Zhida Xu

University of Illinois, Urbana-Champaign, Urbana, Illinois, United States

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Publications (30)78.65 Total impact

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    ABSTRACT: The guest editorial team would like to thank the authors for their contributions to this special issue and the reviewers for their time and dedication.
    Full-text · Article · Aug 2015 · Journal of Nanomaterials
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    ABSTRACT: A bifunctional ultrasensitive nanoplasmonic sensor is demonstrated with combined surface plasmon resonance (SPR) and surface-enhanced Raman spectroscopy (SERS) sensing capabilities. Unlike traditional extraordinary transmission (EOT) devices, nano Lycurgus cup array (nanoLCA) contains a hybrid configuration of periodic quasi-3D nanostructure array and dense sidewall metal nanoparticles within each nanostructure, which enables both refractive index sensing and SERS chemical identification on the same device with high sensitivity. The visible plasmon resonance sensitivity of nanoLCA is measured to be as high as 796 nm/RIU with the figure of merit (FOM) of 12.7 so that the device is applied for colorimetric liquid sensing with an ordinary microscopic system. Moreover, the SERS enhancement of the very same nanoLCA for liquid sample is calculated to be 2.8 × 107, which is the highest among all reported EOT-based SERS devices. The urea concentration detection has been demonstrated to show the complementary rapid colorimetric screening and precise SERS identification functions provided by nanoLCA plasmonic sensor for chemical analysis or biological diagnostics in a resource-limited environment.
    No preview · Article · Jul 2015 · Advanced Optical Materials
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    ABSTRACT: Colorimetric techniques provide a useful approach for sensing application because of their low cost, use of inexpensive equipment, requirement of fewer signal transduction hardware, and, above all, their simple-to-understand results. Colorimetric sensor can be used for both qualitative analyte identification as well as quantitative analysis for many application areas such as clinical diagnosis, food quality control, and environmental monitoring. A gap exists between high-end, accurate, and expensive laboratory equipment and low-cost qualitative point-of-care testing tools. Here, we present a label-free plasmonic-based colorimetric sensor fabricated on a transparent plastic substrate consisting of about one billion nanocups in an array with a subwavelength opening and decorated with metal nanoparticles on the side walls, to bridge that gap. The fabrication techniques of the plasmonic sensor, integration to portable microfluidic devices for lab on chip applications, demonstration of highly sensitive refractive-index sensing, DNA hybridization detection, and protein-protein interaction will be reviewed. Further, we anticipate that the colorimetric sensor can be applied to point-of-care diagnostics by utilizing proper surface functionalization techniques, which seems to be one of the current limiting factors. Finally, the future outlook for the colorimetric plasmonic sensors is discussed.
    Preview · Article · Feb 2015 · Journal of Nanomaterials
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    ABSTRACT: We have implemented a multifunctional optofluidic sensor that can monitor changes in the refractive index and pressure of biofluid simultaneously and can detect free-solution molecular interaction in situ. In this Letter, we demonstrate two major improvements of this sensor proven by both simulation and experiments. One improvement is the broader measurement range of refractive index by making the diffraction grating with high-index polymer. The other improvement is the separation of refractive index sensing from opacity sensing by using the relative power ratio of diffraction orders. This simple, compact and low-cost multifunctional optofluidic sensor has the potential to be used for in situ biofluid monitoring.
    No preview · Article · Oct 2014 · Optics Letters
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    Zhida Xu · Logan Liu
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    ABSTRACT: We have implemented a multi-functional optofluidic sensor that can monitor changes in the refractive index and pressure of biofluid simultaneously and can detect free-solution molecular interaction in-situ. In this paper, we demonstrate two major improvements of this sensor proven by both simulation and experiments. One improvement is the broader measurement range of refractive index by making the diffraction grating with high-index polymer. The other improvement is the separation of refractive index sensing from opacity sensing by using the relative power ratio of diffraction orders. This simple, compact and low-cost multi-functional optofluidic sensor has the potential to be used for in-situ biofluid monitoring.
    Preview · Article · Oct 2014
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    ABSTRACT: Wafer-scale nano-mushroom sensor was demonstrated with the refractive index sensitivity of 373 nm/RIU, resulting in significant color shift detectable by eye. It also works for surface-enhanced Raman spectroscopy with the enhancement factor of 10^7.
    No preview · Conference Paper · Jun 2014
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    ABSTRACT: Recently developed classes of monocrystalline silicon solar microcells (u-cell) can be assembled into modules with characteristics (i.e., mechanically flexible forms, compact concentrator designs, and high-voltage outputs) that would be impossible to achieve using conventional, wafer-based approaches. In this paper, we describe a highly dense, uniform and non-periodic nanocone forest structure of black silicon (bSi) created on optically-thin (30 um) u-cells for broadband and omnidirectional light-trapping with a lithography-free and high-throughput plasma texturizing process. With optimized plasma etching conditions and a silicon nitride passivation layer, black silicon u-cells, when embedded in a polymer waveguiding layer, display dramatic increases of as much as 65.7% in short circuit current, as compared to a bare silicon device. The conversion efficiency increases from 8% to 11.5% with a small drop in open circuit voltage and fill factor.
    Preview · Article · Jun 2014 · Nanotechnology
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    Zhida Xu · Logan Liu
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    ABSTRACT: We demonstrate gold coated polymer surface enhanced Raman scattering (SERS) substrates with a pair of complementary structures--positive and inverted pyramids array structures fabricated by multiple-step molding and replication process. The uniform SERS enhancement factors over the entire device surfaces were measured as 7.2 X 10^4 for positive pyramids substrate while 1.6 X 10^6 for inverted pyramids substrate with Rhodamine 6G as the target analyte. Based on the optical reflection measurement and FDTD simulation result, the enhancement factor difference is attributed to plasmon resonance matching and to SERS "hot spots ". With this simple, fast and versatile complementary molding process, we can produce polymer SERS substrates with extremely low cost, high throughput and high repeatability.
    Preview · Article · Feb 2014 · Journal of Nanophotonics
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    Zhida Xu · Logan Liu
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    ABSTRACT: We demonstrated fabrication of black silicon with slanted nanocone array on both planar and 3D micro and meso scale structures produced by a high-throughput lithography-free oblique-angle plasma etching process. Nanocones with gradual change in height were created on the same piece of silicon. The relation between the slanted angle of nanocones and incident angle of directional plasma is experimentally investigated. In order to demonstrate the monolithic integration of nanostructures on micro and meso scale non-planar surfaces, nanocone forest is fabricated on non-planar silicon surfaces in various morphologies such as silicon atomic force microscopy (AFM) tips and pyramidal pits. By integrating nanocones on inverse silicon micro-pyramid array devices, we further improved the surface enhanced Raman scattering (SERS) enhancement property of this optimized commercial SERS substrate by several folds even when using 66% less noble metal coating. We investigated the length gradient dependence and asymmetric properties of SERS effects for slanted nanocone with polarized excitation. This versatile and angle-controllable nanocone fabrication and monolithic 3D nano-micro-meso integration method provides new dimensions for production and optimization of SERS and other nanophotonic sensors.
    Preview · Article · Feb 2014 · The Journal of Physical Chemistry C
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    Zhida Xu · Logan Liu
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    ABSTRACT: We demonstrate surface plasmon-induced enhancements in optical imaging and spectroscopy on silver coated silicon nanocones which we call black silver substrate. The black silver substrate with dense and homogeneous nanocone forest structure is fabricated on wafer level with a mass producible nanomanufacturing method. The black silver substrate is able to efficiently trap and convert incident photons into localized plasmons in a broad wavelength range, which permits the enhancement in optical absorption from UV to NIR range by 12 times, the visible fluorescence enhancement of ~30 times and the NIR Raman scattering enhancement factor up to ~108. We show a considerable potential of the black silver substrate in high sensitivity and broadband optical sensing and imaging of chemical and biological molecules.
    Preview · Article · Feb 2014 · Applied Physics Letters
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    Zhida Xu · Logan Liu
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    ABSTRACT: Anomalous surface enhanced Raman scattering (SERS) peaks are identified for liquid sample stored in polypropylene centrifuge tubes (PP tube) for months. We observed the unexpected Raman peaks during experiments for Thiamine Hydrochloride aqueous solution stored in PP tube for two months. In order to identify the contaminants we have performed SERS experiments for de-ionized water (DI water) stored in polypropylene centrifuge tube for two months and compared them with fresh DI water sample. We have also carried out Ultra Violet (UV) absorption spectra for both fresh and contaminated water. We believe that the water is contaminated because of chemicals leaching from the PP tube. From the GC-MS data the main contaminant was found to be Phthalic acid and its derivatives. Further SERS and UV absorption experiment for Phthalic acid correlates well with the anomalous peaks identified earlier. We qualitatively confirmed the identification and quantitatively estimated the concentration of suspect contaminants as between 1uM and 10uM with both SERS and UV absorption spectroscopy. With UV absorption spectroscopy, we precisely estimate the concentration as 2.1uM. We have shown that sample in PP tube can be contaminated due to leaching chemicals upon long term storage and suggested SERS and UV-absorption spectroscopy as two quick and simple techniques to detect the contamination
    Preview · Article · Feb 2014
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    Zhida Xu · Logan Liu
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    ABSTRACT: We present an optofluidic sensor based on an elastomeric two-dimensional (2D) grating integrated inside a hemispherical fluid chamber. Laser beam is diffracted before (reflection) and after (transmission) going through the grating and liquid in the dome chamber. The sensing mechanism is investigated and simulated with a finite difference time domain (FDTD) based electromagnetic (EM) method. For experiment, by analyzing the size, power and shape of the 2D diffraction patterns, we can retrieve multiple parameters of the liquid including the refractive index, pressure and opacity with high sensitivity. We demonstrate that glucose concentration can be monitored when mixed in different concentrated phosphate buffered saline (PBS) solution. The free-solution binding of bovine serum albumin (BSA) and anti-BSA IgG is detected with this optical sensor. This low-cost, multifunctional and reliable optofluidic sensor has the potential to be used as monitor of biofluid such as blood in hemodialysis.
    Preview · Article · Feb 2014
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    ABSTRACT: Portable low-cost sensors and sensing systems for the identification and quantitative measurement of bacteria in field water are critical in preventing drinking water from being contaminated by bacteria. In this article, we reported the design, fabrication and testing of a low-cost, miniaturized and sensitive bacteria sensor based on electrical impedance spectroscopy method using a smartphone as the platform. Our design of microfluidics enabled the pre-concentration of the bacteria which lowered the detection limit to 10 bacterial cells per milliliter. We envision that our demonstrated smartphone-based sensing system will realize highly-sensitive and rapid in-field quantification of multiple species of bacteria and pathogens.
    Preview · Article · Dec 2013 · Sensors and Actuators B Chemical
  • Zhida Xu · Xinhao Wang · Kevin Han · Shuo Li · G Logan Liu
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    ABSTRACT: We present an optofluidic sensor based on an elastomeric two-dimensional (2D) grating integrated inside a hemispherical fluid chamber. A laser beam is diffracted before (reflection) and after (transmission) going through the grating and liquid in the dome chamber. The sensing mechanism is investigated and simulated with a finite-difference time-domain-based electromagnetic method. For the experiment, by analyzing the size, power, and shape of the 2D diffraction patterns, we can retrieve multiple parameters of the liquid, including the refractive index, pressure, and opacity with high sensitivity. We demonstrate that the glucose concentration can be monitored when mixed in a different concentrated phosphate-buffered saline solution. The free-solution binding of bovine serum albumin (BSA) and anti-BSA IgG is detected with this optical sensor. This low-cost, multifunctional, and reliable optofluidic sensor has the potential to be used as a monitor of biofluid, such as blood in hemodialysis.
    No preview · Article · Dec 2013 · Journal of the Optical Society of America A
  • Zhida Xu · Yuan Yao · Ralph Nuzzo · Gang L. Liu
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    ABSTRACT: After made black by a reactive ion etching process and SiNx deposition, unprecedentedly, the efficiency of ultrathin solar microcell was improved from 8% to 11.5% and short circuit current increased by 65.7%.
    No preview · Conference Paper · Nov 2013
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    ABSTRACT: The performance of infrared (IR) sensing bimaterial cantilevers depends upon the thermal, mechanical and optical properties of the cantilever materials. This paper presents bimaterial cantilevers that have a layer of black silicon nanocone arrays, which has larger optical absorbance and mechanical compliance than single crystal silicon. The black silicon consists of nanometer-scale silicon cones of height 104–336 nm, fabricated using a three-step O2–CHF3–Ar + Cl2 plasma process. The average cantilever absorbance was 0.16 over the 3–10 μm wavelength region, measured using a Fourier transform infrared (FTIR) microspectrometer. The measured cantilever responsivity to incident IR light compares well to a model of cantilever behavior that relate the spectral absorbance, heat transfer, and thermal expansion. The model also provides further insights into the influence of the nanocone height on the absorbance and responsivity of the cantilever. Compared to a cantilever with smooth single crystal silicon, the cantilever with black silicon has about 2× increased responsivity. The nanocone array fabrication technique for silicon bimaterial cantilevers presented here could be applied to other IR sensors.
    No preview · Article · Sep 2013 · Sensors and Actuators A Physical
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    ABSTRACT: We demonstrated fabrication of black silicon with slanted nanocone array on both planar and 3D micro- and mesoscale structures produced by a high-throughput lithography-free oblique-angle plasma etching process. Nanocones with gradual change in height were created on the same piece of silicon. The relation between the slanted angle of nanocones and incident angle of directional plasma is experimentally investigated. In order to demonstrate the monolithic integration of nanostructures on micro- and mesoscale nonplanar surfaces, nanocone forest is fabricated on nonplanar silicon surfaces in various morphologies such as silicon atomic force microscopy (AFM) tips and pyramidal pits. By integrating nanocones on inverse silicon micropyramid array devices, we further improved the surface-enhanced Raman scattering (SERS) enhancement property of this optimized commercial SERS substrate by severalfold even when using 66% less noble metal coating. We investigated the length gradient dependence and asymmetric properties of SERS effects for slanted nanocone with polarized excitation. This versatile and angle-controllable nanocone fabrication and monolithic 3D nano-micro-meso integration method provides new dimensions for production and optimization of SERS and other nanophotonic sensors.
    No preview · Conference Paper · Nov 2012
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    Zhida Xu · Jing Jiang · Gang Logan Liu
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    ABSTRACT: A high-density and -uniformity sub-100 nm surface-oxidized silicon nanocone forest structure is created and integrated onto the existing texturization microstructures on a photovoltaic device surface by a one-step high-throughput plasma-enhanced texturization method. We suppressed the broadband optical reflection on chemically textured grade-B silicon solar cells for up to 70.25% through this nanomanufacturing method. The performance of the solar cell is improved with the short-circuit current increased by 7.1%, fill factor increased by 7.0%, and conversion efficiency increased by 14.66%. Our method demonstrates the potential to improve the photovoltaic device performance with low-cost and high-throughput nanomanufacturing technology.
    Preview · Article · Jul 2012 · Applied Optics
  • Zhida Xu · Xinhao Wang · G.L. Liu
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    ABSTRACT: Flexible 2D grating with nano-pyramids array used for in-situ and highly sensitive measurement of refractive index and pressure of fluid is demonstrated for potential application in blood monitoring in dialysis.
    No preview · Conference Paper · Jan 2012
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    ABSTRACT: Asymmetric assembly of nanomaterials has attracted broad interests because of their unique anisotropic properties that are different from those based on the more widely reported symmetric assemblies. Despite the potential advantages, programmable fabrication of asymmetric structure in nanoscale remains a challenge. We report here a DNA-directed approach for the assembly of asymmetric nanoclusters using Janus nanoparticles as building blocks. DNA-functionalized spherical gold nanoparticles (AuNSs) can be selectively attached onto two different hemispheres of DNA-functionalized Janus nanoparticle (JNP) through DNA hybridization. Complementary and invasive DNA strands have been used to control the degree and reversibility of the assembly process through programmable base-pairing interactions, resulting in a series of modular and asymmetric nanostructures that allow systematic study of the size-dependent assembly process. We have also shown that the attachment of the AuNSs onto the gold surface of the Janus nanoparticle results in red shifting of the UV-vis and plasmon resonance spectra.
    No preview · Article · Dec 2011 · ACS Nano