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: despot@ipmt-hpm.ac.ru (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.