Y.B. Hahn

Growth of aligned ZnO nanorods and nanopencils on ZnO/Si in aqueous solution: growth mechanism and structural and optical properties

Well-aligned ZnO nanorod and nanopencil arrays were synthesized in a high density on ZnO/Si substrate by a low-temperature aqueous solution technique. Detailed structural characterizations revealed that the as-synthesized nanorods and nanopencils were single crystalline, with a hexagonal phase, and with growth along the [0001] direction. The room-temperature photoluminescence spectra showed a strong ultraviolet emission at 381 nm, a weak blue band at 460 nm, and a broad green emission at 580 nm. A detailed growth mechanism has been proposed for the formation of nanorods and nanopencils based on the different crystallographic habits of wurtzite hexagonal ZnO.

ZnO nanosheet networks and hexagonal nanodiscs grown on silicon substrate: growth mechanism and structural and optical properties

Two-dimensional ZnO nanosheet networks composed of many thin and uniform hexagonal-shaped ZnO nanosheets and ZnO nanodiscs were prepared in a large scale on silicon substrate through thermal evaporation using ZnCl2 and O2 as source materials for Zn and oxygen, respectively, without the use of metal catalysts or additives. Detailed structural studies indicated that the synthesized products are single crystalline with wurtzite hexagonal structure. Raman scattering of the synthesized products confirmed that the as-grown structures have good crystal quality with a hexagonal wurtzite phase. Room temperature photoluminescence spectra showed a strong green band with a suppressed UV emission from the ZnO nanosheet networks, but on the other hand a dominant and strong near band edge emission with a much suppressed deep level emission was observed in the nanodiscs. The growth mechanism of these structures is also discussed in detail.

Highly-sensitive cholesterol biosensor based on well-crystallized flower-shaped ZnO nanostructures.

This paper reports the fabrication of highly-sensitive cholesterol biosensor based on cholesterol oxidase (ChOx) immobilization on well-crystallized flower-shaped ZnO structures composed of perfectly hexagonal-shaped ZnO nanorods grown by low-temperature simple solution process. The fabricated cholesterol biosensors reported a very high and reproducible sensitivity of 61.7 microA microM(-1)cm(-2) with a response time less than 5s and detection limit (based on S/N ratio) of 0.012 microM. The biosensor exhibited a linear dynamic range from 1.0-15.0 microM and correlation coefficient of R=0.9979. A lower value of apparent Michaelis-Menten constant (K(m)(app)), of 2.57 mM, exhibited a high affinity between the cholesterol and ChOx immobilized on flower-shaped ZnO structures. Moreover, the effect of pH on ChOx activity on the ZnO modified electrode has also been studied in the range of 5.0-9.0 which exhibited a best enzymatic activity at the pH range of 6.8-7.6. To the best of our knowledge, this is the first report in which such a very high-sensitivity and low detection limit has been achieved for the cholesterol biosensor by using ZnO nanostructures modified electrodes.

Aligned hexagonal coaxial-shaped ZnO nanocolumns on steel alloy by thermal evaporation

Single-crystalline, hexagonal coaxial-shaped ZnO nanocolumns have been synthesized in a large quantity on steel alloy substrate via catalyst-free thermal evaporation of metallic zinc powder at 490°C. The optical-phonon E2 mode at 437cm−1 in Raman spectrum and a strong UV emission at 381nm in photoluminescence spectrum showed that the synthesized ZnO nanocolumns have a good crystal quality, with a wurtzite hexagonal phase and exhibited a good optical property. A layer-by-layer three-step growth mechanism was proposed for the formation of hexagonal coaxial-shaped ZnO nanocolumns.

Flower-shaped ZnO nanostructures obtained by cyclic feeding chemical vapour deposition: structural and optical properties

Flower-shaped ZnO nanostructures were grown on Si(100) and Si(111) substrates by cyclic feeding chemical vapour deposition without the use of a metal catalyst. The structures obtained on the Si(100) substrate exhibited triangle-shaped leaves with lengths and diameters in the ranges 300-400 and 90-130 nm, respectively. Uniformly shaped leaves with hexagonal facets originating from one centre were observed on the flower-shaped structures grown on the Si(111) substrate. Transmission electron microscopy and selected area electron diffraction patterns showed that these structures are highly crystalline, with the wurtzite hexagonal phase, and preferentially oriented in the c-axis direction. Unlike star-shaped ZnO nanostructures grown on Au-coated silicon substrates, the flower-shaped nanostructures showed sharp and strong UV emission at 378 nm and broad and weak green emission at 520 nm, indicating a good crystal quality and few structural defects.

Behavior of n-ZnO nanorods/p-Si heterojunction devices at higher temperatures

This work explores the temperature dependent heterojunction behavior of n-type zinc oxide (ZnO) nanorods/ZnO∕p-Si diodes. The as-grown ZnO nanorod structures on ZnO coated p-Si substrates are single crystalline and grown along the [001] direction. The p-n diode showed an excellent stability over the temperature range of 20–150°C due to highly doped p-type Si substrate. The turn-on and breakdown voltage of the device slightly decreased with an increase of temperature whereas the saturation current of the device increased from 0.42to0.67μA. The device behavior at different temperatures in forward as well as reverse biased conditions are studied and reported.

Selection of non-alloyed ohmic contacts for ZnO nanostructure based devices

Zinc oxide (ZnO) nanorods were synthesized using a simple and economic aqueous solution method on ZnO coated p-type Si substrates. Their physical properties have been examined using appropriate techniques. These investigations showed that the as-grown ZnO nanorods on ZnO/Si are single crystalline and grown along the [001] direction. These nanostructures exhibited the rms surface roughness and activation energy of ~16.7?nm and ~18?meV, respectively. Stable ohmic contacts are essential for reliable operation of any electronic device. To select such contacts for ZnO nanostructures, metal contacts (M = Ag, Al, In and Sn) were deposited on perfectly cleaned ZnO nanostructures (M/ZnO) using a thermal evaporation technique. From current versus voltage measurements, it is observed that at room temperature the M/ZnO structures except Al/ZnO exhibited nearly ohmic behaviour. With increasing temperature, the contact resistance of all M/ZnO structures except In/ZnO increased due to the formation of metallic bonds between metal and ZnO nanostructures. The annealing effect on M/ZnO structures was also studied, and their behaviour has been discussed and reported.

Efficient blue light-emitting diodes with InGaN/GaN triangular shaped multiple quantum wells

InGaN/GaN triangular shaped multiple quantum wells (QWs) grown by grading In composition with time were adopted as an active layer of blue light-emitting diodes (LEDs). Compared to the LEDs with conventional rectangular QW structures, the triangular QW LEDs showed a higher intensity and a narrower linewidth of electrical luminescence (EL), a lower operation voltage, and a stronger light-output power. EL spectra of the triangular-QW-based LEDs also showed that the peak energy is nearly independent of the injection current and temperature, indicating that the triangular QW LED is more efficient and stable than the rectangular one.

Synthesis and Characterization of Nanocrystalline SnS Films Grown by Thermal Evaporation Technique

The SnS films were grown on glass substrates using the thermal evaporation technique at different substrate temperatures

Growth Mechanism and Optical Properties of Aligned Hexagonal ZnO Nanoprisms Synthesized by Noncatalytic Thermal Evaporation

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