Nanoionics of advanced superionic conductors - … · nanoionics of advanced superionic conductors...
-
Upload
hoangkhuong -
Category
Documents
-
view
219 -
download
0
Transcript of Nanoionics of advanced superionic conductors - … · nanoionics of advanced superionic conductors...
306 Ionics 11 (2005)
Nanoionics of Advanced Superionic Conductors
A.L. Despotul i , A.V. Andreeva and B. Rambabu* Institute of Microelectronics Technology & High Purity Materials RAS, 142432 Chernogolovka,
Moscow Region, Russia *Southern University and A&M College, Baton Rouge, Louisiana, 70813 USA
~E-mail: [email protected] (A.L. Despotuli)
Abstract. New scientific direction - nanoionics of advanced superionic conductors (ASICs) was
proposed. Nanosystems of solid state ionics were divided onto two classes differing by an opposite
influence of crystal structure defects on the ionic conductivity oi (energy activation E): 1) nanosystems on the base compounds with initial small o~ (large values of E); and II) nanosystems
of ASICs (nano-ASICs) with E = 0.1 eV. The fundamental challenge of nanoionics as the conservation of fast ion transport (FIT) in
nano-ASICs on the level of bulk crystal was first recognized and for the providing of FIT in nano-
ASICs the conception of structure-ordered (coherent) ASIC//indifferent electrode (IE) hetero- boundaries was proposed. Nano-ASIC characteristic parameter P = d/Xo (d is the thickness of ASIC
layer with the defect crystal structure at the heteroboundary, and Ao is the screening length of charge for mobile ions of the bulk of ASIC) was introduced. The criterion for a conservation of FIT in
nano-ASIC is P = 1. It was shown that at the equilibrium conditions the contact potentials V at the ASIC//IE coherent heterojunctions in nano-ASICs are V << keT/e. Interface engineering approach
"from advanced materials to advanced devices" was proposed as fundamentals for the development of applied nanoionics. The possibility for creation on the base of ASIC//IE coherent
heterojunctions of the efficient energy and power devices (sensors and supercapacitors with specific capacity ~10 -~ F/cm 2 and maximal frequencies 10~-109 Hz,) suited for micro(nano)electronics,
microsystem technology and 5 Gbit DRAM was pointed out.
1. Introduct ion
Dispersoids of ionic conductors [1-3] and ionic con-
ductor//electronic conductor heterojunctions [4] are classic
objects of solid state ionics and, at the same time, the
objects of nanoionics, as by structure they are nano-
systems. The term and conception of nanoionics as a new
branch of science devoted to a fast ion transport (FIT) in
solid nanosystems was first introduced by [5] in 1992.
Main applications of nanoionics relate to the creation of
new materials, functional structures and devices suited for
the storage and conversion energy and information. In the
latest years, the term "nanoionics" ("nano-ionics") came
into wide use in scientific articles and denotes also the
area of interests of the scientific societies and organiza-
tions [6].
In the present article the new scientific direction -
nanoionics of advanced superionic conductors (ASIC) is
introduced and some appropriate fundamentals are for-
mulated. Key role of interface design in nanoionics of
ASICs is pointed. It is expected that in next decade the
nanoionic devices will find a wide application in the
sphere of wireless sensor networks (multitude auto-
nomous sensors that coordinate among themselves and
revolutionize information gathering in any type of terrain
and conditions).
2. T w o Classes of Sol id State Ionic N a n o -
s y s t e m s and T w o F u n d a m e n t a l l y D i f f e r e n t
N a n o i o n i c s
Solid state ionic conductors (SSIC) with a high level of
unipolar ionic conductivity (Q > 0.001 f2-~cm -~ and the
level of electronic conductivity G, is arbitrary) are called
superionic conductors (SIC), and solids with oi )> 4,
identified as solid electrolytes (SE). Intersection of SIC f3
Ionics 11 (2005)
Fig. 1. Different types of solid state ionic conductors (SSIC) on the cr i- o e diagram [8-10]: SE are the solid electrolytes where the ionic conductivity ~ >> electronic one o~; SIC are the superionic conductors, ~ > 0.001 f2-Lcm ~, and o~ is arbitrary; ASIC are the advanced superionic conductors oi > 0.1 ~ fcm -~, and cr is arbitrary; SIC n SE are the superionic conductors & solid electrolytes, ~ > 0.001 if2-lcm ~, and oi >~ q; ASIC fq SE are the advanced superionic conductors & solid electrolytes, cr~ > 0.1 if2- lcm ~, and o, ~> %
SE is a group of SIC & SE, simultaneously. Among the
SICs there is a subgroup with a record high level of
unipolar ~ . This subgroup can be called as advanced
superionic conductors (ASIC). There is a subgroup ASIC
fq SE, i.e. compounds with o, ~ % (examples are: ct-
AgI, ct-RbAg4Is, CsAg4Br3_xIz§ Rb4Cu16IvCl13 and some
others). For instance, the Rb4Cu16IvC113 is ASIC & SE
with recorded high crj (~ 0.34 if2-~crn -~ at 300 K, and
activation energy of ionic conductivity E = 0,1 eV) [7].
All types of SSICs are presented on the ~,-cr~ diagram
(Fig. 1).
On the boundaries of ionic crystals the double electric
layers (DEL) with a high concentration of defects always
exist. It is a consequence of different values of work
function for different kind of ions [11]. The thickness of
DEL is an order of Debye length )~ and is defined by a
concentration of mobile ions ne. In the work [1] an en-
hanced ionic conductivity cr i in nanocomposites (dis-
persoids) with components o f small initial crg w a s
discovered. Such an effect is due to the high density of
DELs with high values of Oe.
Two classes of SSICs can be distinguished. In the
class o f substances with small ~ ("poor" ionic
conductors), for instance, LiI (at 300 K er e ~ 5 . 5 x 1 0 -7
f2-~cm -1, and energy activation E > 0.4 eV [12]), the
value of ~,o is about ~60 nm that is an order of a grain
3O7
size in nanocomposi tes . At the sizes of crystallites
comparable with a thickness of DEL, the integral values
of cri in nanocomposites of "poor" ionic conductors are
much higher than in component substances. However, the
ion-transport properties (G, E, ni) in the nanocomposites
of "poor" ionic conductors are significantly worse (for
example, the energy of activation E is 4-8 times large)
than in ASICs (c~-AgI, RbAg4Is). Crystal structure of
ASICs is close to an optimal one for FIT (oi =, 0.3
~-lcm-1 at 300 K, E = 0.1 eV). Therefore, the defects of
crystal structure should violate almost everywhere in
ASIC conditions for FIT. Thus, at the high concentration
of defects in nanosystem of "poor" ionic conductors the
integral ~ arises, but in ASICs the influence of defects is
opposite.
In [5], a general approach for description of properties
o f ionic nanosystem was proposed (nanoionics con-
ception). It is based on the using of the P = d / L ~ 1
dimensionless parameter (d is the thickness of boundary
domain of SSIC with the peculiar properties, and the L -
characteristic size of the SSIC nanostructure). For nano-
systems of "poor" ionic conductors with DEL it can be d
~- )~D and P = )~o/L. However, if DEL is absent then
others characteristic values should be used instead of 3, D.
Two examples can be mentioned. In a fuel cell the effec-
tive functioning of catalyst demands that the diffusion
length of proton (d) be comparable with the size of ca-
talyst particle (L). Also, at the solid-state synthesis of
new compounds in the nano-physical-chemical systems
[6,13-16], dissolution of metals in SSICs is accompanied
by the simultaneous insertion of electrons and ions and
local electro-neutrality in the layer with peculiar pro-
perties remains. Therefore, instead of ~.D it is necessary to
use, for example, a critical radius of a new phase crystal
nucleus, the average size of crystallite and others similar
values.
The calculation of ~-D for the c~-RbAg4I ~ ASIC (300
K, and concentration of mobile ions ~ 1 0 28 m -3) according
to the Debye formula:
)~o ~ (eeo k8 T/e2 ni) 1/2, (1)
(e0 = 8.8x 10 -~2 F/m; e is the dielectric constant, 1 for
vacuum; kB is the Boltzmann's constant, 1.4x10 -23 jK-l;
T = 300 K; e is the electronic charge, 1.6x10 -19 C)
results in value for )~D ~ 0.05 nm, which is less than the
size of Ag+-ion. It indicates the need for another formula
(see, for example, [17]) for the calculation o f the
screening length (/LQ).
308 Ionics 11 (2005)
In the RbAg4I 5 ASIC, the oscillation frequencies of
mobile Ag+-ions in the potential minimums of crystalline
relief are ~10 J2 s -~ and the Ag+-ions jump over the
potential barriers (= 0.1 eV) between neighboring
crystallographic positions with the frequencies ~101~ s -~.
According to [18], in the RbAg4I 5 ASIC the concentration
of Ag+-ions in the state of flying over potential barriers is
of the order of ~ 1 0 26 m -3. For these ions the Debye's
formula yields ~,o ~ 0.5 nm (the size of ion is several
times less) and (1) can be using for estimation of
screening length. If the potential of indifferent electrode
(IE) at the RbAg4Is/IE heterojunction changes then the
Ag+-ions (flying under potential barriers) will form the
DEL with the thickness ~-D ~ 0.5 nm during the time
interval At ~ 10 -j~ s. For the time interval A t ~ 10 -1~ s in
the formation of DEL, all Ag+-ions with the concen-
tration about 1028 m -3 will take part and a screening
length )~e should be less than 0.5 rim. For the description
of nano-ASICs ()v 0 < 0.5 nm), dimensionless parameters
were not used because the characteristic values comparable
with so small )v 0 were unknown. In our opinion, such a
situation limites greatly the development of nanoionics.
Currently, the influence of DELs on ~,. in nano-
systems of "poor" ionic conductors (mesoscopic effects)
is investigated in considerable detail (see, for example,
[19-27]), whereas the works on the properties of DELs in
nano-ASICs are presented much worse [6,8-10,28-32]. On
the base of the above mentioned analysis, we draw a
conclusion that the defects of the crystalline structure
increase ~ in nanosystem of "poor" SSICs but in nano-
ASICs the defects violate the conditions for FIT. Thus,
the considered nanosystems should be divided into two
classes requiring the principal different structure design for
the achievement of FIT: (i) nanosystem on the base of
substances with small o~ and characteristic parameter P =
d / L ~ 1; and (ii) nano-ASICs for which the characteristic
parameters were unknown. This conclusion leads to a
supposition about existence of two fundamentally diffe-
rent nanoionics and unknown one is nanoionics of
ASICs.
3. S t r u c t u r e - O r d e r e d ( C o h e r e n t ) Hetero-
boundaries in ASICs The SSIC/electrode heterojunctions are classical objects
of electrochemistry and solid state ionics. In the work [4]
for the interpretation of slow relaxation of SSIC/electrode
processes the conception of ion adsorption was intro-
duced. The idea of surface defect structure of boundary was
formulated in [33], and the conception of effective
thickness of transition defect layer was introduced in [34].
The work [35] was devoted to the development of ad-
sorption relaxation model (relaxation of DEL in ASICs).
In this model, the slow diffusion processes (large values
of energy activation E) on electrode are attributed to the
movement of ionic defects. However, all above-mentioned
models and conceptions are phenomenological and macro-
scopic and they do not take into account the concrete
atomic structure of heteroboundaries. Such structures in a
number of cases can have a size comparable with ~e in
the volume of ASIC.
Atomic structure of homo- and heterophase boundaries
and the analysis of appropriate processes are in a focus of
material science during decades. Terms and approaches as
"crystal engineering of grain boundaries" and "interface
design" were for the first time introduced in [36] by T.
Watanabe and then successfully applied for creation of
advanced micro(nano)structured composite materials with
significantly improved mechanical properties [36-38], and
then in recent years - for creation of advanced electronic
and magnetic materials [39-41]. Unlike other disciplines,
only in the most recent works [42-44] the solid state
electrochemistry began to focus an attention on the
crystallochemistry and crystallography of coherent and
semi-coherent boundaries with the aim to predict and
control electrochemical processes.
Modern technology provides wide possibilities for
application of the interface engineering in the sphere
creation of electronic and opto-electronic devices based on
the "ideal" lattice-matched (coherent) heterojunctions. The
creation of the "Man Made Crystals" - semiconductor
"super-lattices" (L. Esaki) and a series of artificial lattice-
matched interfaces (Zh.I. Alferov) are of great had social
significance [45]. That is why the achievements in the
area of science & technology of semiconductor artificial
coherent interfaces were marked by several Nobel Prizes.
Nanoionics should also progress towards molecular
manufacturing for creation of new advanced devices. The
absolute necessity of interface engineering for the deve-
lopment of nanoionics was pointed to in our recent works
[8-10,31,32,46,47]. And what is important, advanced
ionic devices should be based on the advanced materials.
As fundamentals for the nano-ASIC science & technology
the new interface engineering approach " f i 'om a d v a n c e d
m a t e r i a l s to a d v a n c e d d e v i c e s " was introduced and fitted
to thin-film supercapacitors and sensors based on ASICs
in [8,9].
Fundamental scientific tasks of nano-ASICs are the
theoretical and experimental nano-design of new
Ionics 11 (2005)
materials, functional structures and devices on the base of
ASICs with the conservation of FIT. The proposal to
form the structure-ordered (coherent) ASIC/IE hetero-
boundaries with a defined arrangement of "channels of
FIT" at the interface of ASIC (where the potential barriers
for mobile ions should be ~0.1 eV) for conservation of
FIT was proposed in [6,8-10,31,46,47].
First experiments on the creation of the ASIC//IE
interfaces with the equal values of lattice parameters of
ASIC (RbAgals-family compounds) and IE (metallic
alloys) were done by A.L. Despotuli and V.S.
Khraposhin in the IMT RAS within the period 1991-
1992. The aim was to conserve a crystal structure of
ASIC and FIT at the ASIC//IE heteroboundaries. Due to
the absence of proper experimental conditions for con-
tinuation of research and the existence of patent infor-
mation restrictions the original experimental data on
capacitive properties of the ASIC//IE heterojunctions
were first published only in 2003 in [8-10,31]. In 2002,
the research on the ASIC//IE heterojunctions was reviwed
by A.L. Despotuli and A.V. Andreeva and in the papers
[8-10,31] the new conception of coherent ASIC//IE elec-
trode interfaces, interface design of ASIC nanosystems
were introduced and a strong mathematical methods of
modern theory of interfaces ware applied to experimental
results.
Methodological base for a nano-design of ASICs
should include the synergetic principles: the effective
management of nonequilibrium system can be realized
under the condition of adequacy (the resonance) of external
controlling influences and internal collective properties of
the system (the result of self-organization) [48]. The in-
ternal parameters of investigated ASIC nanosystems are:
crystallochemistry of heteroboundaries, lattice con-
jugating, boundary polarization of chemical bonds, zone
and electronic characteristic heterojunctions and so on.
The external parameters are the change of chemical com-
position, and symmetry of external (deformation, electric,
magnetic and others) fields.
Theoretical and experimental investigations of the
influence of boundary design factors on the synthesis of
nano-ASICs and the analysis of controlling external
effects matching with the anisotropic internal properties
and processes of system self-organization should allow
one to produce a model generation and experimental se-
lection of the nanosystems with FIT and unique pro-
perties (thin-film ASIC//electrode heterostructures, multi-
layer and powder electrode compositions on the base
ASIC and others).
309
From the point of view thermodynamics and
crystallography of boundaries, an adsorption relaxation
phenomenon [4] means the forming of more dense
packaged (low-energy) boundaries. If boundary generates a
high concentration of defects then the condition for FIT in
nano-ASIC is violated. A sharp increase of activation
energy E in the RbAg415 thin-films with a thickness less
than 50 nm was observed in [49]. It was explained by the
violation of ASIC crystal structure in the layers adjacent
to the support plates (fused quartz or glass). The data [49]
indicate the depression of FIT in the systems of ASICs
with a large density of non-ordered boundaries (boundaries
of general type). Therefore for providing of FIT in nano-
ASICs the structure-ordered heteroboundaries are required.
Coherent ASIC//electrode heterojuctions should have a
high DEL capacitance and record small relaxation time at
the change of electrode potential. From the crystallo-
graphic point of view, the low-energy coherent boundaries
are remarkable for: (i) a presence of common translation
and point elements of symmetry, and (ii) an extremum
energy defined by symmetry [50-53].
The example of a metal (Pd) - ferroelectric (SrTiO3)
well matched couple with coherent boundary (lattice
mismatch less than 1.5%) was presented in [54]. Ultra-
thin layers with coherent boundaries were prepared in [55]
where in the UHV-conditions @10 -7 Pa) by the hetero-
epitaxy method at the grow rate about one monolayer/min
were prepared the films (1-5 monolayer) of LiC1 (001)
onto the Cu (001) plates (mutual rotation of crystals was
about 45 degrees for better lattice matching). Local
electronic structure at the heteroboundaries can be defined
by means of the EELS method [55]. According to the
EELS data, at the presence of coherentness, first layers
dielectric (which was epitaxially grown onto an oriented
metal support) may have a bulk electron structure. Thus,
the topical tasks of nanoionics are a searching of con-
ditions for the formation of coherent heterojunctions and
investigation of the properties of ASIC-based nano-
objects.
4. FIT C o n s e r v a t i o n Parameter and L e v e l i n g
of Fermi Levels in Nano ion ic s of ASICs
As stated above, the characteristic parameters were absent
in nanoionics of ASIC and in the present work for nano-
ASICs we are introducing the appropriate parameter. Let
d be the thickness of transition layer with a defect crystal
structure at the ASIC//IE heteroboundary, and ~.Q - the
screening length of charge for mobile ions for a bulk
ASIC. A value d is an order of a lattice parameter of
310 Ionics 11 (2005)
ASIC a (~ 0,5 nm) and a value ~.Q for ASICs (c~-AgI, ct-
RbAg4Is) is less than ~0.5 rim. Then the relationship P =
d/3.Q is the parameter for which a range of values with the
practical significance is limited by only a few. For P = 1,
a crystal structure of ASIC is violated only in the first
monolayer at the boundary. The formation of hetero-
junct ions with P = 1 is a complex challenge including
theoretical and experimental tasks. This case is a most
interesting for application (conservation of FIT) since at
more large values the defect domain stretches over several
lattice parameter a. Note, that in general case an elastic
strain (which is a lways present on the epitaxial raised
heteroboundary) should influence the FIT in nano-ASICs.
However , if a strain is not great (that determine by
matching of lattice parameters) and does not significantly
change the size and distribution of FIT channels in the
boundary structure of ASIC then the influence of strain
will be less as compared with the ones from local surface
defects, phase precipitates and so on.
After the creat ion of ASIC/ / IE heterojunction, the
levelling of Fermi levels of both materials and transfer of
electrons through an interface take place. For bulk ASICs
the ion-electron processes at the heteroboundaries were
considered in [56]. Here, we show for the first time that
in nanoionics of ASICs (contrary to bulk ASICs) the
level l ing o f Fermi levels is not accompanied by an
appearance of the noticeable contact potentials (noticeable
bends of conduction and valence zones). Let us first
consider the contact between an IE electronic conductor
and the RbAgaIs ASIC. If the work function of IE exceeds
the one of ASIC (4~ m > ~ a s t c ) , then the electrons of
ASIC transfer on IE and create the contact potential with
maximum values [56]
V = ( ~ 1 ~ : - q)aslc)/e. (2)
Simultaneously, at the interface of ASIC the charge of
mobile ions with the opposite signs is induced. The depth
of electric field penetration in ASIC and the thickness of
arising DEL are ~Zo" This value is determined by the
concentration of mobile ions n~ at the boundary of ASIC
(for the RbAg4IJIE coherent interface nAg ~ 10 28 m 3).
Electrical capacitance per unit area of DEL is C ~ e e(/3. o.
Using the formula C = 6 / V and (2), it can be found that
the surface density of ionic charge on a facing of DEL
is
6 ~ e e o (Pro - cI)astc) / e Z o . (3)
If the concentration of electronic carriers in ASIC has
the values similar to the wide zone semiconductor mate-
rials, i.e. n~ ~ 10 21 - 10 22 m -3, then for the creation of
surface charge with the density 6 on IE, the sample ASIC
should have the thickness l which satisfies the equation:
6 = l n e e. (4)
From eqs. (3) and (4), it follows
l ~ eeo (491e - q)AS~C) / n,. e 2 )~o" (5)
If contact potential V= (~t , E l - CI)Astc)/e is about 0.3 V,
then 1 ~ 1-10 mm. It is absolutely incredible for micro
and nanoelectronics.
Thus, the main statement of heterojunction physics
(the levelling of Fermi levels at equil ibrium state) trans-
forms for the case of nano-ASIC where such a levelling
occurs without an appearance of noticeable contact po-
tentials ( e V ~ k~T) and bend of electronic zones. It is a
consequence of small values of l (nanosizes) and ~.Q (very
high concentration of mobile ions).
If in thin-fi lm structure (l ~< 1-10 mm) q)lE < ~AS~C
then a transfer of electrons from IE to ASIC forms a
surface posit ive charge on IE. The transferred electrons
distribute uniformly in the thin-film ASIC and rise Fermi
level of ASIC to that of IE (again without an appearance
of noticeable contact potentials). Penetration of electric
field into the ASIC screens by a withdrawal of mobile
ions from the AS1C surface (appearance of DEL with the
thickness )vQ < ~0.5 nm for RbAg4Is). Here the change of
electron concentration in the ASIC thin-f i lm Ane is
compensated by increasing of mobile ion concentration
Ani. It does not noticeably change the ion-transport pro-
perties of nano-ASIC because, for example, AnAg = Ane ~
n a g ~- 1 0 28 m 3 (RbAg4is)"
The character is t ic t ime r for the es tabl ishment of
equil ibr ium states in nano-ASICs can be evaluated by
using the data [53] for the diffusion coefficient of self-
trapped electrons De in RbAg4I 5 ( D e ~ 10 -8 cm2/s at 300
K). For nano-sized ASIC (1 ~ 10 5 cm) the value of T is of
the order of 12/D,, ~ 10 2 c.
5. C r e a t i o n o f New Types of Nanoionic Devices It has been shown above that: (1) screening lengths ZQ in
ASICs (ct-RbAg4Is, 300 K, nag ~ 1 0 28 m -3) may be less
than ~ 0.5 nm; (2) FIT in DEL should be conserved at the
A S I C / / I E coherent interfaces because of the concentration
Ionics 11 (2005)
nAg and potential barrier height for the jumps of mobile
ions are close to the bulk values. Due to such features,
new types of nanoionic devices (supercapacitors and
sensors with high specific characteristics and operation
frequencies) can be created on the base of coherent hetero-
junctions [8-10,31). Specific capacity and maximal ope-
ration frequencies for nanoionic supercapacitors based on
the RbAg415 ASIC were first evaluated in [8-10,31]. It
was shown that the mobile ions with the concentration
Flag ~ 10 26 m -3 flying over potential barriers should form
DEL with the thickness )~o ~ 0.5 nm and specific capacity
C ~ e/~,o ~ 20 g F / c m 2 (e is dielectric constant, 1 for
vacuum) in the time A t ~ 10 -1~ s. Total concentration of
Ag+-ions in the RbAgals is about 1028 m 3 and at the
jump frequency ~ 101~ s -~ all Ag+-ions take part in the
forming of DEL with the thickness less than )~o ~ 0.5 nm
during At > 10 -1~ s. From this it follows that the coherent
heterojunctions allow one to create the high frequency
DEL capacitors (supercapacitors) with specific capacity
higher than ~ 20 g F / c m 2. The specific capacity ~1000
~tF/cm 2 can be reached if a system of the nano-sized
crystallographic steps and (or) facets on the IE surface
(unbroken coherent structure of heteroboundary) is created.
Maximum energy which can be stored in such a thin-film
supercapacitor at the voltage 0.5 V is C ' V 2 / 2 ~ 10 -3 F
(0.5 V)2/2 = 10~* J/cm 2. If the thickness of device
structure is ~ 10 -5 cm and the average density of 5 g/cm 3,
then the specific energy is about ~2 J/g. This value is
about 20 times less than the specific energy of advanced
bulk supercapacitors [8,9,57] based on the distributed
nanostructured carbon electrode materials, liquid electro-
lyte and 3 V work voltage. But in the tablet type devices,
high specific characteristics (F/g, J/g, W/g) are con-
ditioned by the rational using of device volume. However,
in micro(nano)devices the "surface/volume" ratio is
103-105 times greater than in the tablet ones. Therefore,
for thin-film devices the rational using of interfaces is the
only way to increase the specific characteristics. Fabrica-
tion of heterojunctions with the coherent boundaries is
the key point for the creation of new types of nanoionic
devices (sensors, effective microsources of energy and
power). Such devices are urgent for the development of
micro(nano)electronics and autonomous micro(nano)-
electro-mechnical-systems, i.e. MEMS and NEMS [6,8-
10,31,46,47]. In next decade microsystem technology
will be among main breakthrough factors in the trans-
forming of civilization. Principal way of microsystem
technology is the wireless integrated microsensor and
microrobot networks [58]. Such networks may include
311
Fig. 2. Electron-microscopic image of matrix cells (100 • 100 nm) which have been formed by the direct electron beam lithography method in the RbAg415 ASIC thin-films on the carbon support (V.I. Nikolaichik, A.L. Despotuli).
thousands autonomous nodes. Each node should contain
efficient source of energy and power. The creation of such
sources is now recognized as challenge. We expect that
the nanoionic sources should find a wide application in
this problem area.
Figure 2 shows electron-microscopic image of matrix
cells (100 x 100 nm) which have been formed by the
direct electron beam lithography method in the RbAg415
ASIC thin-films (on the carbon supports) with thickness
about 40 nm [5]. On the base of such nanostructures the
high-density matrix of nanoionic supercapacitors can be
made. It is well known [59], that for a normal operation
of matrix capacitor memory, the capacity of separate cell
should be no less than 25 fF. In the 5 Gbit matrix
DRAM the area of separate cell should be less than 0.1
~tm 2. Therefore, a specific capacity of cells in such
DRAM should be higher than 250 f F / ~ t m 2 (25 gF/cm2).
As a result of long years R&D of the large-scale
specialized firms and world electronic corporations (NEC,
SAMSUNG, MITSUBISHI, TOSHIBA and others), the
attained maximum values of specific capacity in ferro-
electric memory are about 150 fF/gm 2 (15 ~tF/cm2). The
ASIC/IE coherent heterojunctions can provide more high
values. The data of reviews [59,60] and press-releases [61]
of the hi-tech companies show the absence of noticeable
progress in the creation of high-density ferroelectric
DRAM within 1995-2003. In the area o f matrix ferro-
electric memory the Moore's law was broken a long time
ago as the area of a separate capacitor cell exceeded 0.5
~un 2. In a sub-micron nanoionic cell, the thickness of the
ASIC layer may be easily done about ~10 -5 cm. Such a
layer of the RbAg4I 5 ASIC (o, ~- 0.3 ff2-tcm -l at 300 K)
with 1 cm 2 area has the resistance about 3x104 Ohm.
312 Ionics 11 (2005)
Fig. 3. Specific energy and power of different types of sources and projected nanoionic supercapacitors.
Then at the specific capacity of 100 ~F/ cm 2 the time
constant of cell is of the order of 3 • 10 -9 s. It corresponds
to maximal operation frequency o f - 3 0 0 MHz. The only
possibility to decrease the time constant of nanoionic cell
and to reach the operation frequency of - 2 GHz is to
reduce the thickness of ASIC film to -10 nm in a sand-
wich capacitive structure. Thus, the ASIC//IE coherent
heterojunctions with specific capacity ~10 -4 F/cm 2 and
operational frequency ~10~-109 Hz are promising struc-
tures for the creation of devices suited to high power and
energy applications and to capacitor DRAM with the
density larger than 5 Gbit.
Due to the record high operation frequencies, the
nanoionic supercapacitors with coherent heterojunctions
will provide mach more specific power than tablet-type
current supercapacitors [8]. The "specific power - specific
energy" diagram (Fig. 3) shows the possibilities of diffe-
rent sources and projected thin-film nanoionic super-
capacitors to provide different requirements of practice. In
Fig. 3, the domain of projected nanoionic supercapacitors
is shown as ellipse (1: the specific capacity is 300
gF/cm 2, Vwork ~- 0.5 V, and maximal operation frequency
is 1 MHz; 2: the specific capacity is 300 ~tF/cm 2, Vwork
0.5 V, and maximal operation frequency is 10 MHz; 3:
the specific capacity is 300 gF /cm 2, V~o,-k ~" 3 V, and
maximal operation frequency is 1 MHz). On the diagram,
the nanoionic supercapacitor ellipse overlays the unassi-
milated area of the parameters (extending on several de-
cimal orders) that indicates the existence of large potential
needs in the devices of this new class.
To make way for chemical sources in the value of
specific energy, the supercapacitors have large advantages
in specific power and stability charge-discharge charac-
teristics. This opens the possibilities for the creation of
hybrid sources. Active stage functioning of hybrid source
(requiring of high power) provides by nanoionic super-
capacitor. In next period supercapacitor re-charges from
low-power device (piezoelectric element, thermoelectric
battery, fuel cell, photoelectric cell and so on). The key
issue in setting up and running wireless sensor networks
is the amount of power required by each of the node for
its radio transmission as the power of signal falls as 1 / f ".
In an ideal situation m = 2. However, due to various
environmental factors such as building materials, street
layouts, etc. m value may be more than 6. It implies
strongly the diameter of network - power used of node
(distance-power) relationship and high power micrsources
are much required.
Thus, we are sure that nanoionics of AS1Cs and
interface engineering of coherent heteroboundaries in
ASICs are the way towards new discoveries and appli-
cations.
6. S u m m a r y
1. New scientific direction - the nanoionics of advanced
superionic conductors (ASIC) was proposed (it
implies the existence of two fundamentally different
nanoionics).
2. Nanosystems of solid state ionic conductors were
divided into two classes: (i) nanosystems on the base
of compounds with small ionic conductivity ~ and
parameter P = d/L ~ 1 (d is the thickness of boundary
domain with specific properties, and L is the
characteristic size of nanostructure). For nanosystems
with double electric layers (DEL) d ~ AD, where Z D is
the Debye's length, and P = )~D/L; (ii) nanosystems
on the base of ASICs.
3. For nanoionics of AS1Cs were introduced: (a) nano-
ASIC characteristic parameter P = d/AQ (d is the
thickness of layer with defect crystal structure at the
boundary of ASIC, and ~.Q is the screening length for
mobile ions in the volume of ASIC; and (b) criterion
of conservation of fast ion transport (FIT) at the
ASIC//electrode boundary (P ~- 1).
4 . Fundamental task o f nanoionics of ASICs was
formulated as theoretical and experimental interface
engineering of new materials, structures and devices
on the base ASICs with conservation of FIT.
Ionics 11 (2005) 313
5. For the solution of fundamental task (conservation of
FIT in the nano-ASICs), the idea to form structure-
ordered (coherent) ASIC//indifferent electrode (IE)
heterojunctions was proposed.
6. It was shown that at the equilibrium conditions
in nano-ASICs the contact potentials V at the
ASIC//IE coherent heterojunctions should be V ~
kBT/e.
7. It was shown that the ASIC//IE coherent hetero-
junctions should provide high specific capacity and
record small relaxation time on the change of
electrode potential (record high operation frequencies).
This opens possibilities for the creation of new types
nanoionic devices such as the cells memory for 5
Gbit DRAM, supercapacitors for hybrid power &
energy sources and thin-film sensors.
8. Interface engineering approach "from advanced mate-
rials to advanced devices" was proposed as funda-
mentals for the development of applied nanoionics.
9. The principal direction of microsystem technology -
wireless micro-sensor networks was indicated as a
main mass user of nanoionic devices: supercapacitors
and sensors.
7. Acknowledgements Authors thank V.V. Aristov and P.P. Malsev for the
support of investigations.
8. References [1] C.C. Liang, J. Electrochem. Soc. 1 2 0 , 1289
(1973).
[2] K. Shahi, J.B. Wagner, Appl. Phys. Lett. 37, 757
(1980).
[3] J. Maier, Ber. Bunsenges. Phys. Chem. 88, 1057
(1984).
[4] D.O. Raleigh, H.R. Crowe, J. Electrochem. Soc.
118, 79 (1971).
[5] A.L. Despotuli, V.I. Nikolaichik, Solid State
Ionics 60, 275 (1993).
[6] A.L Despotuli, A.V. Andreeva, e-publication,
http://preprint.chemweb.com/physchem/0309001
(2003).
[7] B. Owens, J. Power Sources 90, 2 (2000).
[8] A.L Despotuli, A.V. Andreeva, Microsystem
engineering (Rus) 11, 2 (2003).
[9] A.L Despotuli, A.V. Andreeva, Microsystem
engineering (Rus) 12, 2 (2003).
[10] A.L. Despotuli, A.V. Andreeva, e-publication,
http://preprint.chemweb.com/physchern/0306011
(2003)
[11] K.J. Lehovec, J. Chem. Phys. 21, 1123 (1953).
[12] S. Chandra, Superionic Solids, North-Holland
Publishing Company, 1981, p. 404.
[13] A.L. Despotuli, L.A. Despotuli, Phys. Solid State
(Rus) 39, 1544 (1997).
[14] A.L. Despotuli, in: New Trends in Intercalation
Compound for Energy Storage. NATO-SCIENCE
SERIES. Volume 61 (C. Julien et al., Eds.)
Kluwer Academic Publishers, Dordrecht-Boston-
London, 2002, p. 455.
[15] A.L. Despotuli, V.I. Levashov, e-publication,
http://preprint.chemweb.com/inorgchem/0208001
(2002).
[16] A.L. Despotuli, V.I. Levashov, L.A. Matveeva,
Electrochemistry (Rus) 39,526 (2003).
[17] P. Keblinski, J. Eggebrecht, D. Wolf, S.R.
Phillpot, J. Chem. Phys. 113,282 (2000).
[18] A.A. Volkov, G.V. Kozlov, G.I. Mirzoev, V.G.
Goffman, Letters in JETP (Rus) 38, 182 (1983).
[19] J. Maier, Solid State Ionics 86-88, 55 (1996).
[20] J.-S. Lee, St. Adams, J. Maier, Solid State Ionics
136-137, 1261 (2000).
[21] J. Maier, Solid State Ionics 131, 13 (2000).
[22] J. Maier, Solid State Ionics 154-155,291 (2002).
[23] J. Maier, Solid State Ionics 157,327 (2003).
[24] J. Maier, Solid State Ionics 148,367 (2002).
[25] J. Maier, Z. Phys. Chem. 217 (4), 415 (2003).
[26] N. Sata, K. Eberman, K. Eberl, J. Maier, Nature
408,946 (2000).
[27] J. Jamnik, J. Maier, Phys. Chem. Chem. Phys. 5,
5215 (2003).
[28] A.L. Despotuli, A.A. Shestakov, N.V. Lichkova,
Solid State Ionics 70/71, 130 (1994).
[29] J.H. Choy, N.G. Park, Y.I. Kim, S.H. Hwang, J.
Phys. Chem. 99, 7845 (1995).
[30] J.H. Choy, Y.I. Kim, S.J. Hwang, J. Phys. Chem.
B 102, 9191 (1998).
[31] A.L. Despotuli, A.V. Andreeva, in: Proceeding of
International Workshop "Micro Robots, Micro
Machines and Micro Systems", Institute for
Problems in Mechanics RAS, Moscow, April 24-
25, 2003, p. 129.
[32] A.L. Despotuli, A.V. Andreeva, in: Book of
Abstracts "7th International Meeting Fundamental
Challenges of Solid State Ionics", Chernogolovka,
June 16-18, 2004, p. 22.
314 Ionics 11 (2005)
[33] I.M. Lifshitz, Y.E. Geguzin, Phys. Solid State (Rus) 7, 62 (1965).
[34] V.N. Chebotin, L.M. Solov'eva, Electrochemistry
(Rus) 4,858 (1968). [35] E.A. Ukshe, N.G. Bukun, Electrochemistry (Rus)
26, 1373 (1990). [36] T. Watanabe, Res. Mechanica 11, 47 (1984).
[37] T. Watanabe, Acta Mater. 47, 4171 (1999). [38] T. Watanabe, in: Book of abstracts "International
Conference "Interfaces in advanced materials", Chernogolovka, May 26-30, 2003, p. 2.
[39] A.I. II'in, A.V. Andreeva, B.N. Tolkunov, Mat.
Sci. Forum. 206,625 (1996). [401 O.V. Kononenk, A.V. Andreeva, A.I. Win, V.N.
Matveev, in: MRS-Proceedings, 2002, p. 574. [41] A.V. Andreeva, N.M. Talijan et al., e-publication.
http://preprint.chemweb.com/inorgchem/0302001
(2003). [42] M. Backhaus-Ricoult, M.-F. Trichet, Solid State
lonics 150, 143 (2002). [43] R. R6ttger, H. Schmalzried, Solid State Ionics
150, 131 (2002). [44] D.M. Kolb, Surface Science 500,722 (2002).
[45] Zh.I. Alferov, Uspehi Phys. Sci. 172, 1068
(2002). [46] A.V. Andreeva, A.L. Despotuli, in: Book of
abstracts "International Conference Interfaces in advanced materials", Chernogolovka, May 26-30,
2003, p. 32. [47] A.L. Despotuli, A.V. Andreeva, in: Book of
extending abstracts. International Conference
"INTERMATIC-2003", Moscow, June 9-12, 2003,
p. 156. [48] A.V. Andreeva, in: Proceeding of 5th Russian
Conferene on Physicochemistry of Ultra-Dispersoid
System (V.F. Petrunin, Ed.) MEPI, Moscow,
2000, p. 32.
[49] A.L. Despotuli, N.V. Lichkova, N.A. Minenkova, S.V. Nosenko, Electrochemistry (Rus) 2 6, 1524
(1990). [50] A.V. Andreeva, Surface: Physics, Chemistry,
Mechanics 46, 117 (1990). [51] A.V. Andreeva, A.A. Firsova, Preprint of IMT AN
USSR, Chernogolovka, 1990, p. 44. [52] A.V. Andreeva, Mat. Sci. Forum 69, 111 (1991).
[53] A.V. Andreeva, D.L. Meiler, Crystal properties and
preparation 35-38,358 (1991). [54] T. Ochs, S. K6stlmeier, C. Els~isser, Integr.
Ferroelectrics 32,959 (2000).
[55] M. Kiguchi, H. Inoue, T. Sasaki et al., Surf. Sci.
522, 84 (2003). [56] S. Bredikhin, T. Hattori, M. Ishigame, Phys. Rev.
B 50, 2444 (1994). [57] http://www.skeleton-technologies.com [58] P. Bergamo, S. Asgari, H. Wang, D. Maniezzo, L.
Yip, R. Hudson, K. Yao, D. Estrin, IEEE Transactions on Mobile Computing 3, 211 (2004).
[59] S. Ezhilvalavan, T. Tseng, Materials Chemistry
and Physics 65,227 (2000). [60] R.E. Jones, P. Zurcher, P. Chu et al., Micro-
electronic Engineering 29, 11 (1995).
[61] http://www.iapplianceweb.cotrdstory/oeg20030624
s0046.htm
The paper is dedicated to the memory of Prof. E.A. Ukshe who had supported the ideas of nanoionics in 1992.
Paper presented at the Patras Conference on Solid State
lonics - Tran.sport Properties, Patras, Greece, Sept. 14 -
18, 2004.
Manuscript rec. Sept. 27, 2004; acc. June 16, 2005.