Tuning of the Thermoelectric Properties of One-Dimensional Material

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Letter pubs.acs.org/NanoLett

Tuning of the Thermoelectric Properties of One-Dimensional Material Networks by Electric Double Layer Techniques Using Ionic Liquids Kazuhiro Yanagi,*,† Shouhei Kanda,† Yuki Oshima,† Yoshimasa Kitamura,† Hideki Kawai,† Takahiro Yamamoto,‡ Taishi Takenobu,§ Yusuke Nakai,† and Yutaka Maniwa† †

Department of Physics, Tokyo Metropolitan University, Tokyo 192-0397, Japan Department of Liberal Arts (Physics), Tokyo University of Science, Tokyo 125-8585, Japan § Department of Applied Physics, Waseda University, Tokyo 169-8555, Japan ‡

S Supporting Information *

ABSTRACT: We report across-bandgap p-type and n-type control over the Seebeck coefficients of semiconducting singlewall carbon nanotube networks through an electric double layer transistor setup using an ionic liquid as the electrolyte. All-around gating characteristics by electric double layer formation upon the surface of the nanotubes enabled the tuning of the Seebeck coefficient of the nanotube networks by the shift in gate voltage, which opened the path to Fermi-levelcontrolled three-dimensional thermoelectric devices composed of one-dimensional nanomaterials. KEYWORDS: Thermoelectrics, electric double layer, ionic liquid, semiconducting, single-wall carbon nanotubes

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gating by the formation of electric double layers on the surfaces of the nanotubes. The Seebeck coefficient showed peaks that reflect van Hove singularities due to the one-dimensional nature of the semiconducting SWCNTs, and across-bandgap, ptype and n-type were also controlled. The thermoelectric performance of materials can be evaluated using the figure of merit at a temperature T: ZT = σS2Tκ−1, where S, σ, and κ are the Seebeck coefficient and the electrical and thermal conductivities of the materials, respectively. Researchers hypothesized that a fundamental limit exists for improved thermoelectric performance because the three parameters of bulk materials exhibit mutually contradictory properties, for example, an increase of σ leads to a decrease in S and an increase κ;14 however, to overcome this limitation, Hicks and Dresselhaus clarified the importance of using low-dimensional materials or reducing the dimensionality of the materials.1,2 In addition, these researchers also noted the importance of tuning the Fermi level of materials1,2 because a great enhancement in thermoelectric properties is achieved at a proper Fermi level in the density of state (DOS). Various types of chemical doping approaches were reported for tuning the Fermi level; however, in the chemical doping approaches, fine-tuning of the Fermi level is clearly quite difficult.

hermoelectrics are a very important technology to efficiently convert waste heat into electric power. Hicks and Dresselhaus proposed two important approaches to innovate the performance of thermoelectric devices.1,2 One approach involves using low-dimensional materials, and the other approach involves properly tuning the Fermi level because the Seebeck coefficient, which is one of the essential parameters for characterizing the performance of thermoelectric materials, strongly depends on the Fermi level. Therefore, to clarify the relationship between the Seebeck coefficient and the Fermi level, it is important to achieve the highest thermoelectric performance from low-dimensional materials. Various types of chemical doping approaches were proposed to control the value and the sign, p-type and n-type, of the Seebeck coefficient by changing the species of the chemical dopants.3 However, in chemical doping approaches, it is difficult to finetune the Fermi level. Another approach is to control the Fermi level using an electric field.4−11 Previously, the thermoelectric properties of single nanowires, nanotubes, and nanodots were tuned using a back-gating approach;4,7−11 however, with this approach, the tuning of thermoelectric properties in their bulk networks was impossible because the gating was affected on a single nano-object. Here, we report continuous p-type and ntype control over the Seebeck coefficients of one-dimensional nanomaterial networks through an electric double layer transistor (EDLT) setup using an ionic liquid as the electrolyte. In this study, the Seebeck coefficient of semiconducting singlewall carbon nanotube (SWCNT) networks was continuously altered by the shift in the gate voltage due to the all-around © XXXX American Chemical Society

Received: August 4, 2014 Revised: October 9, 2014

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Figure 1. (a) Schematic illustration of the experimental setup to control the Seebeck coefficient of the semiconducting SWCNTs through an electric double layer using an ionic liquid. S, D, R, and G represent the source, drain, and reference and gate electrodes, respectively. (b) Schematic illustrations of the electron injection state (upper panel) and hole injection state (lower panel) induced by shifting of the gate voltage and of the corresponding Fermi level shift in the DOS of the semiconducting SWCNT. (c) Optical absorption spectra as a function of potential shift in the semiconducting SWCNT film (versus Ag/Ag+ of the reference electrode).

imide (TMPA-TFSI, Kanto Kagaku Co.) was used as the ionic liquid. Thermocouples were directly attached to the SWCNT film using silver paste, and these thermocouples were covered with insulating pastes (GC-P100, Sumitomo 3M Co.) to avoid electrochemical reactions. One end of the film was heated with a small heater attached to the film, and the actual voltage applied to the sample was measured as the channel voltage from the reference electrode. All of the measurements were performed under vacuum. The measurement details are provided in the Methods section. As shown in a schematic illustration of Figure 1, panel b, electrons or holes were injected by the shift in the potential of the gate voltages; consequently, the p-type and n-type behavior of the Seebeck coefficient was controlled. Indeed, the optical absorption spectra of the semiconducting SWCNT changed as a function of potential shift, which was measured using a typical optical−electrochemical setup22 with TMPA-TFSI as the electrolyte (Figure 1c). The details of these measurements, which were similar to those in ref 22, are described in the Methods section. The disappearance of the S11 and S22 bands (here, Sii represents the ith optical transition between the van Hove singularity) due to both the positive and negative shifts in the potential of the semiconducting SWCNT film, which correspond to negative and positive potential shifts in gate voltage, respectively, were observed. The shifts clearly corresponded to the Pauli blocking of the optical transition due to carrier injections upon the first van Hove singularities by the shift of Fermi level, as illustrated in Figure 1, which is consistent with the results of our previous report.22 Figure 2 presents the experimental results of the Seebeck coefficient of the semiconducting SWCNT samples. The transport data obtained using the Seebeck coefficient measurement setup are presented in Figure 2, panel a. The data clearly indicated ambipolar behavior, which suggests that electrons and holes could be injected by a shift in the gate voltage. Figure 2, panel b plots the measured Seebeck coefficient as a function of the shift of gate voltage (the data are plotted by the channel voltage to evaluate the actual applied voltage). As observed in the figure, the p-type and n-type behavior of the Seebeck coefficient were clearly controlled by the shift in the potential. We performed measurements on four other samples (see

Carrier injections by electric double layer formation using an electrolyte in a field effect transistor setup were used to tune the Fermi level of materials; therefore, not only the conductance of semiconducting materials,15 but also the phase transitions such as superconducting states,16,17 insulator−metal transitions,18 etc. have been controlled via a change in the gate voltage. In this study, we attempted to tune the thermoelectric properties, particularly the Seebeck effect, using the EDLT technique. For this purpose, we selected semiconducting SWCNTs as the thermoelectric material. SWCNTs are one-dimensional rolled graphitic materials, and because of their low-dimensionality, their thermoelectric properties have attracted interest and been investigated.3,4,9,12,13,19 A very large Seebeck coefficient was observed in the semiconducting types of SWCNT networks,19 and tuning of their thermoelectric properties, especially p-type and n-type control, is currently of great interest. As previously mentioned, determination of the Seebeck coefficient as a function of the Fermi level is very important to correctly tune thermoelectric properties. Control of the shift in Fermi energy using an electric double layer has resulted in various modifications of the electrical and optical properties of SWCNTs, such as conductance,20 optical absorption,21,22 and electroluminescense,23 but not of the thermoelectric properties. In this study, we attempted to tune the thermoelectric properties of semiconducting SWCNT networks using EDLT techniques. Figure 1, panel a presents a schematic illustration of the experimental setup to control the Seebeck effect through EDLT (a photograph of the device is presented in Figure S1 of the Supporting Information). A thin film of high-purity semiconducting SWCNTs with a diameter of 1.4 nm, which was prepared using typical density gradient techniques,24−26 was used as the channel between the gold source and drain electrodes. The details of the sample preparations are provided in the Methods section. A parylene layer (thickness of approximately 10 μm, parylene HT, Parylene Japan) was formed on the surface of the polyimide substrate for thermal isolation because of its extremely low thermal conductivity, and the device was formed upon the parylene layer. N,N,Ntrimethyl-N-propylammonium bis(trifluoromethane sulfonyl)B

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p-doped) semiconducting SWCNTs uncontrolled and served as an essential factor that produced the asymmetrical values of the n-type and p-type behavior in the Seebeck coefficient and power factor. Next, we discuss the channel−voltage dependence of the Seebeck coefficient using the phenomenological Mott’s formula:28 S(E F ) = −

2 π 2 kB T d ln[σ(E)] 3 e dE

E = EF

(1)

where σ(E) is the electrical conductivity at carrier energy E, and the Fermi level EF was approximated as EF = eVchannel using the channel voltage Vchannel. Figure 3 compares the observed

Figure 2. (a) Transport properties of the device (at 0.3 V source− drain voltage), (b) Seebeck coefficient, and (c) power factor of the semiconducting SWCNTs as a function of channel voltage. The dotted line was inserted for the guide to identify the regions of p-type and ntype behavior.

Supporting Information), and the averaged peak value of the ptype was 139 ± 29 μV K−1, and that of the n-type was −92 ± 27 μV K−1. These values are consistent with those of bismuth antimony telluride alloys, typically ±100−200 μV K−1,27 which indicates the good potential of semiconducting SWCNTs as thermoelectric materials. The power factor σS2 values are presented in Figure 2, panel c; these values were estimated from the observed Seebeck coefficient and the transport measurement data. In this calculation, the neutral points in carrier injection, which are the voltage point at the minimum current in the transport and the zero point at the Seebeck coefficient measurement, were adjusted to be the same because in the transport measurements, there was potential shift due to the source−drain voltage in addition to the gate voltage. As observed in Figure 2, panel c, peak structures were present in the power factor, which clearly indicate the importance of appropriately tuning the Fermi level of the semiconducting SWCNTs to achieve good performance of thermoelectric devices. We assume that the asymmetrical peak structure of the p-type and n-type behavior would be caused by the experimental limitation, not by an intrinsic one, for the following reason. In the channel of our device, there was a nondoping region in which the silver pastes were covered by insulating pastes to avoid electrochemical reactions. The presence of this undoped region left initially doped (usually

Figure 3. (a) Comparison of Seebeck coefficient (red) and the calculated line-shape based on Mott’s formula (green). Capacitance of the semiconducting SWCNT film as a function of reference voltage (squares), and (b) the DOS of (11,10) SWCNT calculated using the tight-binding model.

Seebeck coefficient and that calculated using Mott’s formula (here, similar to the power factor calculations, the neutral points were adjusted to be the same). The phenomenological expression in eq 1 is known to be suitable only for metallic states with a smooth DOS. In the present case, eq 1 is useful to capture the behavior of the Seebeck coefficient, particularly in the high electron and hole injection regions. In these regions, the Seebeck coefficient decreases when the |Vchannel| increases, and this behavior agrees well with that obtained from Mott’s formula in eq 1 (Figure 3). In addition, the sign of the Seebeck coefficient that corresponds to the sign of the majority carrier (i.e., electron or hole) coincides with the result from Mott’s formula in eq 1. Thus, our experimental result is theoretically supported and is physically reasonable. Here, we discuss the behavior of the Seebeck coefficient around the semiconducting gap region in more details. By using the simple relationship σ(E) ∝ g(E), the Seebeck coefficient can be written as being proportional to the derivative of DOS, S C

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∝ (∂g(E))/(∂E)|EF. The DOS of SWCNTs exhibits van Hove singularities because of their one-dimensional nature. Therefore, the Seebeck coefficient will possess a singularity value at the Fermi level near the van Hove singularities of SWCNTs. To understand the relationship between the DOS and the Seebeck coefficient, the Seebeck coefficients are compared with the capacitance of the semiconducting SWCNT film as a function of the reference voltage in Figure 3. The capacitance of the semiconducting SWCNTs was measured in an electrochemical setup (the details are described in the Methods section). The −1 total capacitance of the film Ctot can be written as C−1 tot = Cst + 29,30 C−1 . Here, C is the structural capacitance, and C q st q is quantum capacitance of the SWCNTs. Cq represents the DOS, g(EF), of the SWCNTs:30

Figure 4. Seebeck coefficients as a function of gate voltage under the freezing temperature of the ionic liquid. First, the p-type (upper panel) and n-type (lower panel) behaviors were tuned by setting the gatevoltage (Vg) to −0.3 V and 1.2 V, respectively, at room temperature, and then the devices were cooled down to a temperature lower than the freezing temperature of the ionic liquid.

Cq = e 2g (E F)

Therefore, when the Fermi level was located between the band gap of the first van Hove singularities, the total capacitance was dominated by the quantum capacitance term, which reflects the smallness of the DOS of the semiconducting SWCNTs. Therefore, the position of the first van Hove singularities can be deduced from the capacitance measurements. As observed in the figure, a clear dip structure was observed in the capacitance (Figure 3), and the width of this capacitance dip, approximately 0.65 V, corresponds well to the expected energy gap, 0.6 eV. This value was calculated using the tight-binding approximation with γ = 3.0 eV between the first van Hove singularities of (11,10) semiconducting SWCNTs (see Figure 3b), which was considered to be one of main chiralities of our semiconducting sample.31 There were other chiralities such as (13,8) and (14,6); however, their band gaps are similar to that of (11,10); therefore, the line-shape of DOS in our sample can be well approximated by the DOS of the (11,10) chirality. The potential gap between the n-type and the p-type Seebeck peaks was estimated to be approximately 0.70 ± 0.08 V for all the samples we measured (Supporting Information). The value was in good agreement with the width of the capacitance dip; therefore, we concluded that the observed Seebeck peaks reflected the one-dimensional nature of the DOS of the semiconducting SWCNTs. The tuning of the thermoelectric properties can be achieved by the electric double layer formation on the surfaces of the nanotubes. If the double layer was fixed by freezing the motion of the ionic liquid molecules, the tuned thermoelectric properties would be preserved even when the gate voltages were removed or changed. To test this hypothesis, we investigated the thermoelectric properties at a temperature lower than the freezing point of the ionic liquid. Figure 4 shows the Seebeck coefficient of the p-type and n-type tuned semiconducting SWCNT networks at approximately 260 K as a function of the gate voltage. First, the p-type and the n-type networks were tuned by the shift of the gate-voltage to −0.3 V and 1.2 V at room temperature, respectively, and then the devices were cooled down to a temperature lower than the freezing temperature of the ionic liquid. As observed in Figure 4, the tuned thermoelectric properties were well preserved at the same value even when the gate voltages were changed, which indicates a fabrication route to Fermi-level controlled thermoelectric devices. In summary, the Seebeck coefficients of semiconducting SWCNTs with diameters of 1.4 nm were tuned using the EDLT techniques with an ionic liquid and exhibited p-type and

n-type behaviors. The Seebeck coefficient exhibited relatively large absolute values, and if the bandgap of the semiconducting SWCNTs is enlarged as their diameters decrease, the Seebeck coefficient will be substantially enlarged in both the p-type and n-type systems, as expected based on theoretical calculations.32 Therefore, we expect that SWCNTs will have great potential as thermoelectric materials. As demonstrated in this study, EDLT using ionic liquid is an excellent technique to tune the value of the Seebeck coefficient through continuous shift of the Fermi level. Our results demonstrated the wide tuning from acrossbandgap p-type and n-type thermoelectric properties of onedimensional material networks. In addition, the tuned thermoelectric properties are preserved when the electric double layers were fixed by freezing the ionic liquid, which cannot be achieved by back-gate bias approaches. It is noteworthy that the electric double layer will be fixed using polymerization approaches,33 thus our findings imply various future potential applications. Moreover, this study indicates that ensembles of one-dimensional thermoelectric systems (rather than simply a single nanomaterial) can be tuned by an all-around gating through electric double layer formation upon the surfaces of all the nanomaterials that compose the bulk networks (rather than back-gating on a single nanomaterial). Thus, in principle, this technique is applicable even to three-dimensional arrays of nanowires34/nanotubes embedded in polymers, which opens the path to Femi-level-controlled three-dimensional nanowire/ nanotube devices. Recently, carrier injection techniques using electric double layers were known to be useful for the control of various electronic properties of materials such as metal− insulator transitions, superconducting states, and so on. In this study, we revealed that this technique is applicable for the control of thermoelectric properties of one-dimensional material networks. Note: During the submission process of this manuscript, the authors became aware of a related paper35 that was recently submitted to the Japanese Journal of Applied Physics. In the paper, n-type modulation of ZnO two-dimensional crystals was demonstrated by electric double-layer gating using KClO4 as the electrolyte. The results are consistent with our findings that thermoelectric properties can be tuned by electric double layer approaches. Methods. Sample Preparation. Semiconducting singlewall carbon nanotubes with diameters of 1.4 nm were prepared using typical purification procedures using density gradient D

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Techno Co.). Unless otherwise noted, the measurements were performed at 300 K.

centrifugation from SWCNTs produced by the arc-discharge method (Arc SO, Meijyo Nano Carbon Co.).24−26 The purity of the nanotubes was evaluated from the optical absorption of the sample and transport measurements. A typical optical absorption spectrum of the samples obtained during purification is presented in Figure S2 of the Supporting Information, and its transport property in field-effect transistor setup (not Seebeck coefficient measurement setup) is shown in Figure S3 of the Supporting Information. Electrochemical Doping Measurements: Optical Absorption Changes as a Function of Potential Shift and Capacitance Measurements. The optical absorption spectra as a function of potential shift of the semiconducting SWCNT film (Figure 1c) were measured in a manner similar to that in ref 22, but at low temperature (about −20 °C) using a homemade low-temperature measurement system with a mixture of the ionic liquid and an electrolyte solution (TMPA-TFSI/propylene−carbonate = 1:2) to avoid the freezing of the solution. A Ag/Ag+ electrode (RE-7, BAS Co.) was used as the reference electrode. The capacitance of the semiconducting SWCNTs with diameters of 1.4 nm (Figure 3) was measured in a setup similar to that used for the optical absorption measurements using a potentiostat (Als Model 611 ES, BAS Co.) at low temperature. In Figure 3, the potential shift of the sample (versus the reference electrode) is plotted as a function of minus voltage (versus Ag/Ag+) for direct comparison with the channel voltage. The capacitance measurements were performed at 50 mHz. Experimental Setup and Details of the Seebeck Measurements. A photograph of the device for the Seebeck measurements is presented in Figure S1 of the Supporting Information. The semiconducting SWCNT film was formed on the substrate as described in ref 22. The typical film size was approximately 10 mm (length) × 5 mm (width) × 100 nm (thickness). The ionic liquid (TMPA-TFSI) was pooled by silicon rubber on the substrate. The thermocouples (KFT-50− 100−050, ANBE SMT Co.) were attached directly to the semiconducting SWCNT film using silver paste (D-500 DOTITE, Fujikura Kasei Co.). The silver paste parts and thermocouples were covered with insulating pastes (GC-P100, Sumitomo 3M Co.) to avoid electrochemical reactions. A heater (KFR-02N-120-C1−16, Kyowa Dengyo Co.) was also attached. The temperature was evaluated using the thermocouples and measured using a digital multimeter (2000, Keithley Co.). The thermoelectric voltage was measured using a nanovoltmeter (2182A, Keithley Co.) through the Alumel line of the thermocouples; then, the Seebeck coefficient from the Alumel line was removed. The source, gate voltages, and heater currents were adjusted using a source meter (2614B and 2636A, Keithley Co.). When the gate voltage was shifted, we waited for several tens of minutes before measurements of the Seebeck coefficient for correct evaluation and took the offset voltage to be zero. After the temperature was increased by the heater, by approximately 1 K, we also waited for several minutes before data was collected. We used the measured reference voltages as the channel voltages. We measured the Seebeck coefficients of five samples, and the data of four other samples are presented in Figure S4 of the Supporting Information. The linearity of the induced thermoelectric voltage by the difference of temperature was verified as shown in Figure S5 of the Supporting Information. All of the Seebeck measurements were performed under vacuum using a vacuum- and low-temperature prober station (Grail 10, Nagase

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ASSOCIATED CONTENT

S Supporting Information *

Experimental setups in the Seebeck measurements, characterizations of samples, the Seebeck coefficient of four other samples, and linearity check of thermoelectric voltage. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

* E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was partially supported by grants-in-aid from KAKENHI from JSPS and MEXT, Japan.

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REFERENCES

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