We studied a molecular junction with arylalkane self-assembled monolayers sandwiched between two graphene contacts. The arrangement of graphene-based molecular junctions provides a stable device structure with a high yield and allows for extensive transport measurements at 78 K. We observed a temperature-independent current density–voltage (J–V) characteristic and the exponential dependency of the current density on the molecular length, proving that the charge transport occurs by non-resonant tunneling through the molecular barrier. Based on the Simmons model, the bias-voltage dependence of the decay coefficient and barrier height was extracted from variable-length transport characterizations. The J–V data measured were simulated by the Simmons model, which was modified with the barrier lowering induced by the bias voltage. Indeed, there isno need for adjustable fitting parameters. The resulting simulation was in remarkable consistency with experimental measurements over a full bias range up to |V| ≤ 1.5 V for the case of graphene/arylalkane/graphene heterojunctions. Our findings clearly showed the demonstration of stable and reliable molecular junctions with graphene contacts and their intrinsic charge transport characteristics, as well as justifying the application of the voltage-induced barrier lowering approximation to the graphene-based molecular junction.

We demonstrate the fabrication and complete characterization of vertical molecular tunneling junctions based on graphene heterostructures, which incorporate a control series of arylalkane molecules acting as charge transport barriers. Raman spectroscopy and atomic force microscopy were employed to identify the formation of the molecular monolayer via an electrophilic diazonium reaction on a pre-patterned bottom graphene electrode. The top graphene electrode was transferred to the deposited molecular layer to form a stable electrical connection without filamentary damage. Then, we showed proof of intrinsic charge carrier transport through the arylalkane molecule in the vertical tunneling junctions by carrying out multiprobe approaches combining complementary transport characterization methods, which included length- and temperature-dependent charge transport measurements and transition voltage spectroscopy. Interpretation of all the electrical characterizations was conducted on the basis of intact statistical analysis using a total of 294 fabricated devices. Our results and analysis can provide an objective criterion to validate molecular electronic devices fabricated with graphene electrodes and establish statistically representative junction properties. Since many of the experimental test beds used to examine molecular junctions have generated large variation in the measured data, such a statistical approach is advantageous to identify the meaningful parameters with the data population and describe how the results can be used to characterize the graphene-based molecular junctions.

In this study, we investigated the high-temperature electronic transport behavior of spin-coated PEDOT:PSS top-contact molecular ensemble junctions based on self-assembled monolayers (SAMs) of oligophenylene dithiols. We observed irreversible temperature-dependent charge transport at the high-temperature regime over 320 K. The effective contact resistance and normalized resistance decreased with increasing temperature (320 to 400 K), whereas the tunneling attenuation factor was nearly constant irrespective of temperature change. These findings demonstrate that the high-temperature transport properties are not dominated by the integrity of SAMs in molecular junctions, but rather the PEDOT:PSS/SAMs contact. Transition voltage spectroscopy measurements indicated that the contact barrier height of the PEDOT:PSS/SAMs is lowered at elevated temperatures, which gives rise to a decrease in the contact resistance and normalized resistance. The high-temperature charge transport through these junctions is also related to an increase in the grain area of PEDOT cores after thermal treatment. Moreover, it was found that there was no significant change in either the current density or normalized resistance of the annealed junctions after 60 days of storage in ambient conditions.