Reui-San Chen

Ultrahigh photocurrent gain in m-axial GaN nanowires

An ultrahigh photocurrent gain has been found in the ultraviolet-absorbed GaN nanowires with m-directional long axis grown by chemical vapor deposition. The quantitative results have shown the gain values at 5.0×104–1.9×105 of the GaN nanowires with diameters from 40to135nm are near three orders of magnitude higher than the values of 5.2×101–1.6×102 estimated from the thin film counterparts. The intensity-dependent gain study has shown that the gain value is very sensitive to the excitation intensity following an inverse power law and no gain saturation observed in this investigated intensity range from 0.75to250W∕m2. This behavior has strongly suggested a surface-dominant rather than trap-dominant high gain mechanism in this one-dimensional nanostructure. The strong carrier localization effect induced by the surface electric field in the GaN nanowires is also discussed.

High photocurrent gain in SnO2 nanowires

Huge photocurrent gains with a value of 10 000 detected in air and 100 000 detected in vacuum have been obtained from SnO2 nanowires. Unlike previous reports that emphasized on carrier trapping by charge oxygen molecules in metal oxide nanostructures, we point out that spatial separation of photoexcited electrons and holes due to built-in electric field induced by surface charge defects does plays a significant role especially for nanostructured materials with a large surface-to-volume ratio. The study shown here provides a useful guideline to achieve photodetectors based on nanostructured materials with high sensitivity.

Field emission from vertically aligned conductive IrO2 nanorods

We report on the preparation and field-emission properties of vertically aligned conductive IrO2 nanorods. The unique geometrical features of IrO2 nanorods, including nanosized structure and self-assembled sharp tip, exhibit a strong effect on field enhancement (β∼40 000), which result in a low threshold field (Eth∼0.7 V/μm) defined at the beginning of emission. A low turn-on field for driving a current of 10 μA/cm2 is about 5.6 V/μm, which is comparable with the carbon nanotube, diamond, and amorphous carbon. The potential of using IrO2 nanorods as an emitter material has been demonstrated.

On-Chip Fabrication of Well-Aligned and Contact-Barrier-Free GaN Nanobridge Devices with Ultrahigh Photocurrent Responsivity**

In the last two decades, a wide range of semiconductor nanowires have been synthesized and used as building blocks for the development of a new generation of electronic and optoelectronic devices. [1–8] The integration of these nanowires into the thin-film-based microchip has become a critical problem in the practical application of the nanomaterial properties and for industrial manufacture. Single-wire-based devices have been shown to possess novel properties and provide a major platform for fundamental research. [9–13] However, the conventional fabrication of single-wire devices by the ‘‘pick and place’’ method is rather complicated and uneconomic, which is unsuitable for large-scale manufacturing. In contrast, devices based on an ensemble of nanowires are much easier to fabricate, thus reducing the barriers to practical applications. A method based on the bridging concept and showing the potential to directly integrate an ensemble of nanowires onchip was first demonstrated by Haraguchi et al. [14] Different from the traditional ‘‘bottom-up growth and then top-down processing’’, the nanowires are laterally grown across a trench and suspended between two film posts as nanobridges (NBs). [14–26] As the main device architecture and electrodes can be designed and prepared prior to NB growth, nanomaterial deterioration due to the post treatment in the

Growth control and characterization of vertically aligned IrO2 nanorods

Iridium dioxide (IrO2) nanorods with pointed tips have been grown on Si(100) and transition-metal-coated-Si(100) substrates, via metal–organic chemical vapor deposition (MOCVD), using (MeCp)Ir(COD) as the source reagent. The as-deposited nanorods were characterized using field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). FESEM micrographs revealed that the majority of the nanorods are a wedge shape in cross section and converge at top; occasionally several of them pack into a column of a spiral tip. The vertical alignment and packing density are significantly improved by prior deposition of a thin layer of Ti on Si. TEM and XRD results indicate that the sputtered Ti thin layer erects the nanorods in the c-axis direction. XPS spectra show that iridium in IrO2 nanorods also exist in a higher oxidation state.

High-gain photoconductivity in semiconducting InN nanowires

We report on the photoconductivity study of the individual infrared-absorbing indium nitride (InN) nanowires. Temperature-dependent dark conductivity measurement indicates the semiconducting transport behavior of these InN nanowires. An enhanced photosensitivity from 0.3 to 14 is observed by lowering the temperature from 300to10K. A calculated ultrahigh photoconductive gain at around 8×107 at room temperature is obtained from the low-bandgap nitride nanowire under 808nm excitation.

One-dimensional conductive IrO2 nanocrystals

We review the results of the synthesis of IrO2 nanocrystals (NCs) on different substrates via metal-organic chemical vapour deposition (MOCVD) using (MeCp)(COD)Ir as the source reagent. The surface morphology, structural and spectroscopic properties of the as-deposited NCs were characterized using field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), selected-area electron diffractometry (SAD), x-ray diffractometry (XRD), x-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The roles of different substrates for the formation of various textures of nanocrystalline IrO2 are studied. Several one-dimensional (1D) nanostructures have evolved by decreasing the degree of interface instability. The morphological evolution occurs from triangular/wedged nanorods via incomplete/scrolled nanotubes to square nanotubes and square nanorods (NRs), with increasing morphological stability. The results show that the three-dimensional (3D) grains composing traditional film belong to the most stable form as compared to all the 1D NCs, and the sequential shape evolution has been found to be highly correlated to a morphological phase diagram based on the growth kinetics. In addition, area selective growth of IrO2 NRs has been demonstrated on sapphire(012) and sapphire(100) substrates which consist of patterned SiO2 as the nongrowth surface. The initial growth of IrO2 nuclei is studied. Selectivity, rod orientation, and other morphological features of the nanorod forest can find their origins in the nucleation behaviour during initial growth. XPS analyses show the coexistence of higher oxidation states of iridium in the as-grown IrO2 NCs. The usefulness of the experimental Raman scattering together with the modified spatial correlation (MSC) model analysis as a residual stress and structural characterization technique for 1D IrO2 NCs has been demonstrated. The field emission properties of the vertically aligned IrO2 NRs are studied and demonstrated as a high-performance and robust field emitter material owing to its low work function, low resistivity and excellent stability against oxygen.

Molecule-modulated photoconductivity and gain-amplified selective gas sensing in polar GaN nanowires

We report the strong molecular effects on the surface-dominant photoconductivity with high-gain transport in the polar GaN nanowires. Both the transient and steady-state photocurrents are sensitive and selective to the adsorptions of oxygen and hydrogen. The surface band bending of GaN nanowires is proposed to be effectively reduced or enhanced by oxygen or hydrogen, respectively, as a donorlike or acceptorlike surface state. The molecular effect, corroborated with the high-gain photoresponse nature of GaN nanowires is found to amplify the molecule-selective photocurrent signal by near three orders of magnitude higher than its counterpart in dark current. The molecule-tunable photoconductivity, as demonstrated here, would benefit a variety of applications, ranging from the high-gain optoelectronic devices, photoelectric energy transducer, as well as gas and chemical sensors.

Photoconductivity in single AlN nanowires by subband gap excitation

Photoconductivity of individual aluminum nitride (AlN) nanowires has been characterized using different subband gap excitation sources. It is interesting that both positive (under 1.53 and 2.33 eV excitations) and negative (under 3.06 and 3.81 eV excitations) photocurrent responses are observed from the wide band gap nitride nanowires. The negative photoconductivity, which is attributed to the presence of electron trap and recombination center in the bulk of AlN, is capable to be inversed by a strong positive photoconductive mechanism of surface while changes the ambience from the atmosphere to the vacuum. An oxygen molecular sensitization effect is proposed to be the reason resulting in the enhancement of positive photocurrent and the inversion of negative photoresponse in the vacuum. Understanding of the diverse photoconductivity and its molecular effect is of great importance in the development of energy-selective and highly sensitive nanowire photodetector of AlN in the visible and ultraviolet ranges.

Raman scattering and field-emission properties of RuO2 nanorods

We report Raman scattering and field emission properties of rutile RuO2 nanorods obtained by metalorganic chemical vapor deposition. The RuO2 nanorods have lengths up to several micrometers and diameters in the range of 10–50 nm. The nanosize dependencies of the peak shift and the broadening of the three first-order Raman modes agree well with those calculated on the basis of the phonon confinement model. The unique geometrical features of RuO2 nanorods exhibit a strong effect on field enhancement (β∼1153), which results in a low threshold field (Eth∼4.9V∕μm) defined at the beginning of emission. The low turn-on field for driving a current of 10μA∕cm2 is about 10.3V∕μm, which is comparable with amorphous carbon film. Our results indicate that RuO2 nanorods provide an excellent alternative for field emitter due to several advantages, including nanometer structure, natural conductor, enhanced resistance to oxidation, and long-term stability.