We measure conductance and thermopower of single Au-4,4'-bipyridine-Au junctions in distinct low and high conductance binding geometries accessed by modulating the electrode separation. We use these data to determine the electronic energy level alignment and coupling strength for these junctions, which are known to conduct through the lowest unoccupied molecular orbital (LUMO). Contrary to intuition, we find that, in the high-conductance junction, the LUMO resonance energy is further away from the Au Fermi energy than in the low-conductance junction. However, the LUMO of the high-conducting junction is better coupled to the electrode. These results are in good quantitative agreement with self-energy corrected zero-bias density functional theory calculations. Our calculations show further that measurements of conductance and thermopower in amine-terminated oligophenyl-Au junctions, where conduction occurs through the highest occupied molecular orbitals, cannot be used to extract electronic parameters as their transmission functions do not follow a simple Lorentzian form.

Designing materials to function in harsh environments, such as conductive aqueous media, is a problem of broad interest to a range of technologies, including energy, ocean monitoring and biological applications. The main challenge is to retain the stability and morphology of the material as it interacts dynamically with the surrounding environment. Materials that respond to mild stimuli through collective phase transitions and amplify signals could open up new avenues for sensing. Here we present the discovery of an electric-field-driven, water-mediated reversible phase change in a perovskite-structured nickelate, SmNiO. This prototypical strongly correlated quantum material is stable in salt water, does not corrode, and allows exchange of protons with the surrounding water at ambient temperature, with the concurrent modification in electrical resistance and optical properties being capable of multi-modal readout. Besides operating both as thermistors and pH sensors, devices made of this material can detect sub-volt electric potentials in salt water. We postulate that such devices could be used in oceanic environments for monitoring electrical signals from various maritime vessels and sea creatures.

We modulate the conductance of electrochemically inactive molecules in single-molecule junctions using an electrolytic gate to controllably tune the energy level alignment of the system. Molecular junctions that conduct through their highest occupied molecular orbital show a decrease in conductance when applying a positive electrochemical potential, and those that conduct though their lowest unoccupied molecular orbital show the opposite trend. We fit the experimentally measured conductance data as a function of gate voltage with a Lorentzian function and find the fitting parameters to be in quantitative agreement with self-energy corrected density functional theory calculations of transmission probability across single-molecule junctions. This work shows that electrochemical gating can directly modulate the alignment of the conducting orbital relative to the metal Fermi energy, thereby changing the junction transport properties.

A central characteristic of living beings is the ability to learn from and respond to their environment leading to habit formation and decision making. This behavior, known as habituation, is universal among all forms of life with a central nervous system, and is also observed in single-cell organisms that do not possess a brain. Here, we report the discovery of habituation-based plasticity utilizing a perovskite quantum system by dynamical modulation of electron localization. Microscopic mechanisms and pathways that enable this organismic collective charge-lattice interaction are elucidated by first-principles theory, synchrotron investigations, ab initio molecular dynamics simulations, and in situ environmental breathing studies. We implement a learning algorithm inspired by the conductance relaxation behavior of perovskites that naturally incorporates habituation, and demonstrate learning to forget: a key feature of animal and human brains. Incorporating this elementary skill in learning boosts the capability of neural computing in a sequential, dynamic environment.

Point defects, such as oxygen vacancies, control the physical properties of complex oxides, relevant in active areas of research from superconductivity to resistive memory to catalysis. In most oxide semiconductors, electrons that are associated with oxygen vacancies occupy the conduction band, leading to an increase in the electrical conductivity. Here we demonstrate, in contrast, that in the correlated-electron perovskite rare-earth nickelates, RNiO3 (R is a rare-earth element such as Sm or Nd), electrons associated with oxygen vacancies strongly localize, leading to a dramatic decrease in the electrical conductivity by several orders of magnitude. This unusual behavior is found to stem from the combination of crystal field splitting and filling-controlled Mott–Hubbard electron–electron correlations in the Ni 3d orbitals. Furthermore, we show the distribution of oxygen vacancies in NdNiO3 can be controlled via an electric field, leading to analog resistance switching behavior. This study demonstrates the potential of nickelates as testbeds to better understand emergent physics in oxide heterostructures as well as candidate systems in the emerging fields of artificial intelligence.

Thermoelectrics are famously challenging to optimize, because of inverse coupling of the Seebeck coefficient and electrical conductivity, both of which control the thermoelectric power factor. Inorganic−organic interfaces provide a promising route for realization of the strong electrical and thermal asymmetries required for thermoelectrics. In this work, transport properties of inorganic−organic interfaces are probed and understood at the molecular scale using the STMbreak junction measurement technique, theory, and a class of newly synthesized molecules. We synthesized a series of disubstituted thiophene derivatives varying the length of alkylthio-linkers and the number of thiophene rings. These molecules allow the systematic tuning of electronic resonances within the junction. We observed that these molecules have a decreasing Seebeck coefficient with increasing length of the alkyl chain, while oligothiophene junctions show an increasing Seebeck coefficient with length. We find that thiophene−Au junctions have significantly higher Seebeck coefficients, compared to benzenedithiol (in the range of 7−15 μV/K). A minimal tight-binding model, including a gateway state associated with the S−Au bond, captures and explains both trends. This work identifies S−Au gateway states as being important and potentially tunable features of junction electronic structure for enhancing the power factor of organic/inorganic interfaces.

Solid-state ion shuttles are of broad interest in electrochemical devices, nonvolatile memory, neuromorphic computing, and biomimicry utilizing synthetic membranes. Traditional design approaches are primarily based on substitutional doping of dissimilar valent cations in a solid lattice, which has inherent limits on dopant concentration and thereby ionic conductivity. Here, we demonstrate perovskite nickelates as Li-ion shuttles with simultaneous suppression of electronic transport via Mott transition. Electrochemically lithiated SmNiO (Li-SNO) contains a large amount of mobile Li located in interstitial sites of the perovskite approaching one dopant ion per unit cell. A significant lattice expansion associated with interstitial doping allows for fast Li conduction with reduced activation energy. We further present a generalization of this approach with results on other rare-earth perovskite nickelates as well as dopants such as Na The results highlight the potential of quantum materials and emergent physics in design of ion conductors.

Recent experiments have shown that transport properties of molecular-scale devices can be reversibly altered by the surrounding solvent. Here, we use a combination of first-principles calculations and experiment to explain this change in transport properties through a shift in the local electrostatic potential at the junction caused by nearby conducting and solvent molecules chemically bound to the electrodes. This effect is found to alter the conductance of 4,4'-bipyridine-gold junctions by more than 50%. Moreover, we develop a general electrostatic model that quantitatively predicts the relationship between conductance and the binding energies and dipoles of the solvent and conducting molecules. Our work shows that solvent-induced effects are a viable route for controlling charge and energy transport at molecular-scale interfaces.PACS numbers: 31.15. A-,73.30.+y,73.63.-b,85.65.+h Single-molecule junctions, individual molecules contacted with macroscopic electrodes, provide unique insight into the nanoscale physics of charge, spin, and energy transport [1][2][3][4]. To date, the most robust and reproducible approach to assemble single-molecule junctions is the scanning tunneling microscope-based break junction (STM-BJ) technique [5,6], allowing statistically significant measurements of molecular junction conductance [5-9], thermopower [10][11][12], mechanical properties [13][14][15], and binding mechanisms [15]. Previouslydeveloped theoretical approaches have led to quantitative agreement with experiment for molecular junctions, given a good approximation to the junction geometry and a good estimate of the differences in energy ∆E between the junction Fermi energy, E F and the orbital energy of the frontier orbital, either the highest occupied or lowest unoccupied molecular orbital (HOMO or LUMO, respectively) [16]. Theoretical works focusing on this level alignment [17,18] have led to increased understanding and control of molecular junction conductance and thermopower in terms of junction level alignment, with significant impact on experiments [10][11][12]19].Commonly, these experiments take place at room temperature in a non-conductive solvent [5][6][7][8], which has recently been shown to influence both junction formation probability and, in some cases, to alter the conductance [20]. Despite its practical importance, the impact of these solvents on conductance has not yet been fully understood or explained by theory, in part due to the large computation cost [21], and, therefore, continues to be elusive to control. Previous theoretical works have focused on the effect of solvent on the average [22,23] and dynamical [24] molecular junction geometries, and how they affect level alignment and modify the conductance [25]. Another focus has been the coupling of transmission channels due to intermolecular hopping between conducting molecules [26], in the case solvent would influence the formation of multiple simultaneous junctions. However a detailed physical picture and quantitative framework for understandin...