Temperature Dependent Barrier Crossover Regime in Tunneling Single Molecular Devices Based on the Matrix of Isolated Molecules

Abstract

This paper reports analysis of direct (low bias regime that corresponds to Simmons model) and field emission, Fowler-Nordheim (FN) tunneling, and a crossover between these regimes in molecular nanojunction. We have fabricated molecular devices based on a heterogenius mixture of molecular wires of 2-[4-(2mercaptoethyl)-phenyl]ethanethiol (Me-PET) as self-assembled monolayer (SAM) molecules incorporated into the matrix of molecular insulator spacers [penthane 1-thiol (PT)] at a concentration ratio of r ) 10 -6 wires/spacers. The monolayer is sandwiched between two gold (Au) electrodes. A temperature-depended conductivity in this structure was analyzed at both low and elevated biases using models of direct tunneling (Simmons model) and field emission (FN) regime. A crossover voltage, V trans , between these two regimes was determined at different temperatures. Comparison of temperature-dependent Simmons and FN barriers, (Φ B Sim (T) and Φ B FN (T)), and transition bias (V trans (T)) reveals an anomaly in position of V trans (T) with respect to Φ B Sim (T) and Φ B FN (T) at low temperatures (15-100 K). The change in slopes of Φ B Sim (T), Φ B FN (T), andV trans (T) at different temperatures pointed to the switching of the transport mechanism in the system as well. Activated by temperature, the observed phenomenon can be attributed to the existence of an additional transport barrier, which operates in series with the molecular barrier. As candidate for this additional barrier, the injection barrier must be considered. In this context, the transport was controlled by the injection barrier or injection barrier regime (IBR) at low temperatures (15-100 K), and by molecular barrier or molecular barrier regime (MBR) at high temperatures (150-294 K). A molecular diode, with the same structure (Me-PET/PT r ) 10 -6 ), but with Al electrodes, was fabricated to check this assumption. While devices with Au electrodes have a low tunneling barrier (Φ B Sim ≈ 1.2 eV), devices with Al electrodes have a high tunneling barrier (Φ B Sim ≈ 3 eV). Only direct tunneling could be observed in measured bias range (V ) (2 V). Nevertheless the "crossover behavior" was observed in the device with Al electrodes at low temperatures (15-100 K). Therefore, observed "crossover" in the device with Al electrodes is related to the transport processes, which occurs at the electrode due to the injection barrier, rather than the transition from direct to FN tunneling in the molecule at low temperatures. In this case a transition point, V trans , characterizes the IBR at low temperatures, the MBR at high temperatures, and the BTR between these two regimes.

Poly(3-hexylthiophene)/Multiwalled Carbon Hybrid Coaxial Nanotubes: Nanoscale Rectification and Photovoltaic Characteristics

“…5c. 43 The rectification effect for the hybrid P3HT/MWCNT single NT can originate from the temperature independent tunneling behavior.…”

Molecular-Scale Electronics: From Concept to Function

“…Furthermore, for nonconjugated systems, the frontier orbitals are expected to be strongly localized at the end groups of the molecules, which leads to a voltage drop at the interfaces and thus the length-independent V trans . Additionally, it was determined that the voltage drop in the molecular junctions was affected by the temperature . Using the TVS investigations of 2-[4-(2-mercaptoethyl)-phenyl] ethanethiol (Me-PET)-based molecular junctions with either Au or Al electrodes, the voltage drop at low temperatures occurred at the contact interfaces using the injection barrier dominated transport, whereas at high temperatures, the voltage drop occurred along the entire molecule using the molecular barrier dominated transport.…”

Transition voltages of vacuum-spaced and molecular junctions with Ag and Pt electrodes

“…Recently Beebe et al introduced transition voltage spectroscopy (TVS) as a measure of the energy alignment between the frontier molecular orbitals and the electrodes' E F . 3,4 Due to its simplicity and sensitivity, TVS is becoming an increasingly popular spectroscopic tool for molecular devices [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] and has been even extended to characterize vacuum-spaced junctions such as Au/vacuum/Au. 21 It has been shown that the transition voltage of an oligo(phenylene ethynylene) molecule attached with a thiol(isocyanide) linker group to four different metal electrodes, Ag, Pd, Au, and Pt, decreases (increases) as the electrode work function gets larger, and that the change of the measured transition voltage is smaller than the corresponding change in work function.…”