Showing papers by "Paddy K. L. Chan published in 2017"

Solution-Processed Monolayer Organic Crystals for High-Performance Field-Effect Transistors and Ultrasensitive Gas Sensors

Abstract: This work innovatively develops a dual solution-shearing method utilizing the semiconductor concentration region close to the solubility limit, which successfully generates large-area and high-performance semiconductor monolayer crystals on the millimeter scale. The monolayer crystals with poly(methyl methacrylate) encapsulation show the highest mobility of 10.4 cm2 V−1 s−1 among the mobility values in the reported solution-processed semiconductor monolayers. With similar mobility to multilayer crystals, light is shed on the charge accumulation mechanism in organic field-effect transistors (OFETs), where the first layer on interface bears the most carrier transport task, and the other above layers work as carrier suppliers and encapsulations to the first layer. The monolayer crystals show a very low dependency on channel directions with a small anisotropic ratio of 1.3. The positive mobility–temperature correlation reveals a thermally activated carrier transport mode in the monolayer crystals, which is different from the band-like transport mode in multilayer crystals. Furthermore, because of the direct exposure of highly conductive channels, the monolayer crystal based OFETs can sense ammonia concentrations as low as 10 ppb. The decent sensitivity indicates the monolayer crystals are potential candidates for sensor applications.

Marangoni-Effect-Assisted Bar-Coating Method for High-Quality Organic Crystals with Compressive and Tensile Strains

Abstract: Low-cost solution-shearing methods are highly desirable for deposition of organic semiconductor crystals over a large area. To enhance the rate of evaporation and deposition, elevated substrate temperature is commonly employed during shearing processes. However, the Marangoni flow induced by a temperature-dependent surface-tension gradient near the meniscus line shows negative effects on the deposited crystals and its electrical properties. In the current study, the Marangoni effect to improve the shearing process of 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene for organic field-effect transistor (OFET) applications is utilized and regulated. By modifying the gradient of surface tension with different combinations of solvents, the mass transport of molecules is much more favorable, which largely enhances the deposition rate, reduces organic crystal thickness, enlarges grain sizes, and improves coverage. The average and highest mobility of OFETs can be increased up to 13.7 and 16 cm2 V−1 s−1. This method provides a simple deposition approach on a large scale, which allows to further fabricate large-area circuits, flexible displays, or bioimplantable sensors.

Phonon thermal transport in silicene-germanene superlattice: a molecular dynamics study.

Abstract: Two-dimensional (2D) hybrid materials have drawn enormous attention in thermoelectric applications. In this work, we apply a molecular dynamics (MD) simulation to investigate the phonon thermal transport in silicene-germanene superlattice. A non-monotonic thermal conductivity of silicene-germanene superlattice with period length is revealed, which is due to the coherent-incoherent phonon conversion and phonon confinement mechanisms. We also calculate the thermal conductivity of a Si-Ge random mixing monolayer, showing a U-shaped trend. Because of the phonon mode localizations at Ge concentration of 80%, thermal conductivity varies dramatically at low doping regions. By changing the total length (L total), the infinite-length thermal conductivities of pure silicene, pure germanene, silicene-germanene superlattice, and Si-Ge random mixing monolayer are extracted as 16.08, 15.95, 5.60 and 4.47 W/m-K, respectively. The thermal boundary conductance (TBC) of the silicene-germanene is also evaluated, showing a small thermal rectification. At L total = 274.7 nm, the TBC of silicene to germanene is 620.49 MW/m2-K, while that of germanene to silicene is 528.76 MW/m2-K.

Molecular dynamics study of thermal transport in a dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) organic semiconductor

Abstract: The thermal transport in a high-mobility and air-stable small molecule organic semiconductor, dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT), is simulated by using non-equilibrium molecular dynamics. We find that the thermal conductivity of DNTT has a strong dependence on crystal size and orientation directions (a*, b* and c*). The bulk thermal conductivities of DNTT along the a*, b* and c* directions are 0.73, 0.33 and 0.95 W m−1 K−1, respectively. The polycrystalline nature of the DNTT thin film in the experiment means that it is essential to consider the effects of thermal boundary resistance (TBR) and vacancy on the thermal conductivity. The TBRs across different interfaces are calculated as 7.00 ± 0.26, 6.15 ± 0.13 and 3.20 ± 0.09 × 10−9 m2 K W−1 for the a*–b*, a*–c* and b*–c* interfaces, respectively. On the other hand, the thermal conductivities of DNTT with a vacancy concentration of 6% can be reduced by 44%, 33% and 35% in the a*, b* and c* directions. Our findings indicate that the boundary and defect scattering of phonons has significant effects on the thermal conductivity of organic semiconductors. This work contributes fundamental knowledge to control the thermal properties of organic semiconductors in organic electronic devices.

Ambipolar Organic Field-Effect Transistors Based on a Dual-Function, Ultrathin and Highly Crystalline 2,9-didecyldinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (C10-DNTT) Layer

Abstract: Ultrathin organic semiconductor thin films have great value in the investigation of carrier transport behavior in organic field-effect transistors (OFETs). Here, the dual solution shearing (DSS) method is adopted to deposit ultrathin and closely packed 2,9-didecyldinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (C10-DNTT) films. This smooth, ultrathin and highly crystalline DSS-processed C10-DNTT layer can be utilized in p–n stacking bilayer ambipolar OFETs. It has two major functions in the bilayer OFET: (i) it acts as the p-channel active layer; and (ii) it serves as the growth template of the upper n-type semiconductor layer. The closely packed alkyl side chains of the C10-DNTT molecules behave like long alkyl chains in the self-assembled monolayer and can assist the packing and orientation of the following layer. The F16CuPc layer grown on DSS-processed C10-DNTT shows crystallized lamellae structure. The smooth C10-DNTT surface can suppress the trap states and enhance the charge transfer between the p–n layer interface. The drain-source current (IDS) in the p-channel and n-channel shows a threefold and fivefold increase compared with two-step thermal evaporation. These findings demonstrate the potential of using solution-processed ultrathin organic semiconductor in multilayer organic electronics, which cannot be easily achieved by the conventional thermal evaporation approach.

Investigation of interfacial thermal transport across graphene and an organic semiconductor using molecular dynamics simulations

Abstract: The interfacial thermal transport across graphene and an organic semiconductor, dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT), is investigated using molecular dynamics simulations. The average thermal boundary resistance (TBR) of graphene and DNTT is 4.88 ± 0.12 × 10-8 m2 K W-1 at 300 K. We find that TBR of a graphene-DNTT heterostructure possesses as high as 83.4% reduction after the hydrogenation of graphene. Moreover, as the graphene vacancy increases from 0% to 6%, the TBR drops up to 39.6%. The reduction of TBR is mainly attributed to the coupling enhancement of graphene and DNTT phonons as evaluated from the phonon density of states. On the other hand, TBR keeps a constant value while the vacancy in the DNTT layer increases. The TBR would decrease when the temperature and coupling strength increase. These findings provide a useful guideline for the thermal management of the graphene-based organic electronic devices, especially the large area transistor arrays or sensors.

Highly Sensitive Glucose Sensor Based on Organic Electrochemical Transistor with Modified Gate Electrode.

Abstract: An organic electrochemical transistor (OECT) with a glucose oxidase (GOx) and poly(n-vinyl-2-pyrrolidone)-capped platinum nanoparticles (Pt NPs) gate electrode was successfully integrated with a microfluidic channel to act as a highly sensitive chip-based glucose sensor. The sensing mechanism relies on the enzymatic reaction between glucose and GOx followed by electrochemical oxidation of hydrogen peroxide (H2O2) produced in the enzymatic reaction. This process largely increases the electrolyte potential that applies on PEDOT:PSS channel and causes more cations penetrate into PEDOT:PSS film to reduce it to semi-conducting state resulting in lower electric current between the source and the drain. The extremely high sensitivity and low detection limit (0.1 μM) of the sensor was achievable due to highly efficient Pt NPs catalysis in oxidation of H2O2. Pt NPs were deposited by a bias-free two-step dip coating method followed by a UV-Ozone post-treatment to enhance catalytic ability. A polydimethylsiloxane (PDMS) microfluidic channel was directly attached to the OECT active layer, providing a short detection time (~1 min) and extremely low analyte consumption (30 μL). Our sensor has great potential for real-time, noninvasive, and portable glucose sensing applications due to its compact size and high sensitivity.