reaction sites for incoming precursors of ZnO, and thus improved nucleation and growth of ZnO.

For achieving best improvement in the mass gain of ZnO, three types of modifications were applied simultaneously to enhance the growth of incoming ZnO synergistically, and the result was outstanding since the mass gain was almost 2.5 times to the one without any modification. Moreover, the electrical conductivity of PEDOT/HZO superlattice with various modifications was measured to evaluate efficacy of

modifications, the measured electrical conductivities are summarized in Table 4.2. The result tallied with the observation by QCM analyses in previous discussions. It could be seen that electrical mobility was enhanced significantly indicating the reformed quality of HZO and interface between each layers that were originated from the improvement of the nucleation and growth of ZnO of the surface of PEDOT.

In summary, the interface-engineering was explored to integrate ALD-HZO and MLD-PEDOT processes to deposit HZO/PEDOT superlattice. The

interface-engineering included the modification of PEDOT process to ensure the growth of PEDOT under few cycles of deposition and the modification of PEDOT/ZnO interfaces to improve the quality of ZnO (HZO). The modifications of PEDOT process included

switching pulse sequence (SbCl5 first), abrogating the exposure time of SbCl5 and utilizing multiple pulse of EDOT monomers. Besides, two modifications were applied before the deposition of ZnO using normal parameters: the dose of H2O2 to increase the density of surface hydroxyl groups and 5-cycle nucleation process of ZnO that

substituted 5 cycles with normal parameters to promote nucleation.

Table 4.2: The electrical conductivity of PEDOT/HZO superlattice with various modifications in the structure of 6×(4 periods mix 19:1 HZO + 1-cyle PEDOT)

n

(×10¹⁹ cm^-3)

μ (cm² V^-1 s^-1)

σ (S cm^-1)

Without 9.86 2.51 39.4

Type A 12.65 3.10 62.9

Type A+B 11.12 7.87 140.2

Type A+B+C 12.72 13.75 280.2

Figure 4.4: The in-situ QCM results of the deposition of PEDOT on the surface of ZnO using two kinds of pulse sequences.

Figure 4.5: The cross sectional TEM images of deposited films in the structure of 6×(80-cycle ZnO/ 1-cyle PEDOT).

Figure 4.6: The mass gain of ZnO in steady-growth region and that on the surface of inserted PEDOT with various modifications.

Figure 4.7: The mass gain of H2O on surface covered by redundant SbCl5.

Steady Without

4.1.3 Superlattice deposition and thermoelectric performance

After finishing the study of interface-engineering for the deposition of HZO/PEODT superlattice, there was still one remaining issue to address, the

determination of “super ZnO”, which possessed the best electrical performance as stated before, to integrate with inserted PEDOT. Because of the slight variation in deposition condition between our ALD and MLD systems, e.g. working pressure and flow field of precursors, the “super ZnO” had to be determined once again. Therefore, we followed the approaches stated in section 3.2.1, tuning the insertion periodicity of mixed doping layers to acquire the best electrical performance, and then concluded that mix 19:1 HZO was the “super ZnO” in MLD system based on the highest power factor listed in Table 4.3. Thus, mix 19:1 HZO was a host material in the section to study the effects of composition and structure on the thermoelectric performance of HZO/PEDOT

superlattice. However, it is noteworthy that the electrical mobility of HZO deposited by MLD system was higher than that deposited by ALD system with the same insertion periodicity of mixed doping layers. This property was attributed to the higher

preferential-growth in (002) direction as XRD patterns revealed in Figure 4.8. The high preferential-growth in a conductive film yielded fast transportation paths for electrical carriers, and thus the electrical mobility of the film could be much higher than that with lower preferential-growth.

Next, we first clarified the effect of insertion periodicity of PEDOT on the thermoelectric performance. Table 4.4 summarizes the thermoelectric properties of HZO/PEDOT superlattices composed of alternating mix 19:1 HZO host and 1-cycle PEDOT, where the mix 19:1 HZO were of 2 periods (19 ZnO cycles/1 mixed doping cycle/19 ZnO cycles), 4 periods (19/1/19/1/19/1/19), 6 periods

(19/1/19/1/19/1/19/1/19/1/19), or 8 periods (19/1/19/1/19/1/19/1/19/1/19/1/19/1/19); the total number of cycles of HZO in superlattice with 4 kinds of period lengths were 468, 474, 476, and 477, respectively. It is noteworthy that these superlattices were deposited without type-C modification, i.e. the dose of H2O2 before 5-cycle ZnO nucleation layers on the surface of inserted PEDOT. Although the thermoelectric performance was not optimized currently, the effects of insertion periodicity were still evident. At long insertion periodicity, i.e. 6 and 8 periods, the electrical mobility and thermal

conductivity were close to the value of mix 19:1 HZO, since the distances between two inserted PEDOT layers (about 25 and 33 nm for 6 periods and 8 periods, respectively) were longer than the distance between scattering centers for electrical carriers and phonons in mix 19:1 HZO, such as dislocations, impurities, and grain boundaries especially. Based on the (002) XRD peak of mix 19:1 HZO presented in Figure 4.9, the grain size was ~18 nm, which was calculated by Scherrer equation,

D=0.9λ/βcosθ, where D is mean grain size, λ is the wavelength of incident X-ray=0.154

nm, β is the FWHM (Full width at half maximum), and θ is the Bragg angle. Thus, the transportation of electrical carriers and phonons were almost maintained, and the thermoelectric performance was unaffected. On the other hand, the period length of inserted PEDOT layers in the superlattice composed of 2 periods of mix 19:1 and 1-cycle PEDOT was much shorter than the distance between scattering centers of mix 19:1, and thus the inserted PEDOT layers became the major scattering centers for both carriers (heat and electric). Besides, short distance between two adjacent PEDOT layers confined the growth of ZnO crystals; therefore, the transportation of electrical carriers and phonons were retarded extremely even the nucleation and growth of ZnO on PEDOT were improved through interface-engineering. Luckily, the concurrent suppression of electrical mobility and thermal conductivity could be decoupled at medium insertion periodicity of PEDOT layers, i.e. 4 periods in our results. Since the distance between adjacent PEDOT layers in this structure was around 16 nm, which was slightly smaller than the grain size of mix 19:1 HZO, and therefore the suppression of grain growth was retarded. Furthermore, the conductive nature of PEDOT offers

conducting paths for electrical carriers; thus, the electrical mobility could be maintained partially. As for thermal conductivity, the inserted PEDOT layers that were the main scattering centers for phonons in this structure could reduce the thermal conductivity obviously due to the extremely heterogeneous interface between ZnO and PEDOT.

Overall, the thermoelectric performance of superlattice composed of alternating 4-period mix 19:1 HZO host and 1-cycle PEDOT had a potential to be further improved through thickening the inserted PEDOT even though the ZT of this superlattice was not the highest one among four superlattices on this front.

Further, the inserted PEDOT was thickened gradually and type-C modification, i.e. the dose of H2O2 before 5-cycle ZnO nucleation layers, was applied to optimize the performance of superlattices. The thermoelectric properties of deposited superlattices are listed in Table 4.5. The enhancement of electrical mobility (conductivity also) through the usage of type-c modification, which improved the growth and/or nucleation of ZnO, was stated in section 4.2.1. Besides, the slight increment of thermal

conductivity was also attributed to modified interfaces between ZnO and PEDOT, by which the scattering of phonons was reduced. Regarding the dependence of ZT

enhancement on the thickness of inserted PEDOT, we started from the reduced thermal conductivity. It could be seen that the thermal conductivity decreased with the cycle (thickness) of inserted PEDOT. This result was originated from the formation of complete and clear interfaces between ZnO and PEDOT gradually as the cycle of inserted PEDOT increased, hence the interface-scattering of phonons was enhanced due to the significant discrepancy in the type of bonds and the microstructure between ZnO and PEDOT, i.e. covalent bonds and disordered amorphousness of PEDOT versus ionic

bonds and ordered crystallinity of ZnO. The gradually-formed interfaces could be observed and confirmed by the comparison of cross-sectional TEM images presented in Figure 4.10. Moreover, the inserted PEDOT layers also offered a considerable serial thermal resistance due to its ultra-low thermal conductivity, 0.19 W m^-1 K^-1. Thus, the reduced thermal conductivity was obtained. However, the decrement of thermal conductivity was unsatisfactory. Based on the simple series-resistance model, which is illustrated in Figure 4.11, the theoretical thermal conductivity could be expressed as below. It is noteworthy that there were assumptions we made to simplify the calculation:

(1) Cross-sectional area was unity; (2) Thermal conductance G at interfaces were the same. (GHZOPEDOT=GPEDOTHZO=G=0.2 GW m^-2 K^-1, which is based on the result of Mark D. Losego et al.¹⁰⁶); (3) The transportation of phonons inside HZO was

unchanged even with the insertion of PEDOT.

𝑹𝑹_{𝒕𝒕𝒕𝒕𝒕𝒕𝒕𝒕𝒕𝒕}= 𝒅𝒅𝒇𝒇𝒇𝒇𝒕𝒕𝒇𝒇

Therefore, the estimated thermal conductivity of each superlattice should be 1.49, 1.31, 1.15 and 0.95 for 1-cycle, 2-cycle, 4-cycle and 6-cycle, respectively. The

estimated thermal conductivity was much lower than the measured value in all

superlattices. This result indicated the lack of distinct interface and the existence of fast-transportation route for phonons in the inserted PEDOT layers. In fact, it indeed took time (or cycles) for PEDOT to form a complete layer, hence the interface

在文檔中 原子層沉積之金屬氧化物及高分子/金屬氧化物超晶格複合材料熱電性質及氣體滲透率研究 (頁 123-0)