Chaiwat Engtrakul

Near-perfect conduction through a ferrocene-based molecular wire

Here we describe the design, single-molecule transport measurements, and theoretical modeling of a ferrocene-based organometallic molecular wire, whose bias-dependent conductance shows a clear Lorentzian form with magnitude exceeding 70% of the conductance quantum

Hole doping in Al-containing nickel oxide materials to improve electrochromic performance.

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Single-wall carbon nanotube coating on a pyroelectric detector

. We attribute this unprecedented level of single-molecule conductance to a manifestation of the low-lying molecular resonance and extended orbital network long predicted for a conjugated organic system. A similar-in-length, all-organic conjugated phenylethynyl oligomer molecular framework shows much lower conductance.

n-Type transparent conducting films of small molecule and polymer amine doped single-walled carbon nanotubes.

Electrochromic materials exhibit switchable optical properties that can find applications in various fields, including smart windows, nonemissive displays, and semiconductors. High-performing nickel oxide electrochromic materials have been realized by controlling the material composition and tuning the nanostructural morphology. Post-treatment techniques could represent efficient and cost-effective approaches for performance enhancement. Herein, we report on a post-processing ozone technique that improves the electrochromic performance of an aluminum-containing nickel oxide material in lithium-ion electrolytes. The resulting materials were studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), ultraviolet-visible-near-infrared (UV-vis-NIR) spectroscopy, and X-ray absorption spectroscopy (XAS). It was observed that ozone exposure increased the Ni oxidation state by introducing hole states in the NiO(6) octahedral unit. In addition, ozone exposure gives rise to higher-performing aluminum-containing nickel oxide films, relative to nickel oxide containing both Al and Li, in terms of switching kinetics, bleached-state transparency, and optical modulation. The improved performance is attributed to the decreased crystallinity and increased nickel oxidation state in aluminum-containing nickel oxide electrochromic films. The present study provides an alternative route to improve electrochromic performance for nickel oxide materials.

In situ crystallization of high performing WO3-based electrochromic materials and the importance for durability and switching kinetics

Carbon single-wall nanotubes (SWNTs) are studied as the thermal-absorption coating on a large area pyroelectric detector. The SWNTs were produced by a laser vaporization method and dispersed onto the detector surface by use of a simple airbrush technique. The detector was based on a 1-cm-diameter, 60-microm-thick lithium tantalate disk having nickel electrodes. We report the spectral responsivity of the detector ranging from 600 to 1800 nm, as well as the spatial and directional uniformity at 850 nm. Using Drude and Lorentzian dielectric functions and an effective medium approximation to obtain the indices of refraction of semiconductor and metallic SWNTs, we compared the expected theoretical relative responsivity for the two types of tube with the measured relative responsivity of the detector. Values of thermal conductivity, specific heat, and damage threshold obtained from the literature are compared with properties of alternatives for thermal coatings such as gold-black and carbon-based paint.

Lithiation of silica through partial reduction

In this report, we investigate the electrical and optical properties of thin conducting films of SWNTs after treatment with small molecule and polymeric amines. Among those tested, we find hydrazine to be the most effective n-type dopant. We use absorbance, Raman, X-ray photoelectron, and nuclear magnetic resonance spectroscopies on thin conducting films and opaque buckypapers treated with hydrazine to study fundamental properties and spectroscopic signatures of n-type SWNTs and compare them to SWNTs treated with nitric acid, a well-characterized p-type dopant. We find that hydrazine physisorbs to the surface of semiconducting and metallic SWNTs and injects large electron concentrations, raising the Fermi level as much as 0.7 eV above that of intrinsic SWNTs. Hydrazine-treated transparent SWNT films display sheet resistances nearly as low as p-type nitric-acid-treated films at similar optical transmittances, demonstrating their potential for use in photovoltaic devices as low work function transparent electron-collecting electrodes.

Protonation effects on the branching ratio in photoexcited single-walled carbon nanotube dispersions.

A chemical self-assembled synthesis was used to prepare disordered porous semicrystalline WO3-based cathodic electrochromic films. The resulting films were characterized with transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), N2 physisorption, Fourier transform infrared spectroscopy (FTIR) and near-edge X-ray absorption fine structure (NEXAFS). The electrochromic performance was evaluated in a Li-ion electrolyte (i.e., 1 M LiClO4 dissolved in propylene carbonate) with cyclic voltammetry (CV), galvanostatic cycling with potential limitation (GCPL) and chronoamperometry (CA) techniques. It is demonstrated that the crystallinity of WO3 thin films can be readily tuned with the variation of annealing temperatures and TiO2 addition. The results demonstrated excellent stability and durability (i.e., 1500 GCPL cycles in 32 days) for WO3 electrode annealed at 350 °C, ultrafast switching kinetics for WO3–TiO2 electrode (i.e., bleaching and coloration times are 5.5 s and 4.2 s, respectively) and excellent charge reversibility (%R ≈ 100%). Electrochemical, TEM and Raman spectroscopy studies suggest that a change in degree of crystallinity in WO3 occurs during the extended durability test, which then influences the durability and switching kinetics.

Solid-State 13C NMR Assignment of Carbon Resonances on Metallic and Semiconducting Single-Walled Carbon Nanotubes

We demonstrate the reversible lithiation of SiO2 up to 2/3 Li per Si, and propose a mechanism for it based on molecular dynamics and density functional theory simulations. Our calculations show that neither interstitial Li (no reduction), nor the formation of Li2O clusters and Si–Si bonds (full reduction) are energetically favorable. Rather, two Li effectively break a Si–O bond and become stabilized by oxygen, thus partially reducing the SiO2 anode: this leads to increased anode capacity when the reduction occurs at the Si/SiO2 interface. The resulting LixSiO2 (x<2/3) compounds have band-gaps in the range of 2.0–3.4 eV.

Near-infrared Fourier transform photoluminescence spectrometer with tunable excitation for the study of single-walled carbon nanotubes

The ensemble PL quantum yield for raw single-walled carbon nanotubes (SWNTs) dispersed in sodium cholate (SC) is approximately 5 times greater than that for the same raw SWNTs dispersed in sodium dodecyl sulfate (SDS) and approximately 10 times greater than the quantum yield of purified SWNTs dispersed in SC. Absorbance and Raman spectra indicate that purified SC-dispersed SWNTs and raw SDS-dispersed SWNTs are hole-doped by protonation. Experiments comparing PL emission efficiency using E2 and E1 excitation show that protonation significantly affects the E2 --> E1 relaxation process, which has typically been assumed to occur with unit efficiency. The E2 --> E 1 relaxation is 5 times more efficient in producing E 1 PL when SWNTs are unprotonated and protected by the SC surfactant. The results provide clear evidence that extrinsic factors, such as residual acids and the specific nature of SWNT-surfactant and SWNT-solvent interactions, can significantly affect measured SWNT luminescence quantum yields.

Infrared responsivity of a pyroelectric detector with a single-wall carbon nanotube coating.

Solid-state (13)C NMR spectroscopy was used to investigate the chemical shift of nanotube carbons on m- and s-SWNTs (metallic and semiconducting single-walled nanotubes) for samples with widely varying s-SWNT content, including samples highly enriched with nearly 100% m- and s-SWNTs. High-resolution (13)C NMR was found to be a sensitive probe for m- and s-SWNTs in mixed SWNT samples with diameters of approximately 1.3 nm. The two highly enriched m- and s-SWNT samples clearly exhibited features for m- and s-SNWT (13)C nuclei (approximately 123 and 122 ppm, respectively) and were successfully fit with a single Gaussian, while five mixed samples required two Gaussians for a satisfactory fit.