Unveiling the structural origin of the high carrier mobility of a molecular monolayer on boron nitride

Very recently, it was demonstrated that the carrier mobility of a molecular monolayer dioctylbenzothienobenzothiophene $\left({\mathrm{C}}_8\text{−}\mathrm{BTBT}\right)$ on boron nitride can reach $10\mathrm{c}{\mathrm{m}}^2/\mathrm{Vs}$, the highest among the previously reported monolayer molecular field-effect transistors. Here we show that the high-quality single crystal of the ${\mathrm{C}}_8\text{−}\mathrm{BTBT}$ monolayer may be the key origin of the record-high carrier mobility. We discover that the ${\mathrm{C}}_8\text{−}\mathrm{BTBT}$ molecules prefer layer-by-layer growth on both hexagonal boron nitride and graphene. The flatness of these substrates substantially decreases the ${\mathrm{C}}_8\text{−}\mathrm{BTBT}$ nucleation density and enables repeatable growth of large-area single crystal of the ${\mathrm{C}}_8\text{−}\mathrm{BTBT}$ monolayer. Our experimental result indicates that only out-of-plane roughness greater than 0.6 nm of the substrates could induce disturbance in the crystal growth and consequently affect the charge transport. This information would be important in guiding the growth of high-quality epitaxy molecular film.

Figure 1

(a)–(e) Sequential AFM snapshots of ${\mathrm{C}}_8\text{−}\mathrm{BTBT}$ layers on $\mathrm{graphene}/\mathrm{Si}{\mathrm{O}}_2$ during a 4-minute growth. The scale bars are 2 μm. The thickness of the molecular film and the duration of growth are marked on each image. (b), (c) Positions indicated by the white arrows and IL are regions only covered by the IL. (f) Schematic diagram of the molecular structure of ${\mathrm{C}}_8\text{−}\mathrm{BTBT}$ layers on graphene or BN. The thicknesses of the IL, 1L, and 2L on both substrates are 0.60 ± 0.08 nm, 1.70 ± 0.09 nm, and 3.00 ± 0.15 nm, respectively. (g) Roughness of ${\mathrm{C}}_8\text{−}\mathrm{BTBT}$ layers as a function of the coverage. The green curve is calculated according to Eq. (1). The red solid circles and blue symbols are experimental data on graphene recorded at the growth temperature 110 °C and 100 °C, respectively. The yellow triangles are experimental data of ${\mathrm{C}}_8\text{−}\mathrm{BTBT}$ layers on BN.

Figure 2

(a) A large-area STM image of graphene surface on Cu foil ($V_{\mathrm{sample}}=-1\mathrm{V}$ and $I=16.8\mathrm{pA}$). (b) A large-area STM image of ${\mathrm{C}}_8\text{−}\mathrm{BTBT}\mathrm{IL}$ ($V_{\mathrm{sample}}=-1.29\mathrm{V}$ and $I=9.08\mathrm{pA}$). (c) A higher magnification STM image of the ${\mathrm{C}}_8\text{−}\mathrm{BTBT}$ interface layer on graphene/Cu foil ($V_{\mathrm{sample}}=-0.91\mathrm{V}$ and $I=12.1\mathrm{pA}$). The angle between the carbon chains and the benzothiophene is measured to be 140°. The arrangement of molecules is shown in the inset of (c) and (d). Angle between the carbon chains and the benzothiophene is 134°, calculated by DFT. The angle between the benzothiophene plane and the graphene substrate is ∼10°. (e) Section lines along the green lines in (c) and (d). (f) Section lines along the lines in (c) and (d). We can see a $\pi /2$ phase ascending from both lines $a$ to $c$ and lines $a^{\prime }$ to $c^{\prime }$. (g) A typical $dI/dV\text{−}V$ curve recorded at the IL in Fig. 2.

Figure 3

(a) A typical STM image showing multiple domains of the IL ($V_{\mathrm{sample}}=1.04\mathrm{V}$ and $I=10.5\mathrm{pA}$). Green arrows denote the orientations of three different domains. The relative angles between these domains are 21.5°, 98.0°, and 60.5°. The blue and red arrows point to out-of-plane roughness of the substrate. The growth of the IL is disturbed by the out-of-plane roughness, indicated by the red arrows but not by those indicated by the blue arrows. (b), (c)Two typical section lines at the positions of the blue and red lines. (d) Number of steps that do (not) affect the growth of the IL [red bars (blue bars)] versus their height measured in our experiment.