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Gwo Research Group
Research Highlights

III-Nitrides / Plasmonics / Nanosensing



Giant colloidal silver crystals for low-loss linear and nonlinear plasmonics


Nature Communications 6, 7734 (2015)

Chun-Yuan Wang, Hung-Ying Chen, Liuyang Sun, Wei-Liang Chen, Yu-Ming Chang, Hyeyoung Ahn, Xiaoqin Li & Shangjr Gwo

Structural properties of giant colloidal silver single crystals.

The development of ultrasmooth, macroscopic-sized silver (Ag) crystals exhibiting reduced losses is critical to fully characterize the ultimate performance of Ag as a plasmonic material, and to enable cascaded and integrated plasmonic devices. Here we demonstrate the growth of single-crystal Ag plates with millimetre lateral sizes for linear and nonlinear plasmonic applications. Using these Ag crystals, surface plasmon polariton propagation lengths beyond 100 μm in the red wavelength region are measured. These lengths exceed the predicted values using the widely cited Johnson and Christy data. Furthermore, they allow the fabrication of highly reproducible plasmonic nanostructures by focused ion beam milling. We have designed and fabricated double-resonant nanogroove arrays using these crystals for spatially uniform and spectrally tunable second-harmonic generation. In conventional ‘hot-spot’-based nonlinear processes such as surface-enhanced Raman scattering and second-harmonic generation, strong enhancement can only occur in random, localized regions. In contrast, our approach enables uniform nonlinear signal generation over a large area.



All-Color Plasmonic Nanolasers with Ultralow Thresholds: Autotuning Mechanism for Single-Mode Lasing


Nano Lett., 14 (8), pp 4381–4388 (2014)

Yu-Jung Lu, Chun-Yuan Wang, Jisun Kim, Hung-Ying Chen, Ming-Yen Lu, Yen-Chun Chen, Wen-Hao Chang, Lih-Juann Chen, Mark I. Stockman, Chih-Kang Shih, and Shangjr Gwo*

We report on the first demonstration of broadband tunable, single-mode plasmonic nanolasers (spasers) emitting in the full visible spectrum. These nanolasers are based on a single metal–oxide–semiconductor nanostructure platform comprising of InGaN/GaN semiconductor nanorods supported on an Al2O3-capped epitaxial Ag film. In particular, all-color lasing in subdiffraction plasmonic resonators is achieved via a novel mechanism based on a property of weak size dependence inherent in spasers. Moreover, we have successfully reduced the continuous-wave (CW) lasing thresholds to ultrasmall values for all three primary colors and have clearly demonstrated the possibility of “thresholdless” lasing for the blue plasmonic nanolaser.


Dynamic Visualization of Axial p–n Junctions in Single Gallium Nitride Nanorods under Electrical Bias


ACS Nano, 7 (9), pp 7640–7647 (2013)

Yu-Jung Lu, Ming-Yen Lu, Yu-Chen Yang, Hung-Ying Chen, Lih-Juann Chen, and Shangjr Gwo*

We demonstrate a direct visualization method based on secondary electron (SE) imaging in scanning electron microscopy for mapping electrostatic potentials across axial semiconductor nanorod p–n junctions. It is found that the SE doping contrast can be directly related to the spatial distribution of electrostatic potential across the axial nanorod p–n junction. In contrast to the conventional SE doping contrast achieved for planar p–n junctions, the quasi-one-dimensional geometry of nanorods allows for high-resolution, versatile SE imaging under high accelerating voltage, long working distance conditions. Furthermore, we are able to delineate the electric field profiles across the axial nanorod p–n junction as well as depletion widths at different reverse biases. By using standard p–n junction theory and secondary ion mass spectroscopy, the carrier concentrations of p- and n-regions can be further extracted from the depletion widths under reverse biasing conditions. This direct imaging method enables determination of electrostatic potential variation of p–n junctions in semiconductor nanorod and nanowire devices with a spatial resolution better than 10 nm.


Ultrasensitive Surface Acoustic Wave Detection of Collective Plasmonic Heating by Close-Packed Colloidal Gold Nanoparticles Arrays


J. Phys. Chem. C, 2013, 117 (6), pp 2442–2448

Chi-Shun Chiu , Hung-Ying Chen , Chun-Fang Hsiao , Meng-Hsien Lin , and Shangjr Gwo *

A novel electrical detection method based on surface acoustic wave (SAW) is introduced to investigate the heat generation from a two-dimensional (2D) or three-dimensional (3D) close-packed gold nanoparticle (AuNP) array exhibiting tunable collective surface plasmon resonance (CSPR). We have found that the SAW sensor can detect the CSPR-induced photothermal heating effect. The conversion of absorbed light around the CSPR frequency into heat and subsequent transfer from the AuNP array to the lithium niobate (LiNbO3) substrate allows a dual delay-line SAW system to dynamically measure the resonant frequency change of the SAW oscillator coated with a 2D or 3D AuNP array. The experimental findings indicate that SAW detection provides an ultrasensitive (detection limit of temperature change <1 × 10–3 °C) electrical readout scheme for measuring the collective plasmonic heating of close-packed AuNP arrays with a linear dynamic range of incident light intensity and a designable CSPR spectral response.





Plasmonic Nanolaser Using Epitaxially Grown Silver Film


Yu-Jung Lu1, Jisun Kim2, Hung-Ying Chen, Chihhui Wu, Nima Dabidian, Charlotte E. Sanders, Chun-Yuan Wang, Ming-Yen Lu, Bo-Hong Li, Xianggang Qiu, Wen-Hao Chang, Lih-Juann Chen, Gennady Shvets, Chih-Kang Shih, Shangjr Gwo

A nanolaser is a key component for on-chip optical communications and computing systems. Here, we report on the low-threshold, continuous-wave operation of a subdiffraction nanolaser based on surface plasmon amplification by stimulated emission of radiation. The plasmonic nanocavity is formed between an atomically smooth epitaxial silver film and a single optically pumped nanorod consisting of an epitaxial gallium nitride shell and an indium gallium nitride core acting as gain medium. The atomic smoothness of the metallic film is crucial for reducing the modal volume and plasmonic losses. Bimodal lasing with similar pumping thresholds was experimentally observed, and polarization properties of the two modes were used to unambiguously identify them with theoretically predicted modes. The all-epitaxial approach opens a scalable platform for low-loss, active nanoplasmonics.




Bottom-Up Assembly of Colloidal Gold and Silver Nanostructures for Designable Plasmonic Structures and Metamaterials

Langmuir, 28 (24), pp 8902–8908 (2012)

Shangjr Gwo, Meng-Hsien Lin, Chieh-Lun He, Hung-Ying Chen, and Toshiharu Teranishi




We report on bottom-up assembly routes for fabricating plasmonic structures and metamaterials composed of colloidal gold and silver nanostructures, such as nanoparticles (“metatoms”) and shape-controlled nanocrystals. Owing to their well-controlled sizes/shapes, facile surface functionalization, and excellent plasmonic properties in the visible and near-infrared regions, these nanoparticles and nanocrystals are excellent building blocks of plasmonic structures and metamaterials for optical applications. Recently, we have utilized two kinds of bottom-up techniques (i.e., multiple-probe-based nanomanipulation and layer-by-layer self-assembly) to fabricate strongly coupled plasmonic dimers, one-dimensional (1D) chains, and large-scale two-dimensional/three-dimensional (2D/3D) nanoparticle supercrystals. These coupled nanoparticle/nanocrystal assemblies exhibit unique and tunable plasmonic properties, depending on the material composition, size/shape, intergap distance, the number of composing nanoparticles/nanocrystals (1D chains), and the nanoparticle layer number in the case of 3D nanoparticle supercrystals. By studying these coupled nanoparticle/nanocrystal assemblies, the fundamental plasmonic metamaterial effects could be investigated in detail under well-prepared and previously unexplored experimental settings.




Plasmonic Green Nanolaser Based on a Metal–Oxide–Semiconductor Structure

Chen-Ying Wu, Cheng-Tai Kuo, Chun-Yuan Wang, Chieh-Lun He, Meng-Hsien Lin, Hyeyoung Ahn, and Shangjr Gwo


 Abstract Image 

Realization of smaller and faster coherent light sources is critically important for the emerging applications in nanophotonics and information technology. Semiconductor lasers are arguably the most suitable candidate for such purposes. However, the minimum size of conventional semiconductor lasers utilizing dielectric optical cavities for sustaining laser oscillation is ultimately governed by the diffraction limit ((λ/2n)3 for three-dimensional (3D) cavities, where λ is the free-space wavelength and n is the refractive index). Here, we demonstrate the 3D subdiffraction-limited laser operation in the green spectral region based on a metal–oxide–semiconductor (MOS) structure, comprising a bundle of green-emitting InGaN/GaN nanorods strongly coupled to a gold plate through a SiO2 dielectric nanogap layer. In this plasmonic nanocavity structure, the analogue of MOS-type “nanocapacitor” in nanoelectronics leads to the confinement of the plasmonic field into a 3D mode volume of 8.0 × 10–4 μm3 (0.14(λ/2n)3).





Far-Field Optical Imaging of a Linear Array of Coupled Gold Nanocubes: Direct Visualization of Dark Plasmon Propagating Modes

Hung-Ying Chen, Chieh-Lun He, Chun-Yuan Wang, Meng-Hsien Lin, Daisuke Mitsui, Miharu Eguchi, Toshiharu Teranishi, and Shangjr Gwo

 Abstract Image

Plasmonic nanoantenna arrays hold great promise for diffraction-unlimited light localization, confinement, and transport. Here, we report on linear plasmonic nanoantenna arrays composed of colloidal gold nanocubes precisely assembled using a nanomanipulation technique. In particular, we show the direct evidence of dark propagating modes in the plasmon coupling regime, allowing for transport of guided plasmon waves without far-field radiation losses. Additionally, we demonstrate the possibility of plasmon dispersion engineering in coupled gold nanocube chains. By assembling a nanocube chain with two sections of coupled nanocubes of different intercube separations, we are able to produce the effect of a band-pass nanofilter.



Layer-by-Layer Assembly of Three-Dimensional Colloidal

Supercrystals with Tunable Plasmonic Properties

Meng-Hsien Lin, Hung-Ying Chen, and Shangjr Gwo



We present a simple and efficient method for synthesizing large-area (>1 cm2), three-dimensional (3D) gold and silver nanoparticle supercrystal films. In this approach, Janus nanoparticle (top face solvent-phobic and bottom face solvent-philic) films with an arbitrary number of close-packed nanoparticle monolayers can be formed using layer-by-layer (LbL) assembly from suspensions of thiolate-passivated gold or silver colloids. Furthermore, we demonstrate that these films can act as true 3D plasmonic crystals with strong transverse (intralayer) and longitudinal (interlayer) near-field coupling. In contrast to conventional polyelectrolyte-mediated LbL assembly processes, this approach allows multiple longitudinal coupling modes with a conspicuous spectral dependence on the layer number. We have found a universal scaling relation between the spectral position of the reflectance dips related to the longitudinal modes and the layer number. This relation can be understood in terms of the presence of a plasmonic Fabry−Pérot nanocavity along the longitudinal direction that allows the formation of standing plasmon waves under plasmon resonance conditions. The realization of 3D plasmonic coupling enables broadband tuning of the collective plasmon response over a wide spectral range (visible and near-IR) and provides a pathway to designer plasmonic metamaterials.





Experimental Determination of Electron Affinities for InN and GaN Polar Surfaces

Shih-Chieh Lin, Cheng-Tai Kuo, Xiaoge Liu, Li-Yen Liang, Ching-Hung Cheng, Chung-Huang Lin, Shu-Jung Tang, Lo-Yueh Chang, Chia-Hao Chen, and Shangjr Gwo


 Abstract Image 

We have measured the electron affinities of clean, stoichiometric InN and GaN polar surfaces via ultraviolet photoelectron spectroscopy. The electron affinities of InN were measured to be 4.7 and 4.6 eV for In- and N-polar surfaces, respectively. In contrast, the electron affinities of GaN vary greatly with the film polarity, i.e., 3.8 and 3.3 eV for Ga- and N-polar surfaces, respectively. We propose that the difference between polar surfaces originates from the spontaneous polarization effect. Furthermore, it's closely related to the film carrier concentration. With the measured electron affinities, we are able to confirm the known polar heterojunction band alignments. ©2012 The Japan Society of Applied Physics





Single InGaN nanodisk light emitting diodes as full-color subwavelength light sources

Appl. Phys. Lett. 98, 233101 (2011)

Yu-Jung Lu (呂宥蓉), Hon-Way Lin (林弘偉), Hung-Ying Chen (陳虹穎), Yu-Chen Yang (楊右丞), and Shangjr Gwo (果尚志)


Subwavelength electroluminescent sources with spatial, spectral, and polarization controlling capabilities are critical elements for optical imaging and lithography beyond the diffraction limit. Here, we show that the electroluminescence from single, strain-free InGaN nanodisks embedded in self-assembled GaN p-n nanorods can span the entire visible spectrum with a large linear polarization ratio ( ∼ 0.85). Furthermore, this unique nanodisk-in-nanorod geometry enables the realization of the ultrasmall footprint light-emitting diodes (LEDs) to be used as subwavelength light sources. Using these nano-LEDs, we are able to demonstrate near-field, subwavelength photolithography by controlling the exposure time and light intensity from single InGaN nanodisks at chosen wavelengths.


Highlights in

PHYSORG, June 17, 2011               Nature Asia materials, October 11,2011





Is electron accumulation universal at InN polar surfaces?

Appl. Phys. Lett. 98, 052101 (2011)

Cheng-Tai Kuo (郭承泰), Shih-Chieh Lin (林詩傑), Kai-Kuen Chang (張凱焜), Hung-Wei Shiu (許紘瑋), Lo-Yueh Chang (張羅嶽), Chia-Hao Chen (陳家浩), Shu-Jung Tang (唐述中), and Shangjr Gwo (果尚志)



Recent experiments indicate the universality of electron accumulation and downward surface band bending at as-grown InN surfaces with polar or nonpolar orientations. Here, we demonstrate the possibility to prepare flatband InN 0001¯ surfaces. We have also measured the surface stoichiometry of InN surfaces by using core-level photoelectron spectroscopy. The flatband InN 0001¯ surface is stoichiometric and free of In adlayer. It implies that the removal of In adlayer at the InN 0001¯ surface leads to the absence of downward surface band bending. On the other hand, the stoichiometric InN 0001 surface still exhibits surface band bending due to the noncentrosymmetry in the wurtzite structure.



 InGaN/GaN nanorod array white light-emitting diode

 Hon-Way Lin (林弘偉), Yu-Jung Lu (呂宥蓉), Hung-Ying Chen (陳虹穎), Hong-Mao Lee (李弘
 貿), and Shangjr Gwo (果尚志)


Conventional InGaN/GaN light-emitting diodes based on planar quantum well structures do not allow for efficient long-wavelength operation beyond the blue region due to a strong quantum confined Stark effect in lattice-mismatched polar InGaN quantum wells. Here we overcome the limitation by using self-assembled GaN nanorod arrays as strain-free growth templates for thick InGaN nanodisks. In combination with enhanced carrier localization and high crystalline quality, this approach allows us to realize full-color InGaN nanodisk emitters. By tailoring the numbers, positions, and thicknesses of polychromatic nanodisk ensembles embedded vertically in the GaN nanorod p-n junction, we are able to demonstrate natural white (color temperature  6000 K) electroluminescence from InGaN/GaN nanorod arrays.


 Highlights in

   Nature Photonics 4, 738       Nature Nanotechnology 5, 695   Semiconductor Today 5, 92       Laser Focus World 46

         November 2010                       Octobor 2010                       September 2010                     October 2010






Site-Selective Biofunctionalization of Aluminum Nitride Surfaces Using Patterned Organosilane Self-Assembled Monolayers

Langmuir 26, pp. 2969–2974 (2010)

Chi-Shun Chiu, Hong-Mao Lee and Shangjr Gwo*


Surface biochemical functionalization of group-III nitride semiconductors has recently attracted much interest because of their biocompatibility, nontoxicity, and long-term chemical stability under demanding physiochemical conditions for chemical and biological sensing. Among Ill-nitrides, aluminum nitride (AlN) and aluminum gallium nitride (AlGaN) are particularly important because they are often used as the sensing surfaces for sensors based oil field-effect transistor or surface acoustic wave (SAW) sensor structures. To demonstrate the possibility of site-selective biofunctionalization oil AlN surfaces, we have fabricated two-dimensional antibody micropatterns oil AlN surfaces by using patterned self-assembled monolayer (SAM) templates. Patterned SAM templates are composed of two types of organosilane molecules terminated with different functional groups (amino and methyl), which were fabricated oil AlN/sapphire substrates by combining photolithography, lift-off process, and self-assembly technique. Because the patterned SAM templates have different surface properties oil the same Surface, clear imaging contrast of SAM micropatterns can be observed by field-emission scanning electron microscopy (FF-SEM) operating at a low accelerating voltage in the range of 0.5-1.5 kV. Furthermore, the contrast in surface potential of the binary SAM microstructures was confirmed by selective adsorption of negatively charged colloidal gold nanoparticles (AuNPs). The immobilization of AuNPs was limited oil the positively charged amino-terminated regions, while they were scarcely found oil the surface regions terminated by methyl groups. In this work, selective immobilization of green fluorescent protein (G FP) antibodies was demonstrated by the specific protein binding of enhanced GFP (EGFP) labeling. The observed strong fluorescent signal front antibody functionalized regions oil the SAM-patterned AlN surface indicates the retained biological activity of specific molecular recognition resulting from the antibody-EGFP interaction. The results reported here show that micropatterning of organosilane SAMs by the combination of photolithographic process and lift-off technique is a practical approach for the fabrication of reaction regions oil AlN-based bioanalytical microdevices.