No abstract

clinicaltrials.gov Identifier: NCT01550406.

values. [10] Therefore, many research groups have proposed emerging methods to improve contact properties, for example, inserting functional layers between 2D semiconductors and electrodes, [11,12] thermal annealing at contact regions, [13] phase engineering with high-energy beam irradiation, [14] and employing graphene electrodes [15,16] low-work function metals, [17,18] or a direct chemical vapor deposition (CVD)-growth method of 2D heterostructures in lateral. [19,20] However, thermal annealing and high-energy beam irradiation may damage 2D materials, and direct CVDgrowth method of lateral 2D heterostructures is difficult to define accurate junction areas between 2D materials. Therefore, more facile approach to improve the contact properties between metal electrodes and 2D semiconductors is desired for the typical 2D field-effect transistor structure. Among these methods, introducing a thin tunneling barrier between the MoS 2 channel and electrodes is an efficient approach to improve contact properties by reducing the activation energy. [12,21,22] For example, graphene or hexagonal boron nitride (h-BN) have been vertically employed between metal contacts and 2D semiconductors as a thin tunneling barrier for the lower contact resistance. [21,22] Moreover, the use of cobalt electrodes facilitated Ohmic contact even in the extremely low temperature regime. [12] However, relatively complex, high-cost, and low-yield transfer processes were required to make vertically stacked 2D heterostructures. [11,12] Additionally, it is difficult to introduce atomically thin tunneling barriers using conventional transfer methods to the exact contact location on flake-type MoS 2 with dimensions of sub-micrometer size. Here, we demonstrate a simple strategy for inserting a thin tunneling barrier by depositing thiol-molecules between MoS 2 semiconductors and conventional metal electrodes. Vaporized thiol-molecules are chemically adsorbed on MoS 2 with covalent bonding. The inserted thiol-molecules at the contact region create additional tunneling paths, resulting in a drastically reduced activation energy; therefore, the primary injection mechanism of the contact-engineered MoS 2 field-effect transistors (FETs) changes from thermionic emission to field emission, allowing better contact properties without a temperature dependency. In addition, by defining contact regions on MoS 2 using conventional lithography, where injection engineering is desirable, a selective introduction of thiol-molecules is feasible with ≈100% yield Although 2D molybdenum disulfide (MoS 2 ) has gained much attention due to its unique electrical and optical properties, the limited electrical contact to 2D semiconductors still impedes the realization of high-performance 2D MoS 2based devices. In this regard, many studies have been conducted to improve the carrier-injection properties by inserting functional paths, such as graphene or hexagonal boron nitride, between the electrodes and 2D semiconductors. The reported strategies, however, require relatively time...

memories, and organic field-effect transistors (OFETs), have various advantages including mechanical flexibility, low cost, solution-processed fabrication, and tunable material functionalities by molecular design compared with silicon-based materials. [1][2][3][4][5][6][7][8][9][10][11][12][13] However, the contact resistance problem arising between organic materials and metal electrodes has been one of the dominant obstacles for adopting organic semiconducting devices instead of silicon-based devices. Diverse attempts, for instance, self-assembled monolayer (SAM) treatment on metal electrodes, [14][15][16][17][18][19] inserting a charge injection layer between OSC and metals, [20][21][22][23][24][25][26][27] choice of metals for better injection properties, [28,29] adopting carbon-based conductor like graphene as electrodes, [30] have been introduced to improve carrier injection across typically a non-ohmic contact. Especially, considering large operation voltages required for OFETs, improving contact properties of organic/metal interface is an essential step for practical applications of OSCs.Contact doping is one of the most effective techniques to reduce contact resistance and has been widely employed in silicon-based devices and recently in OSCs to reduce the contact resistance. [31][32][33][34][35][36] In order to avoid undesirable OFF currents, it needs to be performed selectively, i.e., in localized regions at the source-drain contacts only and not in the channel region. The doped regions have been usually confined to the top surface of the OSC film by depositing a small amount of dopants on the top of the organic film by thermal evaporation. As a result, the position of the gate dielectrics was normally restricted to the top side of devices (i.e., FETs in a top-gate structure) in order to enhance the charge injection from metal electrodes to the accumulation layer formed on the top surface of the polymer. [31,32] Recently, the combination of poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F 4 -TCNQ) as host and dopant material, respectively, has produced a highly conducting polymer that has been studied as a candidate for a synthetic metal and high power-factor thermoelectric material. [37][38][39][40][41] Interestingly, this combination achieved an efficient bulk-doping of PBTTT by solid-state diffusion which implied that the F 4 -TCNQ Organic semiconductors (OSCs) have been widely studied due to their merits such as mechanical flexibility, solution processability, and large-area fabrication. However, OSC devices still have to overcome contact resistance issues for better performances. Because of the Schottky contact at the metal-OSC interfaces, a non-ideal transfer curve feature often appears in the low-drain voltage region. To improve the contact properties of OSCs, there have been several methods reported, including interface treatment by self-assembled monolayers and introducing charge injection layers. Here, a selectiv...