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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 66

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.
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.
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.
Characteristics of output voltage and current of integrated nanogenerators

(Georgia Institute of Technology, 2009-01-15) Yang, Rusen ; Qin, Yong ; Li, Cheng ; Dai, Liming ; Wang, Z. L. (Zhong Lin)

Owing to the anisotropic property and small output signals of the piezoelectric nanogenerators (NGs) and the influence of the measurement system and environment, identification of the true signal generated by the NG is critical. We have developed three criteria: Schottky behavior test, switching-polarity tests, and linear superposition of current and voltage tests. The 11 tests can effectively rule out the system artifacts, whose sign does not change with the switching measurement polarity, and random signals, which might change signs but cannot consistently add up or cancel out under designed connection configurations. This study establishes the standards for designing and scale up of integrated nanogenerators.
Density-controlled growth of aligned ZnO nanowire arrays by seedless chemical approach on smooth surfaces

(Georgia Institute of Technology, 2008-08) Xu, Sheng ; Lao, Changshi ; Weintraub, Benjamin ; Wang, Z. L. (Zhong Lin)

A novel ZnO seedless chemical approach for density-controlled growth of ZnO nanowire (NW) arrays has been developed. The density of ZnO NWs is controlled by changing the precursor concentration. Effects of both growth temperature and growth time are also investigated. By this novel synthesis technique, ZnO NW arrays can grow on any substrate (polymer, glass, semiconductor, metal, and more) as long as the surface is smooth. This technique represents a new, low-cost, time-efficient, and scalable method for fabricating ZnO NW arrays for applications in field emission, vertical field effect transistor arrays, nanogenerators, and nanopiezotronics.
Toward high output-power nanogenerator

(Georgia Institute of Technology, 2008-04-29) Liu, Jin ; Fei, Peng ; Zhou, Jun ; Tummala, Rao R. ; Wang, Z. L. (Zhong Lin)

In this paper, the factors that determine the power output of a piezoelectric nanowire (NW) nanogenerator (NG) have been analyzed. The output current is the sum of those contributed by all of the NWs while the output voltage is determined by the voltage generated by a single NW, the capacitance of the NW array and the system, and the contact resistance. By growing uniform ZnO NWs with diameters of ~100 nm and lengths of ~5 µm, the output current density and output voltage of the NG was improved to ~8.3 µA/cm² and 10 mV, respectively, which are 20–30 times higher than that we previously reported. A power generation density of ~83 nW/cm² is achieved by using a single layer NW NG.