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Chapter III Surface plasmon-exciton 3.pdf · PDF file Chapter III Surface plasmon-exciton transition 74 3.2 Preparation of Ag and AgI thin films Thin Ag films were deposited on borosilicate

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  • Chapter III Surface plasmon-exciton transition

    74

    3.2 Preparation of Ag and AgI thin films

    Thin Ag films were deposited on borosilicate glass substrates by Vacuum thermal

    evaporation method [1-5]. The precursor material namely highly pure Ag Powder

    (Aldrich, 99.999%) placed evaporation source (molybdenum boat) was thermally-

    evaporated on to the cleaned borosilicate glass substrate kept at an ambient temperature at

    a pressure of 5x10-6 Torr. The base pressure of the vacuum chamber was maintained at

    ~3x10-6 Torr throughout the evaporation process. The amorphous glass substrates were

    kept at a distance of 20 cm from the source for deposition [6]. Thin films of pure Ag with

    thickness of 5-20 nm deposited at room temperature and the rate of deposition was

    maintained constant (0.1 Ao

    Iodization: To iodize Ag films, an air/vacuum-tight figure of eight glass chamber was

    fabricated with dimensions of 10 cm height × 6 cm diameter. Iodine kept at the bottom of

    the lower half of the chamber sublimates at room temperature and slowly deposits on the

    Ag films kept at the top of the chamber shown in figure 3.1 [6-10]. Thus iodization was

    carried for selected durations in the range 5 minutes to 60 minutes. These films without

    any post-annealing treatment were characterized by XRD using a PHILLIPS X-ray

    powder diffractometer with Cu Kα (λ= 1.54056 A

    /sec) during the entire process. The above conditions were

    found favorable for the formation of uniform and homogeneous films. The thickness of

    the films was determined by using stylus profiler (Model XP-1, Ambios Technology,

    USA) profilometer.

    Substrate: Prior to the deposition these substrates were cleaned as follows: substrates

    were first immersed boiling 10% soap solution with 90% water, rubbed with cotton in

    cold water to remove weathering, kept in chromic acid up to boiling point for removing

    organic contaminates, washed in cold water to remove surface contaminants followed by

    ultra-sonification in isopropyl alcohol for 5-10 min duration, and, finally these substrates

    were dried in air before loading in to the vacuum chamber for deposition.

    o) radiation. To analyze the surface

    morphology, films were examined by SPA 400 Atomic Force Microscope (AFM) using

    non contact Dynamic Force Mode. JASCO V-570 UV-VIS-NIR double beam

    spectrophotometer has been used for optical absorption studies at 300K in the UV/visible

    range from 300 to 600 nm with scanning rate 4 nm per second. Fluorolog-3

    Spectroflurometer has been used for Photoluminescence studies at 300K in the

    UV/Visible range from 300 to 800 nm with different excitation wavelength.

  • Chapter III Surface plasmon-exciton transition

    75

    Figure 3.1: Iodization chamber 3.3 Crystal Structure of iodized Ag films

    Ag films thickness of 5-20 nm deposited on silicate glass substrates at room

    temperature were characterized by XRD as shown in figure 3.2. Figure 3.2 shows the

    thickness dependent nanocrystalline formation of Ag thin film. Ag films thickness of 5 nm show the formation of

    crystalline fcc structure with (111), (200) reflections. The low angle (111) Ag peaks is the

    most intense in the pattern, which implies a slight preferential orientation of the Ag grain

    along the (111) reflection [11-14]. Upon increased Ag film thickness, the X–ray intensity

    starts decreasing possibly due to decrease of surface to volume ratio of Ag particle. The

    (111) plane in Ag has lowest surface energy, and therefore, equilibrium growth condition

    leads to a (111) orientation. As the film thickness increases, the (111) reflection shifts to

    higher angles and FWHM reduces, which indicates that grain growth has occurred,

    resulting in the partial relief of intrinsic stress within the films. From the FWHM and

    peak position of the (111) peak, the grain size and the film stress are calculated. It shows

    that the grain sizes and the film stress are influenced by the film thickness [15-16]. With

    increasing film thickness, grain sizes increases and the absolute peak intensities decrease

    while film stress decreases. In the θ-2θ mode, only crystallites with lattice planes parallel

    to the surface are measured. Compressive stress parallel to the surface causes vertical

    8 shaped glass container

    Ag films

    1mm hole

    Iodine flake

  • Chapter III Surface plasmon-exciton transition

    76

    expansion of the film and leads to an increase of interplanar spacing, which is parallel to

    the surface. [17-20].

    20 25 30 35 40 45 50 55 60

    (a)

    Int en

    sit y (

    ar b.

    un i.)

    2θ in deg.

    (c)

    (i)

    Ag (2

    00 )

    Ag (1

    11 )

    (d)

    (b)

    Figure 3.2: XRD pattern of as deposited Ag thin films (a) 10 nm (b) 15nm (c) 20 and (d) 25 nm thick deposited on amorphous borosilicate glass substrates at room temperature. The grain sizes in Ag films were calculated according to the Scherrer formula [21]:

    = 0.098λ / βcosθ (3.1)

    where λ is the X-ray wavelength (1.54186Ao

    How does the iodization of these silver films proceed? A controlled flux of

    molecular iodine vapours is realized in a figure of eight or hourglass jig (figure 3.1) with

    a 1 mm opening at the centre for brief durations ~ 5 to 60 minutes. Thus silver

    nanoparticles are gradually converted into AgI nanoparticles with predominant phase of

    γ-AgI (111) reflection (Zincblende structure) and small “impurity” of β-AgI (101)

    reflection (Wurtzite structure) perhaps due to local iodine excess [(Ag/I)

  • Chapter III Surface plasmon-exciton transition

    77

    development of intense low angle (111) peak of γ-AgI is highlighted. As mentioned

    earlier (111) direction in γ-AgI film has the lowest surface energy, therefore equilibrium

    growth conditions leads to a (111) orientation, whereas non-equilibrium condition give

    rise to other grain orientations detected as other XRD peaks such as (101). β-AgI as an

    inevitable impurity phase due to excess iodine present locally in the thin film system

    [5,9-11,22-25]. The percentage of β-phase is observed to increase with increasing film

    thickness. An enormous growth of γ-AgI phase is observed for 15 nm Ag films after 60

    min of iodization. It consists of major peaks of γ-AgI which corresponds to the metastable

    zincblende structure [5,9]. Intensity increases dramatically with respect to iodization time

    something unusual and rare in ultra thin films. It has been observed that the very thin

    evaporated AgI films thickness < 2-10 nm show formation of metastable silver iodide.

    From these observations one may infer that the formation of γ-AgI nanoparticles are not

    reproducible on ultra thin films, suggesting that both zincblende and wurtzite crystal

    structure enhancement in the intensity are possible as these films are grown on

    discontinuous silver films involving especially very large surface to volume ratio. Two

    challenges of discontinuous films are desirable phase instability and poor reproducibility.

    Also note that the intensity of XRD peaks decrease with increasing film thickness

    probably due to a decrease in the surface area of Ag nanoparticles and randomization of

    (111) planes.

    20 22 24 26 28 30

    In te

    n si

    ty (

    ar b

    . u n

    it )

    2θ in degree

    (a) 5 nmβ-AgI (002) γ-AgI (111)

    (b) 10 nm

    γ -A

    gI (1

    11 )

    β -

    A gI

    (1 01

    )

    (c) 15 nm

    Figure 3.3: XRD pattern of as deposited Ag films5, 10 and 15 nm thick iodized for 60 minutes.

  • Chapter III Surface plasmon-exciton transition

    78

    Iodization generally induces strain and defects (Frenkel defects etc) [30] in the AgI films

    making it hard to generalize about particle growth with respect to thickness induced stress

    particularly in AgI films grown on amorphous substrates. Compressive stress may be

    generated due to (i) differences in the expansion coefficients of film and substrate, (ii)

    incorporation of foreign atom or chemical reactions, (iii) variation of the inter-atomic

    spacing with crystal size, (iv) re-crystallization process, and (v) microscopic voids and

    dislocations. From the peak position shift in the XRD pattern one can calculate average

    strain (ε) using the following relation [9-10]:

    ε = [(d obs - d reference)/d reference] x 100% (3.2)

    where dobs and d reference

    The as-deposited Ag and subsequently iodized Ag films show well-defined

    surface morphologies as revealed by Atomic Force Microscope (AFM) show in figure 3.4

    [6]. The iodization process induces AgI grain growth leading to a significant increase in

    lateral grain growth and a striking difference in the morphology between Ag-I bonds

    depending upon iodization time [