Nanoindentation and Raman studies of phase-separated Ag-As-S glasses

Article abstract of DOI:10.1063/1.3651494

Nanoindentation is used to study the mechanical properties at the nanoscale of the ternary Ag_x(As_0.33S_0.67) _100-x glassy system for 0 x 25. Direct evidence for phase separation of a Ag-poor and a Ag-rich phase in the composition range 2 x 20 is provided. The volume of the Ag-rich phase increases with x and percolates at x ≈ 7. The mechanical properties of the Ag-rich phase are comparable to those of the x 25 glass, while those of the Ag-poor phase are closer to (however, quite different from) the ones of the base glass AsS₂. The structure of both phases at short and medium range order is investigated by micro-Raman spectroscopy and is correlated to their mechanical properties.

Full text of DOI:10.1063/1.3651494

Nanoindentation and Raman studies of phase-separated Ag-As-S glasses
K. S. Andrikopoulos, J. Arvanitidis, V. Dracopoulos, D. Christofilos, T. Wagner et al.
Citation: Appl. Phys. Lett. 99, 171911 (2011); doi: 10.1063/1.3651494
View online: http://dx.doi.org/10.1063/1.3651494
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Published by the American Institute of Physics.
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APPLIED PHYSICS LETTERS 99, 171911 (2011)
Nanoindentation and Raman studies of phase-separated Ag-As-S glasses
K. S. Andrikopoulos,1,2,a) J. Arvanitidis,1 V. Dracopoulos,2 D. Christofilos,3 T. Wagner,4
and S. N. Yannopoulos2,5
1Department of Applied Sciences, Technological Educational Institute of Thessaloniki, 57400 Sindos, Greece
2Foundation for Research and Technology Hellas (FORTH/ICE-HT), P.O. Box 1414, GR-26 504 Patras,
Greece
3Physics Division, School of Technology, Aristotle University of Thessaloniki, GR-54124 Thessaloniki, Greece
4Department of Inorganic Chemistry, Faculty of Chemical-Technology, University of Pardubice, Legion´s sq.
565, 53210, Pardubice, Czech Republic
5Department of Materials Science, University of Patras, GR-26 504 Patras, Greece
(Received 5 August 2011; accepted 19 September 2011; published online 28 October 2011)
Nanoindentation is used to study the mechanical properties at the nanoscale of the ternary
Agx(As0.33S0.67 100ꢀx
)
glassy system for 0 ꢁ x ꢁ 25. Direct evidence for phase separation of a
Ag-poor and a Ag-rich phase in the composition range 2 ꢁ x ꢁ 20 is provided. The volume of the
Ag-rich phase increases with x and percolates at x ꢂ 7. The mechanical properties of the Ag-rich
phase are comparable to those of the x ¼ 25 glass, while those of the Ag-poor phase are closer to
(however, quite different from) the ones of the base glass AsS2. The structure of both phases at
short and medium range order is investigated by micro-Raman spectroscopy and is correlated to
C
V
their mechanical properties. 2011 American Institute of Physics. [doi:10.1063/1.3651494]
Non-crystalline chalcogenides, either bulk or thin films,
are promising materials for a wide range of electronics and
photonics applications.1,2 Their optical bandgap falls within the
visible and the near-IR range and can be simply tailored by fine
tuning the alloy’s composition; this semiconducting nature
combined with a high photosensitivity constitute the hallmark
of chalcogenides and lie at the origin of many of their applica-
tions. In most cases, phase separation—a common effect in
melt-quenched glasses—is an undesirable effect for applica-
tions in optics. However, its role has proved quite interesting
when electrical properties are considered. Especially, near the
percolation threshold where the originally fragmented phase
becomes continuous, the electrical conductivity increases
abruptly by several orders of magnitude.3
systems is needed for illuminating various aspects of the
switching mechanisms which still remain unclear.
In this letter, we present a nanoindentation mapping study
of the mechanical properties of selected glasses from the
phase-separated system Agx(As0.33S0.67 100ꢀx
)
combined with
micro-Raman structural characterization of the two phases at
molecular level. Mapping the mechanical properties at the
nanoscale employing nanoindentation testing provides a direct
evaluation of the mechanical properties of these systems that
cannot be explored by other experimental techniques.
Glasses were prepared from proper mixing of high pu-
rity elements at high temperature and melt-quenching. The
ingots were cut into rectangular slabs which were then pol-
ished to optical quality. Details related to sample preparation
can be found elsewhere.8 Nanoindentation measurements
were performed by means of a Hysitron Tribolab instrument
equipped with a Berkovich-type tip. A trapezoidal loading
function was applied to all glasses.9 The mechanical proper-
ties of the two end members of the glassy system of interest
(i.e., the AsS2, x ¼ 0 and the AgAsS2, x ¼ 25) were thor-
oughly studied since these samples served as reference for
the evaluation of the mechanical properties of the phase sep-
arated glasses. The reduced elastic modulus (Er) and nano-
hardness (H) were determined from the experimental
unload-displacement curves using the Oliver and Pharr
model10 and their mean values were calculated after statisti-
cal analysis of several measurements. Raman spectra, with a
spectral resolution of ꢃ 2 cmꢀ1, were excited with the
780 nm laser line of a Ti-Sapphire laser and recorded using a
triple monochromator DILOR XY instrument equipped with
a liquid-nitrogen cooled CCD. A 100ꢄ objective was
employed, providing a spot size of ꢃ1 lm on the sample.
Optical micro-photographs of representative samples from
With respect to the host glass of this investigation, bi-
nary arsenic sulfides, AsxS100ꢀx are amongst the most thor-
oughly studied systems that can easily form macroscopically
homogeneous glasses in the range 0 ꢁ x ꢁ 42.4 However,
these glasses cannot be considered strictly homogenous at
the nanoscale where various local environments such as Sn-
chains, S8-rings, and “AsS3/2” pyramidal units co-exist,
whose concentration depend on x.5
Silver-containing chalcogenides have recently gained
increased interest as emerging solutions in resistive-
switching random access memories employing the configura-
tion of a metal-insulator-metal memory cell.6 The switching
mechanism is actually based on the polarity dependent elec-
trochemical deposition and removal of metal in a thin solid
state electrolyte film, which is usually a chalcogenide glass.
Phase separation creates droplets of a silver-rich conductive
phase within the electrolyte matrix structure, and hence the
switching mechanism can become much faster because a
much shorter percolative pathway needs to be built between
the droplets of the conductive phase.7 A detailed characteri-
zation of the phases involved in such micro-phase separated
the ternary system Agx(As0.33S0.67 100ꢀx
)
are illustrated in Fig.
1. Due to the high quality of the sample’s surface, it was possi-
ble to obtain evidence of single glass phase for samples with
x ¼ 0, 22, 24, and 25. At low Ag concentration, x < 8, the
a)Electronic mail: kandriko@gen.auth.gr.
C
V
0003-6951/2011/99(17)/171911/3/$30.00
99, 171911-1
2011 American Institute of Physics
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171911-2
Andrikopoulos et al.
Appl. Phys. Lett. 99, 171911 (2011)
TABLE I. Mean values and standard deviations of the elastic modulus and
hardness for the samples under investigation obtained from nano-indentation
measurements. Hardness obtained from micro-hardness experiments on sim-
ilar compositions are given in the last column.
Nanoindentation
measurements
Microhardness
measurements4
Sample’s
Modulus
Hardness
(GPa)
Hardness
(GPa)
Composition
(GPa)
a
AsS2
18.2 6 0.45
37.7 6 0.53
28.3 6 1.97
37.2 6 3.46
—
1.56 6 0.060
2.27 6 0.057
1.98 6 0.15
2.33 6 0.3
—
1.05
1.46
—
a
AgAsS2
b,c
Ag8As31S61
Ag8As31S61
b,d
b
—
Ag10As33S57
1.57
aHomogeneous sample.
bPhase separated sample.
cAg–poor phase.
FIG. 1. (Color online) Optical micro-photographs using 50ꢄ and 20ꢄ
objectives for the x ¼ 0, 20, 24 and x ¼ 2, 4, 8, 10, 12, 14 samples,
respectively.
dAg–rich phase.
compared to the dispersion in the homogeneous AsS2 and
AgAsS2 glasses—that can be attributed to: (i) the fact that
within several of the large droplets formed by one phase,
there may exist smaller droplets of the second phase and (ii)
the complex mechanical response of the sphere boundaries.
glasses exhibit phase separation with the phase A, consisting of
distinct spherical droplets, incorporated in the continuous phase
B. When the Ag concentration reaches ꢃ 8 at. % the initially
continuous phase B becomes fragmented and phase A perco-
lates in the glass structure. The size, type, and number of the
droplets depend on the Ag content. Referring to their size, the
largest spheres are observed for the samples with Ag content
x¼ 4, 8, and 10 (their size spans values from ꢃ1 lm to several
tens of lm). X-ray microanalysis8 revealed that phases A and
B are the Ag-rich (AgAsS2) and the Ag-poor (AsS2 with small
concentration of Ag) phases, respectively.
Table I contains parameters related to the mechanical
properties extracted by the nanoindentation measurements;
despite the fact that the absolute values are somewhat differ-
ent than those obtained from micro-hardness experiments
performed on materials with similar compositions4 (last col-
umn of the table), the results appear to be consistent at least
qualitatively. Images captured by scanning the samples’
surfaces with the indenter (AFM mode) revealed inhomoge-
neous patterns of the mechanical properties, for the glasses
with compositions 2 ꢁ x ꢁ 20, in accordance with the optical
images. Figure 2(a) depicts an AFM topography of the
Ag8(As0.33S0.67)92
tion is evident.
sample’s surface, where the phase separa-
The reduced elastic modulus and the nano-hardness val-
ues of the sample region shown in Fig. 2(a) were acquired by
a number of sequential nanoindentations and a subsequent
calculation of Er and H for each measurement. The results
obtained for the elastic modulus are presented as a contour
map in Fig. 2(b) and are in accordance with the scanned
image. Indentations performed in the “dark” area of the A
phase (Ag-rich phase) resulted in mechanical properties sim-
ilar to those of the AgAsS2 glass (Table I); representative
load/unload curves can be seen in Fig. 2(c). On the other
hand, indentations performed in the B phase (Ag-poor phase)
yield significantly lower values for both Er and H. However,
it is interesting to note that these values did not exactly
match those of the binary AsS2 system (base glass), indicat-
ing that the B phase probably contains a small amount of Ag.
The standard deviations in Table I indicate considerable dis-
persion in the extracted values for each of these two sets—
FIG. 2. (Color online) (a) AFM topography of the Ag8(As0.33S0.67)92 sample
at a 24 ꢄ 20 lm region after the accomplishment of 42 sequential indenta-
tions (dashed square on the upper right corner encloses a single nanoindenta-
tion). White circles have been sketched in order to render the interface
between the two phases more visible. (b) Contour map of the elastic modu-
lus. White circles correspond to those in panel (a); Triangles denote approxi-
mate positions of single indentations. (c) Single indentations performed on
AsS2 and AgAsS2 samples (top) and on the x ¼ 8 sample in the Ag-rich and
Ag-poor phase (bottom). Open (solid) symbols demonstrate data collected
upon load (unload) segment. (d) Raman spectra of the AsS2 and AgAsS2
glasses (top) as well as the corresponding spectra obtained from the Ag-poor
and Ag-rich phase of the x ¼ 8 sample (bottom). Thick (thin) lines represent
VV (VH) spectra. Inset: As3S6 structure, the main molecular species existing
in the mineral smithite. Dark (light) spheres represent As (S) atoms.
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171911-3
Andrikopoulos et al.
Appl. Phys. Lett. 99, 171911 (2011)
In order to gain more information on the structure of the
two phases, micro-Raman spectra were recorded from each
one of these. Representative Raman spectra from the two
phases for the x ¼ 8 glass are shown in the lower panel of
Fig. 2(d). Spectra were recorded for two polarization geome-
tries VV and VH where the electric field vectors of the inci-
dent and scattered beams are parallel and orthogonal,
respectively. In contrast to the constant depolarization ratios
measured from macroscopic scattering volumes,8 frequency
dependent depolarization ratios were derived. This result is
indicative of a homogeneous glass phase in the scattering
volume from which the spectra were collected. The Raman
spectra undoubtedly reveal that the structural characteristics
of the two phases at molecular level are different. The spec-
tra of the Ag-free (AsS2) and the AgAsS2 glasses are also
illustrated in Fig. 2(d).
mid (forming corner sharing pyramids) or another S atom
(forming either disulfide bonds or longer sulfur segments
that interconnect two AsS3/2 pyramids). The existence of S-S
vibrations belonging to various environments can be justified
by the bands appearing in the high frequency region of the
spectrum (460–495 cmꢀ1) assigned to: (a) Sn chains, (b) S8
rings, and (c) disulfide bonds of the type S2As-S-S-AsS2.8
Their intensity appears weaker for the Ag-poor phase com-
pared to that for the case of the Ag-free glass spectrum. The
latter designates the comparatively fewer S-S bonds in the
Ag-poor phase with respect to the S-S bonds existing in the
AsS2 glass structure, thus suggesting a lower concentration
of soft atomic arrangements. This finding accounts for the
higher elastic modulus values obtained for the Ag-poor
phase in comparison to those of the AsS2 glass (Table I).
Another explanation for the observed differences in the me-
chanical properties of the two glasses (the one forming the
Ag-poor phase and the AsS2 base glass) may be that the Ag-
poor phase contains some amount of Ag which evidently
enhances its mechanical properties.
The Raman spectrum of the Ag-rich phase exhibits great
similarities with that of the AgAsS2 glass [see upper panel in
Fig. 2(d) and Ref. 8]. In both spectra, the most intense spec-
tral feature is the band appearing at ꢃ375 cmꢀ1. In order to
appreciate the difference between the Raman spectra of the
AsS2 and AgAsS2 glasses, we should consider the short and
medium range structural order of their corresponding crys-
tals. The structure of the AgAsS2 crystal (either smithite or
trechmannite) differs from that of orpiment (As2S3), which is
a rather good representation of the nearly stoichiometric
AsS2 glass, in that the latter contains layers composed of 6-
membered rings (As6S12 groups), while the former (AgAsS2)
is a molecular glass where the molecular fragments are
3-membered rings [As3S6 groups; see inset of Fig. 2(d) for a
planar species building up the smithite allotrope structure].
The units are surrounded by Agþ ions interacting with four
terminal S atoms, St, from four different neighboring As3S6
groups. In this context, the existence of As-St terminal bonds
In summary, we have presented a study of the mechanical
glassy system
properties of the ternary Agx(As0.33S0.67 100ꢀx
)
at the nanoscale. It was shown that the samples are homogene-
ous for x < 2 and x > 20, contrary to the samples of 2 ꢁ x ꢁ 20
composition, which exhibit phase separation with microscopic
inhomogeneities characterized by highly symmetric sphere-
like domains. The latter was directly observed by the AFM
topography as well as by optical microphotographs of the
samples’ surface; it was also indirectly revealed by the con-
tour map of the modulus obtained after successive nano-
indentations. Regardless the sample’s composition, one of the
phases was found to possess similar mechanical properties to
the x ¼ 25 sample, while the mechanical properties of the sec-
ond phase were found to be closer, however not similar, to
those of the AsS2 system. The structure of both phases at short
and medium range order was revealed by micro-Raman spec-
troscopy and was correlated to their mechanical properties.
is indicated by the presence of the peak at ꢃ375 cmꢀ1
,
appearing at higher frequency than the respective stretching
vibrations of the pyramidal polyhedra with no terminal S
atoms. Besides, it possesses the highest depolarization ratio
in the Raman spectrum, as it is expected for a highly polar-
ized vibrational mode of a terminal bond.11 It is interesting
to notice that there is no S excess in the Ag-rich phase and
consequently no S-S vibrations are observed in the corre-
sponding Raman spectrum (vide infra). The latter is in agree-
ment with a recent structural investigation,12 which also
suggested that the short range order of AgAsS2 glass differs
from its crystalline counterpart mostly in that the fourfold
coordination of the silver atoms in the glass includes three S
atoms and one Ag atom. Given that the Raman bands are dic-
tated by the intramolecular vibrations of the As3S6 groups,
intermolecular interactions mediated through the weak, ionic
Ag-S bonding are not reflected in the Raman spectra, and
thus it is precarious to derive arguments about the coordina-
tion of Ag atoms from the present study.
Financial support by the Research Committee of the
ATEI of Thessaloniki (Grant No 80014) is acknowledged.
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V. Kolobov (Wiley-VCH, Weinheim, 2003).
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9The loading-unloading procedure comprised the steps: (i) linear increase of the
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The comparison between the Raman spectra of the Ag-
free glass with that obtained from the Ag-poor phase reveals
also similarities. The main Raman band at ꢃ340 cmꢀ1 origi-
nates from As-S vibrations in “AsS3/2” pyramids where the S
atoms are connected either to an As atom of a nearby pyra-
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