Epitaxial Growth of MOF Thin Film for Modifying the Dielectric Layer in

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Epitaxial Growth of MOF Thin Film for Modifying the Dielectric Layer in Organic Field-Effect Transistors Zhi-Gang Gu,† Shan-Ci Chen,† Wen-Qiang Fu, Qingdong Zheng,* and Jian Zhang* State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China S Supporting Information *

ABSTRACT: Metal−organic framework (MOF) thin films are important in the application of sensors and devices. However, the application of MOF thin films in organic field effect transistors (OFETs) is still a challenge to date. Here, we first use the MOF thin film prepared by a liquid-phase epitaxial (LPE) approach (also called SURMOFs) to modify the SiO2 dielectric layer in the OFETs. After the semiconductive polymer of PTB7-Th (poly[4,8-bis(5-(2-ethylhexyl)thiophene-2-yl)benzo[1,2-b:4,5b′]dithiophene-co-3-fluorothieno[3,4-b]thiophene-2-carboxylate]) was coated on MOF/SiO2 and two electrodes on the semiconducting film were deposited sequentially, MOF-based OFETs were fabricated successfully. By controlling the LPE cycles of SURMOF HKUST-1 (also named Cu3(BTC)2, BTC = 1,3,5-benzenetricarboxylate), the performance of the HKUST-1/SiO2-based OFETs showed high charge mobility and low threshold voltage. This first report on the application of MOF thin film in OFETs will offer an effective approach for designing a new kind of materials for the OFET application. KEYWORDS: organic field-effect transistor, metal−organic frameworks, thin film, dielectric layer, epitaxial growth

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INTRODUCTION Metal−organic frameworks (MOFs), which are composed of metal ions (or metal oxo-clusters) and organic ligands, are very promising candidates for applications in gas storage and separation as well as in catalysis due to their high crystallinity, large specific surface area, and functional diversity.1−3 During the past few years, thin films or membranes of MOFs have been increasingly investigated due to their potential applications in optical sensors, energy, and environmental solutions.4−14 Among the MOF thin film preparation methods, the liquidphase epitaxy (LPE) layer-by-layer approach15−17 provides an efficient method to control the growth orientations, thickness, and homogeneity of MOF thin films.18−21 This method is prepared by immersing the substrate into a metal salt and an organic ligand solution sequentially and repeatedly. Such MOF thin films (called surface mounted MOFs, SURMOFs) will give a good potential application in devices and sensors. As one class of organic electronic devices, organic field-effect transistors (OFETs) have attracted considerable attention in their application of phototransistors,22 light-emitting transistors,23 memory devices, and displays.24−26 A typical OFET has a device configuration consisting of source/drain electrode, active layer, gate insulator, and electrode.27,28 In this type of OFET, the dielectric layers act as insulator electrode, and organic semiconductors serve as an active layer. Since the charge carriers in OFETs transport along a conducting channel between the dielectric layer and organic semiconductor interface, the dielectric layer is beneficial for growing an ordered organic semiconductor layer, resulting in a significant © 2017 American Chemical Society

decrease in interface trap density. Therefore, the performance of OFETs can be optimized substantially by modifying or changing the dielectric layer. So far, self-assembled monolayers (SAMs) are gaining significant attention as gate dielectrics because of their robust insulating properties.29,30 Various SAMs, such as octadecyltrichlorosilane (OTS),31 hexamethyldisilazane (HMDS), 32 and octadecylphosphonic acid (ODPA),33 have been used to modify the dielectrics for improving OFET performance. However, the flexible SAM molecules are easily affected by several factors such as the alkyl chain length of the molecules and the deposition temperature.34,35 Therefore, to obtain high-performance OFETs, the use of an optimal dielectric layer is as important as the choice of the semiconductor layer. When using low-k dielectrics, the field-effect mobility of polymer semiconductors can be enhanced due to the low dielectric constant materials providing the transport channel with a less polarizable environment that reduces energetic disorder.36−39 MOF materials display crystallinity and micropores as well as a low dielectric constant,40−42 which provide a new candidate for modifying the dielectrics layer in OFETs. In particular, the dielectric layers modified by SURMOFs in the OFETs will not only promote the charge carrier mobility but also not be affected by the force from the top layer. Received: November 13, 2016 Accepted: February 9, 2017 Published: February 9, 2017 7259

DOI: 10.1021/acsami.6b14541 ACS Appl. Mater. Interfaces 2017, 9, 7259−7264

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Obtained OFET sample; (b) schematic structure of OFET with the interfacial modification of SiO2 dielectric layers by MOF HKUST1 thin film; (c) chemical structure of semiconducting polymer PTB7-Th; (d) schematic LPE preparation of SURMOF HKUST-1 and the structure of HKUST-1.

Figure 2. AFM surface and height images of SURMOF HKUST-1 with three LPE dipping cycles (a, c) and after coating PBT7-Th (b, d).

potential candidate for improving the performance of OFETs.37,38 SURMOF HKUST-1 with different LPE cycles was grown on the SiO2/Si substrate for the study. The device configuration is shown in Figure 1a,b, where the active layer

For this purpose, here a typical MOF HKUST-1 (also named Cu3(BTC)2, BTC = 1,3,5-benzene tricarboxylate, Figure 1d) was chosen to modify the dielectric layer in OFETs since it has a low dielectric constant (k = 1.7),41,42 which may provide a 7260

DOI: 10.1021/acsami.6b14541 ACS Appl. Mater. Interfaces 2017, 9, 7259−7264

Research Article

ACS Applied Materials & Interfaces

Figure 3. XRD data (a) and IRRAS spectra (b) of SURMOF HKUST-1 prepared by LPE method with one, two, three, and four LPE dipping cycles; SEM surface images of SURMOF HKUST-1 with three LPE dipping cycles (c) and after coating PBT7-Th (d); (e) SEM surface morphology of OFET.

scratching a line from the sample to the substrate.43 The AFM height profiles (Figures S2 and S3) showed that the thickness were ∼6, ∼9, and ∼13 nm for two-, three-, and four-cycle HKUST-1 layers, respectively. All samples grown on the SiO2 /Si substrates were characterized by X-ray diffraction (XRD) measurements, and the XRD data (Figure 3a) showed the weak peaks at (222) and (333) were identical to [111] oriented HKUST-1 in the simulated XRD of powder HKUST-1. In addition, the intensities of XRD were increased with an increase of the LPE cycles from one, two, three, and four cycles (Figure 3a). In order to study the IR absorbance, the MUD-SAMs functionalized Au substrates were used for SURMOF HKUST-1 preparation under the same conditions. The IR vibration absorbance of the −COO group (1377 and 1650 cm−1) in HKUST-1 was increased with the increasing of cycles (one, two, three, and four cycles) of HKUST-1 as shown in the IRRAS spectra (Figure 3b). Three-cycle HKUST-1 thin film on SiO2/Si substrate showed a highly homogeneous and smooth surface as shown in AFM images (Figure 3a,b) and SEM image (Figure 3c). After the semiconductive polymer PTB7-Th (∼15 nm) was coated, the homogeneous PTB7-Th/HKUST-1/ SiO2/Si thin film formed an OFET. The homogeneous surface morphologies with a roughness of ∼5 nn was checked by AFM images (Figure 3b,d) and surface SEM images (Figure 3d,e). The XRD, IRRAS, and AFM images showed that SURMOF HKUST-1 could be grown on the substrates controllably step by step and provided a good candidate to modify the dielectric layer in OFETs. The XRD pattern (Figure S4) of PTB7-Th deposited on the SiO2/Si substrate and PTB7-Th deposited on the SURMOF HKUST-1 modified SiO2/Si substrate indicated the composite layers in OFETs still kept the crystallinity of [111]-oriented HKUST-1. Therefore, this crystalline MOF may provide a template for improving the ordered arrays of

was based on a semiconductive polymer of poly[4,8-bis(5-(2ethylhexyl)thiophene-2-yl)benzo[1,2-b;4,5-b′]dithiophene-2,6diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene)-2carboxylate-2,6-diyl)] (PTB7-Th, Figure 1c). Then the source and drain Au electrodes were directly deposited onto the PTB7-Th layer to form the OFET device. Because of the porous, crystalline, and oriented thin film with controllable thickness and low-k, the charge carrier mobility and on/off current ratio results showed that the SURMOF HKUST-1 thin film can be used to modify the dielectric layer for improving the performance of OFETs. The present study is the first reported on the modification of dielectric layer with MOF thin film in OFETs and will offer an effective approach for designing a new kind of materials for the OFET application.

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RESULTS AND DISCUSSION The SiO2/Si wafers were treated by oxygen plasma for 10 min to remove impurities as well as increase the number of hydroxyl groups on the substrate surface before the sample preparation. SURMOFs HKUST-1 was grown on the pretreated SiO2/Si substrate using the dipping method under ultrasonication treatment, in which the substrates were dipped subsequently into Cu(OAc)2 ethanolic solution (1 mM) and BTC ethanolic solution (0.1 mM). Between the immersion steps, there was an ultrasonication treatment with pure ethanol for 60 s (further details are described in the Supporting Information).18 As a modification layer with several nanometers, the MOF thin film with one, two, three, and four dipping cycles for HKUST-1 can modify the MOF/SiO2 using a number of distinct LPE cycles. The surface profiles of the sample SURMOF HKUST-1 checked by AFM images (Figure 2a,c) showed a homogeneous thin film with a roughness of ∼3 nm. After the epitaxial growth of the SURMOFs, the height profiles in AFM images of the two-, three-, and four-cycle HKUST-1 were performed by 7261

DOI: 10.1021/acsami.6b14541 ACS Appl. Mater. Interfaces 2017, 9, 7259−7264

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Figure 4. Output characteristic (a) and transfer characteristic (b) of OFET device with HKUST-1 (three cycles)/SiO2/Si.

Table 1. Electrical Characteristics of OFETs Based on Bare SiO2 and One, Two, Three, and Four Cycles SURMOF HKUST-1/ SiO2 dielectric layer bare SiO2 HKUST-1(1cycle)/SiO2 HKUST-1(2cycle)/SiO2 HKUST-1(3cycle)/SiO2 HKUST-1(4cycle)/SiO2

μ (cm2/(V s)) (5.17 (8.33 (1.09 (1.15 (1.32

± ± ± ± ±

0.52) 0.47) 0.07) 0.23) 0.20)

× × × × ×

μmax (cm2/(V s)) −3

10 10−3 10−2 10−2 10−2

5.94 8.79 1.16 1.55 1.54

× × × × ×

10−3 10−3 10−2 10−2 10−2

μWC i(VG − Vth)2 2L

Ion/off

no. of devices tested

102 103 103 103 103

8 5 7 7 7

mobility and on/off ratio increased to 8.33 × 10−3 cm2 V−1 s−1 and 103, respectively. At the same time, the threshold voltage reduced to a value less than 10, which means the device can be operated at a low voltage. The results showed that the performance of OFET was improved significantly when there was a film of SURMOF HKUST-1 between the gate dielectrics and organic semiconductors. After the LPE dipping cycle of SURMOF HKUST-1 was increased, the performances of obtained OFETs were improved. However, there was not much difference between these HKUST-1/SiO2-based OFET devices when two, three, and four LPE dipping cycles of SURMOF HKUST-1 were used as the modification layer. The hole mobilities of HKUST-1/SiO2-based OFET devices with two to four cycles of SURMOF HKUST-1 are found to be (1. 09 ± 0.07) × 10−2, (1.15 ± 0.23) × 10−2, and (1.32 ± 0.20) × 10−2, respectively. All of them showed lower threshold voltages and higher on/off ratios than that of OFETs with the bare SiO2 as dielectric layer.

semiconductor layer that can enhance the performance of OFETs. Top-contact, bottom-gate OFETs were fabricated on the SiO2/Si substrate (Figure 1b). The drain voltage was set at −80 V. The field-effect mobility (μ) in the saturation region is calculated according to the following equation: IDS =

Vth (V) −13 to −33 −7 to −10 −3 to −9 −1 to −10 −1 to −8

(1)

Here, W and L are the channel width and channel length, respectively; IDS is the drain current in the saturated regime; Ci is the capacitance per unit area of the dielectric layer (Fcm−2); VG is the gate voltage; and Vth is the threshold voltage. The saturation region mobilities were calculated from the transfer characteristics of the OFETs using the slope derived from the square root of the absolute value of the current as a function of gate voltage between −80 and −30 V. The threshold voltages of the OFETs were derived from the onsets of the transfer curves. As an example, Figure 4 depicted the output and transfer curves of OFET device containing a HKUST-1 (three cycles) layer. Device performance data of OFETs based on bare SiO2 and one-, two-, three-, and four-cycle SURMOF HKUST1 on SiO2 are listed in Table 1. When the bare SiO2 without modification was used as the dielectric layer, the resulting devices showed a low hole mobility of 5.94 × 10−3 cm2 V−1 s−1 (an average value: 5.17 × 10−3 cm2 V−1 s−1) and a high threshold voltage larger than 13 V. In contrast, when a film of SURMOF HKUST-1 with one LPE dipping cycle was applied on the surface of SiO2 to modify the dielectrics layer, the

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CONCLUSIONS

In conclusion, here we first use MOF thin films prepared by a liquid-phase epitaxial approach to modify the SiO2 dielectric layer in OFETs. By controlling the LPE cycles (two, three, and four cycles) of SURMOF HKUST-1 preparation, the charge mobility, threshold voltage, and current on/off ratio can be well-tuned and optimized. The results showed that the performance of the HKUST-1/SiO2-based OFET was much better than that of the bare SiO2 based OFET. The performance enhancement of the device is mainly attributed 7262

DOI: 10.1021/acsami.6b14541 ACS Appl. Mater. Interfaces 2017, 9, 7259−7264

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ACS Applied Materials & Interfaces to the highly crystalline, homogeneous, and low-k thin film grown on the SiO2/Si substrate and the smaller interface trap density in the OFET, which indicates that the SURMOF HKUST-1-modified dielectric layer is beneficial to the design of a novel architecture of OFETs with tunable and improved performance.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NSFC (21425102, 21601189, and 21521061) and the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB20000000).

EXPERIMENTAL SECTION

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Materials and Instrumentation. All reagents and solvents employed were commercially available and used as received without further purification. Powder XRD (PXRD) analysis was performed on a MiniFlex2 X-ray diffractometer using Cu Kα radiation (λ = 0.1542 nm) in the 2θ range of 4−20° with a scanning rate of 0.5° min−1. The samples grown on functionalized Au substrate were characterized with IRRAS. IRRAS data were recorded using a Bruker Vertex 70 FTIR spectrometer with 2 cm−1 resolution at an angle of incidence of 80° relative to the surface normal. SEM images for the morphology of thin films were measured using the JSM6700. In order to improve the resolution of the dielectric sample, thermally deposited gold was used for SEM measurement. AFM surface and height images were recorded with a Dimension ICON. The thickness of semiconductive polymer was measured by profilometer (Bruker, Dektak XT) The electrical characteristics of the OFETs were measured with an Agilent 4155C semiconductor parameter analyzer. Preparation of SURMOF HKUST-1 Layer. The SiO2/Si substrate was pretreated with KOH (1 mM) and H2O2 (KOH/H2O2 = 3:1) under 80 for 20 min. The substrate can be functionalized with an −OH terminal group. The hydroxy-functionalized SiO2/Si substrate was a good candidate for the preparation SURMOF HKUST-1 usingthe LPE method. There are three containers for different solutions (1 mM Cu(OAc)2, 0.1 mM BTC and pure ethanol) as shown in Figure S1. The functionalized SiO2/Si substrate is immersed in each container sequentially, and the ultrasonic treatment is switched on during the ethanol-rinsing step. By subsequently repeating and alternating the immersion, the one-, two-, three-, and four-cycle SURMOF HKUST-1/SiO2-based dielectric layers are prepared successfully. Device Fabrication and Characterization. As shown in Figure 1b, the semiconducting polymers PTB7-Th were deposited on the bare SiO2- and HKUST-1/SiO2-based Si substrates by spin-coating the polymer solution (∼3 mg mL−1 in chlorobenzene) at 3000 rpm for 60 s, and then the gold top contact source and drain electrodes of ∼50 nm were vapor-deposited through a shadow mask. The channel widths and lengths of the devices were 6.0 mm and 300 μm, respectively. The electrical characteristics of the OFETs were measured with an Agilent 4155C semiconductor parameter analyzer, and the data were recorded by the ICS lite software.

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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/acsami.6b14541. Details of AFM scratching experiments and additional figures (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Qingdong Zheng: 0000-0002-6324-0648 Jian Zhang: 0000-0003-3373-9621 Author Contributions †

Z.-G.G. and S.-C.C. contributed equally. 7263

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