Length-dependent high-frequency response of aromatic and aliphatic molecules: predictions from first-principles calculations

Quantum capacitance and quantum transport calculations are conducted using density functional theory in combination with nonequilibrium Green's functional formalism to predict the effect of length on high-frequency response of aromatic molecules with different number of oligophenyl groups (n = 1, 2, 3) sandwiched between gold (111) electrodes through thiol and mercaptomethyl thiol anchoring groups. The results show that the quantum capacitance of the junctions decreases linearly with the length of the molecule regardless of the type of the edge group. The cutoff frequency of the molecular junctions show exponential decay as a function of the length in the THz frequency range. The insertion of the methylene group decreases the cutoff frequency by more than an order of magnitude due to enhanced differential resistance. We have also studied the high frequency response of aliphatic molecular junctions with different lengths and found that the cutoff frequencies of these structures are orders of smaller than the ones for the aromatic molecular junctions. These findings can be of practical importance in developing metal–molecule–metal diodes for high-frequency applications.