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School of Materials Science and Engineering

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College of Engineering

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Now showing 1 - 10 of 113

Depth resolved luminescence from oriented ZnO nanowires

(Georgia Institute of Technology, 2009-12-14) Rosenberg, R. A. ; Haija, M. Abu ; Vijayalakshmi, K. ; Zhou, Jun ; Xu, Sheng ; Wang, Z. L. (Zhong Lin)

We have utilized the limited penetration depth of x-rays to study the near-surface properties of vertically aligned ZnO nanowires. For an energy of 600 eV the penetration depth varies between 3 and 132 nm as the incidence angle changes from 2° to 33°. Thus, by obtaining optical luminescence spectra as a function of incidence angle, it is possible to probe the near-surface region with nanometer-scale resolution. We will present angle dependent optical luminescence data from oriented ZnO nanowires. By fitting the results to a simple model, we extract a depth for the surface defect regions of ~14 nm.
Effect of Permittivity and Permeability of a Flexible Magnetic Composite Material on the Performance and Miniaturization Capability of Planar Antennas for RFID and Wearable Wireless Applications

(Georgia Institute of Technology, 2009-12) Martin, Lara J. ; Ooi, Sooliam ; Staiculescu, Daniela ; Hill, Michael D. ; Wong, C. P. ; Tentzeris, Emmanouil M.

This paper is an investigation of the feasibility of applying a mechanically flexible magnetic composite material to radio frequency identification (RFID) planar antennas operating in the lower ultrahigh-frequency (UHF) spectrum (∼300– 500 MHz). A key challenge is that the magnetic loss introduced by the magnetic composite must be sufficiently low for successful application at the targeted operating frequency. A flexible magnetic composite comprised of particles of Z-phase Co hexaferrite, also known as Co₂Z, in a silicone matrix was developed. To the authors’ knowledge, this is the first flexible magnetic composite demonstrated to work at these frequencies. The benchmarking structure was a quarter-wavelength microstrip patch antenna. Antennas on the developed magnetic composite and pure silicone substrates were electromagnetically modeled in Ansoft High- Frequency Sounder System full wave electromagnetic software. A prototype of the antenna on the magnetic composite was fabricated, and good agreement between the simulated and measured results was found. Comparison of the antennas on the magnetic composite versus the pure silicone substrate showed miniaturization capability of 2.4× and performance differences of increased bandwidth and reduced gain, both of which were attributed in part to the increase in the dielectric and magnetic losses. A key finding of this paper is that a small amount of permeability (μr∼2.5) can provide a substantial capability for miniaturization, while sufficiently low-magnetic loss can be introduced for successful application at the targeted operating frequency. This magnetic composite shows the capability to fulfill this balance and to be a feasible option for RFID, flexible wearable, and conformal applications in the lower UHF spectrum.
Structural colors from Morpho peleides butterfly wing scales

(Georgia Institute of Technology, 2009-10-12) Ding, Yong ; Xu, Sheng ; Wang, Z. L. (Zhong Lin)

A male Morpho peleides butterfly wing is decorated by two types of scales, cover and ground scales. We have studied the optical properties of each type of scales in conjunction with the structural information provided by cross-sectional transmission electron microscopy and computer simulation. The shining blue color is mainly from the Bragg reflection of the one-dimensional photonic structure, e.g., the shelf structure packed regularly in each ridges on cover scales. A thin-film-like interference effect from the base plate of the cover scale enhances such blue color and further gives extra reflection peaks in the infrared and ultraviolet regions. The analogy in the spectra acquired from the original wing and that from the cover scales suggests that the cover scales take a dominant role in its structural color. This study provides insight of using the biotemplates for fabricating smart photonic structures.
Enhanced Electrical Properties of Anisotropic Conductive Adhesive With $pi$ -Conjugated Self-Assembled Molecular Wire Junctions

(Georgia Institute of Technology, 2009-09) Zhang, Rongwei ; Li, Yi ; Yim, Myung Jin ; Moon, Kyoung-Sik ; Lu, Daoqiang ; Wong, C. P.

We have investigated the electrical properties of anisotropic conductive adhesive (ACA) joint using submicrometer- sized ( 500 nm in diameter) silver (Ag) particle as conductive filler with the effect of -conjugated self-assembled molecular wires. The ACAs with submicrometer-sized Ag particles have higher current carrying capability ( 3400 mA) than those with micro-sized Au-coated polymer particles ( 2000 mA) and Ag nanoparticles ( 2500 mA). More importantly, by construction of -conjugated self-assembled molecular wire junctions between conductive particles and integrated circuit (IC)/substrate, the electrical conductivity has increased by one order of magnitude and the current carrying capability of ACAs has improved by 600 mA. The crucial factors that govern the improved electrical properties are discussed based on the study of alignments and thermal stability of molecules on the submicrometer-sized Ag particle surface with surface-enhanced Raman spectroscopy (SERS), providing a fundamental understanding of conduction mechanism in ACA joints and guidelines for the formulation of high-performance ACAs in electronic packaging industry.
Combined polarized Raman and atomic force microscopy: In situ study of point defects and mechanical properties in individual ZnO nanobelts

(Georgia Institute of Technology, 2009-08-04) Lucas, Marcel ; Wang, Z. L. (Zhong Lin) ; Riedo, Elisa

We present a method, polarized Raman (PR) spectroscopy combined with atomic force microscopy (AFM), to characterize in situ and nondestructively the structure and the physical properties of individual nanostructures. PR-AFM applied to individual ZnO nanobelts reveals the interplay between growth direction, point defects, morphology, and mechanical properties of these nanostructures. In particular, we find that the presence of point defects can decrease the elastic modulus of the nanobelts by one order of magnitude. More generally, PR-AFM can be extended to different types of nanostructures, which can be in as-fabricated devices.
Erratum: Effects of piezoelectric potential on the transport characteristics of metal-ZnO nanowire-metal field effect transistor

(Georgia Institute of Technology, 2009-08-03) Gao, Zhiyuan ; Zhou, Jun ; Gu, Yudong ; Fei, Peng ; Hao, Yue ; Bao, Gang ; Wang, Z. L. (Zhong Lin)
Effects of piezoelectric potential on the transport characteristics of metal-ZnO nanowire-metal field effect transistor

(Georgia Institute of Technology, 2009-06-05) Gao, Zhiyuan ; Zhou, Jun ; Gu, Yudong ; Fei, Peng ; Hao, Yue ; Bao, Gang ; Wang, Z. L. (Zhong Lin)

We have investigated the effects of piezoelectric potential in a ZnO nanowire on the transport characteristics of the nanowire based field effect transistor through numerical calculations and experimental observations. Under different straining conditions including stretching, compressing, twisting, and their combination, a piezoelectric potential is created throughout the nanowire to modulate/alternate the transport property of the metal-ZnO nanowire contacts, resulting in a switch between symmetric and asymmetric contacts at the two ends, or even turning an Ohmic contact type into a diode. The commonly observed natural rectifying behavior of the as-fabricated ZnO nanowire can be attributed to the strain that was unpurposely created in the nanowire during device fabrication and material handling. This work provides further evidence on piezopotential governed electronic transport and devices, e.g., piezotronics.
Gigantic enhancement in response and reset time of ZnO UV nanosensor by utilizing Schottky contact and surface functionalization

(Georgia Institute of Technology, 2009-05-11) Zhou, Jun ; Gu, Yudong ; Hu, Youfan ; Mai, Wenjie ; Yeh, Ping-Hung ; Bao, Gang ; Sood, Ashok K. ; Polla, Dennis L. ; Wang, Z. L. (Zhong Lin)

UV response of ZnO nanowire nanosensor has been studied under ambient condition. By utilizing Schottky contact instead of Ohmic contact in device fabrication, the UV sensitivity of the nanosensor has been improved by four orders of magnitude, and the reset time has been drastically reduced from ~417 to ~0.8 s. By further surface functionalization with function polymers, the reset time has been reduced to ~20 ms even without correcting the electronic response of the measurement system. These results demonstrate an effective approach for building high response and fast reset UV detectors.
Low-threshold two-photon pumped ZnO nanowire lasers

(Georgia Institute of Technology, 2009-05-11) Zhang, Chunfeng ; Zhang, Fan ; Xia, Tian ; Kumar, Nitin ; Hahm, Jong-in ; Liu, Jin ; Wang, Z. L. (Zhong Lin) ; Xu, Jian

We report in this communication the two-photon absorption (TPA)-induced room-temperature lasing performance of ZnO nanowires. Under femtosecond pulse-excitation at λ = 700 nm in the infrared regime, a remarkably low threshold of 160μJ/cm2 was observed for the TPA-induced lasing action, which is of the same order of magnitude as that measured for the linear lasing process. Time-resolved photoluminescence characterization of two-photon pumped ZnO nanowires reveals the presence of a fast decay (3–4 ps) in the stimulated emission as compared to the slow decay (50–70 ps) for the spontaneous emission. The TPA process in ZnO nanowires was characterized with the nonlinear transmission measurement, which uncovers an enhanced TPA coefficient, about 14.7 times larger than that of bulk ZnO samples. The observed TPA enhancement in ZnO nanowires accounts for the low threshold lasing behavior, and has been attributed to the intensified optical field confined within the nanowire waveguides.
Near Void-Free Assembly Development of Flip Chip Using No-Flow Underfill

(Georgia Institute of Technology, 2009-04) Lee, Sangil ; Yim, Myung Jin ; Master, Raj N. ; Wong, C. P. ; Baldwin, Daniel F.

The advanced flip-chip-in-package (FCIP) process technology, using no-flow underfill material for high I/O density (over 3000 I/O) and fine-pitch (down to 150 μm) interconnect applications, presents challenges for flip chip processing because underfill void formation during reflow drives interconnect yield down and degrades reliability. In spite of such challenges, a high yield, reliable assembly process (> 99.99%) has been achieved using commercial no-flow underfill material with a high I/O, fine-pitch FCIP. This has been obtained using design of experiments with physical interpretation techniques. Statistical analysis determined what assembly conditions should be used in order to achieve robust interconnects without disrupting the FCIP interconnect structure. However, the resulting high yield process had the side effect of causing a large number of voids in the FCIP assemblies. Parametric studies were conducted to develop assembly process conditions that would minimize the number of voids in the FCIP induced by thermal effects. This work has resulted in a significant reduction in the number of underfill voids. This paper presents systematic studies into yield characterization, void formation characterization, and void reduction through the use of structured experimentation which was designed to improve assembly yield and to minimize the number of voids, respectively, in FCIP assemblies.