2.1 Materials
The TIPS-PEN ink solution was prepared by dissolving TIPS-PEN (2 wt%, ≥99%, Sigma Aldrich Inc.) in 1,2,3,4-tetrahydronaphthalene (Tetralin, ≥97%, Sigma Aldrich Inc.) solvent. The C60 ink solution was prepared by dissolving C60 powder (1 wt%, 99.5%, Sigma Aldrich Inc.) in 1,3,5-trichlorobenzene (TCB, 99%, Sigma Aldrich Inc.). The Ag nanoparticle ink (DGP 40LT-15C) was purchased from Advanced Nano Products. The Ag ink contains silver nanoparticles (20 wt%, particle diameter 40–50 nm) dispersed in methanol solvent. Polyurethane acrylate (PUA) (MINS-ERM, Minuta Tech.) was used to prepare the UV-curable hard molds for producing organic semiconductor wires. Polydimethylsiloxane (PDMS) (Sylgard 184, Dow Corning) was used to prepare thermal-curable molds for producing silver microelectrodes.
2.2 Preparation of substrates
As a typical transferring substrate, the Si substrates, cut from n-type (100) wafers with resistivity in the range of 1–5 Ω · cm, were used. The Si substrates were initially treated by the chemical cleaning process, proposed by Ishizaka and Shiraki [20], which involves degreasing, HNO3 boiling, NH4OH boiling (alkali treatment), HCl boiling (acid treatment), rinsing in deionized water, and blow-drying with nitrogen. A thin oxide layer was grown by placing the Si substrate in a piranha solution (4:1 mixture of H2SO4 : H2O2) for 10–15 min. The substrate was rinsed several times with deionized water (resistivity =18 MΩ · cm), then dried with a stream of nitrogen. For the electrical measurements, a heavily doped p-type Si (0.001–0.003 Ωcm−1, Hissan Inc.) with a 200 nm thick, thermally grown SiO2 layer was used as a thick oxide substrate.
2.3 Growth of single-crystal organic nanowires
Arrays of organic semiconductors were made on an oxidized Si substrate using the LB-nTM method with PUA molds [21]. The PUA molds were fabricated by casting PUA on the masters that are silicon wafers with desired line patterns made by e-beam/photo lithography. In this study four different line patterns were used for the masters—the width of the parallel lines and the spaces between the lines were 50 nm/150 nm, 100 nm/100 nm, 500 nm/200 nm and 10 μm/10 μm, respectively. After UV curing (~10 min), the PUA molds were peeled away from the masters.
In order to generate arrays of organic semiconductors, only recessed channels of the patterned PUA mold were filled with a TIPS-PEN or C60 ink solution using discontinuous dewetting [22]. By dragging a dropped ink solution over the patterned mold with a glass stick or a needle, the meniscus of the ink solution moved over the surface of the mold to fill the channels without leaving any residue on the raised surface. The ink in the channels was next solidified by drying at mild temperatures below 100°C for 30–60 min. The mold containing the dried organic semiconductors was then brought into contact with a substrate surface covered by a thin ethanol layer. The ethanol layer on the substrate formed a liquid bridge (a capillary bridge) between the substrate and the mold. The liquid bridge allowed good conformal contact between the semiconductors contained in the mold and the substrate. As the ethanol evaporated, the attractive capillary force gradually increased, pulling the two surfaces into contact, and providing good conformal contact between them. After drying, separation of the mold from the substrate yielded an organic semiconductor array on the substrate.
2.4 Fabrication of a wafer-scale array of complementary inverters
The fabrication of an array of single-crystal organic nanowire inverters was initiated by generation of a single-crystal TIPS-PEN nanowire array, as active p-channels, on 200 nm-thick SiO2/p+-Si substrates using LB-nTM. Subsequently, an array of single-crystal C60 nanowires as active n-channels was placed alongside the TIPS-PEN nanowire array using LB-nTM. Next, on this substrate with the two printed nanowire arrays, source and drain electrodes of 1.5 μm-thick Ag were prepared and placed by LB-nTM so as to electrically connect the arrays of p-type and n-type FETs, producing an array of complementary inverters.
Here, we used molds of different materials and specially designed patterns for printing organic semiconductor nanowires and Ag microelectrodes. The two semiconductor arrays were prepared using a wafer-scale PUA mold that contains 180 sections of a line pattern—each section contains 50 nanochannels of 100 nm width, 10 μm length and 200 nm depth—with the space between the sections being 2 mm. For the Ag electrodes, a wafer-scale PDMS mold was fabricated by casting PDMS on the master which is a silicon wafer with an embossed pattern—composed of 180 basic units—made by photolithography. After curing at 70°C for 50 min, the PDMS mold was peeled away from the master so as to be used for printing Ag electrodes.
2.5 Characterization
The samples were characterized using a scanning electron microscope (SEM, Hitachi S4800) operated at 15 kV. The crystallinity of the organic semiconductors was examined by selective-area electron diffraction (EM 912 Omega) with a transmission electron microscope run at 120 kV. All current–voltage (I–V) properties of the FETs and inverters were measured with a semiconductor parameter analyzer (HP 4155C, Agilent Technologies) in the dark and in ambient air (relative humidity ~45%) at 20°C. The field-effect mobility (μ) and threshold voltage (Vth) were calculated in the saturation regime (VDS = −50 or 50 V) by plotting the square root of the drain current (IDS) versus the gate voltage (VGS) using IDS = (WC
i
/2 L) μ (VGS-Vth)2, where C
i
is the capacitance per unit area of the gate dielectric layer, and W and L are the channel width and length, respectively.