We report on the interlayer screening effect of graphene using Kelvin probe force microscopy (KPFM). By using a gate device configuration that enables the supply of electronic carriers in graphene sheets, the vertical screening properties were studied from measuring the surface potential gradient. The results show layer-dependence of graphene sheets, as the number of graphene layers increases, the surface potential decreases exponentially. In addition, we calculate the work function-related information of the graphene layers using KPFM.

Conventional chemical doping processes for conjugated polymers (CPs) often degrade the film morphology or cause unsatisfactory doping efficiency owing to the aggregation formation between charged species or insufficient dopant diffusion. In this work, a new strategic doping method, "hybrid doping," is suggested for maximizing the doping efficiency of CPs without hampering the surface morphology of the CP films. The advantage of hybrid doping is that it combines mixture blending and sequential soaking processes. Based on systemic characterizations including spectroscopic, structural, and electrical analyses, it is revealed that hybrid doping enables whole area doping for the crystalline and amorphous regions of CP films, and thus an unprecedentedly high electrical conductivity of up to 81.5 and 639.1 S cm −1 , for poly(3-hexylthiophene) P3HT and poly (2-([2,2′-bithiophen]-5-yl)-3,8-difluoro-5,10-bis(5octylpentadecyl)-5,10-dihydroindolo [3,2-b]indole) (PIDF-BT), respectively, is achieved. Furthermore, the exceptional electrical conductivity compensates a reduced Seebeck coefficient, resulting in excellent power factors up to 26.8 and 76.1 μW m −1 K −2 for thermoelectric devices based on doped-P3HT and PIDF-BT films, respectively, which is among the highest levels for semiconducting CPs. Hybrid doping is a strategic approach for the simultaneous optimization of electrical conductivity and thermoelectric properties of various CPs.

The effect of molecular weight of a series of conjugated polymers (CPs) on the doping efficiency, electrical conductivity, and related thermoelectric properties of doped CPs is studied. Low (L), medium (M), and high (H) molecular weight batches of PDFD-T polymers, based on difluorobenzothiadiazole and dithienosilole moieties, are synthesized and denoted as PDFD-T(L), PDFD-T(M), and PDFD-T(H), respectively. Furthermore, to compare the effects of different donor moieties, donor units of PDFD-T(L) are structurally modified from thiophene to thienothiophene (TT) and dithienothiophene (DTT), denoted as PDFD-TT(L) and PDFD-DTT(L), respectively. After doping the CPs with FeCl 3 , d-PDFD-T(H) exhibits an electrical conductivity of 402.9 S cm −1 , which is significantly higher than those of d-PDFD-T(L), d-PDFD-T(M), d-PDFD-TT(L), and d-PDFD-DTT(L). The highest power factor of 101.1 µW m −1 K −2 is achieved through organic thermoelectric devices fabricated using PDFD-T(H). Through various characterizations, it is demonstrated that CPs with a high molecular weight tend to have a high carrier mobility while maintaining their original crystallinity and good charge transport pathways even after doping. Therefore, it is suggested that optimizing the molecular weight of CPs is an essential strategy for maximal power generation from their doped CP films.

The dielectric constant (ε) is a key parameter to consider when determining the fundamental electrical properties of the charging and screening of charges in optoelectronic devices based on two-dimensional (2D) layered materials with van der Waals interactions. In this study, we report a direct local mapping of the thickness-dependent ε of MoS2 nanoflakes using a nondestructive electrostatic force microscopy (EFM) imaging technique. EFM is used to simultaneously probe the thickness and local ε value of regions with different thicknesses by detecting a cantilever deflection while applying DC bias voltage between the conducting tip and the substrate. The measured ε increases with the thickness and saturates to the bulk value, consistent with previous theoretical and experimental results of dielectric constants for MoS2 at different thicknesses. Our works provide quantitative information pertaining to the thickness-dependent electric permittivity, which can be useful in quantitative design of high-performance and multifunctional nanoelectronic devices based on layered 2D materials.

Understanding the interlayer charge coupling mechanism in a two-dimensional van der Waals (vdW) heterojunction is crucial for optimizing the performance of heterostructure-based (opto)­electronic devices. Here, we report mapping the gate response of a multilayer WSe2/MoS2 heterostructure with locally different degrees of charge depletion through mobile carrier measurements based on electrostatic force microscopy. We observed ambipolar or unipolar behavior depending on the degree of charge depletion in the heterojunction under tip gating. Interestingly, the WSe2 on MoS2 shows gating behavior that is more efficient than that on the SiO2/Si substrate, which can be explained by the high dielectric environment and screening of impurities on the SiO2 surface by the MoS2. Furthermore, we found that the gate-induced majority carriers in the heterojunction reduce the carrier lifetime, leading to the enhanced interlayer recombination of the photogenerated carriers under illumination. Our work provides a comprehensive understanding of the interfacial phenomena at the vdW heterointerface with charge depletion.