W. X. Sun

National University of Singapore, Singapore, Singapore

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Publications (12)25.74 Total impact

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    ABSTRACT: We report on the fabrication of domain-reversed structures in LiNbO3 by means of direct electron beam lithography at room temperature without any static bias. The LiNbO3 crystals were chemically etched after the exposure of electron beam and then, the patterns of domain inversion were characterized by atomic force microscopy (AFM). In our experiment, an interesting phenomenon occurred when the electron beam wrote a one-dimensional (1-D) grating on the negative c-face: a two-dimensional (2-D) dotted array was observed on the positive c- face, which is significant for its potential to produce 2-D and three-dimensional photonic crystals. Furthermore, we also obtained 2-D ferroelectric domain inversion in the whole LiNbO3 crystal by writing the 2-D square pattern on the negative c-face. Such a structure may be utilized to fabricate 2-D nonlinear photonic crystal. AFM demonstrates that a 2-D domain-reversed structure has been achieved not only on the negative c-face of the crystal, but also across the whole thickness of the crystal. Comment: 17 pages, 4 figures
    Journal of Applied Physics 07/2005; · 2.21 Impact Factor
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    ABSTRACT: Irradiance dependence of excitonic nonlinear absorption in Cadmium Sulfide (CdS) nanocrystals has been studied by using Z-scan method with nanosecond laser pulses. The wavelength dependence of nonlinear absorption has also been measured near the excitonic transition of 1S(e)-1S3/2(h). We observe the saturable absorption, which can be described by a third-order and a fifth-order nonlinear process for both 3.0-nm-sized and 2.3-nm-sized CdS nanocrystals. The experimental results show that the excitonic nonlinear absorption of CdS nanocrystals is greatly enhanced with decreasing particle size. A two-level model is utilized to explain both irradiance and wavelength dependence of the excitonic nonlinearity. Comment: 24 pages, 6 figures
    Journal of Applied Physics 07/2005; · 2.21 Impact Factor
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    ABSTRACT: Amorphous boron nitride (a- BN ) was produced by ball milling of hexagonal BN (h- BN ) . Turbostratic BN (t- BN ) and mesographite BN (m- BN ) with different degrees of three-dimensional order ( DTDO ) were subsequently prepared by annealing the a- BN at different temperatures. The photoluminescence ( PL ) of the BN samples with different structures and DTDOs was investigated in the visible region from 460 to 800 nm . The h -, a -, and t- BN samples, which have completely ordered, disordered and two-dimensionally ordered structures, respectively, emitted weak continuums. The m- BN samples with three-dimensionally ordered structures gave discrete PL bands, which we attributed to the presence of N<sub>B</sub> antisites or a complex formed by an Fe atom and a B vacancy in the BN layer. The intensity and energy of the PL bands changed greatly with DTDO and could be tuned by varying the annealing temperature. The strongest PL bands at 593 and 612 nm were observed in one of the m- BN samples produced at an annealing temperature of 1100 ° C </formu- la>, and their intensity was about 1000 times the intensity of h- BN . Unintentional Fe contamination did not affect the PL nature of the m- BN samples but increased the PL intensities of some bands greatly.
    Journal of Applied Physics 09/2004; · 2.21 Impact Factor
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    ABSTRACT: Raman spectroscopy was used to investigate excimer laser annealing and thickness determination of amorphous silicon (a-Si) layers which are less than 20 nm thick. The a-Si layers were produced on silicon (Si) substrates using Si+ ion implantation with an energy of 10 keV and a dose of 1 × 1015 cm−2. Excimer laser annealing was applied to re-crystallize the a-Si layers. The dependence of re-crystallization on laser fluence was investigated using Raman spectroscopy. A threshold laser fluence of 0.4 J cm−2 was required to re-crystallize the a-Si layers. In Raman spectroscopy, the Raman intensity shows a periodical variation with a period of 90° as a function of the angle between the Si orientation and the laser polarization. Based on this phenomenon, a method to determine nanoscale a-Si film thickness was proposed in two ways. One way was carried out without sample rotation to determine the a-Si thickness provided that the reference c-Si and a-Si/c-Si samples are in the same crystal orientation. The other way was carried out with sample rotation to determine the a-Si thickness without knowing the crystal orientation beforehand.
    Nanotechnology 03/2004; 15(5):658. · 3.84 Impact Factor
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    W. X. Sun, Z. X. Shen
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    ABSTRACT: We describe an apertureless near-field scanning Raman microscope in reflection geometry developed by integrating a near-field scanning optical microscope and a Raman spectrometer. This set-up offers some unique features that make it applicable to more samples. The fabrication of the metal tips is also explained in detail. Near-field Raman mappings have been realized on real silicon devices for the first time. The results illustrate the capability of our near-field Raman microscope in 2D Raman imaging. The apertureless configuration is a breakthrough to the limitation set by the low optical throughput of metal-coated optical fiber tips, reducing drastically the integration time for Raman spectra. The reflection scattering geometry makes the system applicable to any samples without preparation. Copyright © 2003 John Wiley & Sons, Ltd.
    Journal of Raman Spectroscopy 08/2003; 34(9):668 - 676. · 2.68 Impact Factor
  • W X Sun, Z X Shen
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    ABSTRACT: The combination of near-field scanning optical microscopy and Raman spectroscopy provides chemical/structural specific information with nanometer spatial resolution, which are critically important for a wide range of applications, including the study of Si devices, nanodevices, quantum dots, single molecules of biological samples. In this paper, we describe our near-field Raman study using apertureless probes. Our system has two important features, critical to practical applications. (1) The near-field Raman enhancement was achieved by Ag coating of the metal probes, without any preparation of the sample, and (2) while all other apertureless near-field Raman systems were constructed in transmission mode, our system works in the reflection mode, making near-field Raman study a reality for any samples. We have obtained the first 1D Raman mapping of a real Si device with 1s exposure time. This is a very significant development in near-field scanning Raman microscopy as it is the first demonstration that this technique can be used for imaging purpose because of the short integration time. In addition, the metal tips used in our set-up can be utilized to make simultaneous AFM and electrical mappings such as resistance and capacitance that are critical parameters for device applications.
    Ultramicroscopy 05/2003; 94(3-4):237-44. · 2.47 Impact Factor
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    ABSTRACT: Laser annealing has been employed to decompose half-metallic CrO2 into insulating Cr2O3 in air ambient. While both Cr2O3 and CrO2 are present, the relative fraction of each phase was controlled by changing the duration and power of laser irradiation. Glancing angle x-ray diffraction and micro-Raman scattering were used to characterize samples before and after laser annealing. The laser-induced decomposition of CrO2 into Cr2O3 that leads to a threefold enhancement of the low-field magnetoresistance and the realization of phase control of the CrO2/Cr2O3 system in selective microregions by laser irradiation implies: (i) optical lithography can be used as a potential method to directly control the magnetotransport properties which are strongly depended on the interface tunneling barrier and (ii) The CrO2 polycrytalline phase could be much more attractive as a high-density magnetic storage medium. © 2003 American Institute of Physics.
    Journal of Applied Physics 03/2003; 93(7):3951-3953. · 2.21 Impact Factor
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    T Yu, Z X Shen, W X Sun, J Y Lin, J Ding
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    ABSTRACT: Half-metallic CrO2 powder compact with rod-shaped nanoparticles was studied by micro-Raman scattering in the presence of an external magnetic field at room temperature (300 K). In the low-field region (H ≤ 250 mT), the frequency and intensity of the Eg mode, an internal phonon mode of CrO2, increase dramatically with increase in the magnetic field, while the corresponding linewidth decreases. The above parameters become constant when the CrO2 powder enters the saturation state at higher magnetic field. The pronounced anomalies of the Raman phonon parameters under a low magnetic field are attributed to the spin–phonon coupling enhanced by the magnetic ordering, which is induced by the external magnetic field.
    Journal of Physics Condensed Matter 03/2003; 15(12):L213. · 2.22 Impact Factor
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    ABSTRACT: Tip characteristics play an important role in the resolution and sensitivity of scanning probe microscopy. Extensive efforts have been devoted to tip fabrication. Most of the research is focused on scanning tunneling microscopy applications, which require sharp and short tips. Long tips that can be bent into cantilevered tips have great potential in atomic force microscopy/apertureless near-field scanning optical microscopy applications. However, the fabrication of such tips has been rarely reported. The present work is carried out with the aim of optimizing the conditions suitable for fabricating long and sharp tungsten tips. Besides topography, optical, and spectroscopic information, electrical and magnetic measurements can also be carried out with such tips obtained with the recipe reported in this article. The long tips also make it possible to measure deep grooves/trenches.
    Review of Scientific Instruments 08/2002; 73(8). · 1.60 Impact Factor
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    ABSTRACT: A MgB2 powder was mechanically milled for different times. X-ray diffraction indicated the formation of an intermediate phase with an unknown structure before formation of amorphous phase. The intermediate structure was superconducting below 18 K, while the amorphous phase was non-superconducting till 4.2 K. After heat treatment at 700 °C, all the mechanically milled samples formed back into the hexagonal MgB2 structure with Tc = 39 K. X-ray and Raman investigations suggested that the intermediate structure was metastable with a lattice compression, which was probably associated with the reduction in transition temperature.
    physica status solidi (a) 06/2002; 191(2):548 - 554. · 1.21 Impact Factor
  • G. Y. Yu, Z. X. Shen, L. Liu, W. X. Sun
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    ABSTRACT: Raman scattering study of a dilute GaAsN epitaxy layer was carried out at variable temperature and pressure. The localization due to the presence of the N atoms is responsible for the small correlation length in the GaAsN alloy, which is also evident from the broadening and asymmetry of the LO mode of the GaAs-like Raman band. The temperature dependence of the correlation length was analyzed. Nitrogen-induced localization also has a strong influence on the pressure dependence of the Born's effective dynamic charge e*.
    Materials Science in Semiconductor Processing 01/2001; 4(6):581-584. · 1.34 Impact Factor
  • Y. Shi, J Ding, Z. X Shen, W. X Sun, L Wang
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    ABSTRACT: High-energy mechanical milling of spinel NiFe2O4 leads to the formation of a disordered wustite-like structure. Cluster glass behavior was found in the Mössbauer study. The investigation suggested ferrimagnetic clusters in an antiferromagnetic matrix. The ferrimagnetic and antiferromagnetic exchange coupling results in a strong uni-directional anisotropy and a coercivity of over 10kOe after magnetic cooling.
    Solid State Communications 01/2000; 115(5):237-241. · 1.53 Impact Factor