Jonas I. Goldsmith

Coulomb blockade and the Kondo effect in single-atom transistors

Using molecules as electronic components is a powerful new direction in the science and technology of nanometre-scale systems. Experiments to date have examined a multitude of molecules conducting in parallel, or, in some cases, transport through single molecules. The latter includes molecules probed in a two-terminal geometry using mechanically controlled break junctions or scanning probes as well as three-terminal single-molecule transistors made from carbon nanotubes, C60 molecules, and conjugated molecules diluted in a less-conducting molecular layer. The ultimate limit would be a device where electrons hop on to, and off from, a single atom between two contacts. Here we describe transistors incorporating a transition-metal complex designed so that electron transport occurs through well-defined charge states of a single atom. We examine two related molecules containing a Co ion bonded to polypyridyl ligands, attached to insulating tethers of different lengths. Changing the length of the insulating tether alters the coupling of the ion to the electrodes, enabling the fabrication of devices that exhibit either single-electron phenomena, such as Coulomb blockade, or the Kondo effect.

Precise Adjustment of Nanometric-Scale Diffusion Layers within a Redox Dendrimer Molecule by Ultrafast Cyclic Voltammetry: An Electrochemical Nanometric Microtome

Performing cyclic voltammetry at scan rates into the megavolt per second range allows the exploration of the nanosecond time scale as well as the creation of nanometric diffusion layers adjacent to the electrode surface. This latter property is used here to adjust precisely the diffusion layer width within the outer shell of a fourth-generation dendrimer molecule decorated by 64 [Ru(II)(tpy)2] redox centers (tpy = terpyridine). Thus the shape of the dendrimer molecule adsorbed onto the ultramicroelectrode surface can be explored voltammetrically in a way reminiscent of an analysis with a nanometric microtome. The quantitative analysis developed here applied to the experimental voltammograms demonstrates that in agreement with previous scanning tunneling microscopy (STM) studies the adsorbed dendrimer molecules are no more spherical as they are in solution but resemble more closely hemispheres resting onto the electrode surface on their diametrical planes. The same quantitative analysis gives access to the apparent diffusion coefficient featuring electron hopping between the [Ru(II)/ Ru(III)(tpy)2] redox centers distributed on the dendrimer surface. Based on the electron hopping rate constant thus measured and on a Smoluchowski-type model developed here to take into account viscosity effects during the displacement of the [Ru(II)/Ru(III)(tpy)2] redox centers around their equilibrium positions, it is shown that the [Ru(II)/Ru(III)(tpy)2] redox centers are extremely labile in their potential wells so that they may cross-talk considerably more easily than they would do in solution at an equivalent concentration.

Carbonate control of H2 and CH4 production in serpentinization systems at elevated P‐Ts

[1] Serpentinization of forsteritic olivine results in the inorganic synthesis of molecular hydrogen (H 2 ) in ultramafic hydrothermal systems (e.g., mid-ocean ridge and forearc environments). Inorganic carbon in those hydrothermal systems may react with H 2 to produce methane (CH 4 ) and other hydrocarbons or react with dissolved metal ions to form carbonate minerals. Here, we report serpentinization experiments at 200°C and 300 bar demonstrating Fe 2+ being incorporated into carbonates more rapidly than Fe 2+ oxidation (and concomitant H 2 formation) leading to diminished yields of H 2 and H 2 -dependent CH 4 . In addition, carbonate formation is temporally fast in carbonate oversaturated fluids. Our results demonstrate that carbonate chemistry ultimately modulates the abiotic synthesis of both H 2 and CH 4 in hydrothermal ultramafic systems and that ultramafic systems present great potential for CO 2 -mineral sequestration.

Electrochemical Analysis of Single-Walled Carbon Nanotubes Functionalized with Pyrene-Pendant Transition Metal Complexes

The noncovalent functionalization of single-walled carbon nanotubes (SWNTs) is important in the development of advanced materials and nanoelectronic sensors and devices. A cobalt-terpyridine transition metal complex with pendant pyrene moieties has been shown to successfully functionalize SWNTs via noncovalent pi-pi stacking interactions. Cyclic voltammetry at SWNT coated platinum electrodes has been utilized to investigate the process of surface modification and provides conclusive evidence of robust surface functionalization. The electrochemical methodology for examining surface functionalization of SWNTs described herein is generalizable to any redox-active system and provides a simple and powerful means for in situ examination of processes occurring at the surface of nanostructured materials.

Toward quantifying the electrostatic transduction mechanism in carbon nanotube molecular sensors.

Despite the great promise of carbon nanotube field-effect transistors (CNT FETs) for applications in chemical and biochemical detection, a quantitative understanding of sensor responses is lacking. To explore the role of electrostatics in sensor transduction, experiments were conducted with a set of highly similar compounds designed to adsorb onto the CNT FET via a pyrene linker group and take on a set of known charge states under ambient conditions. Acidic and basic species were observed to induce threshold voltage shifts of opposite sign, consistent with gating of the CNT FET by local charges due to protonation or deprotonation of the pyrene compounds by interfacial water. The magnitude of the gate voltage shift was controlled by the distance between the charged group and the CNT. Additionally, functionalization with an uncharged pyrene compound showed a threshold shift ascribed to its molecular dipole moment. This work illustrates a method for producing CNT FETs with controlled values of the turnoff gate voltage, and more generally, these results will inform the development of quantitative models for the response of CNT FET chemical and biochemical sensors.

Wiring up single molecules

The possibility of using single molecules as active elements of electronic devices offers a variety of scientific and technological opportunities. In this article, we discuss transistors, where electrons flow through discrete quantum states of a single molecule. First, we will describe molecules, where current flows through one cobalt atom surrounded by two insulating terpyridyl ligands. Depending on the length of the insulating part of the molecules, two different behaviors are observed: Coulomb blockade for a longer molecule and the Kondo effect for a shorter molecule. We will also discuss measurements of the C70 fullerene and its dimer (C140). In C140 devices, the transport measurements are affected by an intercage vibrational mode that has an energy of 11 meV. We observe a large current increase when this mode is excited, indicating a strong coupling between the electronic and mechanical degrees of freedom in C140 molecules.

Differentiating biotic from abiotic methane genesis in hydrothermally active planetary surfaces.

Molecular hydrogen (H2) is derived from the hydrothermal alteration of olivine-rich planetary crust. Abiotic and biotic processes consume H2 to produce methane (CH4); however, the extent of either process is unknown. Here, we assess the temporal dependence and limit of abiotic CH4 related to the presence and formation of mineral catalysts during olivine hydrolysis (i.e., serpentinization) at 200 °C and 0.03 gigapascal. Results indicate that the rate of CH4 production increases to a maximum value related to magnetite catalyzation. By identifying the dynamics of CH4 production, we kinetically model how the H2 to CH4 ratio may be used to assess the origin of CH4 in deep subsurface serpentinization systems on Earth and Mars. Based on our model and available field data, low H2/CH4 ratios (less than approximately 40) indicate that life is likely present and active.

Iridium(III) Bis-Pyridine-2-Sulfonamide Complexes as Efficient and Durable Catalysts for Homogeneous Water Oxidation.

A family of tetradentate bis(pyridine-2-sulfonamide) (bpsa) compounds was synthesized as a ligand platform for designing resilient and electronically tunable catalysts capable of performing water oxidation catalysis and other processes in highly oxidizing environments. These wrap-around ligands were coordinated to Ir(III) octahedrally, forming an anionic complex with chloride ions bound to the two remaining coordination sites. NMR spectroscopy documented that the more rigid ligand frameworks-[Ir(bpsa-Cy)Cl2](-) and [Ir(bpsa-Ph)Cl2](-)-produced C1-symmetric complexes, while the complex with the more flexible ethylene linker in [Ir(bpsa-en)Cl2](-) displays C2 symmetry. Their electronic structure was explored with DFT calculations and cyclic voltammetry in nonaqueous environments, which unveiled highly reversible Ir(III)/Ir(IV) redox processes and more complex, irreversible reduction chemistry. Addition of water to the electrolyte revealed the ability of these complexes to catalyze the water oxidation reaction efficiently. Electrochemical quartz crystal microbalance studies confirmed that a molecular species is responsible for the observed electrocatalytic behavior and ruled out the formation of active IrOx. The electrochemical studies were complemented by work on chemically driven water oxidation, where the catalytic activity of the iridium complexes was studied upon exposure to ceric ammonium nitrate, a strong, one-electron oxidant. Variation of the catalyst concentrations helped to illuminate the kinetics of these water oxidation processes and highlighted the robustness of these systems. Stable performance for over 10 days with thousands of catalyst turnovers was observed with the C1-symmetric catalysts. Dynamic light scattering experiments ascertained that a molecular species is responsible for the catalytic activity and excluded the formation of IrOx particles.

Reply to Lang et al.: Using field analyses to properly address the kinetics of methane genesis in olivine-rich hydrothermal systems

We appreciate the opportunity to clarify the supplementary use of field data used to investigate abiotic CH4 genesis in hydrothermal systems containing olivine (1, 2). We agree that laboratory experiments cannot replicate the complexity of natural systems. However, the scientific value of experimental studies is to simplify complex systems by first examining the most relevant processes guiding the reactions. This is precisely what our manuscript presents. It should also be noted that carbon isotope geochemistry and thermodynamics, which Lang et al. (2) readily cite to justify their argument, have their roots in simple “benchtop” experiments.

Reply to McCollom: Methane generation during experimental serpentinization of olivine with respect to carbonate saturation

We appreciate the opportunity to address McCollom’s comments (1) regarding our research investigating molecular hydrogen and methane generation during serpentinization (2). Given that the origin and composition of olivine, mineral preparation, solutions, pressure/temperature conditions, and experimental approaches are different compared with references cited by McCollom (e.g., refs. 3, 4) as well as other experiments, different results are expected. Here we focus on the role of carbonate chemistry and how it may be used to rationalize the differences noted by McCollom (1).