Vol.2, No.4, 368-372 (2010) Natural Science
Copyright © 2010 SciRes. OPEN ACCESS
Interfacial control on microstructure, morphology and
optics of beta-AgI nanostructures fabricated on
sputter-disordered Ag-Sn bilayers
D. Bharathi Mohan1, C. S. Sunandana2
1 SEG-CEMUC-Department of Mechanical Engineering, University of Coimbra, Coimbra, Portugal; dhanabharathi@yahoo.com
2 School of Physics, University of Hyderabad, Hyderabad, India; csssp75@yahoo.com
Received 26 October 2009; revised 7 December 2009; accepted 10 February 2010.
We report for the first time a non-template based
facile growth of hexagonal (β) AgI nanorods and
nanoplates easily fabricated by rf magnetron
sputtering on Ag/Sn bilayers upon controlled
iodination. The structural and morphological
evolution of the β-AgI nanostructures is cha-
racterized by X-Ray Diffraction, Atomic Force
Microscopy and optical spectroscopy. Sput-
tering induced disorder in precursor Ag films,
high external stress and high defect concentra-
tions at the Sn-AgI interface particularly facili-
tates the development of layered hexagonal
structure of β-AgI nanostructures. Extremely
sensitive room temperature optical absorbance
involving evolution of W1,2 and W3 exciton tran-
sitions and emission spectra involving phonon
replica corroborate the formation of β-AgI na-
nostructures with high defect concentrations,
are aimed at improving the efficiency of photo-
graphic process and looking at microelec-
trodic and optoelectronic applications.
Keywords: Thin Films; Nanostructures; Crystal
Structure; Optical Properties
Motivated by an extreme mesoscopic ionic conductivity
and superior photographic prowess, lately, many re-
searchers have synthesized Ag/AgI, AgI/γ-Al2O3 and
AgI nanostructures in with controlled nano feature sizes
and shapes by routes, including electrochemical, tem-
plate-chemical, Ultrasonic pyrolysis, W/O microemul-
sions and solution methods[1-8]. These works, however,
focused on the formation of highly stable β-AgI phase at
room temperature [9] and its structural disordering, for-
mation of highly conducting interfacial layers (i.e. 7H
and 9R polytype of AgI with stacking fault arrange-
ments), shape dependent properties and quantum con-
finement effects in nanorods. In this work, rf magnetron
sputtering is exploited as an innovative technique to fab-
ricate β-AgI nanostructures with different shapes-that
could lead to miniaturized nanoscale opto-electronic
devices [10]. Sputtering introduces structural disorder in
Ag films while doping introduces extra disorder and
external stress in the host and thus provides localized
states for the nucleation of nanoparticles in an effec-
tively kinetically controlled process [11]. To test and
implement these ideas we fabricated Ag/Sn bilayers by
sputtering where an ultra thin layer (~3.5 nm to 14 nm)
of Sn serves as capping agent for Ag particles that in-
troduces external stress at the Ag/Sn interfaces eventu-
ally controlling nanomorphology of silver iodide. Mor-
eover, doping could stabilize the crystal structure by
strengthening the cation (or anion) sublattice of the iono-
covalent semiconductor (CdS or AgI) and introducing a
certain number of donors/acceptors in the forbidden gap
of the host semiconductor thereby impacting the electri-
cal and optical properties of the host semiconductor [12].
Sn -with valences 2 and 4- was chosen because it is a
covalent metal and mixes well with Ag and could con-
trols the iodization kinetics [13] enabling realization of
desired optimized nanostructure even for a single Ag/Sn
Ag/Sn bilayers were produced using rf magnetron sput-
tering (MagSput-1G2-RF-HOT-UPG) with Sn layer thick-
ness varied from 3.5 nm to 14 nm while Ag layer thick-
ness was fixed as 90 nm. Silver (99.99% purity) and
tin (99.99% purity) targets each ~55 nm diameter and
~3 mm thick was used for sputtering; the base pressure
was always maintained as 1E-6 mbar. At first, Ag films
were sputtered onto commercial float glass substrates
D. B. Mohan et al. / Natural Science 2 (2010) 368-372
Copyright © 2010 SciRes. OPEN ACCESS
under constant Ar flow rate and rf power of 20 sccm
and 10 Watt respectively. Then, Sn of 3.5 nm, 7 nm
and 14 nm thick was successively deposited on Ag films
with rf power: 5 Watt and Ar flow rate: 20 sccm. Sub-
strate rotation and sputtering pressure were maintained
as 10 RPM and 1E-2 mBar respectively. Thus, bi-layers
of Ag (90 nm)/Sn (3.5 nm), Ag (90 nm)/Sn (7 nm) and
Ag (90 nm)/Sn (14 nm) were fabricated and stored under
vacuum in order to prevent surface oxidation. As grown
Ag film and Ag/Sn bilayers were iodized for selected
durations ranging from 3 hrs to 24 hours in a specially
made jig [14]. AMBIOS XP-1 profilometer was used to
measure the thickness and confirmed equal at different
places for the homogeneity. X-ray diffraction patterns
were obtained using INEL X-Ray Diffractometer (XRD)
with Co K
= 1.78897 A0) radiation. Atomic Force
Microscopy (AFM) measurements were performed using
SPA 400 operated in non-contact Dynamic Force Mode
(DFM) mode. Optical absorption and photoluminescence
studies were carried out using SHIMADZU UV-3101
and HITACHI: F-3010 Fluorescence spectrophotometers
Figure 1 shows XRD patterns of Ag film and Ag/Sn
bilayers with increasing thickness of Sn layer. Ag is
characterized by (111), (200), (220) and (311) planes
corresponds to fcc lattice (JCPDS card No. 7440-22-4).
Ag/Sn bilayers exhibit similar pattern that obtained in
undoped Ag despite increasing Sn layer thickness.
However, intensities are decreased in Ag (90 nm)/Sn
(3.5 nm) could be due to the formation of quasi amor-
phous structure as due to Sn induced disorder in Ag.
Increasing Sn layer thickness from 7 nm to 14 nm in-
creases the intensities with significant broadening
attributed to smaller particle size possibly controlled
by Sn atoms. Figure 2 shows the initial iodination of Ag
(90 nm)/Sn (3.5 nm) encourages both
-AgI and
phases simultaneously. With 12 hrs iodination,
phase became stronger while
-AgI growth stops gradu-
-AgI phase develops gradually with increasing Sn
layer thickness which is characterized by (002), (101),
(102), (110), (103), (112), (202), (203), (105), (202), (303)
and (006) crystal planes (JPCPDS card No. 75-1528). A
facile growth of
-AgI phase is observed on Ag (90 nm)/Sn
(14 nm) could be due to the development of hexagonal and
allied structures pointing to the role of Sn in modifying the
stacking of atomic layers by introducing planar defects.
Interestingly, (101), (102), (110), (103) and (112) reflec-
tions are predominant than from other planes possibly due
to the formation of interfacial highly conducting layers i.e.
7H and 9R polytypes of AgI with the stacking fault ar-
rangements. This is expectedly due to high external stress
and high defect concentrations occurring especially
Figure 1. X-ray diffraction patterns of as deposited Ag and
Ag/Sn bilyers with increasing Sn layer thickness.
Figure 2. X-ray diffraction patterns of Ag/Sn bilayers io-
dinated for 24 hrs.
at the Sn/AgI interface [15]. Formations of such poly-
types are responsible for the enhanced mesoscopic room
temperature ionic conductivity, by as much as four or-
ders of magnitude, compared with bulk
-AgI [16]. Ultra
thin ( 20 nm) undoped Ag produces γ-AgI while thick
( 20 nm) Ag films encourage
-AgI growth [14] how-
ever not as neat a structure as observed in bilayers. Lat-
tice parameter increases from 0.408 nm for undoped Ag
to 0.409 nm for bilayers as well increases the lattice pa-
rameters of a (from 0.458 nm to 0.460 nm) and c (from
0.751 nm to 0.752 nm) of
-AgI. Increases in lattice
parameters possibly reflects the difference in covalent
radii of Sn (0.141 nm) and Ag (0.134 nm). Having de-
posited on glass and Ag surfaces, intrinsic strain could
be different for Ag and Sn films as they possess tetrago-
nal and cubic crystal structure respectively. Intrinsic strain
determined for iodinated bilayers using Nelson-Reily
Function (NRF) [17-18], exhibits zigzag patterns re-
flecting the presence of intrinsic strain in
-AgI structure
(Figure 3).
Undoped Ag (Figure 4(a)) reveals an inhomogeneous
surface with the particle size of about 20 (±1) nm. Par-
ticles are aggregated on the surface as due to lack of
thermal energy during deposition. However, Sn layer
evens the silver surface (Figure 4(b)) by filling pores and
D. B. Mohan et al. / Natural Science 2 (2010) 368-372
Copyright © 2010 SciRes. OPEN ACCESS
Figure 3. NRF function shows intrinsic strain of β-AgI
phase increases with increasing Sn layer thickness.
(a) (b)
Figure 4. AFM shows surface morphology of (a) as deposited
Ag (90 nm) and (b) as deposited Ag(90 nm) /Sn(7 nm) bilayer.
covering boundaries due to its poor metallicity and
higher solubility properties. Iodization of undoped Ag
produces spherical shape of AgI particles with the size of
about 150 (±1) nm (Figure 5(a)) while Ag/Sn bilayers
exhibit rod- and plate- shaped β-AgI particles. More-
over the length of rod increases from 437 (±1) nm to
724 (±1) nm upon increasing tin layer thickness from
3.5 nm to 7 nm (Figures 5(b) and 5(c)). Further increase of
doping (14 nm) modifies the morphology from nanorods to
nanoplatelets (358 × 353 (±1) nm) (Figure 5(d)). Ag atoms
need more iodine atoms in order to satisfy the condition
((Ag/I) 1) for the
-AgI formation and that is indirectly
supplied by Sn atoms through unstable SnI4 tetrahedra.
Uniodized Ag reveals uniodized Ag reveals a broad
negative absorption around 320 nm due to Ag reflects
the light particularly in opaque films [14,19]. No appre-
ciable changes observed upon Sn doping except some
variation in the shape and intensity. At an intermediate
stage of iodization process, an evolution of optical ab-
sorption at 420 nm occurs due to the dipole forbidden
4d10-4d95s transition in AgI allowed by the tetrahedral
symmetry of Ag+ ion in the wurtzite AgI, attributed to
W1,2 exciton besides a broad plasmon resonance [14,19,
20] at 500 nm arises due to residual Ag nanoparticles
when the films are partially iodized consisting Ag-AgI
nanocomposites. After 24 hrs of iodization (Figure 6),
plasmon resonance disappears while W1,2 exciton band
enhances alongside a new peak developed at 330 nm due
to spin-orbit split I- valence of the spin orbit interaction
attributed to W3 exciton whose degeneracy is lifted due
to strain field change at the crystallite surface [21].
These unusual observations are significant because the
extremely sensitive room temperature optical absorption
-AgI has recorded the valence band degeneracy of
which is lifted at room temperature which also happens
to be the temperature at which iodization is carried out.
Absorption becomes very intense, broad and red shifted
upon increasing Sn doping [15]. The absorption in-
creases as the length of the nanorods increases from 437
(±1) to 724 (±1) nm however absorption band edge re-
main same. Surprisingly, absorption is four times inten-
sive for AgI nanoplates as compared to AgI nanospheres.
Absorption band is much wider for nanoplates. Band gap
[14] decreases from 2.87 eV to 2.83 eV when β-AgI par-
ticles change shape from nanospheres to nanoplatelets.
The observed red shift arises from not only the different
polymorphms of AgI nanoparticles but also due to an
increase of Sn layer thickness.
Emission spectra of 24 hrs iodized undoped Ag and
Ag/Sn bilayers were performed with the excitation wa-
velengths 325, 335, 345, 350 and 360 nm. The photo-
induced carrier radiative recombination rate is higher for
the excitation wavelengths 345 nm and 350 nm. Fig-
ure 7 shows the photoluminescence spectra excited at
350 nm.
Figure 5. AFM of (a) Ag, (b) Ag(90 nm)/Sn(3.5 nm), (c)
Ag(90 nm)/Sn(7 nm) and (d) Ag(90 nm)/Sn(14 nm) iodi-
nated for 24 hrs.
D. B. Mohan et al. / Natural Science 2 (2010) 368-372
Copyright © 2010 SciRes. OPEN ACCESS
Figure 6. Optical absorbance of 24 hrs iodinated (a) Ag; (b)
Ag(90 nm)/Sn(2 nm); (c) Ag(90 nm)/Sn(3.5 nm); (d)
Ag(90 nm)/Sn(7 nm); (e) Ag(90 nm)/Sn(14 nm) bilayers.
Figure 7. Emission spectra’s of 24 hrs iodinated (a) Ag; (b)
Ag(90 nm)/Sn(3.5 nm); (c) Ag(90 nm)/Sn(7 nm); (d)
Ag(90 nm)/Sn(14 nm) excited at 350 nm.
A sudden jump appears at 426 nm matching with the
wavelength of absorbance of the Z1,2 exciton [22]. The
phonon emission accompanying PL (phonon replica)
occurs at 437.8, 450.9, 467.0, 482.5 and 492.2 nm
among them the most intense peak centered at 467.0 nm.
PL indicates photoexcited electrons at the conduction
band edge do not recombine with holes immediately.
Instead they undergo many transitions at the shallow trap
states or intrinsic near-band edge states slightly below
the conduction band involving exciton-phonon and mul-
tiphonon interactions. Intrinsic Frenkel defects and im-
purities could be involved in the formation of trapping
states for the recombination. A fundamental reason for
the enhancement of probabilities of phonon assisted op-
tical transitions is the essential non-adiabaticity of exci-
ton-phonon systems in quantum dots [23]. The recom-
bination rate increases in nanorods while it is not too
high in nanoplates. The relaxation process is apparently
slow suggesting that the radiative life time of an exciton
is smaller than the time of relaxation between the exci-
ton energy levels. The enhanced trapping of the shallow
and deep trap states and the limit of saturation can be
visualized from the increase in the full width at half
maximum of the inhomogeneously broadened subbands.
Accordingly, maximum binding of almost all surface
defect sites at low Sn concentration and quenching of
radiative emission [24] at higher Sn concentration takes
place. Thus, the strong PL features with red shift and
multiphonon structure suggests a smaller radiative life
time and higher recombination rate with respect to bulk.
Reduction in intensity with increasing Sn concentrations
saturating the initial traps could further quench the ra-
diative emission, but did not affect the lifetimes effec-
tively. Above a certain limit, Sn effectively blocks charge
recombination and decreases the fluorescence quantum
efficiency at higher concentrations but does not affect
the decay characteristics at all concentrations. This is in
accordance with the fact that the presence of higher
valency dopant cations strongly reduces the iodination
rate of silver under normal conditions. This work there-
fore has implications for opto-electronic applications.
A non-template based facile growth of hexagonal (β) AgI
nanostructures were fabricated on rf magnetron sputtered
Ag/Sn bilayers upon controlled iodination.
-AgI phase
was strongly observed on Ag (90 nm)/Sn (14 nm) as due
to the development of hexagonal and allied structures
that eventually proved the possibility of the formation of
interfacial highly conducting layers i.e. 7H and 9R
polytypes of AgI with the stacking fault arrangements.
Shapes of the nanoparticles are tailored with respect to
the amount of Sn doping onto Ag upon controlled iodi-
zation. Evolutions of W1,2 and W3 exciton transitions and
phonon replica from absorption and emission spectra
respectively corroborates the formation of β-AgI nanos-
tructures with high defect concentrations.
Sincere thanks are due to the University of Hyderabad for the award of
research fellowship to D. Bharathi Mohan under UPE programme and
for sanctioning publication charges for this paper.
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