The Stark Effect: A Tool for the Design of High-Performance Molecular Rectifiers

Quantitative Characterization and Influencing Factors for Electrode-Molecule-Electrode Junction Stability

Molecular electronics has made considerable progress in recent decades. The construction of a stable "electrode-molecule-electrode" junction is critical for the study of molecular electronics, as the stability can promote the exploration of the electrical properties of individual molecules and enable the prolonged observation of physical and chemical phenomena at the single-molecule scale. However, dispersed discussions and conflated concepts hinder our understanding of molecular junction stability. In this review, we systematically discuss the stability of molecular junctions from both thermodynamic and kinetic perspectives, summarize key quantitative parameters and their interrelationships, and provide an overview of the influencing factors at the molecule-electrode interface, as well as the experimental and theoretical analysis methods. We anticipate that this review will contribute to a thorough understanding of the stability of molecular junctions and offer valuable insights for the design of molecular devices based on molecular junctions.

A Robust Molecular Rectifier Based on Ferrocene-Functionalized Bis(diarylcarbene) on Gold

While thiol-based adsorbates have achieved significant success in surface modification and molecular electronics, their thermal and storage instability has hindered long-term commercial viability. Carbene-based thin films offer a promising alternative due to their enhanced stability; however, their molecular electronic properties after postfunctionalization remain to be investigated. In this study, by attaching a ferrocene (Fc) unit to a bis(diarylcarbene)-modified gold surface via carbodiimide coupling, the system exhibits diode behavior with a current rectification ratio of ∼100. This diode behavior is highly temperature-dependent, indicating that hopping is the dominant mechanism for current rectification. Notably, the system demonstrated excellent electrical stability under ambient storage conditions for over 6 months. Our work highlights the potential for designing carbene-based molecular junctions through postmodification and for developing durable functional molecular electronic devices.

Efficient Molecular Rectification in Metal-Molecules-Semimetal Junctions

Molecular rectification is expected to be observed in metal-molecule-metal tunnel junctions in which the resonance levels responsible for their transport properties are spatially localized asymmetrically with respect to the leads. Yet, effects such as electrostatic screening and formation of metal induced gap states reduce the magnitude of rectification that can be realized in such junctions. Here we suggest that junctions of the form metal-molecule(s)-semimetal mitigate these interfacial effects. We report current rectification in junctions based on the semimetal bismuth (Bi) with high rectification ratios (>102) at 1.0 V using alkanethiols, molecules for which rectification has never been observed. In addition to the alleviation of screening and surface states, the efficient rectification is argued to be related to symmetry breaking of the applied bias in these junctions because of a built-in potential within the Bi lead. The significance of this built-in potential and its implications for the future and other applications are discussed.

Dynamically blocking leakage current in molecular tunneling junctions

Molecular tunneling junctions based on self-assembled monolayers (SAMs) have demonstrated rectifying characteristics at the nanoscale that can hardly be achieved using traditional approaches. However, defects in SAMs result in high leakage when applying bias. The poor performance of molecular diodes compared to silicon or thin-film devices limits their further development. In this study, we show that incorporating "mixed backbones" with flexible-rigid structures into molecular junctions can dynamically block tunneling currents, which is difficult to realize using non-molecular technology. Our idea is achieved by the interaction between interfacial dipole moments and electric field, triggering structured packing in SAMs. Efficient blocking of leakage by more than an order of magnitude leads to a significant enhancement of the rectification ratio to the initial value. The rearrangement of supramolecular structures has also been verified through electrochemistry and electroluminescence measurements. Moreover, the enhanced rectification is extended to various challenging environments, including endurance measurements, bending of electrodes, and rough electrodes, thus demonstrating the feasibility of the dynamic behavior of molecules for practical electronic applications.

Gaining insight into molecular tunnel junctions with a pocket calculator without I-V data fitting. Five-thirds protocol

The protocol put forward in the present paper is an attempt to meet the experimentalists' legitimate desire of reliably and easily extracting microscopic parameters from current-voltage measurements on molecular junctions. It applies to junctions wherein charge transport dominated by a single level (molecular orbital, MO) occurs via off-resonant tunneling. The recipe is simple. The measured current-voltage curve I = I(V) should be recast as a curve of V5/3/I versus V. This curve exhibits two maxima: one at positive bias (V = Vp+), another at negative bias (V = Vp-). The values Vp+ > 0 and Vp- < 0 at the two peaks of the curve for V5/3/I at positive and negative bias and the corresponding values Ip+ = I(Vp+) > 0 and Ip- = I(Vp-) < 0 of the current is all information needed as input. The arithmetic average of Vp+ and |Vp-| in volt provides the value in electronvolt of the MO energy offset ε0 = EMO - EF relative to the electrode Fermi level (|ε0| = e(Vp+ + |Vp-|)/2). The value of the (Stark) strength of the bias-driven MO shift is obtained as γ = (4/5)(Vp+ - |Vp-|)/(Vp+ + |Vp-|) sign (ε0). Even the low-bias conductance estimate, G = (3/8)(Ip+/Vp+ + Ip-/Vp-), can be a preferable alternative to that deduced from fitting the I-V slope in situations of noisy curves at low bias. To demonstrate the reliability and the generality of this "five-thirds" protocol, I illustrate its wide applicability for molecular tunnel junctions fabricated using metallic and nonmetallic electrodes, molecular species possessing localized σ and delocalized π electrons, and various techniques (mechanically controlled break junctions, STM break junctions, conducting probe AFM junctions, and large area junctions).