Co-reporter:Hyungseok Kang, Yeontae Kim, Siuk Cheon, Gi-Ra Yi, and Jeong Ho Cho
ACS Applied Materials & Interfaces September 13, 2017 Volume 9(Issue 36) pp:30779-30779
Publication Date(Web):August 18, 2017
DOI:10.1021/acsami.7b09839
We developed a method of chemically welding silver nanowires (AgNWs) using an aqueous solution containing sodium halide salts (NaF, NaCl, NaBr, or NaI). The halide welding was performed simply by immersing the as-coated AgNW film into the sodium halide solution, and the resulting material was compared with those obtained using two typical thermal and plasmonic welding techniques. The halide welding dramatically reduced the sheet resistance of the AgNW electrode because of the strong fusion among nanowires at each junction while preserving the optical transmittance. The dramatic decrease in the sheet resistance was attributed to the autocatalytic addition of dissolved silver ions to the nanowire junction. Unlike thermal and plasmonic welding methods, the halide welding could be applied to AgNW films with a variety of deposition densities because the halide ions uniformly contacted the surface or junction regions. The optimized AgNW electrodes exhibited a sheet resistance of 9.3 Ω/sq at an optical transmittance of 92%. The halide welding significantly enhanced the mechanical flexibility of the electrode compared with the as-coated AgNWs. The halide-welded AgNWs were successfully used as source–drain electrodes in a transparent and flexible organic field-effect transistor (OFET). This simple, low-cost, and low-power consumption halide welding technique provides an innovative approach to preparing transparent electrodes for use in next-generation flexible optoelectronic devices.Keywords: sheet resistance; silver nanowire; sodium halide; transparent conductive electrode; welding;
Co-reporter:Jia Sun, Min Je Kim, Myeongjae Lee, Dain Lee, Seongchan Kim, Jong-Hyun Park, Sungjoo Lee, BongSoo Kim, and Jeong Ho Cho
The Journal of Physical Chemistry C November 2, 2017 Volume 121(Issue 43) pp:24352-24352
Publication Date(Web):October 6, 2017
DOI:10.1021/acs.jpcc.7b08798
In this manuscript, the fabrication of polymer nonvolatile memory cells based on one-transistor–one-transistor (1T1T) device geometries is reported. A spin-coated diketopyrrolopyrrole (DPP)-based polymer semiconductor was used as the active channel layer for both the control transistor (CT) and memory transistor (MT); thermally deposited gold nanoparticles (Au NPs) were inserted between the tunneling and blocking gate dielectrics as a charge-trapping layer of the MT. In the 1T1T memory cell, the source electrode of the CT was connected to the gate electrode of the MT, while the drain electrode of the MT was connected to the gate electrode of the CT. The reading and writing processes of the memory cells operated separately, which yielded a nondestructive read-out capability. The fabricated 1T1T polymer memory cells exhibited excellent device performances with a large memory window of 16.1 V, a high programming–erasing current ratio >103, a long retention of 103 s, a cyclic stability of 500 cycles, and a 2-bit data storage capability. The proposed device architecture provides a feasible method by which to achieve high-performance organic nonvolatile memory.
Co-reporter:Ajjiporn Dathbun, Youngchan Kim, Seongchan Kim, Youngjae Yoo, Moon Sung Kang, Changgu Lee, and Jeong Ho Cho
Nano Letters May 10, 2017 Volume 17(Issue 5) pp:2999-2999
Publication Date(Web):April 17, 2017
DOI:10.1021/acs.nanolett.7b00315
We demonstrated the fabrication of large-area ReS2 transistors and logic gates composed of a chemical vapor deposition (CVD)-grown multilayer ReS2 semiconductor channel and graphene electrodes. Single-layer graphene was used as the source/drain and coplanar gate electrodes. An ion gel with an ultrahigh capacitance effectively gated the ReS2 channel at a low voltage, below 2 V, through a coplanar gate. The contact resistance of the ion gel-gated ReS2 transistors with graphene electrodes decreased dramatically compared with the SiO2-devices prepared with Cr electrodes. The resulting transistors exhibited good device performances, including a maximum electron mobility of 0.9 cm2/(V s) and an on/off current ratio exceeding 104. NMOS logic devices, such as NOT, NAND, and NOR gates, were assembled using the resulting transistors as a proof of concept demonstration of the applicability of the devices to complex logic circuits. The large-area synthesis of ReS2 semiconductors and graphene electrodes and their applications in logic devices open up new opportunities for realizing future flexible electronics based on 2D nanomaterials.Keywords: chemical vapor deposition (CVD); large area; logic gate; ReS2; transistor;
Co-reporter:Jae Hoon Park, Jong Su Kim, Young Jin Choi, Wi Hyoung Lee, Dong Yun Lee, and Jeong Ho Cho
ACS Applied Materials & Interfaces February 1, 2017 Volume 9(Issue 4) pp:
Publication Date(Web):December 29, 2016
DOI:10.1021/acsami.6b15301
One-dimensional (1D) nano/microwires have attracted considerable attention as versatile building blocks for use in diverse electronic, optoelectronic, and magnetic device applications. The large-area assembly of nano/microwires at desired positions presents a significant challenge for developing high-density electronic devices. Here, we demonstrated the fabrication of cross-stacked pn heterojunction diode arrays by integrating well-aligned inorganic and organic microwires fabricated via evaporative assembly. We utilized solution-processed n-type inorganic indium–gallium–zinc-oxide (IGZO) microwires and p-type organic 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-PEN) microwires. The formation of organic TIPS-PEN semiconductor microwire and their electrical properties were optimized by controlling both the amounts of added insulating polymer and the widths of the microwires. The resulting cross-stacked IGZO/TIPS-PEN microwire pn heterojunction devices exhibited rectifying behavior with a forward-to-reverse bias current ratio exceeding 102. The ultrathin nature of the underlying n-type IGZO microwires yielded gate tunability in the charge transport behaviors, ranging from insulating to rectifying. The rectifying behaviors of the heterojunction devices could be modulated by controlling the optical power of the irradiated light. The fabrication of semiconducting microwires through evaporative assembly provides a facile and reliable approach to patterning or positioning 1D microwires for the fabrication of future flexible large-area electronics.Keywords: blade-coating; diode; gate; light; pn heterojunction;
Co-reporter:Qijun Sun;Dong Hae Ho;Caofeng Pan;Yongsuk Choi;Do Hwan Kim;Zhong Lin Wang
ACS Nano December 27, 2016 Volume 10(Issue 12) pp:11037-11043
Publication Date(Web):November 28, 2016
DOI:10.1021/acsnano.6b05895
We report the development of a piezopotential-programmed nonvolatile memory array using a combination of ion gel-gated field-effect transistors (FETs) and piezoelectric nanogenerators (NGs). Piezopotentials produced from the NGs under external strains were able to replace the gate voltage inputs associated with the programming/erasing operation of the memory, which reduced the power consumption compared with conventional memory devices. Multilevel data storage in the memory device could be achieved by varying the external bending strain applied to the piezoelectric NGs. The resulting devices exhibited good memory performance, including a large programming/erasing current ratio that exceeded 103, multilevel data storage of 2 bits (over 4 levels), performance stability over 100 cycles, and stable data retention over 3000 s. The piezopotential-programmed multilevel nonvolatile memory device described here is important for applications in data-storable electronic skin and advanced human-robot interface operations.Keywords: multilevel data storage; nanogenerator; nonvolatile memory; piezopotential; transistor;
Co-reporter:Dain Lee, Seongchan Kim, Yeontae Kim, and Jeong Ho Cho
ACS Applied Materials & Interfaces August 9, 2017 Volume 9(Issue 31) pp:26357-26357
Publication Date(Web):July 14, 2017
DOI:10.1021/acsami.7b07077
Taking advantage of the superlative optoelectronic properties of single-layer MoS2, we developed a one-transistor–one-transistor (1T1T)-type MoS2 optoelectronic nonvolatile memory cell. The 1T1T memory cell consisted of a control transistor (CT) and a memory transistor (MT), in which the drain electrode of the MT was connected electrically to the gate electrode of the CT, whereas the source electrode of the CT was connected electrically to the gate electrode of the MT. Single-layer MoS2 films were utilized as the channel materials in both transistors, and gold nanoparticles acted as the floating gates in the MT. This 1T1T device architecture allowed for a nondestructive read-out operation in the memory because the writing (programming or erasing) and read-out processes were operated separately. The switching of the CT could be controlled by light illumination as well as the applied gate voltage due to the strong light absorption induced by the direct band gap of single-layer MoS2 (∼1.8 eV). The resulting MoS2 1T1T memory cell exhibited excellent memory performance, including a large programming/erasing current ratio (over 106), multilevel data storage (over 6 levels), cyclic endurance (200 cycles), and stable retention (103 s).Keywords: memory; MoS2; nondestructive read-out; one-transistor−one-transistor (1T1T); optoelectronic;
Co-reporter:Jun Beom Kim, Yongsuk Choi, Ajjiporn Dathbun, Seongchan Kim, Dongun Lim, Panuk Hong, Jeong Ho Cho
FlatChem 2017 Volume 5(Volume 5) pp:
Publication Date(Web):1 October 2017
DOI:10.1016/j.flatc.2017.06.009
•Low-voltage complementary inverters are fabricated using ReS2 and BP flakes.•The high capacitance of ion gel provides low-voltage operation below 2 V.•The long-range polarizability of ion gel allows a coplanar-gate geometry of inverter.We demonstrated the preparation of low-voltage complementary inverters based on transistors made of ion gel-gated 2D materials. Mechanically-exfoliated ReS2 was utilized as an n-type semiconductor. The ultrahigh capacitance (6 μF/cm2) and long-range polarizability of the ion gel gate dielectric layer provided low-voltage operation below 2 V and allowed a coplanar-gate configuration of the transistors. The ion gel-gated ReS2 transistors exhibited excellent device performance including an electron mobility of 6.7 cm2/Vs and an on-off current ratio of ∼104. Both the charge-transport mechanism and the contact properties of the device were investigated systematically by measuring the temperature-dependent electrical properties. Mechanically-exfoliated black phosphorous (BP) or WSe2 was employed as the p-type counterpart semiconductors to fabricate the complementary inverter. The resulting 2D complementary inverter exhibited low-voltage operation below 2 V with clear signal inversion. The proposed low-voltage ion gel-gated complementary inverter based on 2D materials opens up new opportunities for realizing future electronics based on 2D materials.Download high-res image (119KB)Download full-size image
Co-reporter:Jae Hoon Park, Kyung Jin Park, Tao Jiang, Qijun Sun, Ji-Hyeok Huh, Zhong Lin Wang, Seungwoo Lee, Jeong Ho Cho
Nano Energy 2017 Volume 38(Volume 38) pp:
Publication Date(Web):1 August 2017
DOI:10.1016/j.nanoen.2017.05.062
•Light-transformable and -healable TENGs were fabricated using azobenzene polymer.•The azopolymer surfaces were sculpted to form surface reliefs via photofluidizaton.•The damaged surface reliefs could be erased and transformed in a reversible fashion.Triboelectric nanogenerators (TENGs) have attracted significant interest due to their energy harvesting performances using relatively versatile and robust device architectures (i.e., a pair of stacked distinct layers connected to electrodes). The materials and device architectures of TENGs are highly compatible with emerging wearable technologies. Recently, rational surface morphology designs were considered as a key to enhancing the output efficiency. Surface relief structures can increase an effective surface area, enhance the mechanical contact area between two stacked materials, and therefore enhance the output voltage of a TENG. Unfortunately, surface reliefs in soft polymeric materials generally degrade under repetitive mechanical contact stress. This work introduces a TENG that is transformable and healable via light irradiation-induced material migration (also called by photofluidization). This technique deterministically maximizes the energy harvesting performance and remotely repairs morphological damage. As a representative example, the photo-transformable azobenzene polymer (azopolymer) was implemented in a TENG. The azopolymer surfaces were sculpted to form surface reliefs with a controlled dimensionality and periodicity. The output voltage and current obtained from the TENGs were linearly proportional to the surface area, as expected. During TENG operation, however, the surface relief collapsed and flattened. As a result, the output signals of the TENGs degraded continuously. The light-powered reversible transformation perfectly healed the surface morphology and the resultant TENG performance. The light-powered transformation and healing of the TENGs provides a facile, large-area, and reliable method for designing future flexible energy-harvesting devices.Download high-res image (264KB)Download full-size image
Co-reporter:Seongchan Kim;Young Jin Choi;Yongsuk Choi;Moon Sung Kang
Advanced Functional Materials 2017 Volume 27(Issue 30) pp:
Publication Date(Web):2017/08/01
DOI:10.1002/adfm.201700651
The fabrication of all-transparent flexible vertical Schottky barrier (SB) transistors and logic gates based on graphene–metal oxide–metal heterostructures and ion gel gate dielectrics is demonstrated. The vertical SB transistor structure is formed by (i) vertically sandwiching a solution-processed indium-gallium-zinc-oxide (IGZO) semiconductor layer between graphene (source) and metallic (drain) electrodes and (ii) employing a separate coplanar gate electrode bridged with a vertical channel through an ion gel. The channel current is modulated by tuning the Schottky barrier height across the graphene–IGZO junction under an applied external gate bias. The ion gel gate dielectric with high specific capacitance enables modulation of the Schottky barrier height at the graphene–IGZO junction over 0.87 eV using a voltage below 2 V. The resulting vertical devices show high current densities (18.9 A cm−2) and on–off current ratios (>104) at low voltages. The simple structure of the unit transistor enables the successful fabrication of low-power logic gates based on device assemblies, such as the NOT, NAND, and NOR gates, prepared on a flexible substrate. The facile, large-area, and room-temperature deposition of both semiconducting metal oxide and gate insulators integrates with transparent and flexible graphene opens up new opportunities for realizing graphene-based future electronics.
Co-reporter:Jong Su Kim;Young Jin Choi;Hwi Je Woo;Jeehye Yang;Young Jae Song;Moon Sung Kang
Advanced Functional Materials 2017 Volume 27(Issue 48) pp:
Publication Date(Web):2017/12/01
DOI:10.1002/adfm.201704475
AbstractMonolayer graphene is used as an electrode to develop novel electronic device architectures that exploit the unique, atomically thin structure of the material with a low density of states at its charge neutrality point. For example, a single semiconductor layer stacked onto graphene can provide a semiconductor–electrode junction with a tunable injection barrier, which is the basis for a primitive transistor architecture known as the Schottky barrier field-effect transistor. This work demonstrates the next level of complexity in a vertical graphene–semiconductor architecture. Specifically, an organic vertical p-n junction (p-type pentacene/n-type N,N′-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8)) on top of a graphene electrode constituting a novel gate-tunable photodiode device structure is fabricated. The model device confirms that controlling the Schottky barrier height at the pentacene–graphene junction can (i) suppress the dark current density and (ii) enhance the photocurrent of the device, both of which are critical to improve the performance of a photodiode.
Co-reporter:Juhee Lee, Yuseong Gim, Jeehye Yang, Hyunwoo Jo, Jaehun Han, Hojin Lee, Do Hwan KimWansoo Huh, Jeong Ho Cho, Moon Sung Kang
The Journal of Physical Chemistry C 2017 Volume 121(Issue 9) pp:
Publication Date(Web):February 17, 2017
DOI:10.1021/acs.jpcc.7b01212
Recent attempts to employ colloidal semiconductor nanocrystals (NCs) as the sensitizing materials for hybrid NC–graphene phototransistors have provided a new effective photosensing platform. However, most of these devices are based on NCs containing either lead or cadmium, which would not be the most preferred material candidates for commercialization. Here, we demonstrate the use of colloidal Cu2–xSe NCs that do not contain lead or cadmium as the sensitizers for NCs–graphene hybrid visible phototransistors. Because the long olyelamine ligands originally attached on Cu2–xSe NCs are known to impede electronic process between NCs and graphene, the long ligands are replaced with short amines including octylamine, hexylamine, and butylamine. It is found that the NCs layer with shorter amine ligands yields a more prominent n-doping effect on graphene under illumination, which results in a systematic negative shift in Dirac voltage. More importantly, this leads to devices with larger photocurrent and larger light responsivity. Consequently, from Cu2–xSe NC–graphene hybrid phototransistors attached with butylamine ligands, responsivity as high as 2600 A/W and photocurrent gain as high as 36 000 are achieved at an optical power of 5 × 10–8 W, which are expected be even larger at lower optical powers.
Co-reporter:Ji-Hyun Cha, Jae Hoon Han, Wenping Yin, Cheolwoo Park, Yongmin Park, Tae Kyu AhnJeong Ho Cho, Duk-Young Jung
The Journal of Physical Chemistry Letters 2017 Volume 8(Issue 3) pp:
Publication Date(Web):January 9, 2017
DOI:10.1021/acs.jpclett.6b02763
High-quality and millimeter-sized perovskite single crystals of CsPbBr3 and Cs4PbBr6 were prepared in organic solvents and studied for correlation between photocurrent generation and photoluminescence (PL) emission. The CsPbBr3 crystals, which have a 3D perovskite structure, showed a highly sensitive photoresponse and poor PL signal. In contrast, Cs4PbBr6 crystals, which have a 0D perovskite structure, exhibited more than 1 order of magnitude higher PL intensity than CsPbBr3, which generated an ultralow photoresponse under illumination. Their contrasting optoelectrical characteristics were attributed to different exciton binding energies, induced by coordination geometry of the [PbBr6]4– octahedron sublattices. This work correlated the local structures of lead in the primitive perovskite and its derivatives to PL spectra as well as photoconductivity.
Co-reporter:Dong Hae Ho;Qijun Sun;So Young Kim;Joong Tark Han;Do Hwan Kim
Advanced Materials 2016 Volume 28( Issue 13) pp:2601-2608
Publication Date(Web):
DOI:10.1002/adma.201505739
Co-reporter:Yongsuk Choi;Junmo Kang;Deep Jariwala;Moon Sung Kang;Tobin J. Marks;Mark C. Hersam
Advanced Materials 2016 Volume 28( Issue 19) pp:3742-3748
Publication Date(Web):
DOI:10.1002/adma.201506450
Co-reporter:Jong Su Kim;Beom Joon Kim;Young Jin Choi;Moo Hyung Lee;Moon Sung Kang
Advanced Materials 2016 Volume 28( Issue 24) pp:4803-4810
Publication Date(Web):
DOI:10.1002/adma.201505378
Co-reporter:Youngbin Lee, Hyunmin Kim, Jinhwan Lee, Seong Hun Yu, Euyheon Hwang, Changgu Lee, Jong-Hyun Ahn, and Jeong Ho Cho
Chemistry of Materials 2016 Volume 28(Issue 1) pp:180
Publication Date(Web):November 30, 2015
DOI:10.1021/acs.chemmater.5b03714
We studied the surface-enhanced Raman scattering of an organic fluoropore (Rhodamine 6G, R6G) monolayer adsorbed onto graphene and two-dimensional (2D) molybedenium disulfides (MoS2) phototransistors and compared the results with the Raman scattering of R6G on 2D tungsten diselenides system (WSe2). The Raman enhancement factor of the R6G film adsorbed onto WSe2 was comparable to the corresponding value on graphene at 1365 cm–1 and was approximately twice this value at 615 cm–1. The amplitude of the charge transfer was estimated in situ by measuring the photocurrent produced in a hybrid system consisting of physisorbed R6G layer and the 2D materials. We found that the enhanced Raman scattering of R6G adsorbed onto the 2D materials was closely correlated with the charge transfer between the adsorbed molecules and the 2D materials. We also revealed that the intensity of Raman scattering generally decreased as the layer number of the 2D materials increased. For the R6G on the MoS2 nanosheet, a single layer system provided a maximum Raman enhancement factor, and this value decreased pseudolinearly with the number of layers. By contrast, the Raman enhancement factor of the R6G on WSe2 was greatest for both the mono- and bilayers, and it decreased dramatically as the number of layers increased. We provide qualitative theoretical explanations for these trends based on the electric field enhancement for the multile Fresnel phases and energy band diagrams of both systems.
Co-reporter:Dain Lee, Yongsuk Choi, Euyheon Hwang, Moon Sung Kang, Seungwoo Lee and Jeong Ho Cho
Nanoscale 2016 vol. 8(Issue 17) pp:9107-9112
Publication Date(Web):31 Mar 2016
DOI:10.1039/C6NR02078J
We demonstrated nanofloating gate transistor memory devices (NFGTMs) using mechanically-exfoliated few-layered black phosphorus (BP) channels and gold nanoparticle (AuNPs) charge trapping layers. The resulting BP-NFGTMs exhibited excellent memory performances, including the five-level data storage, large memory window (58.2 V), stable retention (104 s), and cyclic endurance (1000 cycles).
Co-reporter:Youngbin Lee, Jaehyun Yang, Dain Lee, Yong-Hoon Kim, Jin-Hong Park, Hyoungsub Kim and Jeong Ho Cho
Nanoscale 2016 vol. 8(Issue 17) pp:9193-9200
Publication Date(Web):30 Mar 2016
DOI:10.1039/C6NR00654J
We investigated, for the first time, the photoresponse characteristics of solution-synthesized MoS2 phototransistors. The photoresponse of the solution-synthesized MoS2 phototransistor was solely determined by the interactions of the photogenerated charge carriers with the surface adsorbates and the interface trap sites. Instead of contributing to the photocurrent, the illumination-generated electron–hole pairs were captured in the trap sites (surface and interface sites) due to the low carrier mobility of the solution-synthesized MoS2. The photogenerated holes discharged ions (oxygen and/or water) adsorbed onto the MoS2 surface and were released as neutral molecules. At the same time, the photogenerated electrons filled the traps present at the interface with the underlying substrate during their transport to the drain electrode. The filled trap sites significantly relieved the band bending near the surface region, which resulted in both a negative shift in the turn-on voltage and an increase in the photocurrent. The time-dependent dynamics of the solution-synthesized MoS2 phototransistors revealed persistent photoconductance due to the trapped electrons at the interface. The photoconductance was recovered by applying a short positive gate pulse. The instantaneous discharge of the trapped electrons dramatically reduced the relaxation time to less than 20 ms. This study provides an important clue to understanding the photoresponses of various optoelectronic devices prepared using solution-synthesized two-dimensional nanomaterials.
Co-reporter:Jae Hoon Park, Qijun Sun, Yongsuk Choi, Seungwoo Lee, Dong Yun Lee, Yong−Hoon Kim, and Jeong Ho Cho
ACS Applied Materials & Interfaces 2016 Volume 8(Issue 24) pp:15543-15550
Publication Date(Web):May 26, 2016
DOI:10.1021/acsami.6b04340
One-dimensional (1D) nano/microwires have attracted significant attention as promising building blocks for various electronic and optical device applications. The integration of these elements into functional device networks with controlled alignment and density presents a significant challenge for practical device applications. Here, we demonstrated the fabrication of wafer-scale microwire field-effect transistor (FET) arrays based on well-aligned inorganic semiconductor microwires (indium-gallium-zinc-oxide (IGZO)) and organic polymeric insulator microwires fabricated via a simple and large-area evaporative assembly technique. This microwire fabrication method offers a facile approach to precisely manipulating the channel dimensions of the FETs. The resulting solution-processed monolithic IGZO microwire FETs exhibited a maximum electron mobility of 1.02 cm2 V–1 s–1 and an on/off current ratio of 1 × 106. The appropriate choice of the polymeric microwires used to define the channel lengths enabled fine control over the threshold voltages of the devices, which were employed to fabricate high-performance depletion-load inverters. Low-voltage-operated microwire FETs were successfully fabricated on a plastic substrate using a high-capacitance ion gel gate dielectric. The microwire fabrication technique involving evaporative assembly provided a facile, effective, and reliable method for preparing flexible large-area electronics.
Co-reporter:Min Je Kim, Jong Yong Choi, Gukil An, Hyunjung Kim, Youngjong Kang, Jai Kyeong Kim, Hae Jung Son, Jung Heon Lee, Jeong Ho Cho, BongSoo Kim
Dyes and Pigments 2016 Volume 126() pp:138-146
Publication Date(Web):March 2016
DOI:10.1016/j.dyepig.2015.11.022
•A new low band gap polymer of PTTDPP-DT-DTT was successfully synthesized.•The polymer backbone is highly planar and well-conjugated.•The polymer film morphology was effectively controlled by a binary solvent system.•Chloroform:o-dichlorobenzene (v/v 16:1) displayed the best carrier mobility.•Thermal annealing further improves the performance of organic thin film transistor.We report the synthesis of a new low band gap planar polymer of poly(2,5-bis(2-decyltetradecyl)-3-(5-(dithieno[3,2-b:2′,3′-d]thiophen-2-yl)thieno[3,2-b]thiophen-2-yl)-6-(thieno[3,2-b]thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione) (PTTDPP-DT-DTT) for use in organic thin film transistors (OTFTs). The polymer backbone is highly planar and well-conjugated, facilitating interchain stacking. The PTTDPP-DT-DTT-based OTFTs were fabricated and carrier mobilities were improved by using a binary solvent system: chloroform (CF):toluene (Tol), CF:chlorobenzene (CB), and CF:o-dichlorobenzene (DCB). The addition of higher boiling point solvents promoted film crystallinity with more edge-on orientations. Thus, the use of CF:DCB yielded the highest carrier mobility obtained among the devices tested. Thermal annealing further enhanced the mobility of the CF:DCB device. Atomic force microscopy images disclosed the most fibrous feature in the thermally annealed polymer film cast from CF:DCB solution. This work highlights that both the proper selection of a binary solvent and thermal annealing can manage film morphology of rigid planar conjugated polymer semiconductors effectively for high-performance OTFTs.
Co-reporter:Hyunwoo Kim;Beom Joon Kim;Qijun Sun;Moon Sung Kang
Advanced Electronic Materials 2016 Volume 2( Issue 8) pp:
Publication Date(Web):
DOI:10.1002/aelm.201600122
The first use of proton conductors for gating graphene transistors is demonstrated. The proton conductor used in this study, [poly(styrenesulfonic acid)], is added with sodium halide salts in order to improve the capacitive characteristics of the electrolyte gate dielectric. The influence of the added sodium halide salts (NaF, NaCl, NaBr, and NaI), the salt concentration, and the relative humidity on the dielectric properties of the electrolyte are investigated systematically. Substantial enhancement in the device performances of the graphene transistors including carrier mobility, device ON current, Dirac voltage, and device cut-off frequency is attained with the addition of the sodium halide salts. From the optimized conditions based on NaI salts, the graphene transistors exhibit hole and electron mobilities of 1900 and 990 cm2 V–1 s–1, respectively, with Dirac voltage near 0 V. Moreover, the cut-off frequency of the device presenting the dynamic characteristics of a transistor reach up to 100 kHz.
Co-reporter:Woonggi Kang
The Journal of Physical Chemistry C 2016 Volume 120(Issue 6) pp:3501-3508
Publication Date(Web):January 25, 2016
DOI:10.1021/acs.jpcc.5b10240
A ladder-type poly(phenyl-co-methacryl silsesquioxane) (PPMSQ) copolymer was developed for use as a gate dielectric in high-performance organic field-effect transistors (OFETs). The ladder-type PPMSQ copolymer was synthesized via the hydrolysis of two types of monomers, methacryloxypropyltrimethoxysilane and phenyltrimethoxysilane, followed by a condensation polymerization. The phenyl groups in one monomer were introduced to enhance the structural ordering of the overlying organic semiconductors, whereas the methacryloxypropyl groups in the other monomer were introduced to cross-link the polymer chains via thermal- or photocuring. The curing process enhanced the electrical strength of the gate dielectric layer due to the formation of a network structure with a reduced free volume. Thermal curing reduced the surface energy of the gate dielectrics, which improved the structural order of the overlying organic semiconductors and promoted the formation of large grains. The ladder-type PPMSQ was used as a gate dielectric to produce benchmark p- and n-channel OFETs based on pentacene and N,N′-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8), respectively. The resulting OFETs exhibited excellent electrical performances, including a high carrier mobility (0.53 cm2 V–1 s–1 for the p-type pentacene OFET and 0.17 cm2 V–1 s–1 for the n-type PTCDI-C8 OFET) and a high ON/OFF current ratio exceeding 104. The photocured patterned PPMSQ film was successfully used to fabricate complementary OFET-based inverters that yielded high gains. The use of the ladder-type PPMSQ gate dielectrics provides a novel approach to realizing next-generation organic electronics.
Co-reporter:Min Je Kim
The Journal of Physical Chemistry C 2016 Volume 120(Issue 26) pp:13865-13872
Publication Date(Web):June 14, 2016
DOI:10.1021/acs.jpcc.6b01371
The performances of organic thin film transistors (OTFTs) produced by polymer solution casting are tightly correlated with the morphology and chain-ordering of semiconducting polymer layers, which depends on the processing conditions applied. The slow evaporation of a high boiling point (bp) solvent permits sufficient time for the assembly of polymer chains during the process, resulting in improving the film crystallinity and inducing favorable polymer chain orientations for charge transport. The use of high bp solvents, however, often results in dewetting of thin films formed on hydrophobic surfaces, such as the commonly used octadecyltrichlorosilane (ODTS)-treated SiO2 gate dielectric. Dewetting hampers the formation of uniform and highly crystalline semiconducting active channel layers. In this manuscript, we demonstrated the formation of highly crystalline dithienothienyl diketopyrrolopyrrole (TT-DPP)-based polymer films using a flow-coating method to enable the fabrication of ambipolar transistors and inverters. Importantly, unlike conventional spin-coating methods, the flow-coating method allowed us to use high bp solvents, even on a hydrophobic surface, and minimized the polymer solution waste. The crystalline orientations of the TT-DPP-based polymers were tuned depending on the solvent used (four different bp solvents were tested) and the employment of a thermal annealing step. The use of high bp solvents and thermal annealing of the polymer films significantly enhanced the crystalline microstructures in the flow-coated films, resulting in considerable carrier mobility increase in the OTFTs compared to the spin-coated films. Our simple, inexpensive, and scalable flow-coating method, for the first time employed in printing semiconducting polymers, presents a significant step toward optimizing the electrical performances of organic ambipolar transistors through organic semiconducting layer film crystallinity engineering.
Co-reporter:Hyungseok Kang, Iljoong Kang, Jaehun Han, Jun Beom Kim, Dong Yun Lee, Sung Min Cho, and Jeong Ho Cho
The Journal of Physical Chemistry C 2016 Volume 120(Issue 38) pp:22012-22018
Publication Date(Web):August 29, 2016
DOI:10.1021/acs.jpcc.6b06599
We developed a simple methodology for fabricating silver nanowire (AgNW) micropatterns on a plastic substrate using a photocurable polymer. The patterning method began with the lamination of a UV-curable prepolymer film onto the AgNW-coated rigid glass substrate. Selective UV exposure of the UV-curable prepolymer film through a photomask solidified the exposed regions, and the unexposed regions were simply removed by the solvent. AgNW micropatterns of various sizes and shapes could be readily formed across the entire plastic substrate. Importantly, this photopatterning process enabled the embedding of the AgNW structures into the polymer matrix, which dramatically reduced the surface roughness and enhanced the mechanical stability of the AgNW film. The AgNW structures served as transparent anode electrodes in organic light-emitting diodes (OLEDs) that performed well compared to OLEDs fabricated using conventional indium tin oxide (ITO) or conducting polymer electrodes. This simple, inexpensive, and scalable AgNW patterning technique provides a novel approach to realizing next-generation flexible electronics.
Co-reporter:Youngbin Lee;Jeong Kwon;Euyheon Hwang;Chang-Ho Ra;Won Jong Yoo;Jong-Hyun Ahn;Jong Hyeok Park
Advanced Materials 2015 Volume 27( Issue 1) pp:41-46
Publication Date(Web):
DOI:10.1002/adma.201402271
Co-reporter:Qijun Sun;Wanchul Seung;Beom Joon Kim;Soonmin Seo;Sang-Woo Kim
Advanced Materials 2015 Volume 27( Issue 22) pp:3411-3417
Publication Date(Web):
DOI:10.1002/adma.201500582
Co-reporter:Beom Joon Kim;Euyheon Hwang;Moon Sung Kang
Advanced Materials 2015 Volume 27( Issue 39) pp:5875-5881
Publication Date(Web):
DOI:10.1002/adma.201502020
Co-reporter:Sukjae Jang, Euyheon Hwang, Youngbin Lee, Seungwoo Lee, and Jeong Ho Cho
Nano Letters 2015 Volume 15(Issue 4) pp:2542-2547
Publication Date(Web):March 26, 2015
DOI:10.1021/acs.nanolett.5b00105
The advantages of graphene photodetectors were utilized to design a new multifunctional graphene optoelectronic device. Organic semiconductors, gold nanoparticles (AuNPs), and graphene were combined to fabricate a photodetecting device with a nonvolatile memory function for storing photonic signals. A pentacene organic semiconductor acted as a light absorption layer in the device and provided a high hole photocurrent to the graphene channel. The AuNPs, positioned between the tunneling and blocking dielectric layers, acted as both a charge trap layer and a plasmonic light scatterer, which enable storing of the information about the incident light. The proposed pentacene-graphene-AuNP hybrid photodetector not only performed well as a photodetector in the visible light range, it also was able to store the photonic signal in the form of persistent current. The good photodetection performance resulted from the plasmonics-enabled enhancement of the optical absorption and from the photogating mechanisms in the pentacene. The device provided a photoresponse that depended on the wavelength of incident light; therefore, the signal information (both the wavelength and intensity) of the incident light was effectively committed to memory. The simple process of applying a negative pulse gate voltage could then erase the programmed information. The proposed photodetector with the capacity to store a photonic signal in memory represents a significant step toward the use of graphene in optoelectronic devices.
Co-reporter:Yuseong Gim, Boseok Kang, BongSoo Kim, Sun-Guk Kim, Joong-Hee Lee, Kilwon Cho, Bon-Cheol Ku and Jeong Ho Cho
Nanoscale 2015 vol. 7(Issue 33) pp:14100-14108
Publication Date(Web):23 Jul 2015
DOI:10.1039/C5NR03307A
Atomically-thin molecular layers of aryl-functionalized graphene oxides (GOs) were used to modify the surface characteristics of source–drain electrodes to improve the performances of organic field-effect transistor (OFET) devices. The GOs were functionalized with various aryl diazonium salts, including 4-nitroaniline, 4-fluoroaniline, or 4-methoxyaniline, to produce several types of GOs with different surface functional groups (NO2-Ph-GO, F-Ph-GO, or CH3O-Ph-GO, respectively). The deposition of aryl-functionalized GOs or their reduced derivatives onto metal electrode surfaces dramatically enhanced the electrical performances of both p-type and n-type OFETs relative to the performances of OFETs prepared without the GO modification layer. Among the functionalized rGOs, CH3O-Ph-rGO yielded the highest hole mobility of 0.55 cm2 V−1 s−1 and electron mobility of 0.17 cm2 V−1 s−1 in p-type and n-type FETs, respectively. Two governing factors: (1) the work function of the modified electrodes and (2) the crystalline microstructures of the benchmark semiconductors grown on the modified electrode surface were systematically investigated to reveal the origin of the performance improvements. Our simple, inexpensive, and scalable electrode modification technique provides a significant step toward optimizing the device performance by engineering the semiconductor–electrode interfaces in OFETs.
Co-reporter:Seong Hun Yu, Boseok Kang, Gukil An, BongSoo Kim, Moo Hyung Lee, Moon Sung Kang, Hyunjung Kim, Jung Heon Lee, Shichoon Lee, Kilwon Cho, Jun Young Lee, and Jeong Ho Cho
ACS Applied Materials & Interfaces 2015 Volume 7(Issue 3) pp:2025
Publication Date(Web):January 12, 2015
DOI:10.1021/am507854s
We investigated the heterojunction effects of perylene tetracarboxylic diimide (PTCDI) derivatives on the pentacene-based field-effect transistors (FETs). Three PTCDI derivatives with different substituents were deposited onto pentacene layers and served as charge transfer dopants. The deposited PTCDI layer, which had a nominal thickness of a few layers, formed discontinuous patches on the pentacene layers and dramatically enhanced the hole mobility in the pentacene FET. Among the three PTCDI molecules tested, the octyl-substituted PTCDI, PTCDI-C8, provided the most efficient hole-doping characteristics (p-type) relative to the fluorophenyl-substituted PTCDIs, 4-FPEPTC and 2,4-FPEPTC. The organic heterojunction and doping characteristics were systematically investigated using atomic force microscopy, 2D grazing incidence X-ray diffraction studies, and ultraviolet photoelectron spectroscopy. PTCDI-C8, bearing octyl substituents, grew laterally on the pentacene layer (2D growth), whereas 2,4-FPEPTC, with fluorophenyl substituents, underwent 3D growth. The different growth modes resulted in different contact areas and relative orientations between the pentacene and PTCDI molecules, which significantly affected the doping efficiency of the deposited adlayer. The differences between the growth modes and the thin-film microstructures in the different PTCDI patches were attributed to a mismatch between the surface energies of the patches and the underlying pentacene layer. The film-morphology-dependent doping effects observed here offer practical guidelines for achieving more effective charge transfer doping in thin-film transistors.Keywords: charge transfer doping; organic field-effect transistor; pentacene; perylene tetracarboxylic diimide; pn-heterojunction
Co-reporter:Jong Yong Choi, Woonggi Kang, Boseok Kang, Wonsuk Cha, Seon Kyoung Son, Youngwoon Yoon, Hyunjung Kim, Youngjong Kang, Min Jae Ko, Hae Jung Son, Kilwon Cho, Jeong Ho Cho, and BongSoo Kim
ACS Applied Materials & Interfaces 2015 Volume 7(Issue 10) pp:6002
Publication Date(Web):March 3, 2015
DOI:10.1021/acsami.5b00747
Bottom-contact bottom-gate organic field-effect transistors (OFETs) are fabricated using a low band gap pDTTDPP-DT polymer as a channel material and single-layer graphene (SLG) or Au source/drain electrodes. The SLG-based ambipolar OFETs significantly outperform the Au-based ambipolar OFETs, and thermal annealing effectively improves the carrier mobilities of the pDTTDPP-DT films. The difference is attributed to the following facts: (i) the thermally annealed pDTTDPP-DT chains on the SLG assume more crystalline features with an edge-on orientation as compared to the polymer chains on the Au, (ii) the morphological features of the thermally annealed pDTTDPP-DT films on the SLG electrodes are closer to the features of those on the gate dielectric layer, and (iii) the SLG electrode provides a flatter, more hydrophobic surface that is favorable for the polymer crystallization than the Au. In addition, the preferred carrier transport in each electrode-based OFET is associated with the HOMO/LUMO alignment relative to the Fermi level of the employed electrode. All of these experimental results consistently explain why the carrier mobilities of the SLG-based OFET are more than 10 times higher than those of the Au-based OTFT. This work demonstrates the strong dependence of ambipolar carrier transport on the source/drain electrode and annealing temperature.Keywords: ambipolar organic field-effect transistor; film crystallinity; high carrier mobility; low band gap polymer; single layer graphene electrode
Co-reporter:Youngbin Lee, Seong Hun Yu, Jiwon Jeon, Hyunmin Kim, Jun Young Lee, Hyungjun Kim, Jong-Hyun Ahn, Euyheon Hwang, Jeong Ho Cho
Carbon 2015 Volume 88() pp:165-172
Publication Date(Web):July 2015
DOI:10.1016/j.carbon.2015.02.071
A hybrid structure comprising organic dye molecules (e.g., rhodamine 6G) and graphene was developed for the realization of high-performance optoelectronic devices. The fabricated photodetector offered a broad spectral photo-response across wavelengths in the infrared, visible, and ultraviolet regions, as well as a high responsivity (∼460 A/W at illumination power of 1 μW). The photocurrent generated in the hybrid photodetector (∼mA) was much higher than that generated in a pristine graphene photodetector (<μA). The performance of the dye-sensitized photodetector relied on enhanced photoabsorption and the implementation of a photocurrent gain arising from the photo-excited charges.
Co-reporter:Wi Hyoung Lee, Seong Jun Lee, Jung Ah Lim and Jeong Ho Cho
RSC Advances 2015 vol. 5(Issue 96) pp:78655-78659
Publication Date(Web):10 Sep 2015
DOI:10.1039/C5RA13573G
We developed printed In-Ga-Zn-O (IGZO) thin film transistors (TFTs) by delivering droplets of a precursor solution using a picoliter fluidic dispensing system. Intensely pulsed white light (IPWL) was then used to sinter the printed deposits. From one to six drops, ring-like deposits with similar dimensions were formed; however, the morphologies and thicknesses of the deposits depended strongly on the droplet number. As the droplet number increased, the thickness of the IGZO thin film increased and the pile-up region at the periphery of the deposit gradually expanded. The electrical properties of the droplet-based IGZO TFT were strongly dependent on the droplet number and displayed the highest electron mobility and bias stability at three drops, which yielded good deposit thickness values both in the center and in the periphery regions of the ring-like deposits. These results will be useful for enhancing the electrical properties of TFTs based on printed IGZO films for use in the low-cost/flexible switching devices in display technologies.
Co-reporter:Jae Hoon Park
The Journal of Physical Chemistry C 2015 Volume 119(Issue 14) pp:7802-7808
Publication Date(Web):March 19, 2015
DOI:10.1021/acs.jpcc.5b00771
Transparent conducting electrodes (TCEs) based on metallic grid structures have been extensively explored for use in flexible and transparent electronics according to their excellent conductivity and flexibility. Previous fabrication methods have been limited by the complexity and expense of their processes. Here, we have introduced a simple and cost-effective flow-coating method for preparing flexible and transparent metallic grid electrodes using silver nanoparticles (AgNPs). The process comprises only two steps, including patterning and sintering the horizontal AgNPs lines, followed by patterning and sintering the longitudinal AgNPs lines. The grid width could be easily controlled by varying the concentration of the AgNP solution and the grid spacing could be controlled by varying the distance moved by a translation stage between intermittent stops. The optimized Ag grid electrode exhibited an optical transmittance at 550 nm of 86% and a sheet resistance of 174 Ω/sq. The resulting Ag grid electrodes were successfully used to prepare a flexible piezoelectric nanogenerator. This device showed good performance, including an output voltage of 5 V and an output current density of 0.5 μA/cm2.
Co-reporter:Yongsuk Choi, Qijun Sun, Euyheon Hwang, Youngbin Lee, Seungwoo Lee, and Jeong Ho Cho
ACS Nano 2015 Volume 9(Issue 4) pp:4354
Publication Date(Web):March 27, 2015
DOI:10.1021/acsnano.5b01791
A customized graphene doping method was developed involving stamping using a chemically functionalized rubber lens as a novel design strategy for fabricating advanced two-dimensional (2D) materials-based electronic devices. Our stamping strategy enables deterministic control over the doping level and the spatial pattern of the doping on graphene. The dopants introduced onto graphene were locally and continuously controlled by directly stamping dopants using a chemically functionalized hemispherical rubber lens onto the graphene. The rubber lens was functionalized using two different dopants: poly(ethylene imine) to achieve n-type doping and bis(trifluoromethanesulfonyl)amine to achieve p-type doping. The graphene doping was systematically controlled by varying both the contact area (between the rubber lens and the graphene) and the contact time. Graphene doping using a stamp with a chemically functionalized rubber lens was confirmed by both Raman spectroscopy and charge transport measurements. We theoretically modeled the conductance properties of the spatially doped graphene using the effective medium theory and found excellent agreement with the experimental results. Finally, complementary inverters were successfully demonstrated by connecting n-type and p-type graphene transistors fabricated using the stamping doping method. We believe that this versatile doping method for controlling charge transport in graphene will further promote graphene electronic device applications. The doping method introduced in this paper may also be applied to other emergent 2D materials to tightly modulate the electrical properties in advanced electronic devices.Keywords: complementary inverter; contact doping; dopant; graphene; rubber lens;
Co-reporter:Yongsuk Choi, Won-Yeong Park, Moon Sung Kang, Gi-Ra Yi, Jun-Young Lee, Yong-Hoon Kim, and Jeong Ho Cho
ACS Nano 2015 Volume 9(Issue 4) pp:4288
Publication Date(Web):March 16, 2015
DOI:10.1021/acsnano.5b00700
We devised a simple transparent metal oxide thin film transistor architecture composed of only two component materials, an amorphous metal oxide and ion gel gate dielectric, which could be entirely assembled using room-temperature processes on a plastic substrate. The geometry cleverly takes advantage of the unique characteristics of the two components. An oxide layer is metallized upon exposure to plasma, leading to the formation of a monolithic source–channel–drain oxide layer, and the ion gel gate dielectric is used to gate the transistor channel effectively at low voltages through a coplanar gate. We confirmed that the method is generally applicable to a variety of sol–gel-processed amorphous metal oxides, including indium oxide, indium zinc oxide, and indium gallium zinc oxide. An inverter NOT logic device was assembled using the resulting devices as a proof of concept demonstration of the applicability of the devices to logic circuits. The favorable characteristics of these devices, including (i) the simplicity of the device structure with only two components, (ii) the benign fabrication processes at room temperature, (iii) the low-voltage operation under 2 V, and (iv) the excellent and stable electrical performances, together support the application of these devices to low-cost portable gadgets, i.e., cheap electronics.Keywords: amorphous metal oxide semiconductors; cheap electronics; monolithic thin-film transistor; plasma-induced metallization;
Co-reporter:Min Je Kim
The Journal of Physical Chemistry C 2015 Volume 119(Issue 29) pp:16414-16423
Publication Date(Web):July 2, 2015
DOI:10.1021/acs.jpcc.5b02308
Co-reporter:Qijun Sun;Do Hwan Kim;Sang Sik Park;Nae Yoon Lee;Yu Zhang;Jung Heon Lee;Kilwon Cho
Advanced Materials 2014 Volume 26( Issue 27) pp:4735-4740
Publication Date(Web):
DOI:10.1002/adma.201400918
Co-reporter:Beom Joon Kim, Soong Ho Um, Woo Chul Song, Yong Ho Kim, Moon Sung Kang, and Jeong Ho Cho
Nano Letters 2014 Volume 14(Issue 5) pp:2610-2616
Publication Date(Web):April 28, 2014
DOI:10.1021/nl500446s
Water, the primary electrolyte in biology, attracts significant interest as an electrolyte-type dielectric material for transistors compatible with biological systems. Unfortunately, the fluidic nature and low ionic conductivity of water prevents its practical usage in such applications. Here, we describe the development of a solid state, megahertz-operating, water-based gate dielectric system for operating graphene transistors. The new electrolyte systems were prepared by dissolving metal-substituted DNA polyelectrolytes into water. The addition of these biocompatible polyelectrolytes induced hydrogelation to provide solid-state integrity to the system. They also enhanced the ionic conductivities of the electrolytes, which in turn led to the quick formation of an electric double layer at the graphene/electrolyte interface that is beneficial for modulating currents in graphene transistors at high frequencies. At the optimized conditions, the Na-DNA water-gel-gated flexible transistors and inverters were operated at frequencies above 1 MHz and 100 kHz, respectively.
Co-reporter:Seong Jun Lee, Young-Hoon Kim, Jung Kyu Kim, Hionsuck Baik, Jae Hoon Park, Jaeki Lee, Jaewook Nam, Jong Hyeok Park, Tae-Woo Lee, Gi-Ra Yi and Jeong Ho Cho
Nanoscale 2014 vol. 6(Issue 20) pp:11828-11834
Publication Date(Web):07 Aug 2014
DOI:10.1039/C4NR03771E
We demonstrate continuous roll-to-roll production of highly conductive silver network films on a plastic substrate via mechanical and chemical welding processes. This process included three essential steps: (i) solvent spraying, (ii) roll compression, and (iii) salt treatment and washing. The sheet resistance of the resulting AgNW film was 5 Ω sq−1 at 92% transmittance, which was the lowest sheet resistance and the highest transparency among the values reported previously for solution-processed AgNW electrodes. Moreover, the strong contacts among the AgNWs dramatically enhanced the mechanical stability of the network film. The resulting AgNW film was successfully applied to various organic electronic devices, such as organic field-effect transistors (OFETs), organic light-emitting diodes (OLEDs), and organic solar cells (OSCs).
Co-reporter:Sukjae Jang, Euyheon Hwang and Jeong Ho Cho
Nanoscale 2014 vol. 6(Issue 24) pp:15286-15292
Publication Date(Web):30 Oct 2014
DOI:10.1039/C4NR04117H
A transparent flexible graphene nano-floating gate transistor memory (NFGTM) device was developed by combining a single-layer graphene active channel with gold nanoparticle (AuNP) charge trap elements. We systematically controlled the sizes of the AuNPs, the thickness of the tunneling dielectric layer, and the graphene doping level. In particular, we propose that the conductance difference (i.e., memory window) between the programming and erasing operations at a specific read gate voltage can be maximized through the doping. The resulting graphene NFGTMs developed here exhibited excellent programmable memory performances compared to previously reported graphene memory devices and displayed a large memory window (12 V), fast switching speed (1 μs), robust electrical reliability (105 s), and good mechanical (500 cycles) and thermal stability (100 °C).
Co-reporter:Woonggi Kang, Nam Hee Kim, Dong Yun Lee, Suk Tai Chang, and Jeong Ho Cho
ACS Applied Materials & Interfaces 2014 Volume 6(Issue 12) pp:9664
Publication Date(Web):June 11, 2014
DOI:10.1021/am5020315
We demonstrated the solution-processed single-walled carbon nanotube (SWNT) source–drain electrodes patterned using a plasma-enhanced detachment patterning method for high-performance organic transistors and inverters. The high-resolution SWNT electrode patterning began with the formation of highly uniform SWNT thin films on a hydrophobic silanized substrate. The SWNT source–drain patterns were then formed by modulating the interfacial energies of the prepatterned elastomeric mold and the SWNT thin film using oxygen plasma. The SWNT films were subsequently selectively delaminated using a rubber mold. The patterned SWNTs could be used as the source–drain electrodes for both n-type PTCDI-C8 and p-type pentacene field-effect transistors (FETs). The n- and p-type devices exhibited good and exactly matched electrical performances, with a field-effect mobility of around 0.15 cm2 V–1 s–1 and an ON/OFF current ratio exceeding 106. The single electrode material was used for both the n and p channels, permitting the successful fabrication of a high-performance complementary inverter by connecting a p-type pentacene FET to an n-type PTCDI-C8 FET. This patterning technique was simple, inexpensive, and easily scaled for the preparation of large-area electrode micropatterns for flexible microelectronic device fabrication.Keywords: inverter; micropattern; organic field-effect transistor; plasma-enhanced detachment patterning; single-walled carbon nanotube; source−drain electrode;
Co-reporter:Woonggi Kang, Minwoo Jung, Wonsuk Cha, Sukjae Jang, Youngwoon Yoon, Hyunjung Kim, Hae Jung Son, Doh-Kwon Lee, BongSoo Kim, and Jeong Ho Cho
ACS Applied Materials & Interfaces 2014 Volume 6(Issue 9) pp:6589
Publication Date(Web):April 7, 2014
DOI:10.1021/am500080p
We characterized the electrical properties of a field-effect transistor (FET) and a nonvolatile memory device based on a solution-processable low bandgap small molecule, Si1TDPP-EE-C6. The small molecule consisted of electron-rich thiophene-dithienosilole-thiophene (Si1T) units and electron-deficient diketopyrrolopyrrole (DPP) units. The as-spun Si1TDPP-EE-C6 FET device exhibited ambipolar transport properties with a hole mobility of 7.3 × 10–5 cm2/(V s) and an electron mobility of 1.6 × 10–5 cm2/(V s). Thermal annealing at 110 °C led to a significant increase in carrier mobility, with hole and electron mobilities of 3.7 × 10–3 and 5.1 × 10–4 cm2/(Vs), respectively. This improvement is strongly correlated with the increased film crystallinity and reduced π–π intermolecular stacking distance upon thermal annealing, revealed by grazing incidence X-ray diffraction (GIXD) and atomic force microscopy (AFM) measurements. In addition, nonvolatile memory devices based on Si1TDPP-EE-C6 were successfully fabricated by incorporating Au nanoparticles (AuNPs) as charge trapping sites at the interface between the silicon oxide (SiO2) and cross-linked poly(4-vinylphenol) (cPVP) dielectrics. The device exhibited reliable nonvolatile memory characteristics, including a wide memory window of 98 V, a high on/off-current ratio of 1 × 103, and good electrical reliability. Overall, we demonstrate that donor–acceptor-type small molecules are a potentially important class of materials for ambipolar FETs and nonvolatile memory applications.Keywords: ambipolar field effect transistor; charge trapping; crystallinity; donor−acceptor-type small molecules; hole mobility; nonvolatile memory;
Co-reporter:Jae Hoon Park, Dong Yun Lee, Young-Hoon Kim, Jung Kyu Kim, Jung Heon Lee, Jong Hyeok Park, Tae-Woo Lee, and Jeong Ho Cho
ACS Applied Materials & Interfaces 2014 Volume 6(Issue 15) pp:12380
Publication Date(Web):July 7, 2014
DOI:10.1021/am502233y
We propose a novel approach to fabricating flexible transparent metallic grid electrodes via evaporative deposition involving flow-coating. A transparent flexible metal grid electrode was fabricated through four essential steps including: (i) polymer line pattern formation on the thermally evaporated metal layer onto a plastic substrate; (ii) rotation of the stage by 90° and the formation of the second polymer line pattern; (iii) etching of the unprotected metal region; and (iv) removal of the residual polymer from the metal grid pattern. Both the metal grid width and the spacing were systematically controlled by varying the concentration of the polymer solution and the moving distance between intermittent stop times of the polymer blade. The optimized Au grid electrodes exhibited an optical transmittance of 92% at 550 nm and a sheet resistance of 97 Ω/sq. The resulting metallic grid electrodes were successfully applied to various organic electronic devices, such as organic field-effect transistors (OFETs), organic light-emitting diodes (OLEDs), and organic solar cells (OSCs).Keywords: evaporative assembly; flow coating; metal grid electrode; organic electronic device; transparent electrode
Co-reporter:Wi Hyoung Lee, Seung Goo Lee, Young-Je Kwark, Dong Ryeol Lee, Shichoon Lee, and Jeong Ho Cho
ACS Applied Materials & Interfaces 2014 Volume 6(Issue 24) pp:22807
Publication Date(Web):December 2, 2014
DOI:10.1021/am507003n
In organic field-effect transistors (OFETs), surface modification of the gate-dielectric is a critical technique for enhancing the electrical properties of the device. Here, we report a simple and versatile method for fabricating an ultrathin cross-linked interlayer (thickness ∼3 nm) on an oxide gate dielectric by using polymeric silsesquiazane (SSQZ). The fabricated siloxane film exhibited an ultrasmooth surface with minimal hydroxyl groups; the properties of the surface were chemically tuned by introducing phenyl and phenyl/fluorine pendent groups into the SSQZ. The growth characteristics of two semiconductors—pentacene (p-type) and N,N′-ditridecyl-3,4,9,10-perylene tetracarboxylic diimide (PTCDI-C13, n-type)—on this ultrathin film were systematically investigated according to the type of pendent groups in the SSQZ-treated gate dielectric. Pentacene films on phenyl/fluorine groups exhibited large grains and excellent crystalline homogeneity. By contrast, PTCDI-C13 films exhibited greater crystalline order and perfectness when deposited on phenyl groups rather than on phenyl/fluorine groups. These microstructural characteristics of the organic semiconductors, as well as the dipole moment of the pendent groups, determined the electrical properties of FETs based on pentacene or PTCDI-C13. Importantly, compared to FETs in which the gate dielectric was treated with a silane-coupling agent (a commonly used surface treatment), the FETs fabricated using the tunable SSQZ treatment showed much higher field-effect mobilities. Finally, surface treatment with an ultrathin SSQZ layer was also utilized to fabricate flexible OFETs on a plastic substrate. This was facilitated by the facile SSQZ deposition process and the compatibility of SSQZ with the plastic substrate.Keywords: N,N′-ditridecyl-3,4,9,10-perylene tetracarboxylic diimide; organic field-effect transistors; pentacene; silsesquiazane; surface modifier
Co-reporter:Do Hwan Kim, Jung Ah Lim, Wonsuk Cha, Jung Heon Lee, Hyunjung Kim, Jeong Ho Cho
Organic Electronics 2014 Volume 15(Issue 10) pp:2322-2327
Publication Date(Web):October 2014
DOI:10.1016/j.orgel.2014.06.022
•We fabricated well-defined TIPS-PEN semiconductor crystal arrays via a confined evaporative capillary flow method.•Ribbon-shaped TIPS-PEN crystals with highly preferred crystal orientation along the (l 0 0) axis were developed.•TIPS-PEN field-effect transistors (FETs) exhibited a good hole mobility of 0.72 cm2/Vs.We fabricated well-defined 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-PEN) crystal arrays for use in electronic applications via a simple but effective method, the confined evaporative capillary flow (CEC) method. This has been accomplished by systematically controlling the contact line pinning at the edge of glass stylus and the outward hydrodynamic flow within the drying droplet with various processing solvents and surface properties of the substrate during solidification. We found that after CEC coating of TIPS-PEN solution dissolved into toluene onto SiO2 surface, ribbon-shaped TIPS-PEN crystals were well developed with a width of 20–100 μm and length of 300 μm – 2 mm, which is presumably owing to optimized capillary evaporation. Specifically, TIPS-PEN crystals present highly preferred crystal orientation along the (l 0 0) axis, which can lead to efficient charge transport in a lateral direction. Thus, TIPS-PEN field-effect transistors (FETs) exhibited a good hole mobility of 0.72 cm2/Vs.Graphical abstract
Co-reporter:Guh-Hwan Lim, Seong Jun Lee, Insung Han, Shingyu Bok, Jung Heon Lee, Jaewook Nam, Jeong Ho Cho, Byungkwon Lim
Chemical Physics Letters 2014 Volume 602() pp:10-15
Publication Date(Web):20 May 2014
DOI:10.1016/j.cplett.2014.04.012
The polyol synthesis of silver (Ag) nanostructures typically involves the rapid injection of Ag precursor to a preheated, ethylene glycol solution containing polymeric stabilizers and other additives. Here we report that Ag nanowires can be synthesized in high yields by applying a heat-up process in the polyol synthesis. Electron microscopy studies revealed that multiple-twinned Ag seeds were generated preferentially during the heat-up procedure, and then grew into nanowires. We also demonstrate that these Ag nanowires can be applied as electrode materials for the fabrication of flexible and transparent organic field-effect transistors with a reasonably high hole-mobility.
Co-reporter:Seong Hun Yu, Youngbin Lee, Sung Kyu Jang, Jinyeong Kang, Jiwon Jeon, Changgu Lee, Jun Young Lee, Hyungjun Kim, Euyheon Hwang, Sungjoo Lee, and Jeong Ho Cho
ACS Nano 2014 Volume 8(Issue 8) pp:8285
Publication Date(Web):July 25, 2014
DOI:10.1021/nn502715h
We fabricated dye-sensitized MoS2 photodetectors that utilized a single-layer MoS2 treated with rhodamine 6G (R6G) organic dye molecules (with an optical band gap of 2.38 eV or 521 nm). The proposed photodetector showed an enhanced performance with a broad spectral photoresponse and a high photoresponsivity compared with the properties of the pristine MoS2 photodetectors. The R6G dye molecules deposited onto the MoS2 layer increased the photocurrent by an order of magnitude due to charge transfer of the photoexcited electrons from the R6G molecules to the MoS2 layer. Importantly, the photodetection response extended to the infrared (λ < 980 nm, which corresponded to about half the energy band gap of MoS2), thereby distinguishing the device performance from that of a pristine MoS2 device, in which detection was only possible at wavelengths shorter than the band gap of MoS2, i.e., λ < 681 nm. The resulting device exhibited a maximum photoresponsivity of 1.17 AW–1, a photodetectivity of 1.5 × 107 Jones, and a total effective quantum efficiency (EQE) of 280% at 520 nm. The device design described here presents a significant step toward high-performance 2D nanomaterial-based photodetector.Keywords: broad spectral photoresponse; MoS2; organic dye; photodetector; photoresponsivity
Co-reporter:Song Hee Koo, Seung Goo Lee, Hyojin Bong, Young-Je Kwark, Kilwon Cho, Ho Sun Lim, Jeong Ho Cho
Polymer 2014 Volume 55(Issue 11) pp:2661-2666
Publication Date(Web):27 May 2014
DOI:10.1016/j.polymer.2014.03.046
We demonstrate a facile and efficient method for fabricating multifunctional superhydrophobic organic–inorganic hybrid macroporous coatings and films with robust environmental stabilities involving the electrospinning of phenylsilsesquiazane (PhSSQZ) in the presence of polystyrene (PS). The resulting freestanding PhSSQZ/PS webs, which featured hierarchical fibrous structures with the unique chemical properties of PhSSQZ, provided a practical material with potential uses in many applications including structural coatings, oil–water separation membranes, and high-performance air filters. The materials maintained their fibrous structures and superhydrophobicity even after heat treatment at 600 °C under an ambient atmosphere, which is among the highest level reported up to date for solution-processed superhydrophobic surfaces with soft materials. The solvent resistance and mechanical strength of the PhSSQZ/PS webs were significantly enhanced through the structured siloxane network due to thermally induced hydrolysis of PhSSQZ and condensation of the resulting silanols. The properties of this novel material suggest that the present approach will advance our knowledge and capability to design and develop multifunctional smart materials with robust superhydrophobicity and macroporosity.
Co-reporter:Hyun Ho Choi;Moon Sung Kang;Min Kim;Haena Kim;Kilwon Cho
Advanced Functional Materials 2013 Volume 23( Issue 6) pp:690-696
Publication Date(Web):
DOI:10.1002/adfm.201201545
Abstract
A novel strategy for analyzing bias-stress effects in organic field-effect transistors (OFETs) based on a four-parameter double stretched-exponential formula is reported. The formula is obtained by modifying a traditional single stretched-exponential expression comprising two parameters (a characteristic time and a stretched-exponential factor) that describe the bias-stress effects. The expression yields two characteristic times and two stretched-exponential factors, thereby separating out the contributions due to charge trapping events in the semiconductor layer-side of the interface and the gate-dielectric layer-side of the interface. The validity of this method was tested by designing two model systems in which the physical properties of the semiconductor layer and the gate-dielectric layer were varied systematically. It was found that the gate-dielectric layer, in general, plays a more critical role than the semiconductor layer in the bias-stress effects, possibly due to the wider distribution of the activation energy for charge trapping. Furthermore, the presence of a self-assembled monolayer further widens the distribution of the activation energy for charge trapping in gate-dielectric layer-side of the interface and causes the channel current to decay rapidly in the early stages. The novel analysis method presented here enhances our understanding of charge trapping and provides rational guidelines for developing efficient OFETs with high performance.
Co-reporter:Yeong Don Park, Boseok Kang, Ho Sun Lim, Kilwon Cho, Moon Sung Kang, and Jeong Ho Cho
ACS Applied Materials & Interfaces 2013 Volume 5(Issue 17) pp:8591
Publication Date(Web):August 12, 2013
DOI:10.1021/am402050p
We demonstrate low-voltage, flexible, transparent pentacene humidity sensors with ultrahigh sensitivity, good reliability, and fast response/recovery behavior. The excellent performances of these devices are derived from an inserted polyelectrolyte (poly[2-(methacryloyloxy)ethyltrimethylammonium chloride-co-3-(trimethoxysilyl)propyl methacrylate] (poly(METAC-co-TSPM)) interlayer, which releases free Cl– ions in the electrolyte dielectric layer under humid conditions and boosts the electrical current in the transistor channel. This has led to extreme device sensitivity, such that electrical signal variations exceeding 7 orders of magnitude have been achieved in response to a 15% change in the relative humidity level. The new sensors exhibit a fast responsivity and a stable performance toward changes in humidity levels. Furthermore, the humidity sensors, mounted on flexible substrates, provided low voltage (<5 V) operation while preserving the unique ultrasensitivity and fast responsivity of these devices. We believe that the strategy of utilizing the enhanced ion motion in an inserted polyelectrolyte layer of an OFET structure can potentially improve sensor technologies beyond humidity-responsive systems.Keywords: fast response; flexible humidity sensor; organic transistor; poly(METAC-co-TSPM); polyelectrolyte interlayer; ultrasensitivity;
Co-reporter:Seong Hun Yu, Beom Joon Kim, Moon Sung Kang, Se Hyun Kim, Jong Hun Han, Jun Young Lee, and Jeong Ho Cho
ACS Applied Materials & Interfaces 2013 Volume 5(Issue 19) pp:9765
Publication Date(Web):September 13, 2013
DOI:10.1021/am402919f
An In/Ga-free doping method of zinc oxide (ZnO) is demonstrated utilizing a printable charge transfer doping layer (CTDL) based on (3-aminopropyl)triethoxysilane (APS) molecules. The self-assembled APS molecules placed on top of ZnO thin films lead to n-type doping of ZnO and filling shallow electron traps, due to the strong electron-donating characteristics of the amine group in APS molecules. The CTDL doping can tune the threshold voltage and the mobility of the ZnO thin-film transistors (TFTs) as one varies the grafting density of the APS molecules and the thickness of the underneath ZnO thin films. From an optimized condition, high-performance ZnO TFTs can be achieved that exhibit an electron mobility of 4.2 cm2/(V s), a threshold voltage of 10.5 V, and an on/off current ratio larger than 107. More importantly, the method is applicable to simple inkjet processes, which lead to produce high-performance depletion load ZnO inverters through selective deposition of CTDL on ZnO thin films.Keywords: carrier mobility; charge transfer doping layer; depletion load inverter; solution-processed zinc oxide; thin-film transistor;
Co-reporter:Wi Hyoung Lee;Hyunmin Hwang;Kyunghwan Moon;Kwonwoo Shin
Fibers and Polymers 2013 Volume 14( Issue 12) pp:2077-2082
Publication Date(Web):2013 December
DOI:10.1007/s12221-013-2077-0
We developed a facile method for increasing the environmental stability of a tungsten bronze near-infrared (NIR)-absorbing window using tetraethyl orthosilicate (TEOS) and 1H, 1H, 2H, 2H-perfluorodecyltriethoxysilane (FDS). The environmental stability of the tungsten bronze NIR-absorbing window could be enhanced by applying a variety of protective layers (i.e., TEOS, fluoropolymer (CYTOP), FDS). The protective characteristics of each layer type are discussed. The protection of tungsten bronze surfaces by TEOS and FDS layers enormously enhanced the environmental stability of the NIR absorbing window, whereas an untreated tungsten bronze film rapidly lost its NIR absorption properties. The protection efficiency followed the order: TEOS/FDS>FDS>TEOS>CYTOP. The improved environmental stability arose from the closely packed structure of FDS, which can self-assemble on an oxide surface, such as the tungsten oxide or silicon oxide surfaces. The method developed here provides a simple, robust, and versatile way to improve the environmental stability of a NIR-absorbing window.
Co-reporter:Yeong Don Park;Keita Anabuki;Sumin Kim;Kyung-Won Park
Macromolecular Research 2013 Volume 21( Issue 6) pp:636-640
Publication Date(Web):2013 June
DOI:10.1007/s13233-013-1066-x
Co-reporter:Joong Suk Lee, Seon Kyoung Son, Sanghoon Song, Hyunjung Kim, Dong Ryoul Lee, Kyungkon Kim, Min Jae Ko, Dong Hoon Choi, BongSoo Kim, and Jeong Ho Cho
Chemistry of Materials 2012 Volume 24(Issue 7) pp:1316
Publication Date(Web):March 14, 2012
DOI:10.1021/cm2037487
We investigated the performance of ambipolar field-effect transistors based on a series of alternating low band gap polymers of oligothiophene and diketopyrrolopyrrole (DPP). The polymers contain oligothiophene units of terthiophene [T3] and thiophene-thienothiophene-thiophene [T2TT] and DPP units carrying branched alkyl chains of 2-hexyldecyl [HD] or 2-octyldodecyl [OD]. The structural variation allows us to do a systematic study on the relationship between the interchain stacking/ordering of semiconducting polymers and their resulting device performance. On the basis of synchrotron X-ray diffraction and atomic force microscopy measurements on polymer films, we found that longer branched alkyl side chains, i.e., OD, and longer and more planar oligothiophene, i.e., T2TT, generate the more crystalline structures. Upon thermal annealing, the crystallinity of the polymers was largely improved, and polymers containing a longer branched alkyl chain responded faster because longer alkyl chains have larger cohesive forces than shorter chains. For all the polymers, excellent ambipolar behavior was observed with a maximum hole and electron mobility of 2.2 and 0.2 cm2 V–1 s–1, respectively.Keywords: ambipolar transistors; crystalline structure; high carrier mobility; low band gap polymers; polymer field-effect transistors;
Co-reporter:Beom Joon Kim, Young-Il Park, Hyo Jung Kim, Kwangseok Ahn, Dong Ryeol Lee, Do Hwan Kim, Se-Young Oh, Jong-Wook Park and Jeong Ho Cho
Journal of Materials Chemistry A 2012 vol. 22(Issue 29) pp:14617-14623
Publication Date(Web):17 May 2012
DOI:10.1039/C2JM31698F
The device performance and stability of n-type organic field-effect transistors (OFETs) based on 1,2,3,7,8,9-hexafluoro-indeno[1,2-b]fluorene-6,12-dione (TriF-IF-dione) were investigated. The electrical characteristics of TriF-IF-dione FETs were optimized by systematically controlling the dielectric surface properties via insertion of organic interlayers, such as self-assembled monolayers (NH2–, CH3–, and CF3–) or polymeric layers (polystyrene, PS) at the semiconductor–SiO2 dielectric interfaces. In particular, a thin PS buffer layer on the SiO2 surface provided a device that performed well, with a field-effect mobility of 0.18 cm2 V−1 s−1 and an on–off current ratio of 4.4 × 106. The improvements in the performance of TriF-IF-dione OFET conveyed by the PS interlayers were examined in terms of the crystalline nanostructure and the charge modulation effects in the channel. These properties were strongly correlated with, respectively, the hydrophobicity and the electron-donating characteristics of the dielectric surface. The TriF-IF-dione FETs with a PS interlayer showed excellent electrical stability attributed to high activation energies for charge trap creation. A complementary inverter comprising both p-type pentacene and n-type TriF-IF-dione was also successfully demonstrated.
Co-reporter:Chang Hwan Lee, Sung Kyung Kang, Jung Ah Lim, Young-Je Kwark, Ho Sun Lim, Jooyong Kim and Jeong Ho Cho
Journal of Materials Chemistry A 2012 vol. 22(Issue 29) pp:14656-14660
Publication Date(Web):21 May 2012
DOI:10.1039/C2JM31718D
We demonstrated multifunctional electrospun polyelectrolyte fabrics with switchable superhydrophobicity as well as oleophobicity. To produce this smart fabric, we used a strategy that combines electrospinning to fabricate nanofibrous templates with nanopores and a simple coating of the polyelectrolyte that can exchange counterions with various hydration energies. The ion exchange of polyelectrolyte embedded in nanoporous fibrous webs leads to switchable wetting behavior in water and oil. This electrospun fabric was also utilized as chemical filters for the efficient removal of sulfur dioxide (SO2) from waste gas streams.
Co-reporter:Chang Hwan Lee, Sung Kyung Kang, Jung Ah Lim, Ho Sun Lim and Jeong Ho Cho
Soft Matter 2012 vol. 8(Issue 40) pp:10238-10240
Publication Date(Web):03 Sep 2012
DOI:10.1039/C2SM26625C
Smart fabrics that reversibly switch between superhydrophobicity and superhydrophilicity were developed using a combination of hierarchical nanostructured fibrous webs and a pH-responsive polymer that experiences pH-dependent conformational transitions, poly[2-(diisopropylamino)ethylmethacrylate-co-3-(trimethoxysilyl)propylmethacrylate].
Co-reporter:Hwa Sung Lee, Moon Sung Kang, Sung Kyung Kang, Beom Joon Kim, Youngjae Yoo, Ho Sun Lim, Soong Ho Um, Du Yeol Ryu, Dong Ryeol Lee, and Jeong Ho Cho
The Journal of Physical Chemistry C 2012 Volume 116(Issue 41) pp:21673-21678
Publication Date(Web):October 9, 2012
DOI:10.1021/jp305820r
We demonstrated that the viscoelasticity of a dielectric surface affected the overlying pentacene crystalline nanostructures and the electrical performances of pentacene-based field-effect transistors (FETs). The surface viscoelasticities of the gate dielectrics were systematically controlled by varying the polymer chain lengths of polystyrene brushes (b-PSs) and the substrate temperature during pentacene deposition. The b-PSs were chosen as a model surface because the glass–liquid transition affected neither the surface energy nor the surface roughness. Moreover, the glass–liquid transition temperature increased with increasing b-PS chain length. The liquid-like b-PS chains disturbed the surface arrangement of the pentacene molecules, which reduced the organization of the crystalline structures, yielding smaller grains during the early stages of pentacene growth. The dramatic changes in the film morphology and crystalline nanostructures above the b-PS glass–liquid transition resulted in noticeable changes in the OFET performance. The systematic investigation of the dielectric surface viscoelasticity presented here provides a significant step toward optimizing the nanostructures of organic semiconductors, and thereby, the device performance, by engineering the interfaces in the OFETs.
Co-reporter:Hyunmin Hwang, Piljae Joo, Moon Sung Kang, Gukmoon Ahn, Joong Tark Han, Byeong-Su Kim, and Jeong Ho Cho
ACS Nano 2012 Volume 6(Issue 3) pp:2432
Publication Date(Web):February 7, 2012
DOI:10.1021/nn2047197
We demonstrate a controlled, systematic method to tune the charge transport in graphene field-effect transistors based on alternating layer-by-layer assembly of positively and negatively charged graphene oxide followed by thermal reduction. Surprisingly, tuning the number of bilayers of thermally reduced graphene oxide multilayer films allowed achieving either ambipolar or unipolar (both n- and p-type) transport in graphene transistors. On the basis of X-ray photoemission spectroscopy, Raman spectroscopy, time-of-flight secondary ion mass spectrometry, and temperature-dependent charge transport measurements, we found that nitrogen atoms from the functional groups of positively charged graphene oxide are incorporated into the reduced graphene oxide films and substitute carbon atoms during the thermal reduction. This nitrogen-doping process occurs in different degrees for graphene multilayers with varying numbers of bilayers and thereby results in the interesting transition in the electrical behavior in graphene multilayer transistors. We believe that such a versatile method to control the charge transport in graphene multilayers will further promote their applications in solution-processable electronic devices based on graphene.Keywords: ambipolar to unipolar transition; graphene; layer-by-layer assembly; nitrogen doping; transistor
Co-reporter:Beom Joon Kim, Seoung-Ki Lee, Moon Sung Kang, Jong-Hyun Ahn, and Jeong Ho Cho
ACS Nano 2012 Volume 6(Issue 10) pp:8646
Publication Date(Web):September 6, 2012
DOI:10.1021/nn3020486
Transparent flexible graphene transistors and inverters in a coplanar-gate configuration were presented for the first time using only two materials: graphene and an ion gel gate dielectric. The novel device configuration simplifies device fabrication such that only two printing steps were required to fabricate transistors and inverters. The devices exhibited excellent device performances including low-voltage operation with a high transistor-on-current and mobility, excellent mechanical flexibility, environmental stability, and a reasonable inverting behavior upon connecting the two transistors.Keywords: coplanar-gate configuration; flexible electronics; graphene transistor; transparent electronics
Co-reporter:Hyo-Sang Lee, Joong Suk Lee, Sanghyeok Cho, Hyunjung Kim, Kyung-Won Kwak, Youngwoon Yoon, Seon Kyoung Son, Honggon Kim, Min Jae Ko, Doh-Kwon Lee, Jin Young Kim, Sungnam Park, Dong Hoon Choi, Se Young Oh, Jeong Ho Cho, and BongSoo Kim
The Journal of Physical Chemistry C 2012 Volume 116(Issue 50) pp:26204-26213
Publication Date(Web):November 26, 2012
DOI:10.1021/jp309213h
We report high-performance of ambipolar organic field-effect transistors (FETs) based on the low band gap copolymers of pDPPT2NAP-HD and pDPPT2NAP-OD. The polymers are composed of electron-rich 2,6-di(thienyl)naphthalene (T2NAP) and electron-deficient diketopyrrolopyrrole (DPP) units with branched alkyl chains of 2-hexyldecyl (HD) or 2-octyldodecyl (OD). The polymers were polymerized via Suzuki coupling, yielding optical band gaps of ∼1.4 eV. In the transistor performance test, we observed good ambipolar transport behavior in both polymer films, and pDPPT2NAP-OD displayed hole and electron mobilities 1 order of magnitude higher than the corresponding properties of pDPPT2NAP-HD. Thermal annealing of the polymer films increased the carrier mobilities. Annealing at 150 °C provided optimal conditions yielding saturated film crystallinity and maximized carrier mobility. The highest hole and electron mobilities achieved in these polymers were 1.3 and 0.1 cm2/(V s), respectively, obtained from pDPPT2NAP-OD. The polymer structure and thermal annealing affected the carrier mobility, and this effect was investigated by fully characterizing the polymer films by grazing incidence X-ray diffraction (GIXD), atomic force microscopy (AFM), and transmission electron microscopy (TEM) experiments. The GIXD data revealed that both polymers formed highly crystalline films with edge-on orientation. pDPPT2NAP-OD, which included longer alkyl chains, showed a higher tendency to form long-range order among the polymer chains. Thermal annealing up to 150 °C improved the polymer film crystallinity and promoted the formation of longer-range lamellar structures. AFM and TEM images of the films were consistent with the GI-XD data. Theoretical calculations of the polymer structures provided a rationale for the relationship between the torsional angle between aromatic rings and the carrier mobility. From the intensive electrical measurements and full characterizations, we find that the chemical structure of polymer backbone and side alkyl chain has a profound effect on film crystallinity, morphology, and transistor properties.
Co-reporter:Min Je Kim, Young Min Heo, Jeong Ho Cho
Organic Electronics (April 2017) Volume 43() pp:41-46
Publication Date(Web):April 2017
DOI:10.1016/j.orgel.2017.01.009
Co-reporter:Chang Hwan Lee, Sung Kyung Kang, Jung Ah Lim, Young-Je Kwark, Ho Sun Lim, Jooyong Kim and Jeong Ho Cho
Journal of Materials Chemistry A 2012 - vol. 22(Issue 29) pp:
Publication Date(Web):
DOI:10.1039/C2JM31718D
Co-reporter:Beom Joon Kim, Young-Il Park, Hyo Jung Kim, Kwangseok Ahn, Dong Ryeol Lee, Do Hwan Kim, Se-Young Oh, Jong-Wook Park and Jeong Ho Cho
Journal of Materials Chemistry A 2012 - vol. 22(Issue 29) pp:NaN14623-14623
Publication Date(Web):2012/05/17
DOI:10.1039/C2JM31698F
The device performance and stability of n-type organic field-effect transistors (OFETs) based on 1,2,3,7,8,9-hexafluoro-indeno[1,2-b]fluorene-6,12-dione (TriF-IF-dione) were investigated. The electrical characteristics of TriF-IF-dione FETs were optimized by systematically controlling the dielectric surface properties via insertion of organic interlayers, such as self-assembled monolayers (NH2–, CH3–, and CF3–) or polymeric layers (polystyrene, PS) at the semiconductor–SiO2 dielectric interfaces. In particular, a thin PS buffer layer on the SiO2 surface provided a device that performed well, with a field-effect mobility of 0.18 cm2 V−1 s−1 and an on–off current ratio of 4.4 × 106. The improvements in the performance of TriF-IF-dione OFET conveyed by the PS interlayers were examined in terms of the crystalline nanostructure and the charge modulation effects in the channel. These properties were strongly correlated with, respectively, the hydrophobicity and the electron-donating characteristics of the dielectric surface. The TriF-IF-dione FETs with a PS interlayer showed excellent electrical stability attributed to high activation energies for charge trap creation. A complementary inverter comprising both p-type pentacene and n-type TriF-IF-dione was also successfully demonstrated.
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