Probing Carrier Transport and Structure-Property Relationship of Highly Ordered Organic Semiconductors at the Two-Dimensional Limit

One of the basic assumptions in organic field-effect transistors, the most fundamental device unit in organic electronics, is that charge transport occurs two dimensionally in the first few molecular layers near the dielectric interface. Although the mobility of bulk organic semiconductors has increased dramatically, direct probing of intrinsic charge transport in the two-dimensional limit has not been possible due to excessive disorders and traps in ultrathin organic thin films. Here, highly ordered single-crystalline mono- to tetralayer pentacene crystals are realized by van der Waals (vdW) epitaxy on hexagonal BN. We find that the charge transport is dominated by hopping in the first conductive layer, but transforms to bandlike in subsequent layers. Such an abrupt phase transition is attributed to strong modulation of the molecular packing by interfacial vdW interactions, as corroborated by quantitative structural characterization and density functional theory calculations. The structural modulation becomes negligible beyond the second conductive layer, leading to a mobility saturation thickness of only $\sim 3\mathrm{nm}$. Highly ordered organic ultrathin films provide a platform for new physics and device structures (such as heterostructures and quantum wells) that are not possible in conventional bulk crystals.

Figure 1

Epitaxial growth of 2D pentacene crystals on BN. (a) Schematic illustration of the molecular packing of WL, 1L, and 2L within $b\text{−}c$ plane. (b) Histogram distribution of the thickness of WL, 1L, and 2L, each taken from over 10 samples. (c) Raman spectrum of the pentacene crystals on BN, taken from a 2L sample. (d)–(f) AFM images of WL, 1L, and 2L pentacene crystals on BN, respectively. The layer numbers are marked on each image. Insets show the height profiles along the dashed lines. The scale bars are $2\mu \mathrm{m}$.

Figure 2

Characterization of 2D pentacene crystals. (a) High-resolution AFM image of a 1L sample. The unit cell is marked. The scale bar is 1 nm. (b) SAED pattern from a few-layer pentacene sample (Supplemental Material, Fig. S4a [18] shows the low-magnification TEM image of the sample). Blue circles mark the BN (100) directions and green circles mark the pentacene (110), (120), and (020) directions. (c) Histogram of lattice constants of 1L (upper panel) and 2L (lower panel) pentacene crystals. Blue and red lines represent $a$- and $b$-axes, respectively. Black lines show the best Gaussian fittings. (d) Molecular packing of 1L (upper panel) and 2L (lower panel) pentacene within $a\text{−}b$ plane. The unit cell is marked by the dashed rectangular box. (e) Polarization-dependent PL spectrum from a 2L sample. The black (blue) curve is taken when the PL intensity is strongest (weakest). Red lines are the fitting results with three Gaussian peaks (Supplemental Material, Fig. S6d [18]). Inset is the spatially resolved PL image of the 2L sample, showing excellent uniformity. The scale bar is $3\mu \mathrm{m}$. (f) Normalized PL intensity of the 2L sample in $\mathbf{e}$ as a function of linear polarization angle.

Figure 3

Temperature-dependent electrical transport of 1L and 2L pentacene OFETs. (a) Room temperature $I_{ds}\text{−}{V}_g$ characteristics ($V_{ds}=-2\mathrm{V}$, black line) and the extracted field-effect mobility as a function of $V_g$ (red symbols) of a 1L device. Inset shows the optical microscope image of the device. The scale bar is $20\mu \mathrm{m}$. (b) $I_{ds}\text{−}{V}_g$ characteristics at different temperatures of the same device under $V_{ds}=-2\mathrm{V}$ (symbols), plotted on a double logarithmic scale. From top to bottom, $T=300$, 250, and 140 K, respectively. The lines are power-law fitting results. Inset shows the extracted power exponent as a function of $1000/T$ (symbols). The linear fitting crosses the origin (line), consistent with the 2D hopping mechanism. $T_0=331\mathrm{K}$ is derived from the linear fitting. (c) Experimental (symbols) and calculated (lines) mobility as a function of $1000/T$ under $V_g=-30$ (purple), $-20$ (blue), and $-10\mathrm{V}$ (orange). The calculations are done with the following parameters: $T_0=331\mathrm{K}$, ${\sigma }_0=1.3\times {10}^6\mathrm{S}/\mathrm{m}$, ${\alpha }^{-1}=8.2\AA$. (d) $I_{ds}\text{−}{V}_g$ characteristics ($V_{ds}=-2\mathrm{V}$) at different temperatures of a 2L device. (e) The extracted mobility as a function of $V_g$ at the same temperatures as in (d). (f) Mobility as a function of temperature under $V_g=-20$ (orange), $-35$ (blue), and $-50\mathrm{V}$ (purple).

Figure 4

Visualized molecular orbitals of the inter-molecular bonding states for 1L and 2L. (a) Side views of the molecular orbitals for 1L-B (blue) and 2L-B (red) in the $b\text{−}c$ plane, illustrated by isosurface contours of $0.00015e/{\text{Bohr}}^3$. (b)–(c) Top views of the molecular orbitals in the slices cleaved in the $a\text{−}b$ plane for 1L-B (b) and 2L-B (c). Yellow dashed lines in (a) indicate the positions of the slices. The top phenyl group of the pentacene molecules are drawn in (b) and (c).