Fabrication of hybrid molecular devices using multi-layer graphene break junctions

Experimental and theoretical studies of the electronic transport of an extended curcuminoid in graphene nano-junctions

Exploiting the potential of curcuminoids (CCMoids) as molecular platforms, a new 3.53 nm extended system (pyACCMoid, 2) has been designed in two steps by reacting a CCMoid with amino-terminal groups (NH2-CCMoid, 1, of 1.79 nm length) with polycyclic aromatic hydrocarbon (PAH) aldehydes. CCMoid 2 contains pyrene units at both ends as anchoring groups to optimize its trapping in graphene nano-junctions created by feedback-controlled electro-burning. The measured I-V characteristics show gate-dependent behaviour at room temperature and 10 K, with increased conductance values compared to shorter CCMoids previously reported, and in agreement with DFT calculations. Our results show that the adjusted molecular design improves the conductance, as system 2 separates the conductive backbone from the anchor groups, which tend to adopt a planar configuration upon contact with the graphene electrodes. DFT calculations using Green functions of a set of different molecular conformations of 2 on graphene electrodes show a direct relationship between the units (e.g. pyrene, amide, etc.), in the molecule, through which electrons are injected and the conductance values; where the size of the spacing between the graphene electrodes contributes but is not the dominant factor, and thus, counter-intuitively the smallest spacing gives one of the lowest conductance values.

Graphene-molecule-graphene single-molecule junctions to detect electronic reactions at the molecular scale

The ability to measure the behavior of a single molecule during a reaction implies the detection of inherent dynamic and static disordered states, which may not be represented when measuring ensemble averages. Here, we describe the building of devices with graphene-molecule-graphene single-molecule junctions integrated into an electrical circuit. These devices are simple to build and are stable, showing tolerance to mechanical changes, solution environment and voltage stimulation. The design of a conductive channel based on a single molecule enables single-molecule detection and is sensitive to variations in physical properties and chemical structures of the detected molecules. The on-chip setup of single-molecule junctions further offers complementary metal-oxide-semiconductor (CMOS) compatibility, enabling logic functions in circuit elements, as well as deciphering of reaction intermediates. We detail the experimental procedure to prepare graphene transistor arrays as a basis for single-molecule junctions and the preparation of nanogapped carboxyl-terminal graphene electrodes by using electron-beam lithography and oxygen plasma etching. We describe the basic design of a molecular bridge with desired functions and terminals to form covalent bonds with electrode arrays, via a chemical reaction, to construct stably integrated single-molecule devices with a yield of 30-50% per chip. The immobilization of the single molecules is then characterized by using inelastic electron tunneling spectra, single-molecule imaging and fluorescent spectra. The whole protocol can be implemented within 2 weeks and requires users trained in using ultra-clean laboratory facilities and the aforementioned instrumentation.

Dielectric behavior of curcuminoid polymorphs on different substrates by direct soft vacuum deposition

Our work examines the structural-electronic correlation of a new curcuminoid, AlkCCMoid, as a dielectric material on different substrates. For this purpose, we show a homemade sublimation method that allows the direct deposition of molecules on any type of matrix. The electronic properties of AlkCCMoid have been evaluated by measurements on single crystals, microcrystalline powder, and sublimated samples, respectively. GIWAXS studies on surfaces and XRD studies on powder have revealed the existence of polymorphs and the effect that substrates have on curcuminoid organization. We describe the dielectric nature of our system and identify how different polymorphs can affect electronic parameters such as permittivity, all corroborated by DFT calculations.

Molecular Approach to Engineer Two-Dimensional Devices for CMOS and beyond-CMOS Applications

Two-dimensional materials (2DMs) have attracted tremendous research interest over the last two decades. Their unique optical, electronic, thermal, and mechanical properties make 2DMs key building blocks for the fabrication of novel complementary metal-oxide-semiconductor (CMOS) and beyond-CMOS devices. Major advances in device functionality and performance have been made by the covalent or noncovalent functionalization of 2DMs with molecules: while the molecular coating of metal electrodes and dielectrics allows for more efficient charge injection and transport through the 2DMs, the combination of dynamic molecular systems, capable to respond to external stimuli, with 2DMs makes it possible to generate hybrid systems possessing new properties by realizing stimuli-responsive functional devices and thereby enabling functional diversification in More-than-Moore technologies. In this review, we first introduce emerging 2DMs, various classes of (macro)molecules, and molecular switches and discuss their relevant properties. We then turn to 2DM/molecule hybrid systems and the various physical and chemical strategies used to synthesize them. Next, we discuss the use of molecules and assemblies thereof to boost the performance of 2D transistors for CMOS applications and to impart diverse functionalities in beyond-CMOS devices. Finally, we present the challenges, opportunities, and long-term perspectives in this technologically promising field.

Photoswitching Molecular Junctions: Platforms and Electrical Properties

Remarkable advances in technology have enabled the manipulation of individual molecules and the creation of molecular electronic devices utilizing single and ensemble molecules. Maturing the field of molecular electronics has led to the development of functional molecular devices, especially photoswitching or photochromic molecular junctions, which switch electronic properties under external light irradiation. This review introduces and summarizes the platforms for investigating the charge transport in single and ensemble photoswitching molecular junctions as well as the electronic properties of diverse photoswitching molecules such as diarylethene, azobenzene, dihydropyrene, and spiropyran. Furthermore, the article discusses the remaining challenges and the direction for moving forward in this area for future photoswitching molecular devices.

Charge injection and transport properties of large area organic junctions based on aryl thin films covalently attached to a multilayer graphene electrode

The quantum interaction between molecules and electrode materials at molecule/electrode interfaces is a major ingredient in the electron transport properties of organic junctions. Driven by the coupling strength between the two materials, it results mainly in the broadening and energy shift of the interacting molecular orbitals. Using new electrode materials, such as the recently developed semi-conducting two-dimensional nanomaterials, has become a significant advancement in the field of molecular/organic electronics that opens new possibilities for controlling the interfacial electronic properties and thus the charge injection properties. In this article, we report the use of atomically thin two-dimensional multilayer graphene films as the base electrode in organic junctions with a vertical architecture. The interfacial electronic structure dominated by the covalent bonding between bis-thienyl benzene diazonium-based molecules and the multilayer graphene electrode has been probed by ultraviolet photoelectron spectroscopy and the results are compared with those obtained on junctions with standard Au electrodes. Room temperature injection properties of such interfaces have also been explored by electron transport measurements. We find that, despite strong variations of the density of states, the Fermi energy and the injection barriers, both organic junctions with Au base electrodes and multilayer graphene base electrodes show similar electronic responses. We explain this observation by the strong orbital coupling occurring at the bottom electrode/bis-thienyl benzene molecule interface and by the pinning of the hybridized molecular orbitals.

Efficient Fabrication of Stable Graphene-Molecule-Graphene Single-Molecule Junctions at Room Temperature

We present a robust approach to fabricate stable single-molecule junctions at room temperature using single-layer graphene as nanoelectrodes. Molecular scale nano-gaps in graphene were generated using an optimized fast-speed feedback-controlled electroburning process. This process shortened the time for creating a single nano-gap to be less than one minute while keeping a yield higher than 97 %. To precisely control the gap position and minimize the effects of edge defects and the quantum confinement, extra-narrow grooves were pre-patterned in the graphene structures with oxygen plasma etching. Molecular junctions were formed by bridging the nano-gaps with amino-functionalized hexaphenyl molecules by taking advantage of chemical reactions between the amino groups at the two ends of the molecules and the carboxyl groups at the edges of graphene electrodes. Electronic transport measurements and transition voltage spectroscopy analysis verified the formation of single-molecule devices. First-principles quantum transport calculations show that the highest occupied molecular orbital of hexaphenyl is closer to the Fermi level of the graphene electrodes and thus the devices exhibit a hole-type transport characteristics. Some of these molecular devices remained stable up to four weeks, highlighting the potential of graphene nano-electrodes in the fabrication of stable single-molecule devices at room temperature.

Single Electron Transistor with Single Aromatic Ring Molecule Covalently Connected to Graphene Nanogaps

We report a robust approach to fabricate single-molecule transistors with covalent electrode-molecule-electrode chemical bonds, ultrashort (∼1 nm) molecular channels, and high coupling yield. We obtain nanometer-scale gaps from feedback-controlled electroburning of graphene constrictions and bridge these gaps with molecules using reaction chemistry on the oxidized graphene edges. Using these nanogaps, we are able to optimize the coupling chemistry to achieve high reconnection yield with ultrashort covalent single-molecule bridges. The length of the molecule is found to influence the fraction of covalently reconnected nanogaps. Finally, we discuss the tunneling nature of the covalent contacts using gate-dependent transport measurements, where we observe single electron transport via large energy Coulomb blockade even at room temperature. This study charts a clear path toward the assembling of ultraminiaturized electronics, sensors, and switches.

Sequential Electron Transport and Vibrational Excitations in an Organic Molecule Coupled to Few-Layer Graphene Electrodes

Graphene electrodes are promising candidates to improve reproducibility and stability in molecular electronics through new electrode-molecule anchoring strategies. Here we report sequential electron transport in few-layer graphene transistors containing individual curcuminoid-based molecules anchored to the electrodes via π-π orbital bonding. We show the coexistence of inelastic co-tunneling excitations with single-electron transport physics due to an intermediate molecule-electrode coupling; we argue that an intermediate electron-phonon coupling is the origin of these vibrational-assisted excitations. These experimental observations are complemented with density functional theory calculations to model electron transport and the interaction between electrons and vibrational modes of the curcuminoid molecule. We find that the calculated vibrational modes of the molecule are in agreement with the experimentally observed excitations.

Graphene nanodevices for DNA sequencing

Fast, cheap, and reliable DNA sequencing could be one of the most disruptive innovations of this decade, as it will pave the way for personalized medicine. In pursuit of such technology, a variety of nanotechnology-based approaches have been explored and established, including sequencing with nanopores. Owing to its unique structure and properties, graphene provides interesting opportunities for the development of a new sequencing technology. In recent years, a wide range of creative ideas for graphene sequencers have been theoretically proposed and the first experimental demonstrations have begun to appear. Here, we review the different approaches to using graphene nanodevices for DNA sequencing, which involve DNA passing through graphene nanopores, nanogaps, and nanoribbons, and the physisorption of DNA on graphene nanostructures. We discuss the advantages and problems of each of these key techniques, and provide a perspective on the use of graphene in future DNA sequencing technology.