Article pubs.acs.org/Langmuir
Facile Peeling Method as a Post-Remedy Strategy for Producing an Ultrasmooth Self-Assembled Monolayer for High-Performance Organic Transistors Xiaosong Chen,†,§ Zeyang Xu,§ Kunjie Wu,§ Suna Zhang,§ Hongwei Li,§ Yancheng Meng,§ Zhongwu Wang,§ Liqiang Li,*,§ and Xueming Ma*,† †
Department of Physics, East China Normal University, Shanghai, 200241, China Advanced Nano-Materials Division, Key Laboratory of Nano-Devices and Applications, Suzhou Institute of Nano-Tech and Nano-Bionics (SINANO), Chinese Academy of Sciences (CAS), Suzhou 215123, China
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S Supporting Information *
ABSTRACT: The modification of dielectric surface with a self-assembled monolayer (SAM) such as octadecyltrichlorosilane (OTS) is a widely used method to tune the electrical property of diverse electronic devices based on organic semiconductors, graphene, transition metal dichalcogenides (TMDs), and so forth. The surface roughness of self-assembled OTS monolayer is a key factor in determining its effect on device performance, but the preparation of an ultrasmooth OTS monolayer is a technologically challenging task. In this work, an ultrasmooth OTS monolayer is prepared via a facile peeling method, which may serve as a postremedy strategy to remove the protuberant aggregates. Such a method has not been reported before. With organic semiconductors as a testing model, ultrasmooth OTS may significantly improve the charge mobility of organic field-effect transistors (OFETs). P-type dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) OFET with an ultrasmooth OTS monolayer yields good reproducibility and unprecendented maximum mobility of 8.16 cm2 V−1 s−1, which is remarkably superior to that of the OFET with a pristine OTS monolayer. This work develops a simple method to resolve the common and significant problem of the quality of OTS modification, which would be highly promising for electronic applications as well as other fields such as surface and interface engineering.
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INTRODUCTION A field-effect transistor (FET) based on novel electronic materials including organic semiconductors, nanocarbon materials, two-dimenional materials, and so forth has been envisioned as one of the essensial components for some nextgeneration electronic products.1−6 In FET, the charge transportation generally takes place primarily at the dielectric/ semiconductor interface, so the properties of the dielectric/ semiconductor interface dominate the device performance.3,7−9 The modification of the dielectric surface with self-assembled monolayers (SAM) may improve the quality of the dielectric/ semiconductor interface, reduce charge trapping and scattering, and therefore significantly improve the performance of diverse electronic devices.10−17 Octadecyltrichlorosilane (OTS, Figure 1b) has been widely used as a SAM to modify the dielectric surface, which has resulted in a great improvement in device performance for organic semiconductors,18−22 graphene,14−16,23−25 and transition-metal dichalogenides (TMDs).26,27 The quality (smoothness and density) of OTS may have a great effect on the device performance.18,19,23,24 For example, in organic field-effect transistors (OFET), the rough OTS surface may deteriorate the assembly and ordering of organic semiconductors, and an © XXXX American Chemical Society
ultrasmooth OTS surface may promote the assembly of organic semiconductors and eventually improve the charge-transporting mobility.24 In a graphene device, a rough OTS surface may induce the distortion of the atomic plane of graphene, which will lower the charge-transporting mobility, but an ultrasmooth OTS surface may avoid such a problem.23,24 However, it is quite difficult to guarantee the quality (especially surface roughness) of OTS, which may result in the poor performance reproducibility of FET devices. For example, the reported mobilities of pentacene OFET with OTS-modified dielectrics range from 0.03 to 6.3 cm2 V−1 s−1,18,19,28−32 and the mobilities of DNTT OFETs with OTS-modified dielectrics range from 0.43 to 3.1 cm 2 V −1 s −1 ,33−36 which shows dreadful reproducibility. In graphene FET, the mobility on an OTSmodified dielectric surface also has a large range of 2500− 47 000 cm2 V−1 s−1.14−16,23−25 Apart from the difference in the quality of electronic materials, equipment, experimental skills, etc., the quality (smoothness and density) of OTS may play an important role in the poor reproducbility of organic and Received: July 14, 2016 Revised: August 22, 2016
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DOI: 10.1021/acs.langmuir.6b02585 Langmuir XXXX, XXX, XXX−XXX
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Figure 1. (a) Schematic illustration of the fabrication of ultrasmooth OTS by the PMMA peeling method. (b) Molecular structure of OTS. (c) Photograph of the mechanical peeling off PMMA with tweezers. (d) Schematic illustration of an organic field-effect transistor and the molecular structure of DNTT and PDI-8CN2. used as received. AFM measurements were obtained from Veeco Dimension 3100. XRD measurement with a θ−2θ scan mode was carried out on a Bruker D8 ADVANCE diffractometer. The static water contact angle was taken on a DCAT 21. OTS Monolayer Deposition. Before deposition OTS, the Si/SiO2 substrate is treated with O2 plasma (100 W, 30 s). An OTS monolayer is formed on the Si/SiO2 substrate surface by the vapor deposition method for 2 h. The deposition temperature is maintained at 60 °C, and the pump works all the time during the deposition. After deposition, the Si/SiO2 substrate is sonicated in chloroform and ethanol, respectively. Hereafter, PMMA dissolved in toluene is dropped onto the Si/SiO2/OTS substrate, and the thickness of the PMMA film is about 40−60 μm. After stiffening, the PMMA is annealed at 120 °C for 15 min. After cooling, the PMMA layer is peeled off with tweezers. Then, the Si/SiO2/OTS substrate is washed with acetone, chloroform, and ethanol, respectively. Device Fabrication and Characterization. A highly doped silicon wafer and a 100 nm thermally oxidized SiO2 layer (Ci ≈ 34.5 nF cm−2) were used as a gate and insulator, respectively. Twenty nanometers DNTT/PDI-8CN2 film were deposited by vacuum thermal evaporation at a rate of ca. 0.05 Å s−1 under 10−4 Pa at a substrate temperature of 65/100 °C. The OFETs are characterized with a Keithley 4200 SCS under the assistance of a probe station system under dark and air conditions. The field-effect characteristics are calculated with eq 1 in the saturation regime.
graphene FET. OTS has three reactive Cl−Si groups, which are very active and easy to hydrolyze and further cross-link to form aggregates. Therefore, it is quite difficult to guarantee the surface roughness of the OTS monolayer with high reproducibility by the conventional solution or vapor modification methods;37−39 that is, OTS molecules always cross-link with each other and form aggregates during surface modifcation. In this regard, it would be a good choice to develop a postremedy strategy to remove the aggregates to produce an ultrasmooth OTS monolayer. Such a postremedy method has not been reported so far. In this article, we develop a simple and facile peeling method with the PMMA resist as a postremedy strategy to remove the redundant and protuberant OTS aggregates or other impurities on the OTS surface. After peeling treatment, an ultrasmooth OTS monolayer on SiO2 is obtained. With such a posttreatment method, it would be unnecessary to precisely control the experimental details during the OTS modification process, which has been proven to be difficult especially for large mass production. That is, this method may amend the defects of the OTS surface that are always produced during the modification process. With OFET as the test model, the ultrasmooth OTS monolayer significantly improves the charge-carrier mobility and reproducibility of dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT). The unprecedented maximum mobility of DNTT OFET with ultrasmooth OTS reaches 8.16 cm2 V−1 s−1, which is remarkably higher than that with pristine rough OTS. This work resolves a common and significant problem about the quality of OTS modification, which would be very meaningful for improving the performance and reproducibility of OFET as well as other electronic devices.
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IDS =
μCiW (VGS − VT)2 2L
(1)
where Ci is the dielectric capacitance, W is the channel width, and L is the channel length. It is worth noting that the dielectric layer is a twolayer system, which is made of SiO2 and OTS. It is known that the capacitance (CSiO2) of 100 nm SiO2 is 34.5 nF cm−2 and the capacitance of the OTS monolayer (COTS) is 1100 nF cm−2.40 So the two-layer system in series contributes to the total capacitance (Ctotal) of 33.5 nF cm−2 based on the equation 1/Ctotal = 1/CSiO2 + 1/COTS, which is the same order of magnitude as the 100 nm SiO2.41
MATERIALS AND METHODS
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Materials and Instruments. OTS, 1H,1H,2H,2H-Perfluorooctyltriethoxysilane, (3-aminopropyl)triethoxysilane (APTES), and DNTT were purchased from Sigma-Aldrich. N,N′-Bis(n-octyl)-dicyanoperylene-3,4:9,10-bis(dicarboximide) (PDI-8CN2) was obtained from Polyera. Acetone, chloroform, and ethanol (HPLC) were obtained from the Aladdin Industrial Corporation. All of these compounds were
RESULTS AND DISCUSSION Peeling Process for an Ultrasmooth OTS Monolayer. The schematic fabrication procedure of the ultrasmooth OTS monolayer is illustrated in Figure 1a. The description of B
DOI: 10.1021/acs.langmuir.6b02585 Langmuir XXXX, XXX, XXX−XXX
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Figure 2. AFM images of OTS monolayers before (a, c) and after (b, d) the PMMA peeling treatment. Scan areas are 5 × 5 μm2 (a, b) and 10 × 10 μm2 (c, d), respectively.
experimental details has been given in the Materials and Methods section. OTS molecules are chemically bonded on the SiO2 surface to form a self-assembled monolayer. However, OTS has three reactive and active Si−Cl groups, which occur easily through the cross-linking reaction to form aggregates. Through the conventional vapor or solution method, the surface of the OTS monolayer is always unsmooth, and dense or sparse aggregation dots are generally formed and somehow are difficult to precisely control. More seriously, these aggregates are physically or chemically adsorbed on the surface of the OTS monolayer, and it is quite difficult to remove them by routine methods such as solution ultrasonication. For this problem, we use the PMMA resist as tape to stick the aggregates. The adhesion of PMMA with the unreacted OTS can be understood by diffusion theory. The theory is that high polymer molecules have two basic features: chainlike structure and flexibility. Under the influence of micro-Brownian movement, the flexible PMMA molecular chains in an elastomeric state penetrate the unreacted OTS aggregates and then form a knitting reaction, which could produce a strong adhesive force. Mechanical peeling-off (Figure 1a) of the PMMA layer may remove most of aggregates, and the underlying chemically bonded OTS monolayer can survive the peeling process. After peeling treatment, most of protuberant aggregates can be removed and the surface of
aggregates OTS monolayer becomes very smooth compared to the pristine OTS surface. Figure 2 shows the AFM images of the OTS surface before and after the PMMA peeling treatment. Figure 2a,c shows typical images of two pristine OTS surfaces prepared under similar experimental conditions. There are dense protuberant aggregates on the surface, but the heights of the aggregates are high and low, respectively. The root-mean-square (rms) roughnesses of these two surfaces are 0.35 and 0.17 nm, respectively. These results indicate the different surface roughness and nonduplicability of the OTS deposition method. In fact, OTS modification is frequently performed under the same conditions in our laboratory, but the surface roughness varies from batch to batch. In previous reports, the roughness of OTS also varies one by one.19,37−39,42 The facts indicate that the roughness of the OTS surface with the conventional deposition method is not easy to control. In fact, the quality of the OTS monolayer is related to many factors, including the quality of the OTS agent, environmental temperature and humidity, dust in the air, the cleanliness of containers and tools, the purity and water content of solvents, and the reaction temperature, vacuum, and time. For example, McGovern et al. found that trace water is very important for OTS modification,43 but it is definitely a difficult task to control the amount of trace water in the reaction system. Theoretically, C
DOI: 10.1021/acs.langmuir.6b02585 Langmuir XXXX, XXX, XXX−XXX
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Figure 3. (a) Output (source-drain current versus source-drain voltage), (b) transfer (source-drain current versus source-gate voltage) curves of DNTT OFET, and (c) mobility distribution of 30 DNTT transistors on ultrasmooth OTS with PMMA peeling treatment. (d) Output, (e) transfer curves of DNTT OFET, and (f) mobility distribution of 30 DNTT transistors on pristine OTS without the PMMA peeling treatment.
These comparative results clearly indicate that the peeling treatment may serve as an efficient and duplicable postremedy strategy to produce an ultrasmooth OTS surface. To testify to the existence of OTS after peeling treatment, a static water contact angle was applied to characterize the hydrophobility (Figure S2). Before applied PMMA, the contact angle of the pristine OTS monolayer is 107.7°, which is consistent with the previous reported value of high-density OTS.44,45 The water contact angle of the OTS surface after the PMMA peeling treatment is about 105.0°, which decreases slightly compared to that of pristine OTS but is still in the range of high-density OTS. This result indicates that the underlying OTS monolayer may survive the peeling treatment because the bare SiO2 surface has a contact angle of
