Switching and rectification of a single light-sensitive diarylethene molecule sandwiched between graphene nanoribbons

Regio- and Steric Effects on Single Molecule Conductance of Phenanthrenes

Here, six phenanthrene (the smallest arm-chair graphene nanoribbon) derivatives with dithiomethyl substitutions at different positions as the anchoring groups were synthesized. Scanning tunneling microscopy break junction technique was used to measure their single molecule conductances between gold electrodes, which showed a difference as much as 20-fold in the range of ∼10^-2.82 G₀ to ∼10^-4.09 G₀ following the trend of G_2,7 > G_3,6 > G_2,6 > G_1,7 > G_1,6 > G_1,8. DFT calculations agree well with this measured trend and indicate that the single molecule conductances are a combination of energy alignment, electronic coupling, and quantum effects. This significant regio- and steric effect on the single molecule conductance of phenanthrene model molecules shows the complexity in the practice of graphene nanoribbons as building blocks for future carbon-based electronics in one hand but also provides good conductance tunability on the other hand.

Quantitative analysis of weak current rectification in molecular tunnel junctions subject to mechanical deformation reveals two different rectification mechanisms for oligophenylene thiols versus alkane thiols

Metal-molecule-metal junctions based on alkane thiol (CnT) and oligophenylene thiol (OPTn) self-assembled monolayers (SAMs) and Au electrodes are expected to exhibit similar electrical asymmetry, as both junctions have one chemisorbed Au-S contact and one physisorbed, van der Waals contact. Asymmetry is quantified by the current rectification ratio RR apparent in the current-voltage (I-V) characteristics. Here we show that RR < 1 for CnT and RR > 1 for OPTn junctions, in contrast to expectation, and further, that RR behaves very differently for CnT and OPTn junctions under mechanical extension using the conducting probe atomic force microscopy (CP-AFM) testbed. The analysis presented in this paper, which leverages results from the previously validated single level model and ab initio quantum chemical calculations, allows us to explain the puzzling experimental findings for CnT and OPTn in terms of different current rectification mechanisms. Specifically, in CnT-based junctions the Stark effect creates the HOMO level shifting necessary for rectification, while for OPTn junctions the level shift arises from position-dependent coupling of the HOMO wavefunction with the junction electrostatic potential profile. On the basis of these mechanisms, our quantum chemical calculations allow quantitative description of the impact of mechanical deformation on the measured current rectification. Additionally, our analysis, matched to experiment, facilitates direct estimation of the impact of intramolecular electrostatic screening on the junction potential profile. Overall, our examination of current rectification in benchmark molecular tunnel junctions illuminates key physical mechanisms at play in single step tunneling through molecules, and demonstrates the quantitative agreement that can be obtained between experiment and theory in these systems.

Prominent nonequilibrium effects beyond the standard first-principles approach in nanoscale electronic devices

The standard density functional theory (DFT) based first-principles approach has been widely used for modeling nanoscale electronic devices. A recent experiment, however, reported surprising transport properties of thiol-terminated silane junctions that cannot be understood using the standard DFT approach, presenting a severe challenge for the current computational understanding of electron transport at the nanoscale. Using the recently proposed steady-state DFT (SS-DFT) for nonequilibrium quantum systems, we found that in silane junctions, underlying the puzzling experimental observations is a novel type of intriguing nonequilibrium effect that is beyond the framework of the standard DFT approach. Our calculations show that the standard DFT approach is a good approximation of SS-DFT when silane junctions are near equilibrium, but the aforementioned nonequilibrium effects could drive the thiol-terminated silanes far away from equilibrium even at low biases of around 0.2 V. Further analysis suggests that these nonequilibrium effects could generally exist in nanoscale devices in which there are conducting channels mainly residing at the source contact and close to the bias window. These findings significantly broaden our fundamental understanding of electron transport at the nanoscale.

Rectifying Performance of Heterojunction Based on α-Borophene Nanoribbons with Edge Passivation

We propose a planar model heterojunction based on α-borophene nanoribbons and study its electronic transport properties. We respectively consider three types of heterojunctions. Each type consists of two zigzag-edge α-borophene nanoribbons (Z αBNR), one is metallic with unpassivated or passivated edges by a hydrogen atom (1H-Z αBNR) and the other is semiconducting with the edge passivated by two hydrogen atoms (2H-Z αBNR) or a single nitrogen atom (N-Z αBNR). Using the first-principles calculations combined with the nonequilibrium Green's function, we observe that the rectifying performance depends strongly on the atomic structural details of a junction. Specifically, the rectification ratio of the junction is almost unchanged when its left metallic ribbon changes from ZBNR to 1H-Z αBNR. However, its ratio increases from 120 to 240 when the right semiconducting one varies from 2H-Z αBNR to N-Z αBNR. This rectification effect can be explained microscopically by the matching degree the electronic bands between two parts of a junction. Our findings imply that the borophene-based heterojunctions may have potential applications in rectification nano-devices.

First principle study of the self-switching characteristics of the guanine based single optical molecular switch using carbon nanotube electrodes

The switching property of an optical single molecular switch based on a single DNA molecule guanine with a single walled carbon nanotube electrode has been investigated using density functional theory along with non-equilibrium Green's function based first principle approach. The semi-empirical model of this single bio-molecular switch has been operated at an ultra-high 25 THz frequency in mid-UV range. This single bio-molecule comprises switching activity upon UV photo-excitation. The influence of the highest occupied molecular orbital and lowest unoccupied molecular orbital gap and the quantum ballistic transmission into the switching activity are discussed in detail in this study. It has been observed that the maximum ON-OFF ratio, i.e. 327 is obtained at +0.8 V bias voltage. Theoretical results show that current through the twisted form is sufficiently larger than the straightened form, which recommends that this structure has smart prospective application in the future generation switching nanotechnology.

Two-dimensional Cu2Si sheet: a promising electrode material for nanoscale electronics

Building electronic devices on top of two-dimensional (2D) materials has recently become one of most interesting topics in nanoelectronics. Finding high-performance 2D electrode materials is one central issue in 2D nanoelectronics. In the current study, based on first-principles calculations, we compare the electronic and transport properties of two nanoscale devices. One device consists of two single-atom-thick planar Cu₂Si electrodes, and a nickel phthalocyanine (NiPc) molecule in the middle. The other device is made of often-used graphene electrodes and a NiPc molecule. Planer Cu₂Si is a new type of 2D material that was recently predicted to exist and be stable under room temperature [11]. We found that at low bias voltages, the electric current through the Cu₂Si-NiPc-Cu₂Si junction is about three orders higher than that through graphene-NiPc-graphene. Detailed analysis shows that the surprisingly high conductivity of Cu₂Si-NiPc-Cu₂Si originates from the mixing of the Cu₂Si state near Fermi energy and the highest occupied molecular orbital of NiPc. These results suggest that 2D Cu₂Si may be an excellent candidate for electrode materials for future nanoscale devices.

Charge transport in germanium doped phosphorene nanoribbons

New two dimensional structures containing phosphorus and germanium atoms are introduced for nanoelectronic applications. Under various bias voltages, electronic transport in the systems has been studied with the non-equilibrium Green’s function formalism. I–V characteristics have been extracted. The density of states (DOS) and transmission spectra, T(E,V_bias), have been investigated and it was shown that charge transport occurs when the bias voltage reaches about 1 V. The negative differential resistance (NDR) appears in zigzag phosphorene nanoribbons (zPNRs) while it is completely suppressed after replacing edge phosphorus atoms with germanium ones. The calculated molecular projected self-consistent Hamiltonian (MPSH) shows that the spatial distribution of orbital levels has been affected by the electrodes. The studied structures have a band-gap of about 0.7 eV which absorbs light in the visible range and thus these structures could be interesting contenders for solar cells applications.

New two dimensional structures containing phosphorus and germanium atoms are introduced for nanoelectronic applications.

Spin-dependent electron transport through a Mn-phthalocyanine molecule — A steady-state density functional theory (SS-DFT) study

We generalize the recently proposed steady-state density functional theory (SS-DFT) to spin-dependent cases and theoretically investigate the electronic and transport properties of a Mn-phthalocyanine molecule sandwiched between two graphene nanoribbon leads. The junction filters spin-up (minority spin) electrons while allowing spin-down (majority spin) electrons to pass with a filtering efficiency of about 99.5% at low biases. The spin-down electrons are found to tunnel through the junction via the HOMO orbital of the Mn-phthalocyanine molecule. Detailed analysis of the spin-dependent electron tunneling mechanism as well as the electronic/magnetic properties of the junction is presented.

High performance current and spin diode of atomic carbon chain between transversely symmetric ribbon electrodes

We demonstrate that giant current and high spin rectification ratios can be achieved in atomic carbon chain devices connected between two symmetric ferromagnetic zigzag-graphene-nanoribbon electrodes. The spin dependent transport simulation is carried out by density functional theory combined with the non-equilibrium Green's function method. It is found that the transverse symmetries of the electronic wave functions in the nanoribbons and the carbon chain are critical to the spin transport modes. In the parallel magnetization configuration of two electrodes, pure spin current is observed in both linear and nonlinear regions. However, in the antiparallel configuration, the spin-up (down) current is prohibited under the positive (negative) voltage bias, which results in a spin rectification ratio of order 10(4). When edge carbon atoms are substituted with boron atoms to suppress the edge magnetization in one of the electrodes, we obtain a diode with current rectification ratio over 10(6).

Stable two-dimensional conductance switch of polyaniline molecule connecting to graphene nanoribbons

Incorporating the characteristics of the single-layer graphene nanoribbon and the polyaniline molecule, we theoretically design a two-dimensional molecular device and investigate its transport properties by applying nonequilibrium Green's functions in combination with density-functional theory. The calculated results reveal that the arrangements of frontier molecular orbitals and the energy gap between the HOMO and the LUMO of an isolated polyaniline molecule are different between its two isolable states: full reduced leucoemeraldine base and full oxidized pernigraniline base. When a polyaniline molecule connects to two graphene nanoribbons as a two-dimensional molecular device, the conductance of its full oxidized pernigraniline base is much higher than the conductance of its full reduced leucoemeraldine base. The switch ratios of two bases' currents almost maintain a constant value before 0.8 V. In other word, the conductance switch behavior in our device is stable in a big bias region which makes it have a broader application in future logic and memory devices.

Surfactant-modified chemically reduced graphene oxide for electrochemical supercapacitors

An easy method to synthesize surfactant-modified graphene for a supercapacitor is demonstrated through the intercalation of graphene oxide (GO) with a triblock copolymer Pluronic F127 (F127).

Constructing metallic nanoroads on a MoS₂ monolayer via hydrogenation

Monolayer transition metal dichalcogenides recently emerged as a new family of two-dimensional materials potentially suitable for numerous applications in electronic and optoelectronic devices due to the presence of a finite band gap. Many proposed applications require efficient transport of charge carriers within these semiconducting monolayers. However, constructing a stable conducting nanoroad on these atomically thin semiconductors is still a challenge. Here we demonstrate that hydrogenation on the surface of a MoS₂ monolayer induces a semiconductor-metal transition, and strip-patterned hydrogenation is able to generate a conducting nanoroad. The band-gap closing arises from the formation of in-gap hybridized states mainly consisting of Mo 4d orbitals, as well as the electron donation from hydrogen to the lattice host. Ballistic conductance calculations reveal that such a nanoroad on the MoS₂ surface exhibits an integer conductance, indicating small carrier scattering, and thus is ideal for serving as a conducting channel or an interconnect without compromising the mechanical and structural integrity of the monolayer.