Letter pubs.acs.org/NanoLett
Assemblies of Colloidal CdSe Tetrapod Nanocrystals with Lengthy Arms for Flexible Thin-Film Transistors Hyeonjun Heo,†,△ Moo Hyung Lee,‡,△ Jeehye Yang,‡ Han Sol Wee,§ Jaehoon Lim,† Donghyo Hahm,† Ji Woong Yu,⊥ Wan Ki Bae,∥ Won Bo Lee,⊥ Moon Sung Kang,*,‡ and Kookheon Char*,† †
The National Creative Research Initiative Center for Intelligent Hybrids, The WCU Program of Chemical Convergence for Energy & Environment, School of Chemical and Biological Engineering, ⊥School of Chemical and Biological Engineering, Seoul National University, Seoul 08826, Korea ‡ Department of Chemical Engineering, Soongsil University, Seoul, 06978, Korea § Department of Chemical and Biomolecular Engineering, Sogang University, Seoul 04107, Korea ∥ Photoelectronic Hybrids Research Center, Korea Institute of Science and Technology (KIST), Seoul 02792, Korea S Supporting Information *
ABSTRACT: Herein, we report unique features of the assemblies of tetrapod-shaped colloidal nanocrystals (TpNCs) with lengthy arms applicable to flexible thin-film transistors. Due to the extended nature of tetrapod geometry, films made of the TpNC assemblies require reduced numbers of inter-NC hopping for the transport of charge carriers along a given channel length; thus, enhanced conductivity can be achieved compared to those made of typical spherical NCs without arms. Moreover, electrical conduction through the assemblies is tolerant against mechanical bending because interconnections between TpNCs can be well-preserved under bending. Interestingly, both the conductivity of the assemblies and their mechanical tolerance against bending are improved with an increase in the length of tetrapod arms. The arm length-dependency was demonstrated in a series of CdSe TpNC assemblies with different arm lengths (l = 0−90 nm), whose electrical conduction was modulated through electrolyte gating. From the TpNCs with the longest arm length included in the study (l = 90 nm), the film conductivity as high as 20 S/cm was attained at 3 V of gate voltage, corresponding to electron mobility of >10 cm2/(V s) even when evaluated conservatively. The high channel conductivity was retained (∼90% of the value obtained from the flat geometry) even under high bending (bending radius = 5 mm). The results of the present study provide new insights and guidelines for the use of colloidal nanocrystals in solution-processed flexible electronic device applications. KEYWORDS: Colloidal semiconductor nanocrystal(s), CdSe tetrapod nanocrystal(s), arm length dependence, flexible thin-film transistors, ion gel gate dielectric(s)
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reinforced electronic coupling between neighboring NCs.14 Introducing metal dopants into the CdSe NC films containing thiocyanate ligands resulted in the field-effect electron mobility as high as 27 cm2/(V s) at room temperature, exhibiting the band-like transport.15 Alternatively, families of conductive inorganic chalcogenides16 or halides17 ligands were developed. For instance, films of CdSe NC with In2Se42− ligands showed the field-effect mobility as high as 38 cm2/(V s),18 also exhibiting the band-like transport behavior, indicative of formation of well-extended energy states through the NC assemblies. An alternative approach includes controlling the morphology of nanocrystals. For instance, closely packed cubic PbS NCs exhibiting high inter-NC coupling yielded enhanced carrier mobility compared to the films based on spherical NCs.19 Also,
olloidal semiconductor nanocrystals (NCs) are attractive material candidates for active layers in advanced thin-film electronic/optoelectronic devices.1−3 Beyond the unique sizetunable physical properties,4 the capability to disperse colloidal NCs in conventional solvents facilitates great potential for their use in large-area, printed electronics.5−8 However, the electrical conduction through the assemblies of these materials is limited by inefficient inter-NC hopping processes.9,10 This is mainly due to the presence of bulky insulating ligand molecules attached to the surfaces of inorganic NC cores, which are critical to attain stable dispersions of the materials. Several approaches have been developed to enhance the electrical transport between NCs and improve the overall conduction through their assemblies.11 Typical methods involve replacing the bulky insulating ligands with short molecules.5,12,13 For example, by adopting thiocyanate ligands, the films of CdSe NCs yielded a field-effect electron mobility as high as 1.5 cm2/ (V s), and those of PbTe NCs yielded a Hall-effect mobility as high as 2.8 cm2/(V s), as a result of reduced spacing and © 2017 American Chemical Society
Received: January 9, 2017 Revised: March 27, 2017 Published: March 28, 2017 2433
DOI: 10.1021/acs.nanolett.7b00096 Nano Lett. 2017, 17, 2433−2439
Letter
Nano Letters the films of rod-shaped ZnO NCs (i.e., nanorods) yielded electron mobility as high as 9 cm2/(V s) when optimally aligned.20 The high electron mobility associated with nanorods can be attributed to the elongated structure of the material with a high aspect ratio that reduces the number of charge hopping necessary for crossing a given channel distance. In addition, employing crystals with even larger aspect ratios, i.e., PbSe nanowires, has facilitated the development of transistors with electron and hole mobilities higher than 10 cm2/(V s) with the capability of selectively tuned polarity of the devices.21 In order to further advance the morphology-control approach, tetrapodshaped colloidal nanocrystals (TpNCs) should have significant benefits.22−24 Of all of the benefits, assemblies of TpNCs require a smaller number of hopping processes for carriers to conduct along a given channel length due to their extended-arm structure. This effect would be more pronounced particularly when TpNCs with lengthy arms are employed. Second, because the arms are extended isotropically for TpNCs, the delicate alignment of the materials would no longer be critical to achieve effective conduction pathways through their assemblies, as long as percolation is guaranteed. Thus, the devices could be fabricated with ease and tolerance, unlike those based on onedimensional nanorods or nanowires. Third, the voids in the assemblies could be infused with functional materials,23,25 such as other semiconductors to form bulk-heterojunctions or electrolytes to modulate the carrier density in TpNCs electrochemically. The extended structure of TpNC should also provide their assemblies with greater tolerance against mechanical bending and stretching, as compared to the networks based on spherical NCs. As a proof-of-concept, we present the use of colloidal CdSe TpNCs with lengthy arms for flexible transistor applications. Assemblies of CdSe TpNC were readily prepared from a simple solution process, and the resulting voids were infused with electrolytes to modulate the conduction through the assemblies by more than 5 orders of magnitude at voltages below 3 V. Importantly, the electron conduction through the assemblies exhibited strong correlation with tetrapod arm length, which led to attaining high conductivity above 20 S/cm from the assemblies of CdSe TpNCs with 90-nm-long arms. In addition, we confirmed that the electrical conduction through the TpNC network is more tolerant against mechanical bending compared to that through the assemblies of spherical NCs, indicating that the tolerance is indeed arm length-dependent. Overall, the results of the present study demonstrate the great promise of morphology-controlled colloidal TpNCs for achieving low-cost, flexible transistors with high device performance. Colloidal CdSe TpNCs were prepared by growing tetrapod arms on spherical CdSe NC cores through the continuous precursor injection (CPI) method.23,26 Briefly, zinc-blende spherical CdSe NC cores were first prepared through the typical hot-injection method based on cadmium oleate and selenium−octadecene precursors in octadecene solvent. Without any purification steps, the CPI method was directly applied to the CdSe NC cores by injecting a mixture of separately prepared cadmium and selenium precursors into the spherical NC core dispersions using a syringe pump. This process led to growing arms onto the cores, yielding the CdSe TpNCs. The length (l) and thickness (or diameter) of the arms could be finely tuned by controlling the growth temperature and precursor injection rate during the CPI step. Detailed description of the synthetic procedure are provided in the Supporting Information (SI) and previous reports.23,26 In
addition, we prepared reference spherical CdSe NCs (l = 0 nm) with a size equivalency with the size of TpNC cores. The resulting CdSe TpNCs were purified through subsequent cycles of precipitation and redispersion steps and finally stored in chloroform before use. Transmission electron microscopy (TEM) images in Figure 1a demonstrate the arm length (l =
Figure 1. (a) TEM images of CdSe TpNCs with different arm lengths (l = 30, 60, 75, and 90 nm) (scale bar = 50 nm). (b) A HR-TEM image of an arm for a CdSe TpNC (l = 90 nm) (scale bar = 5 nm). The inset shows a FFT pattern of the image. (c) UV−vis spectroscopy of CdSe TpNCs dispersed in chloroform. (d) An XRD diffractogram of CdSe TpNCs (l = 90 nm) with reference data.
30−90 nm) controlled CdSe TpNCs. Figure 1b displays a highresolution (HR) TEM image and its fast Fourier transform (FFT) pattern (inset) of an arm from a CdSe TpNC (l = 90 nm), indicating that the arms are single crystalline. The series of TpNCs were carefully synthesized to yield equivalent arm thicknesses of ∼8 nm. This could be confirmed both from the TEM images (Figure 1a) and also from the peak position of the first absorbance maximum (Figure 1c). Attaining such a series with consistent arm thicknesses was critical for systematically investigating the influence of the arm length on charge transport through the NC assemblies. Figure 1d shows a XRD pattern of CdSe TpNCs (l = 90 nm). The pattern closely matches with the Wurzite phase of CdSe, indicating that the arms of the TpNCs are formed in the Wurzite structure. Based on these well-defined CdSe TpNCs, thin-film transistors (TFTs) were fabricated by employing ion gels (i.e., solid electrolytes) as a gate dielectric material. The ion gels used in the present study consists of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]) ionic liquids impregnated within poly(styrene-b-methyl methacrylate-b-styrene) (PS−PMMA−PS) block copolymer matrices.27,28 The ionic liquids served as an active component to induce carriers in contact with the TpNC layer, while the triblock copolymer matrix provided the system with solid-like integrity for practical device applications. The use of electrolyte 2434
DOI: 10.1021/acs.nanolett.7b00096 Nano Lett. 2017, 17, 2433−2439
Letter
Nano Letters
gel dielectric layer. Immediately after the spin-coating process, the ion gel layer was again selectively removed using a cotton swab such that only the gel layer bridging the active channel and the coplanar gate electrode remained (Figure 2a). This step was critical for suppressing the leakage current through electrolyte. The entire device fabrication was performed inside a nitrogen-filled glovebox. These devices were exposed to ambient condition only briefly (
