Taewoo Ha

Reflection terahertz time-domain spectroscopy of RDX and HMX explosives

We report on our study of RDX and HMX, two of the most commonly used explosive materials, in bulk pellets with reflection terahertz time-domain spectroscopy in the frequency range of 0.3–3 THz. The maximum entropy method was utilized to correct our raw reflection data against the phase error due to the relative displacement between the sample and the reference. Both the refractive index n and the extinction coefficient k in the terahertz region were acquired for these two explosives without a Kramers-Kronig analysis. Both RDX and HMX exhibit a series of distinct peaks not quite detectable in the more conventional transmission-type measurements due to their high terahertz absorptivity. Our results are compared with the literature data on powder samples.

Terahertz and optical study of monolayer graphene processed by plasma oxidation

We report on our terahertz and optical study of monolayer graphene grown by chemical vapor deposition and processed by plasma oxidation. The plasma oxidation induces oxygen-related defects, and the resulting disorder increases the sheet resistance of graphene as measured via terahertz spectroscopy. The excitonic absorption peak weakens considerably and blue shifts upon plasma oxidation, resulting in higher transmittance in both the visible and ultraviolet regions. Our oxygen plasma-treated graphene also exhibits a free-carrier doping effect as confirmed by the blue shift of the Raman G band.

An almost transparent image pixel with a pentacene/ZnO photodiode, a pentacene thin-film transistor, and a 6,13-pentacenequinone phosphor layer.

Oxide and organic devices on glass have extensively been studied with prototype forms, such as thin-fi lm transistors (TFTs), light-emitting diodes (LEDs), and organic/inorganic hybrid p–n diodes, due to their distinct advantages and functionalities including low fabrication temperature, transparency, and light emission. [ 1–5 ] One of the outstanding functionalities in glass– substrate devices may be transparency and thus several forms of transparent devices have already been reported. [ 5–7 ] However, among such functional organic and oxide devices fabricated on glass, visible detectors are rare. [ 8–10 ] This is because most oxide or organic semiconductors have a large energy band gap ( E g ) of more than ≈ 2 eV, unlike Si-based semiconductors. Such a relatively large band gap allows only a marginal photoabsorption in the visible range. Nevertheless, the realization of transparent image pixels and visible detectors is probably crucial for more smart and mature transparent electronics since they can be integrated with already-demonstrated components, such as transparent complementary inverters, transparent memories, and transparent display panel. [ 5–7 ] Here, considering solid pentacene to be a visible light receiver with a marginal band gap of 1.97 eV, [ 11 ] we successfully fabricated two different types of transparent image pixels that operate at 3 V. They were composed of a pentacene thin-fi lm transistor (TFT) and a semitransparent pentacene/ZnO photodiode with a transparent top electrode in the interest of a light detectable smart functional window; the light detection is available from the both sides of the glass. To the best of our knowledge, a transparent image pixel has not been reported yet, and here we use 150-nm-thick semitransparent pentacene to secure the absorption and detection of visible photons. One type of the transparent pixels demonstrated here was equipped with an additional 6,13-pentacenequinone phosphor layer deposited on the pentacene/ZnO photodiode. The organic phosphor transmits most of visible photons but absorbs UV photons and converts them to yellow-green photons, so that the emitted yellow-green light excites the pentacene/ZnO diode under a reverse bias state. Our approach to the transparent pixel with such an organic phosphor layer

Terahertz electrodynamics and superconducting energy gap of NbTiN

Terahertz electrodynamics of superconducting NbTiN has been studied in the spectral range of 8-70 cm–1 above and below the critical temperature of Tc = 14.1 K. Our transmission terahertz time-domain spectroscopy technique allows for independent and accurate determination of both the real part σ1 and the imaginary part σ2 of the optical conductivity σ as a function of frequency ω and temperature T without a Kramers-Kronig analysis. A clear signature of the superconducting energy gap Δ(T) is observed in the real part σ1 of the optical conductivity below Tc, with 2Δ(0) = 36.5 cm−1 corresponding to 2Δ(0)/kBTc = 3.72. No indication of strong-coupling effects was observed in the imaginary part σ2 toward zero frequency. Our results, including the temperature dependence of the penetration depth λ(T), are in excellent quantitative agreement with weak coupling BCS theory and Mattis-Bardeen formula.

Ferroelectric property improvement of poly(vinylidene fluoride/trifluoroethylene) polymer exposed to a plasma ambient

We report a dramatic property improvement of ferroelectric polymer poly(vinylidene fluoride/trifluoroethylene) [P(VDF-TrFE)] upon exposure to a plasma ambient. The P(VDF-TrFE) layer for typical nonvolatile memory devices shows only ∼6.5 μC/cm2 and 0.55 MV/cm for its remnant polarization and coercive field, respectively, but the values increase to ∼13 μC/cm2 and ∼1 MV/cm after an optimum period of plasma exposure. Our infrared measurement confirms that the plasma energy creates many CC double bonds in the polymer chain while removing some of CF bonds, which are responsible for ferroelectric polarization. Nevertheless, the strengthened polymer can incorporate higher applied field, thereby aligning electric dipoles much better.

Terahertz time-domain spectroscopy of NiO x thin films

We have measured the terahertz transmittance of NiOx thin films grown on Si by thermal evaporation. The frequency-dependent conductivities were determined without resorting to a Kramers-Kronig analysis. Large changes in these spectral functions occurred due to varying deposition rate and annealing temperature. We observed a direct correlation between these parameters with the electronic and optical properties of NiOx thin films.