Showing papers on "Conductance published in 2022"

Precursors of Majorana modes and their length-dependent energy oscillations probed at both ends of atomic Shiba chains

Abstract: Isolated Majorana modes (MMs) are highly non-local quantum states with non-Abelian exchange statistics, which localize at the two ends of finite-size 1D topological superconductors of sufficient length. Experimental evidence for MMs is so far based on the detection of several key signatures: for example, a conductance peak pinned to the Fermi energy or an oscillatory peak splitting in short 1D systems when the MMs overlap. However, most of these key signatures were probed only on one of the ends of the 1D system, and firm evidence for an MM requires the simultaneous detection of all the key signatures on both ends. Here we construct short atomic spin chains on a superconductor-also known as Shiba chains-up to a chain length of 45 atoms using tip-assisted atom manipulation in scanning tunnelling microscopy experiments. We observe zero-energy conductance peaks localized at both ends of the chain that simultaneously split off from the Fermi energy in an oscillatory fashion after altering the chain length. By fitting the parameters of a low-energy model to the data, we find that the peaks are consistent with precursors of MMs that evolve into isolated MMs protected by an estimated topological gap of 50 μeV in chains of at least 35 nm length, corresponding to 70 atoms.

Performance of Thermal Interface Materials.

Abstract: The thermal interface materials (TIMs) used for improving thermal contacts are considered in terms of the performance, performance consideration criteria, performance evaluation methods, and material development approaches. The performance is described mainly by the thermal contact conductance, which refers to the conductance across the thermal contact surfaces that sandwiches the TIM. This conductance depends on the conformability, thermal conductivity, and small-thickness feasibility. However, the vast majority of published work does not consider this conductance, but only the thermal conductivity within the TIM. The highest TIM performance is exhibited by the thermal pastes and low-melting alloys.

Opening of glutamate receptor channel to subconductance levels

Abstract: Abstract Ionotropic glutamate receptors (iGluRs) are tetrameric ligand-gated ion channels that open their pores in response to binding of the agonist glutamate 1–3 . An ionic current through a single iGluR channel shows up to four discrete conductance levels (O1–O4) 4–6 . Higher conductance levels have been associated with an increased number of agonist molecules bound to four individual ligand-binding domains (LBDs) 6–10 . Here we determine structures of a synaptic complex of AMPA-subtype iGluR and the auxiliary subunit γ2 in non-desensitizing conditions with various occupancy of the LBDs by glutamate. We show that glutamate binds to LBDs of subunits B and D only after it is already bound to at least the same number of LBDs that belong to subunits A and C. Our structures combined with single-channel recordings, molecular dynamics simulations and machine-learning analysis suggest that channel opening requires agonist binding to at least two LBDs. Conversely, agonist binding to all four LBDs does not guarantee maximal channel conductance and favours subconductance states O1 and O2, with O3 and O4 being rare and not captured structurally. The lack of subunit independence and low efficiency coupling of glutamate binding to channel opening underlie the gating of synaptic complexes to submaximal conductance levels, which provide a potential for upregulation of synaptic activity.

Highly conducting single-molecule topological insulators based on mono- and di-radical cations

Abstract: Single-molecule topological insulators are promising candidates as conducting wires over nanometre length scales. A key advantage is their ability to exhibit quasi-metallic transport, in contrast to conjugated molecular wires which typically exhibit a low conductance that decays as the wire length increases. Here, we study a family of oligophenylene-bridged bis(triarylamines) with tunable and stable mono- or di-radicaloid character. These wires can undergo one- and two-electron chemical oxidations to the corresponding mono-cation and di-cation, respectively. We show that the oxidized wires exhibit reversed conductance decay with increasing length, consistent with the expectation for Su–Schrieffer–Heeger-type one-dimensional topological insulators. The 2.6-nm-long di-cation reported here displays a conductance greater than 0.1G0, where G0 is the conductance quantum, a factor of 5,400 greater than the neutral form. The observed conductance–length relationship is similar between the mono-cation and di-cation series. Density functional theory calculations elucidate how the frontier orbitals and delocalization of radicals facilitate the observed non-classical quasi-metallic behaviour. Designing long and highly conducting molecular wires has been a great challenge for decades. It has now been shown that a singly oxidized 2.6-nm-long oligophenylene-bridged bis(triarylamine) can show a single-molecule junction conductance over 0.1G0.

Opening of glutamate receptor channel to subconductance levels

Abstract: Abstract Ionotropic glutamate receptors (iGluRs) are tetrameric ligand-gated ion channels that open their pores in response to binding of the agonist glutamate 1–3 . An ionic current through a single iGluR channel shows up to four discrete conductance levels (O1–O4) 4–6 . Higher conductance levels have been associated with an increased number of agonist molecules bound to four individual ligand-binding domains (LBDs) 6–10 . Here we determine structures of a synaptic complex of AMPA-subtype iGluR and the auxiliary subunit γ2 in non-desensitizing conditions with various occupancy of the LBDs by glutamate. We show that glutamate binds to LBDs of subunits B and D only after it is already bound to at least the same number of LBDs that belong to subunits A and C. Our structures combined with single-channel recordings, molecular dynamics simulations and machine-learning analysis suggest that channel opening requires agonist binding to at least two LBDs. Conversely, agonist binding to all four LBDs does not guarantee maximal channel conductance and favours subconductance states O1 and O2, with O3 and O4 being rare and not captured structurally. The lack of subunit independence and low efficiency coupling of glutamate binding to channel opening underlie the gating of synaptic complexes to submaximal conductance levels, which provide a potential for upregulation of synaptic activity.

pH regulates potassium conductance and drives a constitutive proton current in human TMEM175

Abstract: Human TMEM175, a noncanonical potassium (K+) channel in endolysosomes, contributes to their pH stability and is implicated in the pathogenesis of Parkinson’s disease (PD). Structurally, the TMEM175 family exhibits an architecture distinct from canonical potassium channels, as it lacks the typical TVGYG selectivity filter. Here, we show that human TMEM175 not only exhibits pH-dependent structural changes that reduce K+ permeation at acidic pH but also displays proton permeation. TMEM175 constitutively conducts K+ at pH 7.4 but displays reduced K+ permeation at lower pH. In contrast, proton current through TMEM175 increases with decreasing pH because of the increased proton gradient. Molecular dynamics simulation, structure-based mutagenesis, and electrophysiological analysis suggest that K+ ions and protons share the same permeation pathway. The M393T variant of human TMEM175 associated with PD shows reduced function in both K+ and proton permeation. Together, our structural and electrophysiological analysis reveals a mechanism of TMEM175 regulation by pH.

Quantum Conductance in Memristive Devices: Fundamentals, Developments, and Applications

Abstract: Quantum effects in novel functional materials and new device concepts represent a potential breakthrough for the development of new information processing technologies based on quantum phenomena. Among the emerging technologies, memristive elements that exhibit resistive switching, which relies on the electrochemical formation/rupture of conductive nanofilaments, exhibit quantum conductance effects at room temperature. Despite the underlying resistive switching mechanism having been exploited for the realization of next‐generation memories and neuromorphic computing architectures, the potentialities of quantum effects in memristive devices are still rather unexplored. Here, a comprehensive review on memristive quantum devices, where quantum conductance effects can be observed by coupling ionics with electronics, is presented. Fundamental electrochemical and physicochemical phenomena underlying device functionalities are introduced, together with fundamentals of electronic ballistic conduction transport in nanofilaments. Quantum conductance effects including quantum mode splitting, stability, and random telegraph noise are analyzed, reporting experimental techniques and challenges of nanoscale metrology for the characterization of memristive phenomena. Finally, potential applications and future perspectives are envisioned, discussing how memristive devices with controllable atomic‐sized conductive filaments can represent not only suitable platforms for the investigation of quantum phenomena but also promising building blocks for the realization of integrated quantum systems working in air at room temperature.

Nonlinear electrohydrodynamic ion transport in graphene nanopores

Abstract: Pressure-modulated ion transport through single graphene nanopores reveals a nonlinear electrohydrodynamic coupling phenomenon.

Electronic Transport in Molecular Wires of Precisely Controlled Length Built from Modular Proteins.

Abstract: DNA molecular wires have been studied extensively because of the ease with which molecules of controlled length and composition can be synthesized. The same has not been true for proteins. Here, we have synthesized and studied a series of consensus tetratricopeptide repeat (CTPR) proteins, spanning 4 to 20 nm in length, in increments of 4 nm. For lengths in excess of 6 nm, their conductance exceeds that of the canonical molecular wire, oligo(phenylene-ethylenene), because of the more gradual decay of conductance with length in the protein. We show that, while the conductance decay fits an exponential (characteristic of quantum tunneling) and not a linear increase of resistance with length (characteristic of hopping transport), it is also accounted for by a square-law dependence on length (characteristic of weakly driven hopping). Measurements of the energy dependence of the decay length rule out the quantum tunneling case. A resonance in the carrier injection energy shows that allowed states in the protein align with the Fermi energy of the electrodes. Both the energy of these states and the long-range of hopping suggest that the reorganization induced by hole formation is greatly reduced inside the protein. We outline a model for calculating the molecular-electronic properties of proteins.

Single-molecule field effect and conductance switching driven by electric field and proton transfer

Abstract: Single-molecule junctions (SMJs) offer a novel strategy for miniaturization of electronic devices. In this work, we realize a graphene-porphyrin-graphene SMJ driven by electric field and proton transfer in two configurations. In the transistor configuration with ionic liquid gating, an unprecedented field-effect performance is achieved with a maximum on/off ratio of ~4800 and a gate efficiency as high as ~179 mV/decade in consistence with the theoretical prediction. In the other configuration, controllable proton transfer, tautomerization switching, is directly observed with bias dependence. Room temperature proton transfer leads to a two-state conductance switching, and more precise tautomerization is detected, showing a four-state conductance switching at high bias voltages and low temperatures. Such an SMJ in two configurations provides new insights into not only building multifunctional molecular nanocircuits toward real applications but also deciphering the intrinsic properties of matters at the molecular scale.

Analog Synapses Based on Nonvolatile FETs With Amorphous ZrO2 Dielectric for Spiking Neural Network Applications

Abstract: In this article, an analog synapse based on a nonvolatile field-effect transistor with amorphous ZrO2 dielectrics has been fabricated and demonstrated. The conductance modulation properties of the devices have been systematically evaluated. Due to the polarization switching dynamics of the ferroelectric-like amorphous ZrO2 dielectric, which is attributable to the voltage-driven oxygen vacancies and negative charges dipoles, the proposed ZrO2-based devices exhibit superior synaptic characteristics, including good symmetry and linearity for both potentiation and depression, with small cycle-to-cycle variations. The ratio of maximum conductance and minimum conductance ( ${G}_{max}/{G}_{min})$ of devices reaches 130, with conductance states over 30. Also, spike-timing-dependent plasticity (STDP) has been mimicked in the devices successfully. Furthermore, based on the experimental STDP characteristics and conductance modulation properties of potentiation and depression, a spiking neural network architecture constructed by the proposed ZrO2-based synapses has been simulated. High offline and online learning accuracy of 94% and 87%, respectively, on the handwritten digits dataset, has been achieved.

Selective control of conductance modes in multi-terminal Josephson junctions

Abstract: The Andreev bound state spectra of multi-terminal Josephson junctions form an artificial band structure, which is predicted to host tunable topological phases under certain conditions. However, the number of conductance modes between the terminals of multi-terminal Josephson junction must be few in order for this spectrum to be experimentally accessible. In this work we employ a quantum point contact geometry in three-terminal Josephson devices. We demonstrate independent control of conductance modes between each pair of terminals and access to the single-mode regime coexistent with the presence of superconducting coupling. These results establish a full platform on which to realize tunable Andreev bound state spectra in multi-terminal Josephson junctions.

On-demand large conductance in trivial zero-bias tunneling peaks in Majorana nanowires

Abstract: Motivated by recent experiments that report the almost-generic large-conductance peaks without very extensive fine-tuning, we propose an alternative mechanism through direct theoretical simulations that can explain the large zero-bias conductance peaks being generated on-demand in the nontopological regime in Majorana nanowires by satisfying the following three sufficient conditions: (i) strong potential disorder in the bulk of the nanowire, suppressing the topological regime; (ii) strong suppression of the disorder near the nanowire ends connecting to the tunneling leads, perhaps because of screening by the metallic leads and gates; and (iii) low tunnel barrier strength leading to large tunneling amplitude. The third condition is typically achieved experimentally by fine-tuning the tunnel barrier and the first condition is generic in all existing nanowires by virtue of considerable sample disorder induced by unintentional random quenched charged impurities. The second condition is likely to apply to many samples since the disorder potential would be typically screened more strongly at the wire ends because of the large metallic tunnel pads used experimentally. We show that the resultant tunneling conductance manifests large trivial zero-bias peaks almost on demand, and such peaks could be $\sim 2e^2/h$, when appropriately fine-tuned by the tunnel barrier strength and the temperature, as reported experimentally. Our work not only solves the mystery in recent experiments that the observations of the large zero-bias conductance peaks are generic by proposing a theoretically possible mechanism but also explains why these hypothesized conditions are naturally satisfied in experiments.

Multiparticle scattering and breakdown of the Wiedemann-Franz law at a junction of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>N</mml:mi></mml:math> interacting quantum wires

Abstract: We analyze the charge and thermal transport at a junction of interacting quantum wires close to equilibrium. Within the framework of Tomonaga-Luttinger liquids, we compute the thermal conductance for a wide class of boundary conditions and detail the physical processes leading to the breakdown of the Wiedemann-Franz law at the junction. We show how connecting external reservoirs to the quantum wires affects the conductance tensors close to the various fixed points of the phase diagram of the junction. We therefore distinguish two types of violation of the Wiedemann-Franz law: a ``trivial'' one, independent of the junction dynamics and arising from the breakdown of the Fermi-liquid picture in the wire, and a junction-related counterpart, arising from multiparticle scattering processes at the junction.

Substrate-dependence of monolayer MoS2 thermal conductivity and thermal boundary conductance

Abstract: The thermal properties of two-dimensional (2D) materials, such as MoS2, are known to be affected by interactions with their environment, but this has primarily been studied only with SiO2 substrates. Here, we compare the thermal conductivity (TC) and thermal boundary conductance (TBC) of monolayer MoS2 on amorphous (a-) and crystalline (c-) SiO2, AlN, Al2O3, and h-BN monolayers using molecular dynamics. The room temperature, in-plane TC of MoS2 is ∼38 Wm−1 K−1 on amorphous substrates and up to ∼68 Wm−1 K−1 on crystalline substrates, with most of the difference due to substrate interactions with long-wavelength MoS2 phonons (<2 THz). An h-BN monolayer used as a buffer between MoS2 and the substrate causes the MoS2 TC to increase by up to 50%. Length-dependent calculations reveal TC size effects below ∼2 μm and show that the MoS2 TC is not substrate- but size-limited below ∼100 nm. We also find that the TBC of MoS2 with c-Al2O3 is over twice that with c-AlN despite a similar MoS2 TC on both, indicating that the TC and TBC could be tuned independently. Finally, we compare the thermal resistance of MoS2 transistors on all substrates and find that MoS2 TBC is the most important parameter for heat removal for long-channel (>150 nm) devices, while TBC and TC are equally important for short channels. This work provides important insights for electro-thermal applications of 2D materials on various substrates.

Resistive Switching and Synaptic Characteristics in ZnO/TaON-Based RRAM for Neuromorphic System

Abstract: We fabricated an ITO/ZnO/TaON/TaN device as nonvolatile memory (NVM) with resistive switching for complementary metal-oxide-semiconductor (CMOS) compatibility. It is appropriate for the age of big data, which demands high speed and capacity. We produced a TaON layer by depositing a ZnO layer on a TaN layer using an oxygen-reactive radio frequency (RF) sputtering system. The bi-layer formation of ZnO and TaON interferes with the filament rupture after the forming process and then raises the current level slightly. The current levels were divided into high- and low-compliance modes. The retention, endurance, and pulse conductance were verified with a neuromorphic device. This device was stable and less consumed when it was in low mode rather than high mode.

Principal Component Analysis of Surface-Enhanced Raman Scattering Spectra Revealing Isomer-Dependent Electron Transport in Spiropyran Molecular Junctions: Implications for Nanoscale Molecular Electronics

Abstract: The characterization of single-molecule structures could provide significant insights into the operation mechanisms of functional devices. Structural transformation via isomerization has been extensively employed to implement device functionalities. Although single-molecule identification has recently been achieved using near-field spectroscopy, discrimination between isomeric forms remains challenging. Further, the structure–function relationship at the single-molecule scale remains unclear. Herein, we report the observation of the isomerization of spiropyran in a single-molecule junction (SMJ) using simultaneous surface-enhanced Raman scattering (SERS) and conductance measurements. SERS spectra were used to discriminate between isomers based on characteristic peaks. Moreover, conductance measurements, in conjunction with the principal component analysis of the SERS spectra, clearly showed the isomeric effect on the conductance of the SMJ.

Room-Temperature Single-Molecule Conductance Switch via Confined Coordination-Induced Spin-State Manipulation

Abstract: Open AccessCCS ChemistryRESEARCH ARTICLE1 Apr 2022Room-Temperature Single-Molecule Conductance Switch via Confined Coordination-Induced Spin-State Manipulation Jing Li†, Qingqing Wu†, Wei Xu†, Hai-Chuan Wang†, Hewei Zhang, Yaorong Chen, Yongxiang Tang, Songjun Hou, Colin J. Lambert and Wenjing Hong Jing Li† State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005 †J. Li, Q. Wu, W. Xu, and H.-C. Wang contributed equally to this work.Google Scholar More articles by this author , Qingqing Wu† Department of Physics, Lancaster University, Lancaster LA1 4YB †J. Li, Q. Wu, W. Xu, and H.-C. Wang contributed equally to this work.Google Scholar More articles by this author , Wei Xu† State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005 †J. Li, Q. Wu, W. Xu, and H.-C. Wang contributed equally to this work.Google Scholar More articles by this author , Hai-Chuan Wang† State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005 †J. Li, Q. Wu, W. Xu, and H.-C. Wang contributed equally to this work.Google Scholar More articles by this author , Hewei Zhang State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005 Google Scholar More articles by this author , Yaorong Chen State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005 Google Scholar More articles by this author , Yongxiang Tang State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005 Google Scholar More articles by this author , Songjun Hou Department of Physics, Lancaster University, Lancaster LA1 4YB Google Scholar More articles by this author , Colin J. Lambert Department of Physics, Lancaster University, Lancaster LA1 4YB Google Scholar More articles by this author and Wenjing Hong *Correspondence author: E-mail Address: [email protected] State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.021.202100988 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail The emergence of molecular spintronics offers a unique chance for the design of molecular devices with different spin-states, and the control of spin-state becomes essential for molecular spin switches. However, the intrinsic spin switching from low- to high-spin state is a temperature-dependent process with a small energy barrier where low temperature is required to maintain the low-spin state. Thus, the room-temperature operation of single-molecule devices has not yet been achieved. Herein, we present a reversible single-molecule conductance switch by manipulating the spin states of the molecule at room temperature using the scanning tunneling microscope break-junction (STM-BJ) technique. The manipulation of the spin states between S = 0 and S = 1 is achieved by complexing or decomplexing the pyridine derivative molecule with a square planar nickel(II) porphyrin. The bias-dependent conductance evolution proves that the strong electric field between the nanoelectrodes plays a crucial role in the coordination reaction. The density functional theory (DFT) calculations further reveal that the conductance changes come from the geometric changes of the porphyrin ring and spin-state switching of the Ni(II) ion. Our work provides a new avenue to investigate room-temperature spin-related sensors and molecular spintronics. Download figure Download PowerPoint Introduction Molecular spintronics is an emerging field that aims to exceed the limits of conventional electronics by utilizing the spin of the electrons and has many advantages such as non-volatility, high storage density, low energy consumption, and high response speed.1–5 As a prototype, molecular spin switches, especially intrinsic spin switches, can be switched between at least two stable spin-states by external stimuli such as temperature,6 pressure,7 photons,6,8 electric fields,9 and so on, and has been considered as an essential step toward the development of molecular spintronics.10–13 Recently, several types of single-molecule prototype devices have been demonstrated, such as voltage-triggered,14 stretching-induced,15,16 or electron injection-induced17–19 spin switches. However, as a general rule, low spin is stable at low temperature and high spin is stable at high temperature,20,21 and low-temperature vacuum conditions are usually required to obtain a stable spin switch. Hence, an increased operating temperature to room temperature remains challenging. To realize room-temperature manipulation of spin-state in single-molecule devices, the interfacial interaction between molecule and electrode is another major issue.22–24 After adsorption on electrode surfaces, the intrinsic spin switch functionality may be lost or changed (such as, a lower phase transition temperature), which brings difficulty to the molecular design. To address this issue, coordination-induced spin-state switching (CISSS),25 which is insensitive to interfacial interactions due to its chemical reaction-driven spin trans effect, provides a promising strategy for the control of the spin-state at room temperature. The spin-states of some 3d metal ions25–27 are sensitive to the coordination sphere of metal ions, and thus, the square planar Ni-center diamagnetic low-spin state will be switched to a high spin state (S = 0 → S = 1) by controlling axial ligand coordination to a square pyramidal or octahedral configuration.18,25,28–30 On the other hand, previous studies have also demonstrated that the charge transport through the metal porphyrin can determine the different elements and even charge states of the metal center,31–33 suggesting the control of nickel porphyrin between different spin-states may offer a promising route toward single-molecule spin switches at room temperature via CISSS strategy. In this work, we designed a square planar nickel(II) porphyrin derivative, 5,15-bis(4-(methylthio)phenyl)-10,20-bis(2,3,4,5,6-pentafluorophenyl)-Ni(II) porphyrin ( NiTPPF, Figure 1a), and investigated its spin-induced single-molecule conductance switching using the scanning tunneling microscope break-junction (STM-BJ) technique at room temperature. Without axial ligands, NiTPPF exhibits a diamagnetic low-spin state (S = 0) with an empty d x 2 − y 2 and a fully occupied d z 2 orbital of the central nickel ion. One electron is transferred from the d z 2 orbital to the d x 2 − y 2 orbital leading to a paramagnetic high-spin species (S = 1) upon axial coordination with at least one pyridine derivative. Compared with charge transport through the Zn(II)-analog, a giant conductance switch was achieved by manipulating the spin-states, which is comparable with most of the single-molecule magnetoresistance systems requiring low temperature and ultra-high vacuum. These conductance switchings are further confirmed by density functional theory (DFT) calculation. More importantly, the bias-dependent conductance evolution between NiTPPF and its mixture with pyridines indicates that the strong electric field can enrich the polar molecule in the nanoelectrode gap, thereby promoting the coordination reaction in the confined environment. Our work opens a new way to investigate the in-site single-molecule catalytic process and molecular spintronics. Figure 1 | (a) Chemical structure of NiTPPF molecule. (b) Contact geometries for the NiTPPF series of molecules. The reversible coordination reaction between NiTPPF and 3,5-lutidine in solution, and the related spin switch (S = 0 ↔ S = 1). The H and F atoms in NiTPPF and methyl group in 3,5-lutidine are omitted. (c) Logarithmically binned conductance histograms of NiTPPF (2869 traces out of 3137 traces), NiTPPF-1 (2788 trace out of 3318 traces), and NiTPPF-2 (2377 traces out of 2975 traces) in mesitylene solution at 0.1 V, respectively. Inset: Individual conductance traces of NiTPPF, NiTPPF-1, and NiTPPF-2, respectively. (d) Reversible ON/OFF conductance switches of the NiTPPF system by adding 3,5-lutidine and CF3COOH. Download figure Download PowerPoint Experimental Methods All reagents were purchased from TCI (Shanghai, China) and used without further purification. The 1H NMR spectra were recorded on Bruker Avance II 400M instruments (Germany) and Tetramethylsilane (TMS) was used as an internal standard. UV absorption spectra were obtained on a PerkinElmer Lambda 1050 2D (USA). A Microflex™ (USA) LRF matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) was used for MS analysis. According to previous studies,29,34 an electron-deficient porphyrin ring favors the association of axial ligands. Therefore, pentafluorophenyl porphyrin nickel(II) derivatives are often used to study the ligand CISSS effect.25,28,29,35,36 To fabricate the single-molecule junctions between the two gold electrodes, we modified pentafluorophenyl porphyrin nickel(II), and designed a new porphyrin, TPPFNi, which introduced anchor groups –SMe in the opposite position (Figure 1a). Synthesis of 5-(pentafluorophenyl)-dipyrromethane37 A solution of pentafluorobenzaldehyde (3.92 g, 20 mmol) and an excess amount of pyrrole (50 mL) was degassed with N2 at room temperature. To this, trifluoroacetic acid (TFA) (0.17 mL, 2 mmol) was added, and the reaction mixture was stirred for 30 min at room temperature. After the reaction, the mixture was treated with 0.1 M NaOH solution and then extracted with ethyl acetate three times. The combined solution was concentrated and purified by column chromatography over silica gel using Dichloromethane (DCM)/Hex = 1:2 as the eluent to give the product (1.51 g, 24%) as a white solid. 1H NMR (400 MHz, CDCl3, δ) 5.90 (s, 1H), 6.02 (m, 2H), 6.15–6.17 (m, 2H), 6.73–6.74 (m, 2H), 8.15 (s, br, 2H) ppm. Synthesis of 5,15-bis(4-(methylthio)phenyl)-10,20-bis(2,3,4,5,6-pentafluorophenyl)-21H,23H porphyrin Under nitrogen atmosphere, 4-(methylthio)benzaldehyde (0.624 g, 2 mmol) and 5-(pentafluorophenyl)-dipyrromethane (DPMF) were added to dichloromethane (20 mL) via syringe. Trifluoro acetic acid (0.34 mg, 2 mmol) was dissolved in dichloromethane (5 mL) and added to the solution. The reaction mixture was stirred at 45 °C for 1.5 h. Then 2,3-dicyano-5,6-dichlorobenzoquinone was added and stirred for an additional 1 h. After the reaction, triethylamine (2 mL) was added, and the solvent was removed by evaporation. The mixture was further purified with column chromatography over silica gel using DCM/Hex = 1:3 as the eluent to obtain the crude product. The pure crystal product (234 mg, 26%) was obtained by recrystallization from DCM/MeOH. 1H NMR (400 MHz, CDCl3, δ) −2.83 (s, 2H), 2.77 (s, 6H), 7.65 (d, J = 8.12 Hz, 4H), 8.13 (d, J = 8.12 Hz, 4H), 8.79 (d, J = 4.8 Hz, 4H), 8.97 (d, J = 4.8 Hz, 4H) ppm. MALDI-TOF-MS (m/z): calcd for [M]•+, 885.92; found, 886.13. Synthesis of NiTPPF 5,15-bis(4-(methylthio)phenyl)-10,20-bis(2,3,4,5,6-pentafluorophenyl)-21H,23H porphyrin (TPPF) (71 mg, 0.08 mmol) and Ni(OAc)2·4H2O (60 mg, 0.24 mmol) were dissolved in dimethylformamide (DMF) (10 mL). Then, the mixture was refluxed for 6 h. After the reaction, the mixture was concentrated and washed with water to remove the excess Ni(OAc)2·2H2O. The crude product was further purified with column chromatography over silica gel using DCM/Hex = 1:5 as the eluent to obtain the product as a red solid (27 mg, 36%). 1H NMR (400 MHz, CDCl3, δ) 2.73 (s, 6H), 7.59 (d, J = 8.24 Hz, 4H), 7.95 (d, J = 8.16 Hz, 4H), 8.68 (d, J = 4.8 Hz, 4H), 8.88 (d, J = 5.08 Hz, 4H) ppm. MALDI-TOF-MS (m/z): calcd for [M]•+, 941.91; found, 942.05. Synthesis of 5,15-bis(4-(methylthio)phenyl)-10,20-bis(2,3,4,5,6-pentafluorophenyl)Zn(II) porphyrin (ZnTPPF) TPPF (44 mg, 0.05 mmol) and Zn(OAc)2·2H2O (39 mg, 0.175 mmol) were dissolved in CHCl3/MeOH=2:1 (9 mL). Then, the mixture was refluxed for 3 h. After the reaction, the mixture was concentrated and filtered through a short silica gel column with ethyl acetate as eluent. The solvent was evaporated, and the resulting purple solid was washed with MeOH to obtain a pure product (45 mg, 98%). 1HNMR (400 MHz, Toluene-D8, δ) 2.29 (s, 6H), 7.52 (d, J = 8.12 Hz, 4H), 7.95 (d, J = 8.12 Hz, 4H), 8.68 (d, J = 4.76 Hz, 4H), 8.88 (d, J = 4.6 Hz, 4H) ppm. MALDI-TOF-MS (m/z): calcd for [M]•+, 947.88; found, 948.04. Results and Discussions Single-molecule conductance measurements The conductance of single-molecule junctions was measured using the STM-BJ technique (Figure 1b and Supporting Information Figure S1) to track the coordination reaction between NiTPPF and an electron-rich auxiliary ligand 3,5-lutidine in mesitylene solution (TMB).38,39 Three samples, 0.1 mM pure NiTPPF solution, 0.1 mM pure NiTPPF solution with 1.1 equiv of 3,5-lutidine ( NiTPPF-1), and 0.1 mM pure NiTPPF solution with 2.2 equiv of 3,5-lutidine ( NiTPPF-2), were used for the single-molecule conductance measurements. For each solution, we repeatedly formed and broke gold–gold atom contacts. During the gold tip retraction, a plateau around G0 (2e2/h, 77.6 μS) representing a gold–gold atom contact was first observed, followed by some other plateaus below G0 indicating the formation of Au-molecule-Au junctions. We collected thousands of conductance traces (2869 in 3137, 2788 in 3318, and 2377 in 2957 traces for NiTPPF, NiTPPF-1, and NiTPPF-2, respectively) to analyze the 1D conductance histograms (Figure 1c) by removing the tunneling traces ( Supporting Information Figure S2). All samples showed a prominent low conductance peak and a less prominent high conductance peak.40 The most probable conductance value for each sample was obtained by fitting a Gaussian function to the conductance peak. The high conductance was ∼10−4.1G0 (6.16 ns) in all three samples ( Supporting Information Figure S3), while the low conductance decreased from 10−5.37G0 (0.331 nS) of free NiTPPF to 10−5.66G0 (0.170 nS) of NiTPPF-1, and to a lower value 10−5.80G0 (0.122 nS) of NiTPPF-2 with increasing concentrations of 3,5-lutidine (Figure 1c and Supporting Information Figure S4), indicating that the decrease in conductance stems from the spin states transition after coordination with 3,5-lutidine. Furthermore, the plateau length of high conductance junctions is ca. 0.45 nm, which is nearly half of the whole molecule (ca. 0.95 nm), after adding a 0.5 nm gold–gold snap back distance to the stretching distance ( Supporting Information Figure S4), which suggests that this plateau occurs when one of the gold tips connects with the porphyrin ring. The plateau length corresponding to the low-conductance junctions is ca. 1.4 nm. Hence, the corrected junction length is ca. 1.9 nm, which is in line with the sulfur–sulfur distance of NiTPPF. Therefore, the plateau length of the junctions also reveals that the low conductance peak for each sample contains the core information for studying the CISSS-induced charge transport features. Benefiting from the stability of NiTPPF in a solution of CF3COOH at room temperature,30 the five- or six-coordinated NiTPPF can decomplex to pure NiTPPF by adding excess CF3COOH to completely protonate the 3,5-lutidine. As a result, the high-spin state (S = 1) can be switched back to the low-spin state (S = 0). By adding CF3OOH or 3,5-lutidine in the samples, we observed reversible conductance switching (Figure 1d). To reveal the role of coordinated-induced spin switching in the conductance switch in the NiTPPF system, the single-molecule conductance of the ZnTPPF system was also measured as a control experiment, as it does not change the spin-state after adding an extra-axial ligand ( Supporting Information Figure S5). The UV–vis spectra show that the band of pure ZnTPPF at 425 nm shifts to 431 nm upon the addition of 3,5-lutidine ( Supporting Information Figure S10), indicating the five- or six-coordinated complex was formed.41 The 1D conductance histograms and 2D conductance-displacement cloud maps were similar to NiTPPF. The conductance of the pure ZnTPPF is about 10−5.26G0 (0.426 nS), which is slightly larger than NiTPPF, and consistent with previously reported systems.31 After adding 3,5-lutidine, the conductance of coordinated ZnTPPF decreased to 10−5.31G0 (0.380 ns) for ZnTPPF-1, and 10−5.35G0 (0.347 ns) for ZnTPPF-2. Compared to the slight decrease of about ∼22.8% in Zn(II)-analog, giant conductance changes up to ∼169% were observed in the Ni(II) system, which reveals that the coordination-induced spin-switch behavior is vital. NMR and UV–vis spectrum in ensemble solution To prove the coordinated-induced spin switching behavior in our system, the NMR and UV–vis spectra of NiTPPF and its mixture with the electron-rich auxiliary ligand 3,5-lutidine were collected at room temperature ( Supporting Information Figures S6 and S7). One index from low-spin diamagnetic Ni(II) transitioning to a high-spin state (S = 1) is whether the chemical shift of the pyrrole proton to a low field with larger ppm value gives rise to five- or six-coordinated complexes.25,28–30 From NMR spectroscopy, the chemical shifts of the pyrrole protons in the pure NiTPPF are located at ∼8.8 and 8.5 ppm, which shifted to lower fields after titrating with 3,5-lutidine, indicating the formation of paramagnetic five- and six-coordinated complexes ( Supporting Information Figure S6). On the other side, the UV–vis titration spectra show that the band of pure NiTPPF at 415.8 nm decreases, and a new band arises at 433.1 nm upon the addition of 3,5-lutidine ( Supporting Information Figure S7), which is consistent with reference.29 Through the UV–vis titration experiments, it is assumed that the paramagnetic NiTTPF•(L) and NiTTPF•(L)2 have the same extinction coefficients, hence the equilibrium constants are K1 = 0.92 ± 0.03 L/mol, K2 = 4.83 ± 0.20 L/mol ( Supporting Information Figure S7), which is similar to the reported complexes.29 Both the NMR and UV–vis titration experiments demonstrate the CISSS in NiTPPF and 3,5-lutidine solutions. The difference between free and confined environments According to the equilibrium constants, K1 = 0.92 ± 0.03 L/mol and K2 = 4.83 ± 0.2 L/mol, extracted from the UV–vis titration spectra ( Supporting Information Figure S7), the concentrations of NiTTPF•(L) and NiTTPF•(L)2 in the NiTPPF-1 and NiTPPF-2 solution are exceptionally low (<0.01%). However, in the STM-BJ experiments, the conductance is changed obviously, indicating the axially coordinated components are dominant in the mixture solutions. The difference in the equilibrium constant between the confined and free environment suggests that the confined environment plays an essential role in the coordination reactions. Compared with the confined and free environments, the strong electric field in the confined environment is the main difference, which may be the main source of the difference in equilibrium constants between confined and free environments. As a polar molecule, 3,5-lutidine will gather in the nanogap by electrostatic forces under the strong electric field (108 V/m).42,43 In this situation, the strong electric field acts like a gripper, trapping the 3,5lLutidine molecules between the nanoelectrodes, which causes the concentration in the confined nanogap to be much higher than in the solution ( Supporting Information Figure S11) and promotes the coordination reaction. To prove the electric field effect in the coordination reaction, we first try to study the conductance evolution of the NiTPPF and NiTPPF-2 samples in TMB solvent under different bias voltages (0.02, 0.1, 0.3, and 0.5 V, Supporting Information Figures S12 and S13). The conductance of the NiTPPF sample slightly increases as the bias voltage increases (Figure 2a). However, the conductance of the NiTPPF-2 sample decreases with the increased bias voltage and reaches a balance value around 10−6.1G0 (0.0616 ns) when the bias voltage is higher than 0.3 V. More importantly, the conductance of the NiTPPF-2 sample under 0.02 V is similar to that of the NiTPPF sample. To show a more comprehensive picture, we also studied the CISSS using pyridine without a substituted group, and 4-methoxycarbonylpyridine with an electron-withdrawing group as the free ligands ( Supporting Information Figures S14 and S16). The conductance decreases with the increased bias voltage, ( Supporting Information Figures S15 and S17), which is the same as with NiTPPF-2. However, the conductance of the pyridine and 4-methoxycarbonylpyridine system only reached about 10−5.7 G0 (0.154 ns) under a bias voltage of 0.5 V, which can be attributed to the small equilibrium constants of the coordination with poor basicity of ligands ( Supporting Information Figures S8 and S9 and Table S1). The bias-dependent conductance evolution between NiTPPF and its mixture with the pyridine derivative indicates that the strong electric field enriches the pyridine derivatives in the nanoelectrode gap, thereby promoting the coordination reaction in the confined environment. Figure 2 | (a) The conductance evolution of the NiTPPF and NiTPPF-2 samples in TMB solvent under different bias voltages. (b) The conductance evolution of the NiTPPF and NiTPPF-2 samples in different solvents under 0.1 V. Download figure Download PowerPoint Besides, the electric field between the nanoelectrodes can be partially shielded by the solvent with a high dielectric constant,42,44 which provides another way to prove the electric field effect in our system. When the electric field is partially shielded by solvent, the electrostatic force on the 3,5-lutidine molecule decreases, so the local concentration of the 3,5-lutidine molecule in the confined nanogap will decrease. As a result, the proportion of coordinated complexes in the confined nanogap will decrease, and the conductance will tend toward the NiTPPF molecule. To prove this hypothesis, we performed STM-BJ experiments in trichlorobenzene (TCB) and acetonitrile (MeCN), which both have larger dielectric constants than TMB (εMeCN > εTCB > εTMB). The 2D conductance-displacement histograms and the plateau lengths of NiTPPF and NiTPPF-2 samples in TCB and MeCN solvents are consistent with the ones in TMB solvent, indicating the conductance peak is from the same single-molecule junction ( Supporting Information Figures S18 and S19). The low conductance of NiTPPF gradually decreases as the dielectric constant of the solvent increases (Figure 2b), which can be attributed to the electrostatic gating effect of the molecular levels in the dielectric solvent.45 However, the low conductance of NiTPPF-2 shows the opposite trend, indicating the conductance changes are from spin switch rather than electrostatic gating effect of solvent. Based on the above analysis, the proportion of NiTTPF•(L) and NiTPPF•(L)2 decreased as the dielectric constant increased from the nonpolar TMB to the polar MeCN solvent (Figure 2b), suggesting that the concentration of 3,5-lutidine in the nanogap is controlled by the electrostatic field and decreases as the dielectric constant of solvent increases. Results in different solvents revealed that the electrostatic effect of the strong electric field within the nanogap enriches the local concentration of free reactant molecules, which causes the equilibrium constants of the coordination reaction to be different in the confined environment and the free environment. This also explains why a significant high spin state can be observed in the NiTPPF system after adding 3,5-lutidine in TMB solution. Theoretical calculation To further understand the multiple-states in the experimental results, theoretical simulations and analyses based on DFT and quantum transport theory were carried out.46,47 The DFT calculated spin-resolved projected density of states of the three complexes are summarized in Figure 3 and Supporting Information Figure S21. The calculated results show that the free NiTPPF is in the low-spin ground states (S = 0) with an empty d x 2 − y 2 orbital. One electron is transferred from the d z 2 orbital to the d x 2 − y 2 orbital leading to a paramagnetic high-spin species (S = 1) upon axial coordination with at least one 3,5-lutidine molecule (Figures 1b and 3a–3c), which is in line with the NMR spectroscopy ( Supporting Information Figure S6). Figure 3 | DFT-calculated spin-resolved projected density of states of gas-phase NiTPPF series of molecules, where L represents 3,5-lutidine. Blue and red colors correspond to the spin-up and -down contributions, respectively. The filled curve corresponds to the metal orbital of Ni ion, and their occupations are shown on the right side. The isosurface plots of the star-marked orbitals are given on the left side. Download figure Download PowerPoint The transmission functions of the gold/molecule/gold junctions for the four molecules are presented and analyzed (Figures 4a–4b). It should be noted that the curves of NiTTPF•(L) and NiTTPF•(L)2 shown in Figure 4b are the average transmission spectra over spin up and down components ( Supporting Information Figure S22), so the two neighboring peaks between −2 and −1.5 eV belong to the two spin channels. The curves of free NiTPPF molecules possess wider energy gaps between the peaks around −2 and 0.2 eV compared to those of NiTTPF•(L) and NiTTPF•(L)2 due to the quantum effect of different molecular sizes. The narrow resonances at −1.5 and −1.1 eV are associated with the three 3d orbitals ( d x y , d y z , d x z ) of Ni for the free NiTPPF molecules shown in Supporting Information Figure S20. The tiny one at −1.4 eV in the curve of NiTTPF•(L) involves the d x 2 − y 2 orbital of the spin-up component. Other kinks between −1.3 and −0.5 eV are due to instabilities in the calculations. The very narrow and tiny peaks can be observed both in low- and high-spin states which originate from the localized orbitals on porphyrin rings and Ni atoms and rare weights on the −SMe anchor groups ( Supporting Information Figure S21). Some orbitals are even completely silent, such as the d x 2 − y 2 orbital of spin-up for the NiTTPF•(L)2 molecule. The Fermi energy is close to the lowest unoccupied molecular orbital (LUMO) in all three molecules, suggesting that LUMOs are the dominant charge transport channels (Figure 4b, inset). The transmission coefficients near the Fermi energy level are T ( NiTPPF) > T ( NiTTPF•(L)) > T ( NiTTPF•(L)2). We also calculated the Zn(II)-analog system ( Supporting Information Figures S23 and S24) and demonstrated that the small energy change in electron energy levels after axial coordination can slightly suppress charge transport. However, this effect is weaker than that of the Ni system with spin manipulation, which is in good agreement with the experiments. Figure 4 | (a) Configurations of gold/NiTPPF-series/gold junctions. (b) The transmission functions of the junctions. The curves of NiTTPF•(L) and NiTTPF•(L)2 depict the average transmission functions over the two spin channels where the two molecules have high-spin ground states. Download figure Download PowerPoint Conclusions We demonstrate the reversible switching of spin-state in single-molecule junctions at room temperature using the STM-BJ technique. Based on this study, we suggest two possible ways to improve the device. First, from the theoretical calculations, the contribution of 3d orbitals of nickel ions to electron transmission is suppressed by the highly conjugated porphyrin ring. Hence, taking advantage of chemical tunability to eliminate the conjugation of the ligand and increase the contribution of the metal ion in the conductive channel will hopefully result in a device with a higher conductance difference. Second, the auxiliary ligand can be functionalized so that it can respond to more stimuli to develop a multifunctional device. For example, photon-driven CISSS (PD-CISSS) single-molecule electric devices can be explored by introducing a pho

An improved representation of the relationship between photosynthesis and stomatal conductance leads to more stable estimation of conductance parameters and improves the goodness‐of‐fit across diverse data sets

Abstract: Stomata play a central role in surface–atmosphere exchange by controlling the flux of water and CO2 between the leaf and the atmosphere. Representation of stomatal conductance (gsw) is therefore an essential component of models that seek to simulate water and CO2 exchange in plants and ecosystems. For given environmental conditions at the leaf surface (CO2 concentration and vapor pressure deficit or relative humidity), models typically assume a linear relationship between gsw and photosynthetic CO2 assimilation (A). However, measurement of leaf‐level gsw response curves to changes in A are rare, particularly in the tropics, resulting in only limited data to evaluate this key assumption. Here, we measured the response of gsw and A to irradiance in six tropical species at different leaf phenological stages. We showed that the relationship between gsw and A was not linear, challenging the key assumption upon which optimality theory is based—that the marginal cost of water gain is constant. Our data showed that increasing A resulted in a small increase in gsw at low irradiance, but a much larger increase at high irradiance. We reformulated the popular Unified Stomatal Optimization (USO) model to account for this phenomenon and to enable consistent estimation of the key conductance parameters g0 and g1. Our modification of the USO model improved the goodness‐of‐fit and reduced bias, enabling robust estimation of conductance parameters at any irradiance. In addition, our modification revealed previously undetectable relationships between the stomatal slope parameter g1 and other leaf traits. We also observed nonlinear behavior between A and gsw in independent data sets that included data collected from attached and detached leaves, and from plants grown at elevated CO2 concentration. We propose that this empirical modification of the USO model can improve the measurement of gsw parameters and the estimation of plant and ecosystem‐scale water and CO2 fluxes.

Quantum Interference-Controlled Conductance Enhancement in Stacked Graphene-like Dimers.

Abstract: Stacking interactions are of significant importance in the fields of chemistry, biology, and material optoelectronics because they determine the efficiency of charge transfer between molecules and their quantum states. Previous studies have proven that when two monomers are π-stacked in series to form a dimer, the electrical conductance of the dimer is significantly lower than that of the monomer. Here, we present a strong opposite case that when two anthanthrene monomers are π-stacked to form a dimer in a scanning tunneling microscopic break junction, the conductance increases by as much as 25 in comparison with a monomer, which originates from a room-temperature quantum interference. Remarkably, both theory and experiment consistently reveal that this effect can be reversed by changing the connectivity of external electrodes to the monomer core. These results demonstrate that synthetic control of connectivity to molecular cores can be combined with stacking interactions between their π systems to modify and optimize charge transfer between molecules, opening up a wide variety of potential applications ranging from organic optoelectronics and photovoltaics to nanoelectronics and single-molecule electronics.