Polar Molecule Confinement Effects on Dielectric Modulations of Sr

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Article Cite This: ACS Appl. Electron. Mater. 2019, 1, 836−844

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Polar Molecule Confinement Effects on Dielectric Modulations of SrBased Metal−Organic Frameworks Muhammad Usman,† Pei-Hsuan Feng,‡ Kuan-Ru Chiou,§ Jenq-Wei Chen,§ Li-Wei Lee,† Yen-Hsiang Liu,*,‡ and Kuang-Lieh Lu*,† †

Institute of Chemistry, Academia Sinica, Taipei 115, Taiwan Department of Chemistry, Fu Jen Catholic University, Taipei 24205, Taiwan § Department of Physics, National Taiwan University, Taipei 106, Taiwan Downloaded via UNIV OF SOUTHERN INDIANA on July 18, 2019 at 03:58:49 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: The dielectric behavior of metal−organic frameworks is highly dependent on the polarity of molecules that are confined in their structure. Hence, it is of fundamental importance to examine the influence of polar molecules in a well-designed framework. Herein, we clearly distinguish the role of polar molecular confinement on dielectric modulations in three isostructural Sr-based MOFs [Sr2(1,3-bdc)2(H2O)(DMF)]n (1D), [Sr2(1,3-bdc)2(H2O)2·H2O]n (1W), and their dehydrated analogue [Sr2(1,3-bdc)2]n (1). The synthesis of Sr-based MOF [Sr2(1,3-bdc)2(H2O)(DMF)]n (1D) was performed by the solvothermal reaction of SrCl2·6H2O and benzene-1,3-dicarboxylic acid (1,3-bdc) at 110 °C. The effective dielectric constant (κeff) of the DMF-containing compound 1D was found to be 22.4 (κ = 39.3, where κ is the intrinsic dielectric constant) at 1 MHz (295 K) which serves to highlight the significant function of the coordinated DMF when compared with its isostructural MOF having coordinated and guest H2O molecules (1W; κeff = 7.9, κ = 13.0) and the dehydrated analogue (1; κeff = 2.4, κ = 3.2). The presence of DMF molecules between the 2D layers of compound 1D instead of H2O molecules or a vacuum resulted in a high dielectric constant due to the large kinetic diameter and dipole moment of DMF molecules. The significance of this study is the design of an elegant model with a stable core structure, which can be used to clearly distinguish the role of polar molecules as well as the presence and absence of guest molecules on the dielectric behavior of electronic materials. This is of fundamental significance in chemistry, which will pave the way for the design of dielectric MOFs in the future for the microelectronics applications. KEYWORDS: dielectric constant, metal−organic frameworks, microelectronics, polarity, strontium

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INTRODUCTION Dielectric materials with their energy storing capacity have a plethora of applications particularly in designing capacitors, transformers, resonators, transistors, photovoltaic devices, integrated circuits (ICs), and so on.1,2 With the emerging advancements and innovations in the semiconductor industry, the synthesis of novel dielectric materials has become a topic of great interest, particularly with regard to their use in designing multilevel nanoscale flexible electronic technology.3,4 Because of the shrinkage in the size of electronic components, the efficiency of traditional dielectric materials is a serious concern. Traditional materials such as SiO2, AlO2, and other inorganic metal oxide materials have reached their functional limits.5,6 Hence, low dielectric behavior has been reported for mesoporous silica composites,7−9 fluorine-containing carbon materials,10 porous polybenzoxazole-based films,11−13 poly(pxylylene) polymers, and organometallic compounds,14 while methacrylate polymers, styrenes, poly(4-vinylphenol) (PVP), poly(vinylidene fluoride) (PVDF), and POSS polymers have been used as high dielectric gate polymeric materials.15,16 However, these organic dielectric materials possess some series © 2019 American Chemical Society

issues regarding their thermal stability, mechanical strength, compatibility with other inorganic components of devices, and the fact that they degrade over time.17,18 Very recently, hybrid inorganic−organic dielectric materials have emerged as an alternative to organic as well inorganic dielectrics with unique features of having both organic and inorganic molecules within a single material.19−24 Metal−organic frameworks (MOFs)25 which are constructed through the coordination of inorganic nodes and organic linkers have been considered as potentially possible stable low or high dielectric materials.19−24 In the past decades, MOFs have developed a reputation for use in gas absorption,26−30 sensing,31 chemical separation,32,33 catalysis,34−37 drug delivery, and optical and biomedical imaging.38,39 However, less attention has been paid to their dielectric applications. Because of their high porosity, low charge density, and insulating behavior, MOFs have been reported as low Received: January 4, 2019 Accepted: May 10, 2019 Published: May 10, 2019 836

DOI: 10.1021/acsaelm.9b00007 ACS Appl. Electron. Mater. 2019, 1, 836−844

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ACS Applied Electronic Materials dielectrics, particularly as an interlayer dielectric (ILD) component in the past few years.19,21,24 ZIF-8 has been reported as a low dielectric material which is not only thermally stable but also mechanically robust.40 In addition, some Zn-, Mg-, Ni-, and Pb-based frameworks have also been reported which show low dielectric behavior.24,41,42 The low polarization and charge density in these porous MOFs are key factors that contribute to their low dielectric behavior. A vacuum has the lowest dielectric constant (κ = 1), and as MOF structures have large pore volumes or a void space on their interior, they exhibit low dielectric constants. However, these pores are not always empty but filled with guest molecules which are highly influential for tuning the physical and chemical properties of the parent frameworks. In addition to the intrinsic properties of guest molecules due to its free moment, they also play an important role in altering intermolecular interactions such hydrogen bonding and π−π stacking interactions. Host−guest chemistry has been established to be intrinsically correlated to the dielectric behavior of MOFs. In some cases, the temperature-dependent motion of the dipolar guest molecules can lead to significant dielectric modulations at particular temperatures.43 It has been reported that when polar molecules are present in a porous organic−inorganic hybrid cage with a perovskite-type structure, the MOF acquires switchable dielectric behavior in MOF.44 Polar solvents, particularly H2O and DMF, can generate interfacial polarization effects that can result in very high dielectric constants at room temperature.45 Dielectric measurements of Ni-based supramolecular networks indicated a significant decrease in the dielectric constant value when their guest molecules having high polarity were exchanged by the molecules with low polarity.46 The low dielectric permittivity in a mixed solvent system can be attributed to the polarization of both solvents which cancel each other, thus resulting in a slight increase in dielectric permittivity.47 A few reports have also been appeared in which the effects of H2O molecules on the dielectric properties of MOFs have been investigated.24 However, a comparison has not been drawn to show the effect of the different polar molecules on the dielectric constant of MOFs. Herein, unambiguous dielectric modulations of two commonly used polar molecules (H2O and DMF) in MOFs are highlighted. A model with three isostructural frameworks [Sr2(1,3-bdc)2(H2O)(DMF)]n (1D), ([Sr2(1,3-bdc)2(H2O)2· H2O]n; 1W), and their dehydrated compound [Sr2(1,3-bdc)2]n (1) (1,3-H2bdc = benzene-1,3-dicarboxylic acid) adopting a highly thermal stable core structure was successfully designed (Scheme 1). The results of dielectric studies clearly revealed the role of polar solvents on the dielectric modulations in MOFs. This study is of fundamental importance and will pave the way for the design of future dielectric materials for microelectronic applications.

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Scheme 1. Systematic Diagram Showing the Increase in Dielectric Constant with the Inclusion of Polar Molecules in the Core Framework

PerkinElmer PARAGON 1000 FT-IR spectrometer was operated to perform Fourier transform infrared (FTIR) spectroscopy by using the KBr disk method. N2 adsorption/desorption isotherms were measured by a volumetric method using a Micromeritics ASAP 2020 instrument at 77 K. Synthesis of [Sr2(1,3-bdc)2(H2O)(DMF)]n (1D). A 40.1 mg sample of SrCl2·6H2O (0.15 mmol) was dissolved in 2.0 mL of ethanol solution, and the solution was treated by ultrasonics for 3 min. A 66.5 mg (0.4 mmol) sample of 1,3-benzenedicarboxylic acid (1,3H2bdc) was first dissolved in 3.0 mL of N,N-dimethylformamide (DMF) and then was added to the ethanol solution of SrCl2. Another 2 mL of water was added to the mixture solution, and the resulting solution was vacuum-packed in a Teflon-lined vessel (23 mL) which was placed in an oven for 2 days at 110 °C. The colorless, thin plateshaped crystals of 1D that were obtained were washed with 10 mL of an ethanol solution. Some high quality crystals of 1D were chosen and used for the single crystal X-ray analyses and other characterizations. Yield: 70.6% (based on metal ions (Sr)). Elemental analysis data for 1D: Calcd for C38H34N2O20Sr4: C, 38.38; H, 2.88; N, 2.36. Found: C, 38.33; H, 2.83; N, 2.08. X-ray Crystallography. A suitable single crystal of 1D was chosen for the X-ray crystallographic analysis at low temperature. A Nonius Kappa CCD diffractometer with a built-in graphite monochromated Mo Kα (λ = 0.71073 Å) X-ray source was operated to collect the single crystal crystallographic data at 200 K. The MULTI-SCAN method was applied using SADABS and SAINT for the absorption corrections.48,49 Crystal structural refinement data and the experimental parameters are listed in Table S1. The single crystal structure was resolved using direct methodologies as well as Fourier techniques, whereas SHELXL97 was used to refine structure via full-matrix least squares.50 All the non-hydrogen components were refined anisotropically, whereas the hydrogen (H) of the coordinated water molecule was located by difference Fourier mapping. The hydrogen atoms attached with oxygen atoms were refined using a riding model approximation (where Uiso(H) = 1.5Ueq(O)). Riding model approximations with Uiso(H) = 1.2Ueq(O) were applied for the refinement of the hydrogen atoms which were attached to carbon atoms and were positioned at the defined locations. Selected bond distances and their angles are given in Table S2. CIF data for 1D can be accessed at the Cambridge Crystallographic Data Center, under the deposition number of CCDC 1867216. Electrical Measurements. All the electrical (dielectric constant and impedance) data have been measured by using the HP 4284A Precision LCR Meter interfaced with a Lakeshore temperature controller with a basic accuracy of ±0.05% at all test frequencies with six digit resolution on every range. The standard calibration procedure for the instrument was adopted to ensure its basic accuracy (±0.05%).

EXPERIMENTAL DETAILS

Materials and Methods. All the reagents were obtained from commercial sources and were used as received without additional treatment. A PerkinElmer 2400 analyzer has been used to perform elemental analyses. Powder diffraction measurements for the bulk samples were recorded on a Bruker D2 PHASER powder X-ray diffractometer using the Cu Kα (λ = 1.54056 Å) source in the step mode with a step size of 0.02° in 2θ at 2 s/step speed. Thermogravimetric analyses (TGA) were performed from 30 to 900 °C at a heating rate of 5 °C/min−1 under a nitrogen atmosphere (20 mL/min) with a PerkinElmer TGA thermogravimetric analyzer. A 837

DOI: 10.1021/acsaelm.9b00007 ACS Appl. Electron. Mater. 2019, 1, 836−844

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ACS Applied Electronic Materials Measurements were conducted for the solid-state pellet samples of 1D to make a better comparison with previously reported pellet samples (1W and 1). To minimize the impact of potential voids in the pellet sample, crystals of 1D were finely ground to form uniform nanoscale particles and pressed to form high quality pellets of different thickness (t = 602−730 μm), disk radius of 0.003 m, and pellet area = 2.83 × 10−5 m2 under a high mechanical pressure (30 MPa). The obtained pellets have a density 70% that of the X-ray crystal density. Pellet samples of 1D were coated with silver paste to produce electrical contacts. All measurements were performed in the frequency range of 1 kHz−1 MHz under a vacuum with the temperature range set as 300−80 K. The parallel plate capacitor (Cp) mode was adopted to collect and calculate the capacitance, dielectric loss, impedance, and ac conductivity data by using eqs 1−3. κ = C pd /ε0A

(1)

ε = ε0 tan δ

(2)

σ = (ωC p)(d /A) tan δ

Crystal Structure of Sr-Based MOFs. The solid-state structure of 1D was determined by single crystal X-ray diffraction analysis, and the results revealed that 1D crystallizes in a monoclinic space group C2/c (Table S1). All of the powder X-ray diffraction (PXRD) peaks match well with the pattern obtained from the simulated data which confirmed the purity of as-synthesized crystals (Figure S4). The crystallographic asymmetric unit contains two crystallographically independent Sr2+ ions, two 1,3-bdc, a coordinated DMF molecule, and one coordinated water molecule. Functional groups (carboxylate) on the 1,3-bdc ligands were deprotonated, thus resulting in charge neutrality with the two Sr2+ cations. The bdc ligand serves as a μ2-bridging unit that coordinated with five Sr2+ ions through the four oxygen atoms of the two carboxylate groups. In 1D, there are two crystallographically distinct Sr2+ cations (Figure 1a). One of the Sr(1) ions is coordinated with two chelating carboxylate linkers, three monodentate carboxylate groups from five different 1,3-bdc ligands, and one coordinated DMF molecule (Figure S2a). On the other hand, the Sr(2) ion is also coordinated with two chelating carboxylate linkers and three monodentate carboxylate groups from five different 1,3-bdc linkers, but with one coordinated water molecule (Figure S2b). Both of the Sr ions are surrounded by eight coordinated oxygen atoms that are arranged in a bisdispenoid fashion. It is noteworthy that the Sr(1) and Sr(2) centers are arranged in an alternate fashion and are bridged by two monodentate carboxylate groups between the Sr ions, and eight [SrO8] polyhedrons are edge-shared to form an eight-membered ring unit (Figure S3). These [SrO8] eight-member rings are further linked to form a two-dimensional layered network expanded along the bc-plane. It is noteworthy that the distinct local coordination environment of the Sr(1) and Sr(2) ions by coordinating to different water or DMF molecules may play a central role in regulating the layer-to-layer interactions in the complex. Crystals of 1D were found to be isostructural to the previously reported ([Sr2(1,3-bdc)2(H2O)2·H2O]n; 1W) and its dehydrated analogue [Sr2(1,3-bdc)2]n (1) (1,3-H2bdc = benzene1,3-dicarboxylic acid). All of these compounds possess a highly stable core, composed of bdc ligands and Sr metals, with thermal stability of more than 420 °C.23 Compound 1D

(3)

Here, ε0 is the dielectric permittivity in a vacuum (8.85 × 10 F/m), A is the area of the electrode (m2), d is the thickness (m), and σ is the conductivity of the pellets. 12

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RESULTS AND DISCUSSION Synthesis of Sr-Based MOFs. A Sr-based MOF [Sr2(1,3bdc)2(H2O)(DMF)]n (1D) was synthesized under solvothermal conditions by the reaction of SrCl2·6H2O and benzene1,3-dicarboxylic acid (1,3-H2bdc) in a mixed solvent system composed of ethanol, DMF, and H2O (Scheme 2). After 2 Scheme 2. Synthesis Procedure of Compound 1D

days, thin plate-like colorless crystals of 1D were collected in high yield (70.6%), which were suitable for a single crystal Xray diffraction study. Compounds 1W and 1 were synthesized and characterized following a previously reported procedure.23

Figure 1. (a) A two-dimensional layer network of 1D expanded by [SrO8] building subunits approximately along the bc-plane. (b) A prospective view of the layer-to-layer coordination interactions of DMF in 1D. Color code: green: Sr(1); yellow: Sr(2); blue: N; red: O; gray: C. 838

DOI: 10.1021/acsaelm.9b00007 ACS Appl. Electron. Mater. 2019, 1, 836−844

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ACS Applied Electronic Materials contains no guest molecules between the two adjacent layers as compared with the previously reported compound 1W, which holds guest water molecules between the layers via hydrogen bonding. Compound 1 is a core structure without any coordinated or guest solvent molecule, obtained after the thermal treatment of either 1D or 1W at 140 °C under vacuum. With different guest molecules and no guests, layerto-layer hydrogen-bonding interactions of all three compounds are completely different as shown in Figure 1b. Thermogravimetric analyses (TGA) of 1D revealed a weight loss of 16.1% (calculated 15.3%) from 220 to 570 °C, which can be attributed to the loss of the coordinated water and DMF molecules (Figure S5). The dehydrated sample 1 is found to be highly thermal stable up to 450 °C (Figure S6). Dielectric Investigations of Sr-Based MOFs. To understand the effects of polar molecule confinement, temperature-dependent dielectric measurements were performed in the temperature range 50−300 K, as shown in Figure 2a,b. To make a better comparison with previous studies, a similar dielectrics measurement method using a pelleted sample at 30 MPa was adopted for 1D. To minimize the impact of potential voids, crystals of 1D were finely ground to form uniform nanoscale particles and pressed to form high quality pellets of different thickness. PXRD, FTIR, and BET were performed on the mechanical pressed samples to ensure

that the MOF did not decompose by applying high mechanical pressure (30 MPa). The results of PXRD and FTIR clearly indicate that the MOF is crystalline even after applying a high pressure (Figures S4 and S7). Negligible variations in the BET surface area of as-synthesized and pressed samples of the compounds (1W and 1D) also confirm that MOFs retain their crystallinity (Figures S8). Furthermore, electrical measurements on different pellets samples indicated the dielectric properties of 1D do not display huge variations with standard deviation in the range 1.47−0.47 (Figures S11 and S12). The findings of the dielectric study indicated that when the compound 1D is cooled from 300 to 50 K, the effective dielectric constant (κeff) decreases from 26.5 (1 kHz) to 22 (1 kHz); the decreasing behavior of the dipole moment of thermally vibrant polar molecules (H2O and DMF) confined in the MOF was observed (Figure 3a).23,40−44 The complex relative permittivity [ε′r(ω) + ε″r(ω)] (where ε′r(ω) is the real part and ε″r(ω) is the imaginary part of permittivity) of compound 1D was also examined at various temperatures. Compound 1D indicated an effective dielectric constant of κ = 26.5 at lower frequency (∼1 kHz) which rapidly reduced to κ = 22.4 at high frequency (∼1 MHz) (Figure S9). The relative permittivity (effective dielectric constant) curves displayed a decaying trend with frequencies. Incorporation of polar molecules (DMF) with high dipole moments between the layers is responsible for such decay behavior of dielectric constant (26.5 to 22.4) from 1 kHz to 1 MHz. At higher frequencies, the mobility of the dipoles of DMF in 1D decreases, which results in a significant reduction in the dipole polarization. Hence, a decaying trend was observed in the dielectric behavior of 1D with respect to frequency (Figure S9). A similar trend was also observed for previously reported dielectric MOFs.23,41−44 As the polar molecules are confined within a similar core structure, the dielectric constant is directly related to the dipole moment of those molecules inside the core under an applied field (Ea). The dipole moment of DMF is much higher (D = 3.82) than that for a water molecule (D = 1.85); hence, when the voltage is applied, the dielectric constant for the DMFcontaining compound 1D is higher than that for the watercontaining compound 1W (Table 1).51−54 The delay in the reorientation of the dipole with respect to the applied voltage is measured in the form of dielectric loss. Dielectric loss (0.45 to 0.28) curves also display the decaying behavior with respect to increasing frequency for compound 1D as shown in Figure S10. Several polarization mechanisms as well as the impedance (capacitance and resistance) of the deposited electrodes are responsible for the dielectric loss in pellet samples.51,52 This type of decreasing trend of dielectric curves with respect to frequency has also been reported for some ceramic, inorganic materials, composite compounds, and other MOFs as well.40,55,56 When the size of the molecules in the framework is increased, the vacuum will decrease. As a result, the dielectric constant will be enhanced (Figure 3). Pore or vacuum volume was confirmed using the Platon software which indicated that molecule volume per unit volume of compound 1D is 29.1%, which is almost 3 times higher than that of H2O-containing compound 1W. A vacuum or an air gap have the lowest dielectric constant (κ = 1). It was also proposed by the international technology roadmap for semiconductors that when the more pores or vacuum space are present in a solidstate material, the dielectric constant will be lower. In our case, the presence of DMF molecules in the empty space between

Figure 2. (a) Effective dielectric constant (relative permittivity) as a function of temperature for 1D in the frequency range of 1 kHz−1 MHz. (b) Dielectric loss (D) as a function of temperature for 1D in the frequency range of 1 kHz−1 MHz. 839

DOI: 10.1021/acsaelm.9b00007 ACS Appl. Electron. Mater. 2019, 1, 836−844

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Figure 3. The presence of polar molecules is shown for compound 1D, where two coordinated DMF molecules are present, compound 1W, where two guest H2O molecules exist, while the dehydrated compound 1 contains no molecules.

Furthermore, thermogravimetric analyses (TGA) of both the MOFs (1W and 1D) also revealed a 16.1% weight loss for DMF in 1D, which is comparatively higher than the weight lost of solvent molecules (9.4%) in the water-containing compound (1W) due to the fact that molecular weight of DMF is higher than that for H2O (Figures S5 and S6). Heavier DMF molecules in between the core of MOF structure 1 results in a lower vacuum and, hence, a higher dielectric constant as compared with H2O-containing MOF (1D) and 1. Pores size and their distribution in the core structure have significant roles in terms of understanding the effects of polar molecules in the pores of MOFs, particularly regarding their electrical properties.53,56,57 Hence, the vacuum or porosity in the compounds (1W and 1D) were confirmed using gas sorption measurements at 77 K. The N2 adsorption− desorption isotherms indicated BET surface areas of 25.04 and 10.11 m2/g for 1W and 1D, respectively (Figure 4). The corresponding pore size distribution was calculated by using the Horvath−Kawazoe (HK) method on the N2 desorption isotherms (insets of Figure 4). A pore size distribution of ∼3 to 10 nm (maximum at 3.5 nm) was observed for both the compounds which indicated the presence of microsized pores in the core structure. The observed pore volume and surface area for 1D are comparatively lower than those found for 1W, which is probably due to large molecular size of the coordinated DMF in 1D as compared with the smaller guest molecule of H2O in the core structure of 1W. Analogous gas isotherm behavior and lower surface area for 1D indicated that

Table 1. Comparison between Dielectric Properties of Compounds Based on the Features of the Polar Molecules Present or Absent in Their Structuresa no.

compounds

1

molecule in MOF

2 3

kinetic diameter51 dipole moment (D)51 molecular volumeb effective dielectric constant (κeff)

4 5

6

intrinsic dielectric constant (κ) (EMA)

[{Sr2(1,3bdc)2(H2O)2}· H2O] (1W)

[Sr2(1,3bdc)2(H2O) (DMF)] (1D)

moleculefree framework N/A 0

H2O-containing framework

DMF-containing framework

0.26 nm (H2O) 1.85 (H2O)

0.55 nm (DMF) 3.82 (DMF)

0 2.4 at 1 MHz (300 K) 3.2 at 1 MHz (300 K)

10.5% 7.9 at 1 MHz (300 K)

29.1% 22.4 at 1 MHz (300 K)

13.0 at 1 MHz (300 K)

39.3 at 1 MHz (300 K)

[Sr2(1,3bdc)2] (1)

a

References 23, 48, 49, and 51. bMolecular volume per unit volume of compounds was calculated from PLATON software.

the 2D layers of compound 1D possesses greater kinetic diameters (0.55 nm) than the water molecules (0.26 nm).51−53 Therefore, the diameter of the molecule in the core structure is very high while the vacuum or air gap in the core structure of 1D is lower than that of the H2O-containing compound 1W. Hence, a high molecular volume or lower vacuum results in a higher dielectric constant in 1D when compared with the H2Ocontaining compound 1W whose vacuum is higher (Figure 3).

Figure 4. N2 adsorption−desorption isotherm and pore size distribution curve (inset) of compounds (a) 1D and (b) 1W. 840

DOI: 10.1021/acsaelm.9b00007 ACS Appl. Electron. Mater. 2019, 1, 836−844

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The dielectric constant of the Sr-based MOF is increasing with an increase in the polar molecule per unit volume of the compounds. In addition, polar molecules with a higher dipole moment will also assist in increasing the dielectric constant. A comparison of structural parameters and dielectric constants for all the three compounds is shown in Table 1. Impedance and Electrical Conductivity Measurements. To understand the nature of the electrical behavior of compound 1D, we carried out temperature-dependent impedance spectroscopy (Z′ vs Z″) studies for the mechanically pressed pellet samples (Figure 6). Because of the

it has a lower porosity than 1W. Hence, with low porosity or vacuum, the dielectric constant of 1D is found to be higher when compared with relatively highly porous compound 1W. The relatively high dielectric constant due to low porosity of our compounds can be justified when compared to ZIF-8 and IRMOFs which possess a high porosity and hence low dielectric constants.40,58 Dielectric measurements have been performed on pellet samples of MOFs which contain substantial amount of voids. Hence, the measured dielectric constant has been denoted by effective dielectric constant (κeff) of medium composed of MOF as well as a small volume fraction of external voids with grain size distribution.59 To estimate the intrinsic dielectric constant (κ) of MOF materials, effective medium approximations (EMA) has been applied using Bruggeman effective medium theory (eq 4).59,60 The intrinsic dielectric constant for compound 1D, calculated by using Bruggeman effective medium theory, is found to have a similar tendency with respect to temperature and frequency with the κ value in the range of 46 to 25 (Figure S13). As the inclusion of porosity or vacuum usually decreases the dielectric constant of any material, hence, the relatively high intrinsic dielectric constant of 1D as compared to its effect dielectric constant is justified. Furthermore, intrinsic dielectric constants of 1W and 1 have also been estimated for 1W (κ = 13.0 (1 MHz)) and 1 (κ = 3.2 (1 MHz)) by using the Bruggeman effective medium approximation. f

ε − εeff ε − εeff + (1 − f ) 0 =0 ε + 2εeff ε0 + 2εeff

(4)

where f is the volume fraction of the material, ε is the relative permittivity or intrinsic dielectric constant (κ) of the material, ε0 is the permittivity of a vacuum, and εeff is the effective dielectric constant (κeff) of the pellet sample. A graph showing the increase in effective and intrinsic dielectric constants of Sr-based MOFs with respect to molecular volume in the core structure is shown in Figure 5.

Figure 6. (a) Temperature-dependent impedance spectroscopy studies (Z′ vs Z″) for 1D. (b) Plot of ac conductivity vs temperature for compound 1D.

presence of potential voids in the pellet, the measured value is the electrical conductivity of the pellet sample, composed of a MOF and a small volume fraction of voids. Straight lines for all the frequencies in ln(κefff) vs ln(f) data also suggest that the dielectric behavior is associated with only one contribution which is due to the intrinsic pellet sample (Figure S14).23,40 Dielectric materials require large energy gaps with low electrical conductivity to control the leakage current that passes through the dielectric layer. Hence, insulating behavior would be highly desirable for any dielectric material. To further examine the conducting behavior in compound 1D, the ac conductivity was calculated on the pellet samples using the above-mentioned eq 3. From the data shown in Figure 6, it can be seen that pellet sample of compound 1D exhibited a low electrical conductivity of the order of 10−7 S/cm, even at

Figure 5. Effective dielectric constants (κeff) and intrinsic dielectric constant (κ) of Sr-based MOFs (1, 1D, and 1W) at 1 MHz versus the volume of polar molecules per unit volume of compounds (calculated by the PLATON software). 841

DOI: 10.1021/acsaelm.9b00007 ACS Appl. Electron. Mater. 2019, 1, 836−844

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ACS Applied Electronic Materials

M.U., K.R.C., and J.W.C. measured all the electrical properties. M.U., Y.H.L., and K.L.L. wrote the manuscript with the input from all of the authors.

higher frequencies, which is comparable to previously reported conductivity values (∼10−7−10−9 S/cm) for dielectric MOFs.22,23,40,46,60 The reason for such a low ac conductivity of the MOF is poor overlapping between the d orbital of the metal to ligand which presents a hurdle to flow direct charge. There is no charge transfer path created within the framework; hence, a low conductivity is reported for compound 1D. A similar behavior was reported for compound 1W after removing all of H2O molecules (compound 1). Given its relatively high dielectric constant and highly insulating nature, the reported compound 1D has the potential for use as a dielectric material in gate dielectrics, transistors, and integrated circuits designs. The results presented promise to pave the way to the further design of dielectric MOFs in the future for various microelectronic applications.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge Academia Sinica, Fu Jen Catholic University, and Ministry of Science and Technology, Taiwan, for their financial support.

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CONCLUSIONS A model with three isostructural frameworks [Sr2(1,3-bdc)2(H2O)(DMF)]n (1D), ([Sr2(1,3-bdc)2(H2O)2·H2O]n (1W), and the dehydrated sample [Sr2(1,3-bdc)2]n (1) with a highly thermal stable core structure was successfully designed. The results of a dielectric study clearly revealed the role of polar molecules on the dielectric modulations in MOFs. The findings indicate that the compound 1D possesses a relatively high dielectric constant (κeff = 22.4; κ = 39.2) at 1 MHz (300 K). The higher dynamic size and dipole moment of DMF molecules compared to H2O molecule result in the core frame having fewer voids and hence increase the dielectric constant of 1D compared to the H2O-containing MOF (1W) (κeff = 7.9; κ = 13.0) and solvent-free core framework (1) (κeff = 2.4; κ = 3.2). The dielectric properties of MOFs have been an emerging area of research, and investigating the role of polar solvents is of fundamental importance in terms of understanding the dielectric behavior of MOFs that contain polar molecules in their structures. The foregoing results discussed in this work will lead the way for future research on dielectric MOFs and their applications in integrated circuit designing.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaelm.9b00007. Crystal data, crystal structures for 1D, bond lengths [Å] and angles [deg], hydrogen-bonding distances (Å) and angles (deg), thermogravimetric analysis, powder X-ray diffraction studies, FTIR, frequency-dependent dielectric constant, and electrical measurements (PDF) Crystallograhic data of 1D (CIF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*(K.-L.L.) E-mail [email protected]. *(Y.-H.L.) E-mail [email protected]. ORCID

Muhammad Usman: 0000-0003-4518-8281 Kuang-Lieh Lu: 0000-0002-5529-7126 Author Contributions

Y.H.L. and K.L.L. supervised the overall project. M.U. conceived the project. P.H.F. and Y.H.L. synthesized the crystal and performed the structural characterizations. L.W.L. and Y.H.L. performed the crystallographic measurements. 842

DOI: 10.1021/acsaelm.9b00007 ACS Appl. Electron. Mater. 2019, 1, 836−844

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ACS Applied Electronic Materials

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