SYNTHESIS AND CHARACTERIZATION OF II-IV GROUP AND SILICON RELATED NANOMATERIALS

SYNTHESIS AND CHARACTERIZATION OF II-IV GROUP AND SILICON RELATED NANOMATERIALS ISMATHULLAKHAN SHAFIQ MASTER OF PHILOSOPHY CITY UNIVERSITY OF HONG KONG FEBRUARY 2008

CITY UNIVERSITY OF HONG KONG 香港城市大學 Synthesis and Characterization of II-IV Group and Silicon Related Nanomaterials 與硅相關的二至四組納米材料的合成與特性描述 Submitted to Department of Physics and Materials Science 物理及材料科學學系 in Partial Fulfillment of the Requirements for the Degree of Master of Philosophy 哲學碩士學位 by ISMATHULLAKHAN Shafiq February 2008 二零零八年二月

Abstract i Despite considerable efforts, rational synthesis of ZnO nanostructures with tunable n-type conductivity is a challenging issue. On the other hand, as-synthesized ZnO nanostructures are often randomly oriented, and thus have limited applications in optoelectronic devices. Herein, we report a controlled growth and doping process of well-aligned zinc oxide (ZnO) nanowire (NW) arrays via thermal evaporation. Influence of Gallium (Ga) dopant on the growth direction of ZnO NWs was examined. The growth direction of ZnO NWs was found to depend on the dopant content. Electrical transport properties of ZnO NWs were studied by fabricating and characterizing single nanowire field-effect transistors (FETs). It is shown that the ZnO NW conductivity can be tuned by two orders of magnitude, through the way of doping. Doping is a widely used method to tailor the electrical and optical properties of semiconductors by introducing discrete energy states in the band gap. In this regard, the role of Indium (In) as a luminescence activator and as a compensator of n-type materials is of considerable significance for II VI compound semiconductors. In this work, high quality n-type Indium doped cadmium sulphide (CdS) nanomaterials were fabricated by doping through a simple thermal evaporation method. Photoluminescence studies on the intrinsic and doped nanoribbons reveal the presence of discrete exciton emission bands in doped samples. Studies on field emission properties of doped CdS nanopens and nanopencils were also carried out. It is found that the nanopens with sharp tip has less turn on field compared to the nanopencil samples. Photoconductive response characteristics of single CdS nanoribbon (NR) to various wavelengths were also investigated. It is shown that a single CdS NR

ii photoconductor can be used as one switch in optoelectronic applications, because of its reversible switching ability between high and low conductivities. Manipulation of nanomaterials remains as another major challenge in the field of nanotechnology, despite significant progress. The fabrication of integrated systems using nanomaterial requires the site-specific growth or placement on relevant device platforms. In addition, the formation of complex and multi-component structures are needed for low-dimensional structures and electronic devices. In this dissertation, heteroepitaxial growth of single-crystalline ZnS x Se 1-x nanowire arrays on ZnS nanoribbon substrates by the metal-catalyzed vapor-liquid-solid growth method were carried out. ZnS x Se 1-x nanowire arrays were aligned crosswise to the top surface and vertically grown on side surfaces of ZnS nanoribbon substrates with variable compositions making it having tunable optical properties. Photoluminescence spectroscopy of the nanostructures reveals the lasing emission from the nanowires beyond threshold excitation intensity and exciton emissions below threshold and at low temperatures. Control of channel diameter and branching of a hierarchical tubular nanostructure is important for developing nano-channels or nano-containers for various applications. A simple thermo-evaporation synthesis and two-step method for epitaxial growth of branched silicon oxide (SiO) nanotubes from ZnS/SiO core-shell nanowire heterostructures using zinc sulfide (ZnS) and SiO as sources were studied. ZnS nanowires were synthesized by Au-catalyzed vapor-liquid-solid growth, and served as templates to form amorphous SiO nanotubes via evaporation of the ZnS core. Successive SiO coating and ZnS wire removal was found to graft new branches to the original tube and the diameter of the prepared porous SiO nanotube could be post-processed by electron beam irradiation. The resulting core-shell structures were

iii found to have uniform diameters, which are suitable for heterostructure nanodevices fabrication. Low-temperature photoluminescence studies on the SiO nanotubes sample reveal the visible-light emission centered at 612 nm. The present growth template approach may be extended to assemble nano-fluidic channel network for bioanalytical and chemical separations and branched field effect transistors.

Table of contents Abstract...i Certificate of approval by panel of examiners...iv Acknowledgements...v Table of contents...vi List of figures...ix 1 Chapter-1.Introduction...1 1.1 One-dimensional semiconductor nanomaterials...1 1.2 Silicon based Nanomaterials...3 1.2.1 Overview on Silicon nanowire technology... 3 1.2.2 Properties of silicon nanowires... 4 1.3 Introduction to Nanodevices...5 1.4 Scope of the dissertation...6 1.5 References...9 2 Chapter-2.Methodology...13 2.1 Introduction...13 2.2 X- Ray diffraction...13 2.2.1 X-ray diffractor... 13 2.3 Electron microscopy...14 2.3.1 Scanning electron microscope (SEM)... 16 2.3.2 Transmission electron microscope (TEM)... 17 2.4 Optical spectroscopy...19 2.4.1 Spectroscopy and Raman Effect... 19 2.4.2 Photoluminescence (PL)... 20 2.4.3 Raman Spectroscopy... 21 2.5 Device fabrication...23 2.5.1 Shadow mask method... 23 2.5.2 Photolithography... 24 vi

2.6 Reference...26 3 Chapter-3.Tunable n-type conductivity and transport properties of Gadoped ZnO nanowire arrays...27 3.1 Overview...27 3.2 Experiments...28 3.3 Characterization...30 3.3.1 Electron microscopy characterization... 30 3.3.2 Optical properties... 32 3.3.3 FET based on single ZnO Nanostructures... 34 3.4 Conclusion...39 3.5 References...40 4 Chapter-4.Synthesis and characterization of indium doped cadmium sulfide nanostructure...43 4.1 Introduction...43 4.2 Experiments...44 4.3 Characterization...45 4.3.1 Characterization of low temperature product: Nanoribbons... 45 4.3.2 Characterization of high temperature product: Nanopens and Nanopencils 54 4.4 Conclusion...60 4.5 References...62 5 Chapter-5.Synthesis of ZnS x Se 1-x nanowire arrays grown heteroepitaxially on ZnS nanoribbon substrates and their optical properties...64 5.1 Introduction...64 5.2 Experiments...65 5.3 Results and discussion...67 5.4 Conclusion...77 5.5 References...78 vii

viii 6 Chapter-6.Grafting Branches to Nanotubes via a Templating Process...79 6.1 Overview...79 6.2 Experiments...80 6.3 Characterization...82 6.3.1 SEM and TEM Characterization of SiO Nanotubes... 82 6.3.2 Production and branching of SiO nanotubes... 84 6.3.3 Diameter change of nano-network... 87 6.4 Optical Characterization...87 6.5 Conclusion...88 6.6 References...90 7 Chapter-7.Conclusion...92

List of figures ix Figure 1.1 Silicon nanowire arrays fabricated by E-Beam Lithography technique.--- 6 Figure 2.1 X Ray Diffractor experimental Setup. ------------------------------------------14 Figure 2.2 Figure showing the electron interaction with specimen and the probing depth.----------------------------------------------------------------------------------------------15 Figure 2.3 Construction of a typical Scanning electron microscope. -------------------16 Figure 2.4 Schematic setup of a Transmission electron microscope. -------------------18 Figure 2.5 Schematic illustration of the experimental setup of photoluminescence spectroscopy. ------------------------------------------------------------------------------------21 Figure 2.6 Schematic of the experimental setup of Renishaw in via Raman microscope. -----------------------------------------------------------------------------------------------------22 Figure 2.7 Schematic illustration of the fabrication process by shadow mask method. -----------------------------------------------------------------------------------------------------23 Figure 2.8 Schematic illustration of the experimental setup fabrication process by photolithography method.----------------------------------------------------------------------25 Figure 3.1 (a-b) SEM image of undoped ZnO nanowire arrays. (c) SEM image of Gadoped (1%) ZnO nanowire arrays. The corresponding EDS spectrum measured at NW tips is shown in the inset in (b) and (c). (d) HRTEM image of an undoped ZnO nanowire. (e) HRTEM of a Ga-doped (0.2%) ZnO nanowire. (f) HRTEM of a Gadoped (1%) ZnO NW s. ------------------------------------------------------------------------31 Figure 3.2 (a) Room temperature PL spectra of undoped and Ga-doped ZnO NWs. (b) XPS spectra of Ga-doped (1%) ZnO NWs. The inset shows Ga 2p peak.---------------33 Figure 3.3 I-V curves from 2-probe and 4-probe measurements of Ga-doped (0.2%) ZnO NW FET. The inset is the SEM image of the ZnO NW FET.------------------------35 Figure 3.4 Characteristics of undoped ZnO NWs FET with a diameter of 76 nm and an effective length of about 2 µm. (a) I DS -V DS plots at different V g. The insets show

SEM image of a single nanowire FET (upper) and logarithmic plot of the transfer characteristics at V DS =1V (lower). (b) I DS -V g plots at different V DS. -------------------36 x Figure 3.5 I DS -V DS plots of Ga-doped ZnO NWs at different V g. (a) 0.2 at % of Ga, and the diameter of NW is 62 nm and effective length is 2 µm. (b) 1 at % of Ga, and the diameter of NW is 90 nm and effective length is 2 µm. -------------------------------37 Figure 3.6 Distribution of conductivity values for 45 devices. 15 devices each for Gadoped (0%), (0.2%), and (1%) ZnO NW s.--------------------------------------------------38 Figure 4.1 (a) & (b) SEM images of undoped nanoribbon at lower and higher magnifications respectively. (c) & (d) SEM images of doped nanoribbons at lower and higher magnifications respectively (e) & (f) XRD spectra of undoped and doped CdS nanoribbons respectively. ----------------------------------------------------------------46 Figure 4.2 (a) & (b) TEM and HRTEM images of undoped nanoribbons with corresponding SAED pattern on the inset. (c) & (d) TEM and HRTEM images of doped nanoribbons with corresponding SAED pattern on the inset (e) EDS spectrum of the doped CdS nanoribbon.-----------------------------------------------------------------47 Figure 4.3 (a) Temperature-dependent photoluminescence (PL) spectra of undoped CdS nanoribbons. The spectra are shifted vertically for clarity and the numbers 1 to 10 correspond to the measuring temperatures T = 9, 20, 35, 55, 80, 100, 130, 200, 250 and 293 K, respectively. (b) Temperature-dependent photoluminescence (PL) spectra of In doped CdS nanoribbons. (c) & (d) Temperature dependence of peak energy of band edge emission and full-width half maximum (FWHM) of band edge emission of undoped nanoribbon respectively. (e) Temperature dependence of peak energy of band edge emission and the exciton emission.----------------------------------49 Figure 4.4 (a) I-V curves of a doped CdS single nanoribbon illuminated with light of different wavelength. The light intensity is kept constant at 1.75mW/cm 2. The insets show the optical microscopic image of the single-nanoribbon device. (b) I-V curves of a CdS single nanoribbon under light irradiation of varying intensity at 490nm. An enlarged view of the I-V curve measured in the dark is shown in the inset.------------52

Figure 4.5 Real-time photocurrent of a CdS:In nanoribbon at a 2 V bias to 490nm light illumination with a period of 5 s ON and 5 s OFF. ---------------------------------53 xi Figure 4.6 (a) & (b) SEM Image of doped nanopens at lower and higher magnification respectively. (c) & (d) SEM Image of doped nanorods at lower and higher magnification respectively (e) & (f) SEM Image of doped nanopencils at lower and higher magnification respectively. (g) nanopencils at initial growth stage (h) Schematic of growth of nanopens and nanopencils respectively. ------------------------55 Figure 4.7 (a) & (b) TEM images of doped nanopens and nano penholders respectively (c) TEM Image of gold tip at nano penholders at initial growth stage (d) Schematic setup of the field measurement setup (e) Field-emission measurements of the CdS nanopencils and nanopens respectively, showing current density-electric field characteristics with the inset for the corresponding Fowhler Nordheim (F-N) plot.58 Figure 5.1 (a) SEM image of as-grown ZnS Nanoribbon substrates. (b) SEM image of cross-aligned ZnS x Se 1-x nanowire arrays on ZnS Nanoribbon substrates (c) SEM image showing the angle of cross-aligned growth of ZnS x Se 1-x nanowires. The inset is an EDX spectrum of ZnS x Se 1-x nanowires (d) SEM image of cross-aligned ZnS x Se 1-x nanowire arrays during initial growth stage (e) SEM image showing the vertical growth of ZnS x Se 1-x nanowires on side surfaces of ZnS nanoribbon substrates (f) SEM image of vertical ZnS x Se 1-x nanowire arrays during initial growth stage. (g) and (h) EDX spectrum of ZnS x Se 1-x nanowire arrays corresponding to values of X~0.85 and 0.45 respectively---------------------------------------------------------------------------------68 Figure 5.2 (a) TEM image of as-grown ZnS Nanoribbon substrates. (b) HRTEM image of ZnS Nanoribbon substrates. Inset: SAED pattern ([100] zone axis) inset indicate the wurtzite nature of the single-crystal nanowires. (c) TEM image showing the angle of cross-aligned growth and vertically grown ZnS x Se 1-x nanowires. (d) & e) HRTEM images revealing the [210] and [001] growth directions of the nanowire. Inset: The electron diffraction patterns ([100] zone axis) indicate the wurtzite nature of the single-crystal nanowires. (f) Schematic diagram showing the alignment of nanowires grown on different surfaces of the substrate. ----------------------------------70 Figure 5.3 TEM and HRTEM images of ZnS x Se 1-x nanowires grown on the top ZnS nanoribbon substrates (a) and (c) TEM image of nanowire corresponding to

xii composition of x~0.45 and x~0.85 respectively (b) and (d) corresponding HRTEM image of nanowires shown in (a) and (c) revealing the growth direction of [210].The electron diffraction patterns ([100] zone axis) inset indicate the wurtzite nature of the single-crystal nanowires. ----------------------------------------------------------------------71 Figure 5.4 PL spectra under 266nm excitation for nanowire arrays of different compositions a) x~0.85 b) x~0.76 and c) x~0.45. Main graph: PL spectra of the ZnS x Se 1-x excited under different power densities. (Left Inset: Relation between the PL intensity and corresponding power density revealing the lasing threshold of the nanowire arrays). -------------------------------------------------------------------------------73 Figure 5.5 (a) (c) Normalized PL spectrums of the ZnSxSe1-x nanowire arrays for composition corresponding to X~0.45,0.76, 0.85 in the temperature range of 9K- 293K. (d) & (e) Variation of peak energy P3 and I1 with respect to temperature for the samples corresponding to X~0.45 and 0.85 respectively. ----------------------------75 Figure 6.1 (a) High-temperature tube furnace with two movable sample holders for synthesis and branching of SiO nanotubes. b) Schematic graph showing the temperature profile of the tube furnace during growth.-----------------------------------81 Figure 6.2 SEM images of the as-prepared sample showing (a) ribbon and (b) wirelike nanostructures, (c) and (d) TEM images of the as-prepared sample having branches grown in different directions from the same trunk or connect each other forming a network. ------------------------------------------------------------------------------83 Figure 6.3 a) Low-resolution TEM image and b) High-resolution TEM image of the white-section of the sample. -------------------------------------------------------------------84 Figure 6.4 TEM images of nanotubes containing Au particles.--------------------------86 Figure 6.5 Diameter change of a T-nano-branched pipe (a) before exposure and (b) after exposure to electron beam.--------------------------------------------------------------87 Figure 6.6 Photoluminescence spectrums of a) SiO/ZnS Heterostructures and b) SiO Nanotube at 9K temperature.------------------------------------------------------------------88