Chong-Yang Liu

Electrochemical Characterization of Films of Single-Walled Carbon Nanotubes and Their Possible Application in Supercapacitors

Films of single-wall carbon nanotubes (SWCNTs) were cast from suspensions in several solvents on the surface of a Pt or Au electrode. Cyclic voltammetry of the films in MeCN did not show well-resolved waves (as distinct from films of C_(60) prepared in a similar manner). However, the increase in the effective capacitance of the electrode with a SWCNT film at 0.5 V vs. an AgQRE was 283 F/g, which is about twice that of carbon electrodes in nonaqueous solvents.

Electrostatic electrochemistry at insulators

The identity of charges generated by contact electrification on dielectrics has remained unknown for centuries and the precise determination of the charge density is also a long-standing challenge. Here, electrostatic charges on Teflon (polytetrafluoroethylene) produced by rubbing with Lucite (polymethylmethacrylate) were directly identified as electrons rather than ions by electrochemical (redox) experiments with charged Teflon used as a single electrode in solution causing various chemical reactions: pH increases; hydrogen formation; metal deposition; Fe(CN)(6)(3-) reduction; and chemiluminescence in the system of Teflon(-)/Ru(bpy)(3)(2+)/S(2)O(8)(2-) (analogous to electrogenerated chemiluminescence). Moreover, copper deposition could be amplified by depositing Pd first in a predetermined pattern, followed by electroless deposition to produce Cu lines. This process could be potentially important for microelectronic and other applications because Teflon has desirable properties including a low dielectric constant and good thermal stability. Charge density was determined using Faradays law and the significance of electron transfer processes on charged polymers and potentially other insulators have been demonstrated.

Chemical redox reactions induced by cryptoelectrons on a PMMA surface.

We show that pristine PMMA can spontaneously transfer electrons to species in a liquid, thereby inducing a variety of electron transfer reactions. The electrons that are transferred we call cryptoelectrons; these have a surface density of the order of 5 x 10(13) cm(-2) and are at a considerably more negative reduction potential than the PMMA bonding electrons. For example, metal ions including Ag(+), Cu(2+), and Pd(2+) were reduced and plated on a PMMA surface and Fe(CN)(6)(3-) was reduced to Fe(CN)(6)(4-). Moreover, protons were reduced when PMMA powder was dropped into a slightly acidic solution, resulting in a pH increase and hydrogen generation. Chemiluminescence was produced in a solution containing Ru(bpy)(3)(2+) and S(2)O(8)(2-) with the addition of PMMA powder. These results clearly demonstrate that there are available electrons in PMMA that can participate in redox reactions at a rather negative potential. We also show that contacting PMMA with Teflon depletes this electronic surface charge. However, the PMMA used for a redox reaction or contacted with Teflon that was depleted of the electronic surface charge could be recharged by contacting with a suitable reductant.

Electrochemistry and electrogenerated chemiluminescence with a single faradaic electrode

We describe an electrochemical cell containing a single faradic electrode (a Pt ultramicroelectrode) and a blocked (polarized) electrode (Si with an insulating SiO2 film) that served as a capacitive counter electrode. When a bias was applied between the two electrodes, a current was observed from a faradic process at Pt and a capacitive current at the blocking electrode. A steady charging current was obtained when the solution was moved along the insulator to continuously contact fresh surface (emersing a layer of charged ions and electronic countercharge into the gas phase). Electrogenerated chemiluminescence was clearly observed from a system containing Ru(bpy)(3)2+ in this kind of cell under pulsed excitiation, demonstrating that faradaic reactions can be carried out in an electrochemical cell without generation of any products at a counter electrode. The use of such a system for coulometric addition of desired species in nanosystems and in synthesis is suggested.

Pressure-induced insulator: conductor transition in a photoconducting organic liquid-crystal film

Intermolecular separation determines the extent of orbital overlap and thus the rate of electron transfer between neighbouring molecules in an organic crystal. If such a crystal is compressed, the resistivity decreases owing to a diminishing intermolecular distance. Metal–insulator transitions have been observed by applying hydrostatic pressure to, for example, Langmuir films of metal nanoparticles. But previous attempts to observe a clear transition point in organic crystals, such as anthracene and tetracene, were not successful owing to difficulties with electrically insulating the high-pressure cell. Here we report a different approach by using a sample that is photoconductive and forms an organized film. A cylindrical tip (∼100 μm in diameter) was used to compress the sample instead of a piston/cylinder structure, entirely eliminating the problem of electrical insulation. Furthermore, by illuminating the sample with a laser, the conductivity of the sample is increased by several orders of magnitude. By monitoring the photocurrent with sensitivity at the 10-13 A level, changes in resistivity at very low pressure could be monitored. We observe a sharp increase in current that could indicate a transition from hopping to delocalized conduction.

Highly efficient and bright electroluminescent Ru(bpy)3(ClO4)2∕Alq3 device

Electroluminescent devices of indium tin oxide (ITO)∕Alq3(212nm)∕Ru(bpy)3(ClO4)2(100nm)∕Ga:In were fabricated and showed high external quantum and power efficiencies, of 6.4% and 5.3%, respectively, at an optical output power of 0.4mW∕cm2 under a bias of 2.3 V. At a higher bias voltage (>2.7V), the output power was well above 10mW∕cm2, but with a lower efficiency. Light emission occurred at the interface between Alq3 and Ru(bpy)3(ClO4)2, whose relative energies of both excited and ground states were offset, ideal for confining both charge carriers and minimizing the quenching of the Ru(bpy)32+ excited state. By comparison, in a single layer device without Alq3, the emission zone was located at the ITO interface where excited states were quenched and electron injection from the reduced molecules to the ITO contact produced a unipolar current and thus lower efficiency.

Increased photo- and electroluminescence by zone annealing of spin-coated and vacuum-sublimed amorphous films producing crystalline thin films

Spin-coated and vacuum sublimed amorphous thin films (∼100 nm thick) were converted into single crystalline films with a simple moving-zone-annealing technique, in which a heated metal wire generated a narrow annealed zone in the film as it is moved across the sample. This annealing resulted in both the photoluminescence and electroluminescence to increase dramatically (by 3–4 times), for example with Ru(bpy)3(ClO4)2 and aluminum 8-hydroxyquinoline, (Alq3), as light emitting devices. This technique is of interest in improving the behavior of highly luminescent thin film optoelectronic devices.

Irradiation-induced nanometer-scale surface etching of a CdSe film with a scanning tunneling microscope

Abstract Nanometer size features have been etched in a circa 200 A thick CdSe film held in a scanning tunneling microscope (STM) by irradiation with a He—Ne laser (8 mW) with the STM tip biased to a positive potential. Etching does not occur in the absence of irradiation or with the STM tip biased negative. A tentative mechanism based on electrical field assisted photodecomposition is proposed.

Improving z-tracking accuracy in the two-photon single-particle tracking microscope

Here, we present a method that can improve the z-tracking accuracy of the recently invented TSUNAMI (Tracking of Single particles Using Nonlinear And Multiplexed Illumination) microscope. This method utilizes a maximum likelihood estimator (MLE) to determine the particles 3D position that maximizes the likelihood of the observed time-correlated photon count distribution. Our Monte Carlo simulations show that the MLE-based tracking scheme can improve the z-tracking accuracy of TSUNAMI microscope by 1.7 fold. In addition, MLE is also found to reduce the temporal correlation of the z-tracking error. Taking advantage of the smaller and less temporally correlated z-tracking error, we have precisely recovered the hybridization-melting kinetics of a DNA model system from thousands of short single-particle trajectories in silico. Our method can be generally applied to other 3D single-particle tracking techniques.

Addressing of Optoelectronic Memory of Thin Film Zinc Porphyrin with Crossed 5 μm Indium Tin Oxide Arrays

We report here the preparation and characterization of an optoelectronic memory device based on a single layer of organic thin film ~;0.9 mm thick! of zinc octakis~b-decoxyethyl !porphyrin, sandwiched between two crossed indium tin oxide ~ITO! arrays. The ITO lines in the array were 5 mm wide and were separated from each other by a 5 mm gap. Data~in the form of an electric charge! could be independently stored at and retrieved from an intersection of the crossed ITO lines with irradiation. Each intersection defined one memory pixel (5 3 5 mm) and there was no cross talk with nearby pixels under the test conditions, clearly demonstrating its potential application as an information storage device using a molecular thin film.