Review pubs.acs.org/CR Molecular-Scale Electronics: From Concept to Function † ‡ ⊥ † ⊥ † § † ∥ Dong Xiang, , , Xiaolong Wang, , Chuancheng Jia, Takhee Lee,*, and Xuefeng Guo*, , † Beijing National Laboratory for Molecular Sciences, State Key Laboratory for Structural Chemistry of Unstable and Stable Species, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China ‡ Key Laboratory of Optical Information Science and Technology, Institute of Modern Optics, College of Electronic Information and Optical Engineering, Nankai University, Tianjin 300071, China § Department of Physics and Astronomy, and Institute of Applied Physics, Seoul National University, Seoul 08826, Korea ∥ Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, China ABSTRACT: Creating functional electrical circuits using individual or ensemble molecules, often termed as “molecular-scale electronics”, not only meets the increasing technical demands of the miniaturization of traditional Si-based electronic devices, but also provides an ideal window of exploring the intrinsic properties of materials at the molecular level. This Review covers the major advances with the most general applicability and emphasizes new insights into the development of efficient platform methodologies for building reliable molecular electronic devices with desired functionalities through the combination of programmed bottom-up self-assembly and sophisticated top-down device fabrication. First, we summarize a number of different approaches of forming molecular-scale junctions and discuss various experimental techniques for examining these nanoscale circuits in details. We then give a full introduction of characterization techniques and theoretical simulations for molecular electronics. Third, we highlight the major contributions and new concepts of integrating molecular functionalities into electrical circuits. Finally, we provide a critical discussion of limitations and main challenges that still exist for the development of molecular electronics. These analyses should be valuable for deeply understanding charge transport through molecular junctions, the device fabrication process, and the roadmap for future practical molecular electronics. CONTENTS 3.2.2. Dash-Line Lithography 4352 3.3. Other Carbon-Based Electrodes 4352 1. Introduction 4319 4. Other Electrodes for Molecular Electronics 4354 2. Metal Electrodes for Molecular Electronics 4321 4.1. Silicon-Based Electrodes 4354 2.1. Single-Molecule Junctions 4321 4.2. Polymer-Based Electrodes 4356 2.1.1. Scanning Probe Microscopy Break Junc- 5. Characterization Techniques for Molecular Elec- tion 4321 tronics 4356 2.1.2. Mechanically Controllable Break Junc- 5.1. Inelastic Electron Tunneling Spectroscopy 4356 tion 4326 5.1.1. History and Background 4357 2.1.3. Electromigration Breakdown Junction 4330 5.1.2. IETS Measurement 4357 2.1.4. Electrochemical Deposition Junction 4333 ff 5.1.3. IETS Applications 4359 2.1.5. Surface-Di usion-Mediated Deposition 5.2. Temperature-Length-Variable Transport Junction 4335 Measurement 4360 2.2. Ensemble Molecular Junctions 4335 5.3. Noise Spectroscopy 4362 2.2.1. Lift-and-Float Approach 4336 5.3.1. Thermal Noise and Shot Noise 4363 2.2.2. Liquid Metal Contact 4337 5.3.2. Generation-Recombination and Flicker 2.2.3. Nanopore and Nanowell 4338 Noise 4363 2.2.4. On-Wire Lithography 4339 5.3.3. Noise Spectroscopy Measurements 4364 2.2.5. Transfer Printing Techniques 4340 5.3.4. Application of Noise Spectroscopy 4364 2.2.6. Self-Aligned Lithography 4343 ff 5.4. Transition Voltage Spectroscopy 4367 2.2.7. Bu er Interlayer-Based Junction 4343 5.4.1. Applications of TVS 4368 2.2.8. On-Edge Molecular Junction 4345 5.5. Thermoelectricity 4370 3. Carbon Electrodes for Molecular Electronics 4346 5.6. Optical and Optoelectronic Spectroscopy 4370 3.1. Carbon Nanotube-Based Electrodes 4346 5.6.1. Raman Spectroscopy 4370 3.1.1. Electrical Breakdown 4346 3.1.2. Lithography-Defined Oxidative Cutting 4349 3.2. Graphene-Based Electrodes 4351 3.2.1. Electroburning 4351 Received: November 20, 2015 Published: March 16, 2016 © 2016 American Chemical Society 4318 DOI: 10.1021/acs.chemrev.5b00680 Chem. Rev. 2016, 116, 4318−4440 Chemical Reviews Review 5.6.2. Ultraviolet−Visible Spectroscopy 4372 Acknowledgments 4420 5.6.3. X-ray Photoelectron Spectroscopy 4372 References 4420 5.6.4. Ultraviolet Photoelectron Spectroscopy 4372 6. Theoretical Aspect for Electron Transport through Molecular Junctions 4373 1. INTRODUCTION 6.1. Theoretical Descriptions of the Tunneling What does the future hold for electronic devices? To what Process 4373 extent can their dimensions be reduced in the future? Forty 6.2. Electron Transport Mechanism 4374 years ago, the gate length of a transistor was approximately 10 6.2.1. Coherent Electron Transport through μm; however, over the past few decades, traditional transistors Molecular Junctions 4374 have shrunk dramatically and now reach dimensions of less − ff 6.2.2. Electron Phonon Interaction E ects on than 10 nm in research devices.1,2 The further miniaturization Transport Mechanism 4375 of electronic devices remains extremely challenging, which is 6.3. First-Principles Modeling 4375 primarily due to either technique limitations or lack of 6.3.1. Introduction to Density Functional fundamental understanding of transport mechanisms.3 In this Theory 4375 sense, it is remarkable that chemically identical molecules, with − 6.3.2. Current Voltage Characteristics Calcu- sizes on the order of 1 nm, can be synthesized in bulk while lations 4376 accomplishing a variety of electronic tasks, including conduct- 7. Integrating Molecular Functionalities into Elec- ing wire, rectification, memory, and switching; thus, they have trical Circuits 4378 the potential to partly replace traditional solid-state device 7.1. Wiring toward Nanocircuits 4379 counterparts in the future. Comprehensive experimental 7.1.1. Backbones as Charge Transport Path- findings in electron transport through individual molecules ways 4379 introduce the idea that beyond traditional complementary 7.1.2. Conductance of Single Molecules 4388 metal oxide semiconductor (CMOS) technology, the ultimate fi 7.2. Recti cation toward Diodes 4393 goal for shrinking electrical circuits is the realization of 7.2.1. General Mechanisms for Molecular Rec- molecular-scale/single-molecule electronics because single fi ti cation 4393 molecules constitute the smallest stable structures imagina- fi − 7.2.2. Recti cation Stemming from Molecules 4395 ble.4 7 Molecular-scale electronics, which is the concept of fi ff 7.2.3. Recti cation Stemming from Di erent creating functional electrical circuits on the basis of properties Interfacial Coupling 4397 inherent in individual or ensemble molecules, have several fi 7.2.4. Additional Molecular Recti ers 4400 unparalleled advantages as compared to silicon-based electronic 7.3. Modulation toward Molecular Transistors 4400 devices. First, the extremely reduced size of the molecules in 7.3.1. Back Gating for Novel Physical Phenom- order of 1 nm may enable heightened capacities and faster ena Investigation 4400 performance. Moreover, such small size of the molecule 7.3.2. Side Gating for Electron Transport provides the ability to surpass the limit of conventional silicon Control 4403 circuit integration. Second, the abundant diversity in the ffi 7.3.3. Electrochemical Gating for E cient molecular structures, which can be changed via flexible Gate Coupling 4403 chemical designs, may lead to a direct observation of novel 7.4. Switching toward Memory Devices 4404 effects as well as the fundamental discovery of physical 7.4.1. Conformation-Induced Switch 4405 phenomena that are not accessible using traditional materials 7.4.2. Electrochemically Triggered Switch 4407 or approaches. Third, another attractive feature of this approach 7.4.3. Spintronics-Based Switch 4408 is the universal availability of molecules due to the ease of bulk 7.5. Transduction toward Molecular Sensors 4410 synthesis, thus potentially leading to low-cost manufacturing. 7.5.1. Sensing Based on Chemical Reactions 4411 In fact, molecular-scale electronics is currently a research area 7.5.2. Sensing Based on Biological Interactions 4412 of focus because it not only meets the increasing technical 7.5.3. Sensing Based on Thermoelectrical demands of the miniaturization of traditional silicon-based Conversion 4414 electronic devices but also provides an ideal window of 8. Summary and Outlook 4415 exploring the intrinsic properties of materials at the molecular 8.1. Main Challeges 4416 level. Generally, molecular-scale electronics refers to the use of 8.1.1. In Situ Measurement 4416 single molecules or nanoscale collections of single molecules as − 8.1.2. Device Yield 4416 electronic components.2,8 10 The primary theme in this field is 8.1.3. Device-to-Device Variation and Instabil- the construction, measurement, and understanding of the ity 4416 current−voltage responses of electrical circuits, in which 8.1.4. Integration 4417 molecular systems play an important role as pivotal elements.11 8.1.5. Energy Consumption 4417 Indeed, over the past decade, we observed significant 8.1.6. Addressability 4417 developments achieved in both experiments and theory to 8.1.7. General Strategies To Meet Challenges 4417 reveal the electronic and photonic responses of these − 8.2. Open Questions 4418 conceptually simple molecular junctions.5 12 8.3. Outlook 4419 The history of molecular-scale electronics is surging