Responsive molecular devices are one of the core units for molecular electronics, and dynamic covalent bonds (DCBs) provide the opportunity for the fabrication of responsive molecular devices. Herein we employ a single dynamic acyl hydrazone bond to fabricate tailored molecular devices using the scanning tunneling microscopy break‐junction technique (STM‐BJ) and the eutectic Ga‐In technique (EGaIn). We found that the single‐DCB‐tailored molecular devices exhibited acid‐base and/or photo‐thermal response with three well‐defined molecular conductance states. The reversible switching has the ON/OFF ratio of ≈10 between each state for single‐molecule junctions and ≈3 for the SAMs‐based molecular junctions. Combined with the density functional theory calculations, we revealed that the multiple conductance states of these molecular junctions originate from the dynamic acyl hydrazone bond exchange and C=N isomerization. Our work opens the avenue towards the design of tailored single‐molecule electrical devices by implanting dynamic covalent bonds in molecular architectures.

Molecular electronics is a promising subject to overcome the size limitation of silicon‐based electronic devices. In the past decades, various micro/nanofabrication techniques have been developed for constructing molecular junctions, and a number of breakthroughs are made in the characterizations and applications of the single‐molecule device. The history and progress are reviewed in this article, laying emphasis on the recent works on the combination of micro/nanofabrication techniques with other techniques such as electrochemical deposition and surface‐enhanced Raman spectroscopy (SERS). Some prototypical single‐molecule devices such as molecular transistors are presented. Finally, the challenges and prospects in the fabrication of single‐molecule devices are discussed.

challenging to reveal the evolution of the charge transport mechanism from the conductance characterization of singlemolecule conductance. In the past decade, numerous data analysis methods were introduced to process the conductance traces, including statistical methods, [4][5][6][7] analytical modeling, [8,9] electronic noise analysis, [10][11][12][13][14][15] and machine learning and deep learning methods. [16][17][18] Among these methods, analysis of flicker noise from the single-molecule junctions has suggested to shed new light on the understanding of charge transport mechanisms of singlemolecule junctions. [19][20][21][22] The electronic noise of single-molecule junctions can be categorized into highfrequency noise and low-frequency noise, and the latter, including flicker noise and random telegraph signal (RTS) noise (that is 1/f noise and 1/f 2 noise, respectively due to their frequency dependence nature), is the result of rearrangement of metal atom on the electrode surface and the fluctuation of the microenvironment of molecule junctions. [23] Previous reports have shown that flicker noise exhibits a power-law dependence with conductance that can serve as a probe of the electrode-molecule coupling. [21,22] Specifically, if the dependence exponent is close to 1.0, the electrode-molecule coupling follows the throughbond mechanism, while an exponent close to 2.0 corresponds to a through-space coupling. During the evolution of different junction configurations, the interface coupling may change dramatically with the evolution of electrode-molecule coupling during the break junction process, suggesting that an investigation of the time evolution of flicker noise in molecule junctions can provide a unique tool to understand the evolution of charge transport mechanisms. [24][25][26][27][28][29] However, the time dimension of the conductance traces is hardly deciphered in past studies. Toward the time resolution, time-frequency analysis (TFA) has proven to be an effective tool that combines both time and frequency dimensions in processing signals such as speech signals and polysomnography signals, [30,31] suggesting that TFA can be a promising approach to characterize the time evolution behavior of flicker noise in single-molecule junctions.In this study, we report that flicker noise can serve as an indicator of the evolution of charge transport mechanisms in single-molecule break junctions. By assigning a dependence The electronic noise characterization of single-molecule devices provides insights into the mechanisms of charge transport. In this work, it is reported that flicker noise can serve as an indicator of the time-dependent evolution of charge transport mechanisms in the single-molecule break junction process. By introducing time-frequency analysis, the authors find that flicker noise components of the molecule junction show time evolution behavior in the dynamic break junction process. A further investigation of the power-law dependence of flicker with conductance during the dynamic break junction pro...

The discovery of quantum interference (QI) is widely considered as an important advance in molecular electronics since it provides unique opportunities for achieving single-molecule devices with unprecedented performance. Although some pioneering studies suggested the presence of spin qubit coherence and QI in collective systems such as thin films, it remains unclear whether the QI can be transferred step-by-step from single molecules to different length scales, which hinders the application of QI in fabricating active molecular devices. Here, we found that QI can be transferred from a single molecule to their assemblies. We synthesized and investigated the charge transport through the molecular cages using 1,3-dipyridylbenzene (DPB) as a ligand block with a destructive quantum interference (DQI) effect and 2,5-dipyridylfuran (DPF) as a control building block with a constructive quantum interference (CQI) effect using both single-molecule break junction and large area junction techniques. Combined experiments and calculations revealed that both DQI and CQI had been transferred from the ligand blocks to the molecular cages and the monolayer thin film of the cages. Our work introduced QI effects from a ligand to the molecular cage comprising 732 atoms and even their monolayers, suggesting that the quantum interference could be scaled up within the phase-coherent distance.

No abstract

the reaction dynamics of confined reaction in millimolar concentration scale efficiently. However, NMR, isothermal titration calorimetry, and thermogravimetry are challenging to obtain the dynamic changes during confined reaction at micromolar concentrations (5 × 10 −6 m) relative to conventional reactions, which is due to its complex multimolecular interactions, [10] and the poor solubility results in concentrations in the solution below the detection range of conventional methods. [11] The test methods that can detect the reactants in micromolar concentration include fluorescence spectra and SERS, but fluorescence spectra need to be labeled with fluorescent groups, [12] and SERS requires substrates. [13] Therefore, the application of the above methods in micromolar concentration chemical reaction tracking is still lacking in universality. The single-molecule conductance measurement technique is based on the repeated opening and closing of metal electrodes to form a large number of single-molecule junctions. The electrode forms a nanometer gap during the stretching process, thus achieving the capture of single molecules. [14][15][16] Break junction techniques offer the opportunity to detect the single-molecule electrical signals of the reactants at the singlemolecule scale. [17][18][19][20][21][22][23] Although mass spectrometry can be used to study reactions at low concentrations, this technique cannot reach single-molecule level. Moreover, the high energy of mass spectrometry will affect the weak interaction and reaction. While, this observation based on scanning tunneling microscope-break junction (STM-BJ) technique is lossless and low consumption. Therefore we choose STM-BJ technique to track the confined reaction based on host-guest interaction.The single-molecule conductance measurement of reaction can provide direct evidence for reaction kinetics [24,25] and reaction mechanisms from the molecular level. Recent advances suggested that the single-molecule techniques can track reactions such as Diels-Alder reaction, [26,27] ring cleavage and ring closure reaction, [28,29] cleavage of alkoxyamines, [30] isomerization reactions of cumulenes, [31] aniline coupling, [32] photoswitches of photochromic molecules, [33] and supramolecular interactions are also detected in molecular junctions includeThe host-guest interaction acts as an essential part of supramolecular chemistry, which can be applied in confined reaction. However, it is challenging to obtain the dynamic process during confined reactions below micromolar concentrations. In this work, a new method is provided to characterize the dimerization process of the guest 1,2-bis(4-pyridinyl) ethylene in host cucurbit[8]curil using scanning tunneling microscope-break junction (STM-BJ) technique. The guest reaction kinetics is quantitatively by nuclear magnetic resonance (NMR) and in situ single-molecule junctions. It is found that in the single-molecule conductance measurements, the electrical signals of the reactants with a concentration as low as 5 × 10 −6 m a...

No abstract

Electrodes play an essential role in controlling electrode−molecule coupling. However, conventional metal electrodes require linkers to anchor the molecule. Van der Waals interaction offers a versatile strategy to connect the electrode and molecule without anchor groups. Except for graphene, the potential of other materials as electrodes to fabricate van der Waals molecular junctions remains unexplored. Herein, we utilize semimetallic transition metal dichalcogenides (TMDCs) 1T′-WTe 2 as electrodes to fabricate WTe 2 /metalated tetraphenylporphyrin (M-TPP)/WTe 2 junctions via van der Waals interaction. Compared with chemically bonded Au/M-TPP/Au junctions, the conductance of these M-TPP van der Waals molecular junctions is enhanced by ∼736%. More importantly, WTe 2 /M-TPP/WTe 2 junctions exhibit the tunable conductance from 10 −3.29 to 10 −4.44 G 0 (1.15 orders of magnitude) via single-atom control, recording the widest tunable range of conductance for M-TPP molecular junctions. Our work demonstrates the potential of two-dimensional TMDCs for constructing highly tunable and conductive molecular devices.