Single Junctions Research Articles

Surface enhanced Raman scattering of single 1,4-Benzenedithiol molecular junction

Here, we present simultaneous electronic and optical measurements of a single 1,4-benzenedithiol (BDT) molecular junctions to investigate the electronic and structural details in the molecular junction and to understand the charge transport property at the single molecular scale. The electronic property was investigated by DC conductance measurement while structural property was characterized using surface enhanced Raman scattering (SERS) measurement. The single BDT junctions sandwiched between Au nanogap-electrodes were prepared by the mechanically controllable break junction method at ambient conditions. The simultaneous conductance and SERS measurements demonstrate that ring deformation mode coupled to C–S stretching mode, ring breathing mode, and C=C stretching mode are detectable for the single BDT molecular junctions with electronic conductance of [Formula: see text] [Formula: see text]. The single molecule origin is supported by the characteristic variability of SERS within samples. Time evolution of the conductance and SERS signals indicated that the molecular conductance and the vibrational energy of the ring breathing mode exhibits anti-correlated relationship. This relationship can be mediated by time evolution of structural change in the single molecular junction and corresponding change in strength of metal–molecular coupling. The larger metal–molecular coupling causes higher electronic conductance of the molecular junction while charge transfer effect leads to weakening of molecular bonds and thus a resulting decrease in the vibration energy of the ring breathing mode.

Single Tripyridyl–Triazine Molecular Junction with Multiple Binding Sites

We present an electronic characterization of a single molecular junction of 2,4,6-tris(2′,2″,2‴-pyridyl)-1,3,5-triazine (TPTZ) with multiple metal–molecule binding sites using scanning tunneling microscopy-based break junction method under ambient conditions. The TPTZ molecule consists of a centered triazine moiety and surrounding three 2-pyridyl groups. The benzene rings containing a N atom in TPTZ act as molecular binding sites for bridging a gap between two Au electrodes to form a single molecular junction. Because the N atom at the ortho-position in the 2-pyridyl groups is spatially hidden from the electrode surfaces, the single molecular junction forms via direct metal−π couplings. We demonstrated that the single TPTZ molecular junctions exhibit highly conductive character up to 10–1 G0 (G0 = 2e2/h), which is due to the effect of the direct metal−π coupling. We found three preferential conductance states of ca. 10–1, 10–2, and 10–4 G0, which suggests that the single TPTZ molecular junctions have thre...

ChemInform Abstract: Structure‐Property Relationships in Atomic‐Scale Junctions: Histograms and Beyond

AbstractReview: 41 refs.

Matlab PV solar concentrator performance prediction based on triple junction solar cell model

This paper deals with a generalized triple-junction solar cell model developed by using Matlab in order to predict the performances of a Photo Voltaic Thermal (PVT) solar concentrator. The triple-junction solar cell model represents each single junction included in the entire triple-junction solar cell. The model has been developed considering not only parameters related to the environmental conditions, but also considering the concentration factor. The proposed model allows all single junctions to be considered one by one, and therefore, it is suitable for modeling not only a specific triple junction solar cell, but also two junction solar cells and triple-junction solar cells made by different material. Moreover, the proposed model implementation allows for a thorough analysis under different environmental conditions. The proposed model is general and useful not only to estimate the system loss and solar cell degradation during operation, but also to evaluate the overall performance when solar cells are connected to MPPT (Maximum Power Point Tracking) electronic converters. The theoretical analysis is validated by experimental verification included in the paper.

Inelastic effects in molecular transport junctions: The probe technique at high bias.

We extend the Landauer-Büttiker probe formalism for conductances to the high bias regime and study the effects of environmentally induced elastic and inelastic scattering on charge current in single molecule junctions, focusing on high-bias effects. The probe technique phenomenologically incorporates incoherent elastic and inelastic effects to the fully coherent case, mimicking a rich physical environment at trivial cost. We further identify environmentally induced mechanisms which generate an asymmetry in the current, manifested as a weak diode behavior. This rectifying behavior, found in two types of molecular junction models, is absent in the coherent-elastic limit and is only active in the case with incoherent-inelastic scattering. Our work illustrates that in the low bias-linear response regime, the commonly used "dephasing probe" (mimicking only elastic decoherence effects) operates nearly indistinguishably from a "voltage probe" (admitting inelastic-dissipative effects). However, these probes realize fundamentally distinct I-V characteristics at high biases, reflecting the central roles of dissipation and inelastic scattering processes on molecular electronic transport far-from-equilibrium.

Portable and wireless IV-curve tracer for >5 kV organic photovoltaic modules

The practical design of a wirelessly controlled portable IV-curve tracer based on a capacitive load is described. The design is optimized for the measurement of solar cell modules presenting a high open circuit voltage of up to 6kV and a low short circuit current below 100mA. The portable IV-tracer allows for on-site/in-situ characterization of large modules under real operating conditions and enables fast detection of potential failure of anomalies in electrical behavior. Currently available electronic loads only handle voltages up to around 1kV. To overcome cost and safety issues related to high voltage applications, the design is based on low cost components, battery-based isolated supply and wireless communication. A prototype has been implemented and field tested for characterization of different organic photovoltaic modules (OPV) made according to the infinity concept with a large number of serially connected single junctions (~7.450 single junctions) presenting open circuit voltages up to 5.6kV.

Single-molecule electronics: Cooling individual vibrational modes by the tunneling current.

Electronic devices composed of single molecules constitute the ultimate limit in the continued downscaling of electronic components. A key challenge for single-molecule electronics is to control the temperature of these junctions. Controlling heating and cooling effects in individual vibrational modes can, in principle, be utilized to increase stability of single-molecule junctions under bias, to pump energy into particular vibrational modes to perform current-induced reactions, or to increase the resolution in inelastic electron tunneling spectroscopy by controlling the life-times of phonons in a molecule by suppressing absorption and external dissipation processes. Under bias the current and the molecule exchange energy, which typically results in heating of the molecule. However, the opposite process is also possible, where energy is extracted from the molecule by the tunneling current. Designing a molecular "heat sink" where a particular vibrational mode funnels heat out of the molecule and into the leads would be very desirable. It is even possible to imagine how the vibrational energy of the other vibrational modes could be funneled into the "cooling mode," given the right molecular design. Previous efforts to understand heating and cooling mechanisms in single molecule junctions have primarily been concerned with small models, where it is unclear which molecular systems they correspond to. In this paper, our focus is on suppressing heating and obtaining current-induced cooling in certain vibrational modes. Strategies for cooling vibrational modes in single-molecule junctions are presented, together with atomistic calculations based on those strategies. Cooling and reduced heating are observed for two different cooling schemes in calculations of atomistic single-molecule junctions.

Structure-Property Relationships in Atomic-Scale Junctions: Histograms and Beyond.

Over the past 10 years, there has been tremendous progress in the measurement, modeling and understanding of structure-function relationships in single molecule junctions. Numerous research groups have addressed significant scientific questions, directed both to conductance phenomena at the single molecule level and to the fundamental chemistry that controls junction functionality. Many different functionalities have been demonstrated, including single-molecule diodes, optically and mechanically activated switches, and, significantly, physical phenomena with no classical analogues, such as those based on quantum interference effects. Experimental techniques for reliable and reproducible single molecule junction formation and characterization have led to this progress. In particular, the scanning tunneling microscope based break-junction (STM-BJ) technique has enabled rapid, sequential measurement of large numbers of nanoscale junctions allowing a statistical analysis to readily distinguish reproducible characteristics. Harnessing fundamental link chemistry has provided the necessary chemical control over junction formation, enabling measurements that revealed clear relationships between molecular structure and conductance characteristics. Such link groups (amines, methylsuflides, pyridines, etc.) maintain a stable lone pair configuration that selectively bonds to specific, undercoordinated transition metal atoms available following rupture of a metal point contact in the STM-BJ experiments. This basic chemical principle rationalizes the observation of highly reproducible conductance signatures. Subsequently, the method has been extended to probe a variety of physical phenomena ranging from basic I-V characteristics to more complex properties such as thermopower and electrochemical response. By adapting the technique to a conducting cantilever atomic force microscope (AFM-BJ), simultaneous measurement of the mechanical characteristics of nanoscale junctions as they are pulled apart has given complementary information such as the stiffness and rupture force of the molecule-metal link bond. Overall, while the BJ technique does not produce a single molecule circuit for practical applications, it has proved remarkably versatile for fundamental studies. Measured data and analysis have been combined with atomic-scale theory and calculations, typically performed for representative junction structures, to provide fundamental physical understanding of structure-function relationships. This Account integrates across an extensive series of our specific nanoscale junction studies which were carried out with the STM- and AFM-BJ techniques and supported by theoretical analysis and density functional theory based calculations, with emphasis on the physical characteristics of the measurement process and the rich data sets that emerge. Several examples illustrate the impact of measured trends based on the most probable values for key characteristics (obtained from ensembles of order 1000-10 000 individual junctions) to build a solid picture of conductance phenomena as well as attributes of the link bond chemistry. The key forward-looking question posed here is the extent to which the full data sets represented by the individual trajectories can be analyzed to address structure-function questions at the level of individual junctions. Initial progress toward physical modeling of conductance of individual junctions indicates trends consistent with physical junction structures. Analysis of junction mechanics reveals a scaling procedure that collapses existing data onto a universal force-extension curve. This research directed to understanding the distribution of structures and physical characteristics addresses fundamental questions concerning the interplay between chemical control and stochastically driven diversity.

Perpendicular magnetic tunnel junctions with a synthetic storage or reference layer: A new route towards Pt- and Pd-free junctions.

We report here the development of Pt and Pd-free perpendicular magnetic tunnel junctions (p-MTJ) for STT-MRAM applications. We start by studying a p-MTJ consisting of a bottom synthetic Co/Pt reference layer and a synthetic FeCoB/Ru/FeCoB storage layer covered with an MgO layer. We first investigate the evolution of RKKY coupling with Ru spacer thickness in such a storage layer. The coupling becomes antiferromagnetic above 0.5 nm and its strength decreases monotonously with increasing Ru thickness. This contrasts with the behavior of Co-based systems for which a maximum in interlayer coupling is generally observed around 0.8 nm. A thin Ta insertion below the Ru spacer considerably decreases the coupling energy, without basically changing its variation with Ru thickness. After optimization of the non-magnetic and magnetic layer thicknesses, it appears that such a FeCoB/Ru/FeCoB synthetic storage layer sandwiched between MgO barriers can be made stable enough to actually be used as hard reference layer in single or double magnetic tunnel junctions, the storage layer being now a single soft FeCoB layer. Finally, we realize Pt- or Pd-free robust perpendicular magnetic tunnel junctions, still keeping the advantage of a synthetic reference layer in terms of reduction of stray fields at small pillar sizes.

Optical approaches to improving perovskite/Si tandem cells

ABSTRACTRecently, metal-halide perovskites have demonstrated an extraordinarily rapid advance in single junction cell efficiency to over 20%, while still offering potentially low costs. Since the bandgap is larger than the ideal single-junction value, perovskite-based tandem cells can theoretically offer even higher efficiencies. Instead, however, the record tandem cell performance in experiments to date has come in slightly below that of record single junctions, although slightly higher than the same single junctions. In this work, we consider both how this disconnect can be explained quantitatively, and then devise experimentally feasible, variance-aware approaches to address them. The first stage of our approach is based on reconfiguring dielectric front coatings to help reduce net reflected power and balance junction currents by reshaping the reflection peaks. This method could be applied to post-fabrication stage of perovskite/c-Si tandem cells, and also applicable to cell and module level structures. In the second stage of our approach, we can almost entirely eliminate Fresnel reflection by applying a conformal periodic light trapping structure. In the best case, a short circuit current (Jsc) of 18.0 mA/cm2 was achieved, after accounting for 4.8 mA/cm2 of parasitic loss and 1.6 mA/cm2 reflection loss. Further improvements may require a change in the baseline materials used in perovskite cells.

Cobalt (II) oxide and nickel (II) oxide alloys as potential intermediate-band semiconductors: A theoretical study

Solar cells based on single pn junctions, employing single-gap semiconductors can ideally achieve efficiencies as high as 34%. Developing solar cells based on intermediate-band semiconductors (IBSCs), which can absorb light across multiple band gaps, is a possible way to defy this theoretical limit and achieve efficiencies as high as 60%. Here, we use first principles quantum mechanics methods and introduce CoO and Co0.25Ni0.75O as possible IBSCs. We show that the conduction band in both of these materials is divided into two distinct bands separated by a band gap. We further show that the lower conduction band (i.e., the intermediate band) is wider in Co0.25Ni0.75O compared with CoO. This should enhance light absorption from the valence band edge to the intermediate band, making Co0.25Ni0.75O more appropriate for use as an IBSC. Our findings provide the basis for future attempts to partially populate the intermediate band and to reduce the lower band gap in Co0.25Ni0.75O in order to enhance the potential of this material for use in IBSC solar cell technologies. Furthermore, with proper identification of heterojunctions and dopants, CoO and Co0.25Ni0.75O could be used in multi-color light emitting diode and laser technologies.

Strong Fermi level pinning induces a high rectification ratio and negative differential resistance in hydrogen bonding bridged single cytidine pair junctions.

We propose a high performance single molecule rectifier by sandwiching a deoxycytidine base pair between gold electrodes. The conductance of the single base pair junction can be controlled by its protonation status, with ON/OFF ratios between the protonated (pCC) and deprotonated (CC) junctions of 3-5 orders of magnitude. In the conducting pCC state, we observed a high rectification ratio of two orders of magnitude at bias voltage values around 0.1 V. This rectification ratio surpasses most of the theoretical designs for single molecular rectifiers, while the low working voltage implies significant energy efficiency. Negative differential resistance (NDR) was also witnessed in the protonated state, with a peak to valley ratio of 24. Both the rectifying and NDR effects originate from strong Fermi level pinning effects. The electronic performance offers these single base pair junctions potential applications as a unimolecular rectifier or switch with an NDR effect. The current-voltage response is unique compared with those of the reported canonical A-T and G-C pairs, and provides the possibility to be used for i-motif DNA structure recognition or sequencing.

Flexible and fragmentable tandem photosensitive nanocrystal skins.

We proposed and demonstrated the first account of large-area, semi-transparent, tandem photosensitive nanocrystal skins (PNSs) constructed on flexible substrates operating on the principle of photogenerated potential buildup, which avoid the need for applying an external bias and circumvent the current-matching limitation between junctions. We successfully fabricated and operated the tandem PNSs composed of single monolayers of colloidal water-soluble CdTe and CdHgTe nanocrystals (NCs) in adjacent junctions on a Kapton polymer tape. Owing to the usage of a single NC layer in each junction, noise generation was significantly reduced while keeping the resulting PNS films considerably transparent. In each junction, photogenerated excitons are dissociated at the interface of the semi-transparent Al electrode and the NC layer, with holes migrating to the contact electrode and electrons trapped in the NCs. As a result, the tandem PNSs lead to an open-circuit photovoltage buildup equal to the sum of those of the two single junctions, exhibiting a total voltage buildup of 128.4 mV at an excitation intensity of 75.8 μW cm(-2) at 350 nm. Furthermore, we showed that these flexible PNSs could be bent over 3.5 mm radius of curvature and cut out in arbitrary shapes without damaging the operation of individual parts and without introducing any significant loss in the total sensitivity. These findings indicate that the NC skins are promising as building blocks to make low-cost, flexible, large-area UV/visible sensing platforms with highly efficient full-spectrum conversion.

Electron tunneling through molecule-electrode contacts of single alkane molecular junctions: experimental determination and a practical barrier model.

An advanced understanding of the molecule-electrode contact interfaces of single-molecule junctions is a necessity for real world application of future single-molecule devices. This study aims to elucidate the change in the contact tunnelling barrier induced by junction extension and how this change affects the resulting junction conductance. The contact barrier of Au-octanedithiol/octanediamine-Au junctions was studied under triangle (TRI) mechanical modulations using the modified scanning tunneling microscopy (STM) break junction technique. The experimental results reveal that as the junction separation extends, the contact barrier of octanedithiol follows a unique trend, a linear increase followed by a plateau in barrier height, which is in contrast to that of octanediamine, a nearly rectangle barrier. We propose a modified contact barrier model for the unique barrier shape of octanedithiol, based on which the calculation agrees well with the experimental data. This study shows unprecedented experimental features of the molecule-electrode contact barrier of single-molecule junctions and provides new insights into the nature of contact effect in determining electron transport through single-molecule junctions.

Solvent-dependent conductance decay constants in single cluster junctions.

Single-molecule conductance measurements have focused primarily on organic molecular systems. Here, we carry out scanning tunneling microscope-based break-junction measurements on a series of metal chalcogenide Co6Se8 clusters capped with conducting ligands of varying lengths. We compare these measurements with those of individual free ligands and find that the conductance of these clusters and the free ligands have different decay constants with increasing ligand length. We also show, through measurements in two different solvents, 1-bromonaphthalene and 1,2,4-trichlorobenzene, that the conductance decay of the clusters depends on the solvent environment. We discuss several mechanisms to explain our observations.

Computational design of donor-bridge-acceptor systems exhibiting pronounced quantum interference effects.

Quantum interference is a well-known phenomenon that dictates charge transport properties of single molecule junctions. However, reports on quantum interference in donor-bridge-acceptor molecules are scarce. This might be due to the difficulties in meeting the conditions for the presence of quantum interference in a donor-bridge-acceptor system. The electronic coupling between the donor, bridge, and acceptor moieties must be weak in order to ensure localised initial and final states for charge transfer. Yet, it must be strong enough to allow all bridge orbitals to mediate charge transfer. We present the computational route to the design of a donor-bridge-acceptor molecule that features the right balance between these contradicting requirements and exhibits pronounced interference effects.

Conformations of cyclopentasilane stereoisomers control molecular junction conductance.

Here we examine the impact of ring conformation on the charge transport characteristics of cyclic pentasilane structures bound to gold electrodes in single molecule junctions. We investigate the conductance properties of alkylated cyclopentasilane cis and trans stereoisomers substituted in the 1,3-position with methylthiomethyl electrode binding groups using both the scanning tunneling microscope-based break junction technique and density functional theory based ab initio calculations. In contrast with the linear ones, these cyclic silanes yield lower conductance values; calculations reveal that the constrained dihedral geometries occurring within the ring are suboptimal for σ-orbital delocalization, and therefore, conductance. Theoretical calculations reproduce the measured conductance trends for both cis and trans isomers and find several distinct conformations that are likely to form stable molecular junctions at room temperature. Due to the weakened σ-conjugation in the molecule, through-space interactions are found to contribute significantly to the conductance. This manuscript details the vast conformational flexibility in cyclopentasilanes and the tremendous impact it has on controlling conductance.

High electronic couplings of single mesitylene molecular junctions.

We report on an experimental analysis of the charge transport properties of single mesitylene (1,3,5-trimethylbenzene) molecular junctions. The electronic conductance and the current–voltage characteristics of mesitylene molecules wired into Au electrodes were measured by a scanning tunnelling microscopy-based break-junction method at room temperature in a liquid environment. We found the molecular junctions exhibited two distinct conductance states with high conductance values of ca. 10−1G0 and of more than 10−3G0 (G0 = 2e2/h) in the electronic conductance measurements. We further performed a statistical analysis of the current–voltage characteristics of the molecular junctions in the two states. Within a single channel resonant tunnelling model, we obtained electronic couplings in the molecular junctions by fitting the current–voltage characteristics to the single channel model. The origin of the high conductance was attributed to experimentally obtained large electronic couplings of the direct π-bonded molecular junctions (ca. 0.15 eV). Based on analysis of the stretch length of the molecular junctions and the large electronic couplings obtained from the I–V analysis, we proposed two structural models, in which (i) mesitylene binds to the Au electrode perpendicular to the charge transport direction and (ii) mesitylene has tilted from the perpendicular orientation.

Superconducting transport in single and parallel double InAs quantum dot Josephson junctions with Nb-based superconducting electrodes

We report conductance and supercurrent measurements for InAs single and parallel double quantum dot Josephson junctions contacted with Nb or NbTiN superconducting electrodes. Large superconducting gap energy, high critical field, and large switching current are observed, all reflecting the features of Nb-based electrodes. For the parallel double dots, we observe an enhanced supercurrent when both dots are on resonance, which may reflect split Cooper pair tunneling.

Thermopower distribution of single molecule junctions with different interaction types

The thermopower (S) distribution in single-molecule junctions with different interaction types were investigated by using a scanning tunneling microscope break-junction (STM-BJ) technique. We used 4,4’-bipyridine (BPy) and 1,2-bis(4-pyridyl)ethylene (BPyE) molecules, each having the Van der Waals (vdW) interaction between a pyridine ring and a Au atom and a donor-acceptor (DA) interaction between a nitrogen(N) atom and a Au atom, depending on the different binding geometries formed with the Au electrodes. From the full width at half maximum (FWHM) in the distribution of S, we found that S had a smaller variation for the vdW interaction compared to the DA interaction, due to the high binding stability of vdW interaction. Furthermore, we measured the molecular bonding forces which are in the range of 1.5 nN - 1.8 nN for the vdW interaction and 0.8 nN for the DA interaction. This confirms that the bonding is stronger for the vdW interaction than for the DA interaction, which is consistent with the experimental results for the S distributions as well as those for the molecular bonding stabilities.