Fig. 1

Four-terminal FET measurements for Cl₂-NDI single crystals with different thicknesses. a Schematic diagram of a four-terminal vacuum gap FET configuration on patterned PDMS substrate with a [110]-oriented Cl₂-NDI single crystal. S, D, G represent source, drain, and gate electrodes, respectively. V₁ and V₂ are two voltage sensing channel electrodes. Inset: polarized light microscopy image of a device and chemical structure of Cl₂-NDI. The black bar scale is 150 µm. b Transfer curves (I_D-V_G) for single crystal FETs at different V_D. The extracted electron field effect mobility is ~2.7 cm² V⁻¹ s⁻¹. The channel length and width are 150 and 200 µm, respectively. The crystal thickness is 5.6 µm. c Mobility of 78 crystals (V_D = 10 V) and step density at the surface of 24 crystals as a function of crystal thickness. The three regions (thin, thick, thicker) correspond to mobility >2 cm² V⁻¹ s⁻¹ with crystal thickness less than 10 µm, 1–2 cm² V⁻¹ s⁻¹ with crystal thickness of 10–24 µm and <1 cm² V⁻¹ s⁻¹with crystal thickness exceeding 24 µm, respectively. The number of steps is measured by atomic force microscopy (AFM) and counted along the direction of the crystal’s long axis ([110] direction). d AFM images of step density for single crystals with thicknesses of 4.3 µm (top) and 34.7 µm (bottom), respectively