The present invention relates to a thermoelectric measurement system and a thermoelectric device. More specifically, the present invention relates to a thermoelectric measurement system based on a liquid eutectic gallium-indium electrode and a thermoelectric device based on a liquid eutectic gallium-indium electrode and including a molecular layer formed by self-assembly on an electrode.
Organic thermoelectric materials refer to materials that convert thermal energy into electricity. Organic thermoelectric materials are very interesting from both environmental and scientific points of view in that they recycle thermal energy, which is the final form of energy. Organic thermoelectric materials enable harvesting of eco-friendly energy in situations where a temperature difference exists, have the advantages of bendability and stretchability, and can be processed at low cost.
For the application of such organic materials to thermoelectric devices, it is necessary to elucidate on a molecular level how the chemical and electronic structures of organic active components are related to the thermoelectric performance of the devices.
However, the complex structures of molecules and/or polymers incorporated into most organic thermoelectric devices lead to ill-defined solid-state surface structures and unclear interfacial properties between the molecules and between the molecules and electrodes, making it difficult to achieve the desired results.
The thermal performance of micro- and nano-scale devices is steadily gaining particular importance but understanding of the relationship between structural and thermal properties of micro- and nano-scale devices at the atomic level is still limited. Development of thermoelectric materials with high efficiency and understanding of the structure-property relations for thermoelectric properties are currently the subject of intense research in this field. Ultimately, more studies need to be done at a molecular level to achieve these goals.
Indeed, few studies have been conducted on molecular-scale thermoelectrics than on molecular-scale electronics. Nanoscale platforms that allow high yield, convenience, and ease in thermoelectric measurements with high reliability and reproducibility are required for thermoelectric research on a molecular level. Moreover, the platforms should not damage ultrathin delicate organic films, self-assembled monolayers during device fabrication and thermoelectric measurements.
Therefore, the present invention intends to provide a thermoelectric measurement system based on a liquid eutectic gallium-indium electrode that is efficient enough to meet the above-described requirements and is highly reliable and reproducible in measurement.
The present invention also intends to provide a thermoelectric device including a self-assembled molecular layer and based on a liquid eutectic gallium-indium electrode.
One aspect of the present invention provides a thermoelectric measurement system for measuring a voltage value based on a temperature difference between a liquid metal top electrode and a metal bottom electrode.
The thermoelectric measurement system of the present invention is based on a conical non-Newtonian liquid metal electrode having a surface on which a conductive thin (about 1 nm thick) gallium oxide (Ga2O3) layer is formed by self-passivation.
Specifically, the thermoelectric measurement system of the present invention includes: a top electrode; a bottom electrode opposite to the top electrode; and a junction in contact with the top and bottom electrodes and including a sample whose thermoelectric properties are to be measured, wherein the top electrode is made of a liquid metal.
Any liquid metal may be used for the top electrode as long as it has a low melting temperature, exhibits fluidic behavior, and has intrinsic properties of fluid such as high surface tension. The liquid metal is preferably a harmless and stable eutectic alloy such as eutectic gallium-indium (EGaIn) or eutectic gallium-indium-tin (EGaInSn).
According to one embodiment of the present invention, the top electrode may be an electrode based on a eutectic gallium-indium (EGaIn) alloy.
The top electrode is in the form of a conical tip and has a surface on which a conductive gallium oxide (Ga2O3) layer is formed by self-passivation.
A thermocouple may be provided on the top electrode or the bottom electrode to measure a temperature change based on the Seebeck effect.
The thermoelectric measurement system of the present invention may further include a nanovoltmeter that measures a thermoelectric voltage (ΔV) at the junction.
The thermoelectric measurement system of the present invention may further include a hot chuck that controls the temperature of the bottom electrode and creates a temperature difference (ΔT) at the junction.
The thermoelectric measurement system of the present invention may further include a tungsten (W) tip as a grounding electrode.
The thermoelectric measurement system of the present invention is constructed to measure a voltage value based on a temperature difference between the liquid metal top electrode and the metal bottom electrode. Any thermoelectric material may be used without limitation as the sample. For example, the sample may be an inorganic semiconductor, an organic monomolecular compound, a conductive polymer, a conductive polymer-nanocarbon composite or a conductive polymer-inorganic semiconductor hybrid composite.
According to a specific embodiment of the present invention, the sample may be a self-assembled molecular layer bound to the surface of the bottom electrode.
The present invention also provides a thermoelectric device including a top electrode, a bottom electrode opposite to the top electrode, and a molecular layer formed on the bottom electrode wherein the molecular layer is formed by self-assembly of an oligophenylene thiol represented by S(Ph)n (wherein Ph is a phenyl group and n is an integer from 1 to 10) and the top electrode is an electrode based on a liquid eutectic gallium-indium (EGaIn) alloy.
The thermoelectric performance (Seebeck coefficient) of the thermoelectric device according to the present invention is enhanced as n increases.
The thermoelectric measurement system of the present invention can measure the thermoelectric performance of samples, including large-area molecular layers and thermoelectric materials such as inorganic and organic-inorganic composite materials as well as various organic molecules, with high efficiency and reproducibility without the need for expensive equipment.
In addition, the use of EGaIn as a non-toxic liquid metal for the top electrode enables the measurement of the thermoelectric performance of nano- to micro-scale organic thermoelectric devices while minimizing damage to samples in the form of nano-scale thin films. Therefore, the thermoelectric measurement system of the present invention can be widely used across the thermoelectric device industry.
Furthermore, the thermoelectric measurement system of the present invention can be used to demonstrate the thermoelectric and electrical properties of various organic molecules, thus enabling the development of various organic thermoelectric devices.
The present invention will now be described in more detail.
The present invention is directed to an efficient thermoelectric device based on a large-area junction structure and a thermoelectric measurement system platform including the thermoelectric device. The thermoelectric measurement system platform is based on a conical electrode composed of a eutectic gallium-indium alloy as a non-Newtonian liquid metal and having a surface on which a conductive thin (about 1 nm thick) gallium oxide (Ga2O3) layer is formed by self-passivation.
The thermoelectric measurement system of the present invention is constructed to measure a voltage value based on a temperature difference between a liquid EGaIn top electrode and a metal bottom electrode.
The construction of the EGaIn-based thermoelectric measurement system according to the present invention is shown in (a) of
The thermoelectric measurement system of the present invention essentially includes the following elements: (i) a micromanipulator adapted to form a Ga2O3/EGaIn top electrode in the form of a conical tip and a junction; (ii) a thermocouple adapted to measure a temperature change in the bottom electrode based on the Seebeck effect; (iii) a nanovoltmeter adapted to measure a thermoelectric voltage (ΔV) at the junction; (iv) a hot chuck adapted to control the temperature of the bottom electrode and create a temperature difference (ΔT) at the junction; and (v) a tungsten (W) tip as an grounding electrode.
The thermoelectric measurement system of the present invention will be described with reference to the following exemplary embodiments.
As can be seen in (a) of
Due to the advantageous features of the EGaIn top electrode, a sufficiently large amount of thermoelectric data to draw a statistically robust inference about the relationship between the structure and characteristics of the thermoelectric measurement system can be obtained in a reliable and reproducible manner.
In exemplary embodiments of the present invention, oligophenylene thiolates (S(Ph)n, wherein n is an integer from 1 to 3, see (b) of
The formation of the self-assembled monolayer (SAM) on the AuTS substrate in the thermoelectric measurement system of the present invention minimizes the degree of structural defects caused by the roughness of the substrate. When a temperature difference is created, the ΔV value is measured in μV and the Seebeck coefficient (S, μV/K; S=−ΔV/ΔT) is estimated.
First, the thermopowers of all internal components of the thermoelectric measurement system according to the present invention are measured. In exemplary embodiments of the present invention, a short-circuited junction is formed on the AuTS, as shown in (a) of
That is, the thermopower and temperature profiles of all components of the system circuit are measured according to the previous method reported by a research group led by Segalman and Majumdar, and the measured output voltages are evaluated.
In one exemplary embodiment of the present invention, the ΔV/ΔT measured at the junction is −2.4±0.1 μV/K (see (b) of
In one exemplary embodiment of the present invention, a HOPG//Ga2O3/EGaIn junction is formed in which a van der Waals contact is formed while maintaining the Ga2O3 layer and the thermopower of the Ga2O3 layer on the EGaIn conical tip is measured ((a) of
The surface of the EGaIn conical tip is assumed to be rough from a molecular viewpoint. In the Examples section that follows, the SGa2O3 value was measured at the HOPG when an EGaIn spherical drop having a smoother surface was used instead of the EGaIn conical tip, to determine whether this roughness affected the thermoelectric measurement. As can be seen in (a) of
The thermoelectric device and the thermoelectric measurement system platform of the present invention were verified with oligophenylene thiolates (S(Ph)n).
ΔV values were measured at AuTS/S(Ph)n//Ga2O3/EGaIn large-area junctions according to the following standard protocol. The ΔV values were found to be statistically significant.
First, ˜100 data points (˜50 data points for the monomolecular SAM, SPh) and 10 intersection points per junction were selected at different locations per sample at a specific temperature difference (ΔT). After ΔV values at 3-10 junctions were measured using the EGaIn conical tip, a tip was newly formed to minimize the influence of contamination on the surface of the old tip. Data were obtained by varying the numbers of repeated measurements, junctions, tips, and samples. Statistics reflecting all data were prepared.
The thermoelectric data measured for the junctions are summarized in Table 2.
The yields of the working junctions were 51-81% for the monomolecular SPh and 90-97% for both S(Ph)2 and S(Ph)3. (a) of
ΔV=−(SSAM−SWtip)ΔT (1)
The Seebeck coefficients (SSAM) of S(Ph)n SAM were estimated to be 7.8±0.4 (n=1), 9.8±0.2 (n=2), and 12.9±1.5 μV/K (n=3), as calculated by Equation 1. The positive polarity of the SSAM values suggests that the molecular orbital closest to the Fermi level of Ga2O3/EGaIn (−4.3 eV) is the highest occupied molecular orbital (HOMO). The magnitude and polarity of the SSAM values are consistent with the previous results measured at monomolecular and small-area (101-102 molecules) junctions.
(d) of
S
SAM
=S
C
+n*β
S (2)
where n is the length of the molecule (i.e. the number of the phenylene units), βS is the change rate of thermopower with n, and SC is the thermopower of a hypothetical junction where n is 0 (i.e., a non-short-circuited junction that does not contain SAM).
Equation 2 is derived from the transmission function based on the junction and the Landauer formalism. In the plot of (d) of
As discussed above, the presence of the large-area junction with the microelectrode composed of liquid eutectic gallium-indium alloy ensures high efficiency and reproducibility of the thermoelectric measurement platform system according to the present invention.
The system of the present invention can be constructed to measure a voltage value based on a temperature difference between the liquid metal top electrode and the metal bottom electrode. This construction ensures high reliability and reproducibility of the system.
The present invention will be specifically explained with reference to the following examples. However, these examples are not intended to limit the scope of the present invention.
First, all organic solvents were purchased from Sigma-Aldrich and Daejung and used as supplied. All oligophenylenethiols (HS(Ph)n, where n is an integer from 1 to 3) were purchased from Sigma-Aldrich (purity 97%) and used as supplied. High purity eutectic gallium-indium (EGaIn; 99.99%) was purchased from Sigma-Aldrich and used as supplied. All thiol derivatives were stored under a nitrogen atmosphere at 4° C. Gold thin films (300 nm) were deposited onto silicon thin films (100 mm in diameter, 1-10 Ωcm, 525±50 microns thick) by an electron beam evaporator (ULVAC). Photocurable adhesives were purchased from Norland (NOA81) and used as supplied.
A toluene (anhydrous 99.9%) solution (total concentration=3 mM) containing (HS(Ph)n) was placed in a vial. The solution was sealed and degassed by N2 bubbling through the solution for ˜10 min. A template-stripped gold (AuTS) chip was rinsed with pure toluene and placed in the solution with the exposed metal face up. The vial was then filled with N2. After 3 h incubation at room temperature, the SAM-bound AuTS chip was removed from the solution and rinsed by repeated dipping in clean toluene (3×1 mL). The solvent on the SAM was then evaporated in air for a few seconds.
The characteristics of the SAM were determined through contact angle measurement following the method reported in the literature and by X-ray photoelectron spectroscopy (XPS).
(1) A top electrode and a junction were formed following the procedure reported in the literature. The top electrode was prepared in the form of a conical tip based on EGain as a liquid metal. Briefly, a 10 μL gas-tight syringe was filled with EGaIn (≥99.99%, Aldrich). A drop of EGaIn was pushed to the tip of the syringe needle, the hanging drop was brought into contact with a surface on which the EGaIn could stick (e.g., an oxidized Ag surface), and the needle gently pulled away from the drop to obtain a conical tip.
(2) Thermoelectric measurements were performed under normal atmospheric conditions. The SAM was placed on a hot chuck and the remaining portion was covered with glass to block or minimize heat transfer to the EGaIn tip, which can be seen from the schematic diagram of the inventive thermoelectric measurement system based on EGaIn as a liquid metal shown in (a) of
Then, using a micromanipulator, the EGain conical tip was gently brought into contact with the surface of the SAM. 50-100 points per junction were measured for output voltage. At least 3 samples were prepared. 3-10 different locations per sample were measured for output voltage. After output voltages at 9-10 junctions were measured using the EGaIn conical tip, a new tip was prepared. The yield of each working junction was calculated by the proportion of non-short-circuited junctions in all short-circuited junctions. The short-circuited junction was defined as a junction that shows an SEGaIn of 3.4 μV/K, which is a value obtained for a short-circuited junction of an EGaIn conical tip and AuTS only.
To measure the thermopower of a Ga2O3 layer on the EGaIn conical tip, a junction was formed on highly ordered pyrolytic graphite (HOPG, 1 cm×1 cm) according to the same procedure. The thermopower of the Ga2O3 layer on the EGaIn conical tip were measured and compared with that on the EGaIn spherical drop. The geometric contact area was estimated with an optical microscope to determine whether the measured output voltage was dependent on the contact areas. After formation of the junction, the diameter of the geometric contact area was measured at high magnification. Assuming the circular contact, the area was derived from the measured diameter.
The thermoelectric performance of each junction was analyzed according to the previous method reported by a research group led by Segalman and Majumdar. The measured thermoelectric voltage was attributed to the thermopower of the junction between the EGaIn tip and the exposed substrate (AuTS or HOPG) or the SAM-bound substrate. The thermopower reflects the slope of ΔV versus ΔT. The ΔT occurs at the junction. Without SAM, ΔT occurs between the EGaIn tip and the tungsten (W) grounding electrode (see
The thermoelectric measurement system of the present invention can measure the thermoelectric performance of samples, including large-area molecular layers and thermoelectric materials such as inorganic and organic-inorganic composite materials as well as various organic molecules, with high efficiency and reproducibility without the need for expensive equipment.
In addition, the use of EGaIn as a non-toxic liquid metal for the top electrode enables the measurement of the thermoelectric performance of nano- to micro-scale organic thermoelectric devices while minimizing damage to samples in the form of nano-scale thin films. Therefore, the thermoelectric measurement system of the present invention can be widely used across the thermoelectric device industry.
Furthermore, the thermoelectric measurement system of the present invention can be used to demonstrate the thermoelectric and electrical properties of various organic molecules, thus enabling the development of various organic thermoelectric devices.
Number | Date | Country | Kind |
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10-2018-0123838 | Oct 2018 | KR | national |
Filing Document | Filing Date | Country | Kind |
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PCT/KR2019/010613 | 8/21/2019 | WO | 00 |