A theoretical view of unimolecular rectification

Challenges for Incorporating Optical Switchability in Organic-Based Electronic Devices

Transistors operate by controlling the current flowing from a source to a drain electrode via a third electrode (gate), thus giving access to a binary treatment (ON/OFF or 0/1) of the signal currently exploited in microelectronics. Introducing a second independent lever to modulate the current would allow for more complex logic functions amenable to a single electronic component and hence to new opportunities for advanced electrical signal processing. One avenue is to add this second dimension with light by incorporating photochromic molecules in current organic-based electronic devices. In this Spotlight, we describe different concepts that have been implemented in organic thin films and in molecular junctions as well as some pitfalls that have been highlighted thanks to theoretical modeling.

Rectification and negative differential resistance via orbital level pinning

A donor-acceptor system, 4-thiophenyl-azafulleroid (4TPA-C60), is investigated at the point of HOMO/LUMO resonance and beyond to understand how negative differential resistance (NDR) features may be observed in such systems. Our previous investigation showed that charge transfer between the occupied and unoccupied states at resonance hindered crossing of the HOMO and LUMO levels, thus preventing the formation of an NDR feature. In this work, it is shown that the negative differential resistance feature of 4TPA-C60 can be tailored based on the couplings at the metal/molecule interface. Ab initio calculations show that limited charge extraction from atomically sharp contacts results in a HOMO-LUMO pinning effect which delays the onset of the NDR feature. Subsequent unpinning of the states can only occur when additional charge extraction channels enter the bias window, highlighting an important role which non-frontier states play in charge transport. The proposed charge transfer mechanism is then exploited by introducing a fluorine atom into the C60 cage to tune the energies of the acceptor, and narrow the width of the current peak. These findings not only demonstrate the importance of the metal/molecule interface in the design of molecular electronic architectures but also serve to inform future design of molecular diodes and RTDs.

Programmable Electrochemical Rectifier Based on a Thin-Layer Cell

A programmable electrochemical rectifier based on thin-layer electrochemistry is described here. Both the rectification ratio and the response time of the device are programmable by controlling the gap distance of the thin-layer electrochemical cell, which is easily controlled using commercially available beads. One of the electrodes was modified using a ferrocene-terminated self-assembled monolayer to offer unidirectional charge transfers via soluble redox species. The thin-layer configuration provided enhanced mass transport, which was determined by the gap thickness. The device with the smallest gap thickness (∼4 μm) showed an unprecedented, high rectification ratio (up to 160) with a fast response time in a two-terminal configuration using conventional electronics.

Modulation and Control of Charge Transport Through Single-Molecule Junctions

The ability to modulate and control charge transport though single-molecule junction devices is crucial to achieving the ultimate goal of molecular electronics: constructing real-world-applicable electronic components from single molecules. This review aims to highlight the progress made in single-molecule electronics, emphasizing the development of molecular junction electronics in recent years. Among many techniques that attempt to wire a molecule to metallic electrodes, the single-molecule break junction (SMBJ) technique is one of the most reliable and tunable experimental platforms for achieving metal-molecule-metal configurations. It also provides great freedom to tune charge transport through the junction. Soon after the SMBJ technique was introduced, it was extensively used to measure the conductances of individual molecules; however, different conductances were obtained for the same molecule, and it proved difficult to interpret this wide distribution of experimental data. This phenomenon was later found to be mainly due to a lack of precise experimental control and advanced data analysis methods. In recent years, researchers have directed considerable effort into advancing the SMBJ technique by gaining a deeper physical understanding of charge transport through single molecules and thus enhancing its potential applicability in functional molecular-scale electronic devices, such as molecular diodes and molecular transistors. In parallel with that research, novel data analysis methods and approaches that enable the discovery of hidden yet important features in the data are being developed. This review discusses various aspects of molecular junction electronics, from the initial goal of molecular electronics, the development of experimental techniques for creating single-molecule junctions and determining single-molecule conductance, to the characterization of functional current-voltage features and the investigation of physical properties other than charge transport. In addition, the development of advanced data analysis methods is considered, as they are critical to gaining detailed physical insight into the underlying transport mechanisms.

Dual conductance, negative differential resistance, and rectifying behavior in a molecular device modulated by side groups

We investigate the electronic transport properties for a molecular device model constructed by a phenylene ethynylene oligomer molecular with different side groups embedding in a carbon chain between two graphene electrodes. Using the first-principles method, the unusual dual conductance, negative differential resistance (NDR) behavior with large peak to valley ratio, and obvious rectifying performance are numerically observed in such proposed molecular device. The analysis of the molecular projected self-consistent Hamiltonian and the evolution of the frontier molecular orbitals (MOs) as well as transmission coefficients under various external voltage biases gives an inside view of the observed results, which suggests that the dual conductance behavior and rectifying performance are due to the asymmetry distribution of the frontier MOs as well as the corresponding coupling between the molecule and electrodes. But the NDR behavior comes from the conduction orbital being suppressed at certain bias. Interestingly, the conduction properties can be tuned by introducing side groups to the molecule and the rectification as well as the NDR behavior (peak to valley ratio) can be improved by adding different side groups in the device model.

Rectification in substituted atomic wires: a theoretical insight

Recently, there have been discussions that the giant diode property found experimentally in diblock molecular junctions could be enhanced by the many-body electron correlation effect beyond the mean field theory. In addition, the effect of electron-phonon scattering on an electric current through the diode molecule, measured by inelastic tunneling spectroscopy (IETS), was found to be symmetric with respect to the voltage sign change even though the current is asymmetric. The reason for this behavior is a matter of speculation. In order to clarify whether or not this feature is limited to organic molecules in the off-resonant tunneling region, we discuss the current asymmetry effect on IETS in the resonant region. We introduced heterogeneous atoms into an atomic wire and found that IETS becomes asymmetric in this substituted atomic wire case. Our conclusion gives the other example of intrinsic differences between organic molecules and metallic wires. While the contribution of electron-phonon scattering to IETS is not affected by the current asymmetry in the former case, it is affected in the latter case. The importance of the contribution of the electron-hole excitation to phonon damping in bringing about the current asymmetry effect in IETS in the latter case is discussed.

Electronic transport through zigzag/armchair graphene nanoribbon heterojunctions

The electronic transport properties of a graphene nanoribbon (GNR) are known to be sensitive to its width, edges and defects. We investigate the electronic transport properties of a graphene nanoribbon heterojunction constructed by fusing a zigzag and an armchair graphene nanoribbon (zGNR/aGNR) side by side. First principles results reveal that the heterojunction can be either metallic or semiconducting, depending on the width of the nanoribbons. Intrinsic rectification behaviors have been observed, which are largely sensitive to the connection length between the zGNR and aGNR. The microscopic origins of the rectification behavior have been revealed. We find that the carrier type can alter from electrons to holes with the bias voltage changing from negative to positive; the asymmetrical transmission spectra of electrons and holes induced by the interface defects directly results in the rectification behavior. The results suggest that any methods which can enhance the asymmetry of the transmission spectra between holes and electrons could be used to improve the rectification behavior in the zGNR/aGNR heterojunction. Our findings could be useful for designing graphene based electronic devices.