Al-Amin Dhirani

Competitive transport and percolation in disordered arrays of molecularly-linked Au nanoparticles

We explore the electrical characteristics of disordered films of strongly coupled, molecularly-linked gold nanoparticles (NPs). dc conductivity vs temperature (g vs T) measurements exhibit features that can track a number of competing transport mechanisms. Films with fewer than 6 layers show clear signatures of both activated tunneling and thermionic emission. Our linked NPs admit locally metallic transport, likely through strong quantum interactions, and at room temperature, films with 6 or more layers exhibit a transition to metallic dominated behavior. Observed g vs T dependencies have been modeled treating arrays as disordered resistor networks and using an effective medium approximation (EMA). Our results show that percolation phenomena can play critical roles in transport through NP films, particularly near metal–insulator transitions.

Variable single electron charging energies and percolation effects in molecularly linked nanoparticle films

We study electrical transport in strongly coupled, molecularly linked, gold nanoparticle (NP) films whose bulk dc conductances are governed by percolation phenomena. Films with fewer NPs exhibit current suppression below a threshold voltage, likely due to single-electron charging of NP clusters. In some cases, the thresholds are very large (∼1 V) and suppression persists to room temperature. The thresholds tend to decrease with increasing amounts of NPs in the film, and eventually, metal-like conductance is observed down to at least 10 K. The observed trend toward metal-like conductance, despite the presence of film disorder, is enabled by strong inter-NP coupling and increasing film connectivity. The latter is an inherent property of molecularly linked NP films due to both robust chemical inter-NP linkages provided by alkane dithiol linker molecules, coupled with the ability to grow chains of connected NPs to arbitrary lengths through cyclical Au/dithol treatments. In the case of small thresholds, our da...

Influence of low energy barrier contact resistance in charge transport measurements of gold nanoparticle+dithiol-based self-assembled films.

Gold-thiol self-assembly is a widely employed strategy for engineering electronic devices using molecules and other nanostructures as building blocks. However, device behavior is expected to be governed by both building block architecture and contact effects. In order to elucidate the role of the latter in such devices, we have studied conductance of n-butanedithiol-linked Au nanoparticle (NP) films using different types of electrode configurations, namely, four-probe versus two-probe and break junctions before versus after dielectric break down of contact resistance. We find that contact resistance is governed by transport across a small barrier which can dominate device behavior when temperatures and resistances of the self-assembled devices are low. Accounting for such contact resistance reveals a more precise picture of device behavior in these regimes, including in the present system film properties near the onset of the percolation insulator-to-metal transition and beyond.

A hybrid scanning tunneling–atomic force microscope operable in air

We describe a hybrid scanning tunneling–atomic force microscope (STM–AFM) capable of measuring current and force simultaneously under ambient conditions. In order to reduce meniscus forces, the microscope uses a sharp STM tip as a probe and an AFM cantilever as a sample substrate. This improvement allows use of correspondingly flexible cantilevers enhancing force detection sensitivity. Using the STM–AFM, we have been able to explore a number of phenomena that can occur in nanometer scale tunnel junctions in air, including a correlation between hysteretic changes in contact potential and rapid increases in current at large bias voltages.

Discrete electron forces in a nanoparticle-tunnel junction system

According to the “orthodox” model for single electron tunneling, sudden changes in current–voltage characteristics of nanoparticle (NP)-tunnel junction (TJ) systems [“Coulomb blockade” (CB) and “Coulomb staircase” (CS) phenomena] arise fundamentally due to charge quantization. We have embedded NPs (∼2.5 nm in diameter) in the TJ of a hybrid scanning tunneling-atomic force microscope and have simultaneously measured current and forces generated in the system. We discuss an application to micromechanical switching actuated by single electrons. We also show that CB and CS phenomena are in fact associated with steplike changes in force, directly confirming the discrete charge nature of the phenomena.

Shadow mask fabrication of micron-wide break-junctions and their application in single-nanoparticle devices

We present a robust, shadow mask method for fabricating break-junctions (BJs). The method uses electromigration and results in devices comprising 100 µm wide gold wires separated by nanogaps. By functionalizing the BJs with alkanedithiols and using electrostatic trapping, we incorporate single gold nanoparticles (NPs) in the gaps with high yield. The resulting single-NP devices exhibit single-electron charging thresholds that can be probed using both voltage and temperature measurements. Both voltage thresholds and thermal activation energies scale with NP size as expected.

A rapid, high yield size-selective precipitation method for generating Au nanoparticles in organic solvents with tunably monodisperse size distributions and replaceable ligands

Size-selective precipitation (SSP) is a powerful tool for obtaining monodisperse nanoparticles. Here we report a fast, high yield and tunable SSP procedure via non-solvent addition for producing nearly monodisperse, organic-soluble Au nanoparticles with standard deviations as low as σ < 7%. The addition of excess ligands and judicious choice of ligand head group significantly improve both precipitate yields and nanoparticle monodispersity.

Microfabricated, silicon devices with nanowells and nanogap electrodes: a platform for dielectric spectroscopy with silane-tunable response

Combining the advantages of nanogap devices and impedance spectroscopy can potentially provide a platform for dielectric spectroscopy with widely ranging applications—from fundamental studies at the nanoscale and surfaces to label free and selective sensors. The present study characterizes the impedance response of a microfabricated, silicon-based device with a large array of nanowells surrounded by annular, nanogap detection regions. Device impedance is measured versus frequency over 5 orders in a variety of organic solvents with dielectric constants ranging over 2 orders. The study finds two key results. First, an equivalent R/C circuit model is found to compare favorably with device impedance response over these wide ranges of parameters. Importantly, the model correlates with structure of the nanogap device, which suggests that such a structure-impedance response approach can help guide modeling of other devices geometries. Second, the model points to—and data confirm—correlation between nanogap device response and dielectric constant of materials in the nanogaps, particularly at low frequencies. In addition, the correlation is significantly modified by robust, silane functionalization of the devices due to a large surface-to-volume ratio of the nanogaps. These results demonstrate that nanogap impedance spectroscopy using microfabricated/silanized silicon devices is a robust and versatile platform for dielectric spectroscopy of materials on the nanoscale and on surfaces.

Evaluation of a low cell constant conductance detector for detection of charged species in high-performance liquid chromatography

The present study evaluates high performance liquid chromatography (HPLC) detection based on a commercial conductance detector with a low cell constant of 0.005 cm−1 and a volume of 2 μL and a photodiode array UV-vis detector typically used in HPLC. When detecting a static NaCl solution, the conductance detector yields a limit of detection (LOD, 3 × noise) for NaCl of 39 parts per trillion. When flowing methanol through both conductance and UV-vis detectors and injecting benzoic acid/methanol, the signal-to-noise (S/N) ratio of the conductance detector is 8-fold higher than that of the UV-vis at its optimal wavelength. Using an HPLC with a C-18 column, flowing a 75 : 25 water : methanol solution, and using an acetylsalicylic acid (ASA)/water solution, the conductance detector yielded an ∼18-fold higher S/N ratio. It was found that HPLC system noise reduces the S/N ratio of the conductance detectors. The conductance detector detected non-chromophoric species generated by atmospheric CO2 as well as by decomposition of ASA. Conductance chromatograms yielded ASA peak heights and areas that varied linearly with the ASA concentration from 0.5 ppm to 10 ppm with linear correlation coefficients exceeding 0.999. In view of the sensitivity of conductance detection, its potential application as a sensitive tool for cleaning assessments for pharmaceutical equipment was confirmed by dispersing 50 μg of ASA on a 2′′ × 2′′ stainless steel sheet, swabbing the surface, dissolving the collected material in water and injecting the solution into the HPLC. The results in this study demonstrate that low cell constant conductance detection can be remarkably sensitive to ionized/charged species and thereby has potential to serve as an analytical tool for this important class of molecules.

Oligoazomethine-doped planar tunnel junctions: Correlating molecular structure with junction electrical characteristics

We have investigated electrical properties of planar aluminum/aluminum oxide/silver tunnel junctions modified with phenyl-based azomethine oligomers. Normalized differential conductance, NDC (NDC=σV/σV=0, where σ=dI/dV), of the junctions increases with oligomer length. At a bias of 2 V, azomethines with three phenyl rings exhibit NDCs that are on average more than an order of magnitude greater than those of unmodified oxide junctions. Differential conductances of junctions modified with azomethines increase more rapidly with temperature than those of plain oxide junctions. Our results are consistent with a model in which both increased conjugated length of the sandwiched organic layer and a molecule/metal interface lead to a lowering of the barrier profile outside the aluminum oxide tunnel region.