Electrochemical Deposition of a Single‐Crystalline Nanorod Polycyclic Aromatic Hydrocarbon Film with Efficient Charge and Exciton Transport

Abstract Electrochemical deposition has emerged as an efficient technique for preparing conjugated polymer films on electrodes. However, this method encounters difficulties in synthesizing crystalline products and controlling their orientation on electrodes. Here we report electrochemical film deposition of a large polycyclic aromatic hydrocarbon. The film is composed of single‐crystalline nanorods, in which the molecules adopt a cofacial stacking arrangement along the π–π direction. Film thickness and crystal size can be controlled by electrochemical conditions such as scan rate and electrolyte species, while the choice of anode material determines crystal orientation. The film supports exceptionally efficient migration of both free carriers and excitons: the free carrier mobility reaches over 30 cm2 V−1 s−1, whereas the excitons are delocalized with a low binding energy of 118.5 meV and a remarkable exciton diffusion length of 45 nm.

substrate, the as-grown monolayer graphene film on Pt foil was spin-coated with PMMA (950 kDa molecular weight, 4 wt% in ethyllactate) at 2000 r. p. m. for 1 min, and then cured at 90 °C for 10 min. The PMMA/graphene stack was separated from the Pt foil in a 1 M NaOH aqueous solution under a constant current of 0.2 A, and then collected onto the ITO/quartz substrate and baked at 130 C for 1 h to remove the residual water. After PMMA was removed by acetone at room temperature, the resulting graphene/ITO/quartz was annealed at 300 C for 5 h under a pressure of 10 -5 Pa to further clean the surface of graphene.

Active electrode surface area
The active surface areas (non-apparent area) of glassy carbons were calibrated as follows: the electrodes to be tested were placed in an aqueous solution containing K3Fe(CN)6 (5.0 × 10 -3 mol L -1 ) and KCl (1 mol L -1 ), and the CV curves were recorded in the range of -0. 25

Cottrell equation
The Cottrell equation, [S4] describes the chronoamperometric response, and its integral from t = 0 gives the cumulative charge passed in reducing the diffusing reactant: where Q is electricity, n is transfer electron number, F is faraday constant, A is active surface area of electrodes, C0 is bulk concentration of the electroactive units and t is charging time. The diffusion rate D0 can be found from the linear slope in the time scale of 10 s.

Anson equation
The Anson equation [S5] is also useful to get a fully or partially controlled chronocoulometric response by interface charge transfer kinetics. This goal can be achieved by using a step potential that performs diffusion control electrolysis less aggressively throughout the experimental time domain. In other words, it is necessary to perform a step on the potential in the rising portion of the sampled current voltammogram, which corresponds to the time scale of interest, and the time scale must be short enough that the electrode dynamics control the current for a relatively long period of time. If a step at t = 0 from an initial potential where electrolysis does not occur, Anson equation can be obtained: where Q is electricity, n is transfer electron number, F is faraday constant, A is active surface area of electrodes, C0 is bulk concentration of the electroactive units and t is charging time. The electron transfer rate Kf can be found from the linear slope in the time scale of 0.1 s.

Density-functional theory simulation
The crystalline structures of HBC-6Ph were calculated using density-functional theory (DFT) implemented in the CASTEP module of Materials Studio 7.0. [S6,S7] The generalized gradient approximation (GGA) in the form of Perdew-Burke-Ernzerhof (PBE) [S8,S9] was selected as the exchange-correlation functional. Grimme dispersion correction [S10,S11] was employed in all calculations to describe van der Waals (vdW) and π-stacking interactions. The lattice dimensions were optimized simultaneously with the geometry. A plane wave energy cutoff of 750 eV and the Monkhorst-Pack k-point grid of 1 × 1 × 2 were used. The convergence criteria for energy, force, stress and displacement are 5 × 10 -6 eV/atom, 0.01 eV/Å, 0.02 GPa and 5 × 10 -4 Å, respectively.

Preparation of HBC-6Ph films on PAA for THz measurements
The HBC-6Ph films were firstly deposited on ITO or MLG, and then uniformly coated with an aqueous solution of PAA (average Mw ~250,000, 35 wt% in H2O). After the water evaporated and dried, the PAA on the surface of HBC-6ph films was peeled off, thus the HBC-6ph films were transferred to the PAA substrates from ITO or MLG.

THz spectroscopy
The optical pump-THz probe spectrometer is operated by a regenerative Ti: sapphire femtosecond amplifier system with ~50 fs duration pulsed laser. The center wavelength of pulses is ~ 800 nm and the repetition rate is 1 kHz. A single-cycle THz pulse in the 0-2.5 THz frequency range is generated by an optical rectification process by pumping a 0.5-mm thick ZnTe crystal by 800 nm pulses.
The transient THz electric field can be mapped out via electro-optic sampling by a third pulse (800 nm, 50 fs duration) [S12] by varying the arrival of the sampling pulse to the detection crystal with respect to the THz pulse. In order to monitor the charge carrier dynamics, the sample was photo-excited by 400 nm pulse. The conductivity of photogenerated charge carriers, i.e., photoconductivity can be further measured by tracking the photo-induced THz field absorption.

Calculation of effective mass of HBC-6Ph single crystal
The HBC-6Ph crystal structure was considered as input geometry for computation of the effective mass, with the Quantum

Exciton binding energy
The PL intensity verse temperature plot could be fitted using in which I0 is the PL intensity at 0 K, kB is the Boltzmann constant and EB is exciton binding energy.

Exciton-exciton annihilation analysis
Exciton-exciton annihilation occurs (S1 + S1 → Sn + S0 → S1 + S0 + phonon) under excitation by intermediate or high photo fluxes, where the fusion of two excitations forms ionized or higher-energy state. In the case of singlet-singlet annihilation, the kinetic equation is written by following formula:  Figure S1. Synthesis of HPB-6Ph.