The negative differential resistance mechanism of a molecular device based on double‐cage fluorinated fullerene C20F18(NH)2C20F18: A theoretical study

The electronic transport properties of the molecular device based on double‐cage fluorinated fullerene C₂₀F₁₈(NH)₂C₂₀F₁₈ were studied theoretically. The results show that the device exhibits two negative differential resistance (NDR) peaks in its I‐V curve. The NDR peak under low bias voltage originates from the bias‐induced alignment of the molecular orbitals, and the conduction channel being suppressed at a certain bias voltage is the main reason for the NDR peak under a relatively high bias voltage.     

Nanoscale characterization of redox and acid properties of keggin-type heteropolyacids by scanning tunneling microscopy and tunneling spectroscopy: effect of heteroatom substitution

Nanoscale characterization of acid and redox properties of Keggin-type heteropolyacids (HPAs) with different heteroatoms, H(n)MW(12)O(40) (M = P, Si, B, Co), was carried out by scanning tunneling microscopy (STM) and tunneling spectroscopy (TS) in this study. HPA samples were deposited on highly oriented pyrolytic graphite surfaces to obtain images and tunneling spectra by STM before and after pyridine adsorption. All HPA samples formed well-ordered 2-dimensional arrays on graphite before and after pyridine exposure. NDR (negative differential resistance) peaks were observed in the tunneling spectra. Those measured for fresh HPA samples appeared at less negative voltages with increasing reduction potential of the HPAs and with increases in the electronegativity of the heteroatom, but with decreases in the overall negative charge of the heteropolyanions. These results support the conclusion that more reducible HPA samples show NDR behavior at less negative applied voltages in their tunneling spectra. Introduction of pyridine into the HPA arrays increased the lattice constants of the 2-dimensional HPA arrays by ca. 6 A. Exposure to pyridine also shifted NDR peak voltages of H(n)MW(12)O(40) (M = P, Si, B, Co) samples to less negative values in the TS measurements. The NDR shifts of HPAs obtained before and after pyridine adsorption were correlated with the acid strengths of the HPAs, suggesting that tunneling spectra measured by STM could serve to probe acid properties of HPAs. These results show how one can relate the bulk acid and redox properties of HPAs to surface properties of nanostructured HPA monolayers determined by STM.     

Controlled growth of micropatterned, oriented calcite films on a self-assembled multilayer film

A micropatterned multilayer film, which was fabricated from layer-by-layer electrostatic self-assembly of nitrodiazoresin (NDR)/poly(acrylic acid) (PAA) followed by photolithography, was utilized as a structured template for the biomimetic mineralization of calcium carbonate. Micropatterned CaCO3 films consisting of regularly aligned calcite crystals oriented in the <104> direction were selectively deposited on the patterned NDR/PAA multilayer film.     

Macroscopically uniform nanoperiod alloy multilayers formed by coupling of electrodeposition with current oscillations

The electrodeposition from an acidic solution containing Cu(2+), Sn(2+), and a cationic surfactant gave a negative differential resistance (NDR) and a current oscillation in a narrow potential region of about 20 mV lying slightly more negative than the onset potential for Sn-Cu alloy deposition. Scanning Auger microscopic inspection has indicated that alloy films deposited during the oscillation have a clear alternate multilayer structure composed of two alloy layers of different compositions. The multilayer had the period of thickness of 40-90 nm and was uniform over a macroscopically wide area of about 1 mm x 1 mm. Detailed investigations have revealed that the NDR arises from adsorption of a cationic surfactant (acting as an inhibitor for diffusion of metal ions) on the alloy surface, and the oscillation comes from coupling of the NDR with the ohmic drop in the electrolyte.     

Iron-phthalocyanine molecular junction with high spin filter efficiency and negative differential resistance

We investigate the spin transport properties of iron-phthalocyanine (FePc) molecule sandwiched between two N-doped graphene nanoribbons (GNRs) based on the density functional theory and nonequilibrium Green's function methods. Our calculated results clearly reveal that the FePc molecular junction has high spin-filter efficiency as well as negative differential resistance (NDR). The zero-bias conductance through FePc molecule is dominated by the spin-down electrons, and the observed NDR originates from the bias-dependent effective coupling between the FePc molecular orbitals and the narrow density of states of electrodes. The remarkable high spin-filter efficiency and NDR are robust regardless of the edge shape and the width of GNRs, and the N-doping site in GNRs. These predictions indicate that FePc junction holds great promise in molecular electronics and spintronics applications.     

Switching in molecular transport junctions: polarization response

We discuss several proposed explanations for the switching and negative differential resistance (NDR) behavior seen in some molecular junctions. Several theoretical models are discussed, and we present results of electronic structure calculations on a series of substituted oligo(phenylene ethynylene) molecules. It is shown that a previously proposed polaron model is successful in predicting NDR behavior, and the model is elaborated with image charge effects and parameters from electronic structure calculations. This model now incorporates substituent effects and includes the effects of conformational change, charging, and image charge stabilization.     

Changes of coupling between the electrodes and the molecule under external bias bring negative differential resistance

We report a first-principles study of electrical transport and negative differential resistance (NDR) in a single molecular conductor consisting of a borazine ring sandwiched between two Au(100) electrodes with a finite cross section. The projected density of states (PDOS) and transmission coefficients under various external voltage biases are analyzed, and it suggests that the variation of the coupling between the molecule and the electrodes with external bias leads to NDR. Therefore, we propose that one origin of NDR in molecular devices is caused by the characteristics of both the molecule and the electrodes as well as their cooperation, not necessarily only by the inherent properties of certain species of molecules themselves. The changes of charge state of the molecule have minor effects on NDR in this device because the Mulliken population analysis shows that electron occupation variation on the molecule is very small when different external biases are applied.     

硅表面含二茂铁基团自组装单分子膜制备及电流-电压关系测量

合成了一种头基为二茂铁基团的长链烯烃分子(Fc-CO-NH-(CH₂)₉-CH=CH₂) (FcUA)，并用红外、核磁、质谱等手段对其进行了表征。用微波引发将该化合物嫁接到平面硅的表面，并用X射线光电子能谱、反射红外光谱、原子力显微镜表征了这一过程。最后，通过循环伏安电化学法和电敏感原子力显微镜的导电模式对其进行电学性质测试。结果表明这层单分子膜可以提高硅片的介电常数。同时还观察到了一种不稳定的类似负微分电流现象。     

Electronic Transport Through N24B24 Molecular Junction

We have investigated the electron transport properties of a N₂₄B₂₄ molecule coupled to two metallic contacts with a combination of GW approximation and the non-equilibrium Green's-function technique. The calculations indicate that the four and three resonant tunneling peaks are seen for the density of states (DOS) curves in the cases of single and multiple atomic contacts, respectively. The off state and negative di erential resistance (NDR) effect are observed in the I-V characteristics of the N₂₄B₂₄ molecule. The NDR behavior is also observed in voltages of about ∓4.5, ∓4, ∓4.6, and ∓4.3 V for one, four, six, and eight atomic contacts. Also, the I-V characteristics of N₂₄B₂₄ are in off state at low voltages that is independent of the contact types. The current curves against the gate voltage depend on contact types and indicate that N₂₄B₂₄ molecule behaves as a semiconductor.     

Interaction-induced negative differential resistance in asymmetric molecular junctions

Combining insights from quantum chemistry calculations with master equations, we discuss a mechanism for negative differential resistance (NDR) in molecular junctions, operated in the regime of weak tunnel coupling. The NDR originates from an interplay of orbital spatial asymmetry and strong electron-electron interaction, which causes the molecule to become trapped in a nonconducting state above a voltage threshold. We show how the desired asymmetry can be selectively introduced in individual orbitals in, e.g., oligo(phenyleneethynylene)-type molecules by functionalization with a suitable side group, which is in linear conjugation to one end of the molecule and cross-conjugated to the other end.     

Negative differential resistance and hysteresis through an organometallic molecule from molecular-level crossing

Analogous to a quantum double-dot system, diblock structured molecules could also show negative differential resistance (NDR). Using combined density functional theory and nonequilibrium Green function technique, we show that molecular-level crossing in a molecular double-dot system containing cobaltocene and ferrocene leads to NDR and hysteresis.     

Negative differential resistance in oxidized zigzag graphene nanoribbons

A theoretical study of zigzag graphene nanoribbons (ZGNRs) with an epoxy-pair chain (ZGO) is performed. The electronic transport properties are mainly evaluated by non-equilibrium Green's functions using the TRANSIESTA package. The results indicate that the graphene oxide can have a negative differential resistance (NDR) phenomenon, supported by bias-dependent transmission curves of different spin orientations. Applying non-zero bias voltages makes the density of states (DOS) of the right electrodes shift down. Due to an energy gap between the LUMO and LUMO+1 in ZGOs, with a certain bias, the conduction band of the right electrode cannot match the LUMO of the scattering region, then NDR occurs. With a larger bias, NDR ends when the second conduction band of the right electrode's DOS covers the LUMO of the scattering region. Since most of proposed ZGO systems possess such a gap between the LUMO and LUMO+1, NDR can be widely observed and this discovery may provide great potential applications in NDR-based nanoelectronics by using modified graphene materials.     

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.     

Observation of negative differential resistance in diketopyrrolopyrrole-based copolymer films

Remarkable and repeatable negative differential resistance (NDR) phenomenon was observed in a metal-polymer-metal structure diode based on bishexyloxy-divinyl-benzene-alt-diketopyrrolopyrrole (C6DPPPPV),a type of donor-acceptor (D-A) conjugated copolymer.Thickness dependence of the devices implied that the observed NDR characteristics were bulk-controlled.The device performance was considered to depend on the slow trapping and releasing processes related to the local deep states,which was enhanced by the gr...     

Configuration and photochemical reaction of a bolaamphiphilic diacid with a diazo resin in monolayers and Langmuir-Blodgett films

The monolayer formation of a bolaamphiphile, 1,18-octadecanedicarboxylic acid (ODA), on pure water and the subphase containing a positively charged photoactive 2-nitro-N-methyldiphenylamine-4-diazoniumformaldehyde resin (NDR) have been investigated by pi-A isotherms, pi-t curves, and Brewster angle microscopy (BAM) measurements. It has been revealed that although an unstable monolayer was formed by ODA alone, a stable complex monolayer between ODA and NDR could be formed at the interface through electrostatic adsorption and hydrogen bonding. It has been shown that the ODA formed a U-shaped monolayer at a lower pressure and was converted to a stretched configuration upon compression to a higher surface pressure on the subphase containing NDR. Under UV irradiation at the interface photoreaction can occur in the complex monolayer, which causes shrinkage of the monolayer. Photochemical reactions can also occur in deposited Langmuir-Blodgett films. In reactions occurring at the air/water interface, the two ends of ODA can react with NDR to form an ester containing aromatic rings. This makes the compound more hydrophobic and can easily be stretched without any phase transition upon compression. When the film with U-shaped configuration was deposited onto solid substrates, the configuration could be kept even upon photoirradiation.     

Negative differential resistance (NDR) in similar molecules with distinct redox behaviour

The transport characterization of self-assembled monolayers (SAMs) based on the closed and open-shell forms of a fully conjugated polychlorotrimethylphenyl (PTM) derivative hybridized with the gold substrate reveals that both systems exhibit negative differential resistance (NDR) in their I-V curves which was attributed to similar resonant tunnelling with unoccupied molecular orbitals. This work demonstrates that distinct transport mechanisms can dominate depending on the bias-voltage applied and shows that NDR processes are not influenced here by the redox character of the molecules.     

Attenuating negative differential resistance in an electroactive self-assembled monolayer-based junction

The negative differential resistance (NDR) peak current observed in redox active self-assembled monolayer-based molecular junctions has been attenuated by controlling the composition of the molecular junction. Two approaches studied here include capping the electroactive ferrocenyl groups with beta-cyclodextrin and functionalizing the scanning tunneling microscope tip used to probe the self-assembled monolayer (SAM) with n-alkanethiols of different lengths. These are the first examples of systematic modification of the magnitude of the NDR response in a molecule-based system.     

SCF-Xα-SW model calculations for metal clusters of nickel,copper, and silver and for the oxygen chemisorption on these metals

The oxygen chemisorption on Ni, Cu and Ag is studied by comparing PE spectra of these systems and SCF-Xα scattered-wave cluster models. Consideration of octahedral clusters M₆ (M = Ni, Cu, Ag) shows that they are large enough to reproduce trends in energy differences, such as the width of the d-bands and the distance from the top of the d-bands to the Fermi level, as found in experiment and in bulk energy band calculations. Substrate model clusters for the interaction of oxygen with different metal surfaces are derived from an octahedron by removing one ((100) face) or two adjacent metal atoms ((110) face). Comparing the UPS difference spectrum for O/Ag (110) with several Ag₄O cluster models makes it possible to interpret the peaks above the Ag d-band as O-Ag anti-bonding levels. These peaks are caused by O 2p-Ag 4d and O 2p-Ag 5s interaction. The corresponding bonding levels fall in the Ag d-bands and cannot therefore be identified with confidence in the spectra. The decreasing intensity of the oxygen derived peak below the metal d-band in the UPS spectra when going from Ni to Cu to Ag, and the simultaneously increasing O peaks above the d-band correlate with the changes of the localization of the corresponding bonding and anti-bonding levels in the oxygen sphere and the decreasing strength of the chemisorption bond.     

Au nanoparticle micropatterns prepared from self-assembled films

A kind of hybrid multilayer film based on mercaptobenzoic acid-capped Au nanoparticles (MBA-Au-NPs) and photoreactive nitrodiazoresin (NDR) has been fabricated via electrostatic self-assembly. Upon exposure to UV light, the initial ionic bonds between the layers of the film convert into covalent bonds and the film stability toward polar solvents, salt, or surfactant solutions increases significantly. The micropatterned NDR/MBA-Au-NP film with the covalently linked architecture was formed by selecting exposure of the film through a photomask and later developed in sodium dodecyl sulfate (SDS) aqueous solution. The metallic Au-NP micropatterns, furthermore, are produced by sintering the micropatterned NDR/MBA-Au-NP film at 550 degrees C, at which the organic components are removed completely. The well-defined micropatterns were characterized with atomic force microscopy (AFM), field emission scanning electron microscopy (FE-SEM), microscope with a charge-coupled device (CCD) camera, and X-ray photoelectron spectroscopy (XPS).