306 Ionics 11 (2005)

Nanoionics of Advanced Superionic Conductors

A.L. Despotuli, 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 - of advanced superionic conductors (ASICs) was proposed. Nanosystems of 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 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 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 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. Introduction introduced and some appropriate fundamentals are for- Dispersoids of ionic conductors [1-3] and ionic con- mulated. Key role of interface design in nanoionics of ductor//electronic conductor heterojunctions [4] are classic ASICs is pointed. It is expected that in next decade the objects of solid state ionics and, at the same time, the nanoionic devices will find a wide application in the objects of nanoionics, as by structure they are nano- sphere of wireless sensor networks (multitude auto- systems. The term and conception of nanoionics as a new nomous sensors that coordinate among themselves and branch of science devoted to a fast ion transport (FIT) in revolutionize information gathering in any type of terrain solid nanosystems was first introduced by [5] in 1992. and conditions). Main applications of nanoionics relate to the creation of new materials, functional structures and devices suited for 2. Two Classes of Solid State Ionic Nano- the storage and conversion energy and information. In the systems and Two Fundamentally Different latest years, the term "nanoionics" ("nano-ionics") came Nanoionics into wide use in scientific articles and denotes also the Solid state ionic conductors (SSIC) with a high level of area of interests of the scientific societies and organiza- unipolar ionic conductivity (Q > 0.001 f2-~cm-~ and the tions [6]. level of electronic conductivity G, is arbitrary) are called In the present article the new scientific direction - superionic conductors (SIC), and solids with oi )> 4, nanoionics of advanced superionic conductors (ASIC) is identified as solid electrolytes (SE). Intersection of SIC f3 Ionics 11 (2005) 3O7 size in nanocomposites. 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 Fig. 1. Different types of solid state ionic conductors opposite. (SSIC) on the cri- o e diagram [8-10]: SE are the solid In [5], a general approach for description of properties electrolytes where the ionic conductivity ~ >> electronic one o~; SIC are the superionic conductors, ~ > 0.001 f2-Lcm ~, of ionic nanosystem was proposed (nanoionics con- and o~ is arbitrary; ASIC are the advanced superionic ception). It is based on the using of the P = d/L ~ 1 conductors oi > 0.1 ~ fcm-~, and cr is arbitrary; SIC n SE are dimensionless parameter (d is the thickness of boundary the superionic conductors & solid electrolytes, ~ > 0.001 if2-lcm ~, and oi >~q; ASIC fq SE are the advanced superionic domain of SSIC with the peculiar properties, and the L - conductors & solid electrolytes, cr~ > 0.1 if2-lcm ~, and o, ~>% 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 SE is a group of SIC & SE, simultaneously. Among the others characteristic values should be used instead of 3,D. SICs there is a subgroup with a record high level of Two examples can be mentioned. In a fuel cell the effec- unipolar ~. This subgroup can be called as advanced tive functioning of catalyst demands that the diffusion superionic conductors (ASIC). There is a subgroup ASIC length of proton (d) be comparable with the size of ca- fq SE, i.e. compounds with o, ~ % (examples are: ct- talyst particle (L). Also, at the solid-state synthesis of AgI, ct-RbAg4Is, CsAg4Br3_xIz§ Rb4Cu16IvCl13 and some new compounds in the nano-physical-chemical systems others). For instance, the Rb4Cu16IvC113 is ASIC & SE [6,13-16], dissolution of metals in SSICs is accompanied with recorded high crj (~ 0.34 if2-~crn-~ at 300 K, and by the simultaneous insertion of electrons and ions and activation energy of ionic conductivity E = 0,1 eV) [7]. local electro-neutrality in the layer with peculiar pro- All types of SSICs are presented on the ~,-cr~ diagram perties remains. Therefore, instead of ~.D it is necessary to (Fig. 1). use, for example, a critical radius of a new phase crystal On the boundaries of ionic crystals the double electric nucleus, the average size of crystallite and others similar layers (DEL) with a high concentration of defects always values. exist. It is a consequence of different values of work The calculation of ~-D for the c~-RbAg4I~ ASIC (300 function for different kind of ions [11]. The thickness of K, and concentration of mobile ions ~ 10 28 m -3) according DEL is an order of Debye length )~ and is defined by a to the Debye formula: concentration of mobile ions ne. In the work [1] an en- hanced ionic conductivity cri in nanocomposites (dis- )~o ~ (eeo k8 T/e2 ni) 1/2, (1) persoids) with components of small initial crg was discovered. Such an effect is due to the high density of (e0 = 8.8x 10 -~2 F/m; e is the dielectric constant, 1 for DELs with high values of Oe. vacuum; kB is the Boltzmann's constant, 1.4x10 -23 jK-l; Two classes of SSICs can be distinguished. In the T = 300 K; e is the electronic charge, 1.6x10 -19 C) class of substances with small ~ ("poor" ionic results in value for )~D ~ 0.05 nm, which is less than the conductors), for instance, LiI (at 300 K ere ~ 5.5x10 -7 size of Ag+-ion. It indicates the need for another formula f2-~cm-1, and energy activation E > 0.4 eV [12]), the (see, for example, [17]) for the calculation of the value of ~,o is about ~60 nm that is an order of a grain screening length (/LQ). 308 Ionics 11 (2005) In the RbAg4I5 ASIC, the oscillation frequencies of thickness of transition defect layer was introduced in [34]. mobile Ag+-ions in the potential minimums of crystalline The work [35] was devoted to the development of ad- relief are ~10 J2 s -~ and the Ag+-ions jump over the sorption relaxation model (relaxation of DEL in ASICs). potential barriers (= 0.1 eV) between neighboring In this model, the slow diffusion processes (large values crystallographic positions with the frequencies ~101~ s -~. of energy activation E) on electrode are attributed to the According to [18], in the RbAg4I5 ASIC the concentration movement of ionic defects. However, all above-mentioned of Ag+-ions in the state of flying over potential barriers is models and conceptions are phenomenological and macro- of the order of ~10 26 m -3. For these ions the Debye's scopic and they do not take into account the concrete formula yields ~,o ~ 0.5 nm (the size of ion is several atomic structure of heteroboundaries. Such structures in a times less) and (1) can be using for estimation of number of cases can have a size comparable with ~e in screening length. If the potential of indifferent electrode the volume of ASIC. (IE) at the RbAg4Is/IE heterojunction changes then the Atomic structure of homo- and heterophase boundaries Ag+-ions (flying under potential barriers) will form the and the analysis of appropriate processes are in a focus of DEL with the thickness ~-D ~ 0.5 nm during the time material science during decades. Terms and approaches as interval At ~ 10-j~ s. For the time interval At ~ 10-1~ s in "crystal engineering of grain boundaries" and "interface the formation of DEL, all Ag+-ions with the concen- design" were for the first time introduced in [36] by T. tration about 1028 m -3 will take part and a screening Watanabe and then successfully applied for creation of length )~e should be less than 0.5 rim. For the description advanced micro(nano)structured composite materials with of nano-ASICs ()v0 < 0.5 nm), dimensionless parameters significantly improved mechanical properties [36-38], and were not used because the characteristic values comparable then in recent years - for creation of advanced electronic with so small )v0 were unknown. In our opinion, such a and magnetic materials [39-41]. Unlike other disciplines, situation limites greatly the development of nanoionics. only in the most recent works [42-44] the solid state Currently, the influence of DELs on ~,. in nano- electrochemistry began to focus an attention on the systems of "poor" ionic conductors (mesoscopic effects) crystallochemistry and crystallography of coherent and is investigated in considerable detail (see, for example, semi-coherent boundaries with the aim to predict and [19-27]), whereas the works on the properties of DELs in control electrochemical processes. nano-ASICs are presented much worse [6,8-10,28-32]. On Modern technology provides wide possibilities for the base of the above mentioned analysis, we draw a application of the interface engineering in the sphere conclusion that the defects of the crystalline structure creation of electronic and opto-electronic devices based on increase ~ in nanosystem of "poor" SSICs but in nano- the "ideal" lattice-matched (coherent) heterojunctions. The ASICs the defects violate the conditions for FIT. Thus, creation of the "Man Made Crystals" - semiconductor the considered nanosystems should be divided into two "super-lattices" (L. Esaki) and a series of artificial lattice- classes requiring the principal different structure design for matched interfaces (Zh.I. Alferov) are of great had social the achievement of FIT: (i) nanosystem on the base of significance [45]. That is why the achievements in the substances with small o~ and characteristic parameter P = area of science & technology of semiconductor artificial d/L ~ 1; and (ii) nano-ASICs for which the characteristic coherent interfaces were marked by several Nobel Prizes. parameters were unknown. This conclusion leads to a Nanoionics should also progress towards molecular supposition about existence of two fundamentally diffe- manufacturing for creation of new advanced devices. The rent nanoionics and unknown one is nanoionics of absolute necessity of interface engineering for the deve- ASICs. lopment of nanoionics was pointed to in our recent works [8-10,31,32,46,47]. And what is important, advanced 3. Structure-Ordered (Coherent) Hetero- ionic devices should be based on the advanced materials. boundaries in ASICs As fundamentals for the nano-ASIC science & technology The SSIC/electrode heterojunctions are classical objects the new interface engineering approach "fi'om advanced of electrochemistry and solid state ionics. In the work [4] materials to advanced devices" was introduced and fitted for the interpretation of slow relaxation of SSIC/electrode to thin-film supercapacitors and sensors based on ASICs processes the conception of ion adsorption was intro- in [8,9]. duced. The idea of surface defect structure of boundary was Fundamental scientific tasks of nano-ASICs are the formulated in [33], and the conception of effective theoretical and experimental nano-design of new Ionics 11 (2005) 309 materials, functional structures and devices on the base of From the point of view thermodynamics and ASICs with the conservation of FIT. The proposal to crystallography of boundaries, an adsorption relaxation form the structure-ordered (coherent) ASIC/IE hetero- phenomenon [4] means the forming of more dense boundaries with a defined arrangement of "channels of packaged (low-energy) boundaries. If boundary generates a FIT" at the interface of ASIC (where the potential barriers high concentration of defects then the condition for FIT in for mobile ions should be ~0.1 eV) for conservation of nano-ASIC is violated. A sharp increase of activation FIT was proposed in [6,8-10,31,46,47]. energy E in the RbAg415 thin-films with a thickness less First experiments on the creation of the ASIC//IE than 50 nm was observed in [49]. It was explained by the interfaces with the equal values of lattice parameters of violation of ASIC crystal structure in the layers adjacent ASIC (RbAgals-family compounds) and IE (metallic to the support plates (fused quartz or glass). The data [49] alloys) were done by A.L. Despotuli and V.S. indicate the depression of FIT in the systems of ASICs Khraposhin in the IMT RAS within the period 1991- with a large density of non-ordered boundaries (boundaries 1992. The aim was to conserve a crystal structure of of general type). Therefore for providing of FIT in nano- ASIC and FIT at the ASIC//IE heteroboundaries. Due to ASICs the structure-ordered heteroboundaries are required. the absence of proper experimental conditions for con- Coherent ASIC//electrode heterojuctions should have a tinuation of research and the existence of patent infor- high DEL capacitance and record small relaxation time at mation restrictions the original experimental data on the change of electrode potential. From the crystallo- capacitive properties of the ASIC//IE heterojunctions graphic point of view, the low-energy coherent boundaries were first published only in 2003 in [8-10,31]. In 2002, are remarkable for: (i) a presence of common translation the research on the ASIC//IE heterojunctions was reviwed and point elements of symmetry, and (ii) an extremum by A.L. Despotuli and A.V. Andreeva and in the papers energy defined by symmetry [50-53]. [8-10,31] the new conception of coherent ASIC//IE elec- The example of a metal (Pd) - ferroelectric (SrTiO3) trode interfaces, interface design of ASIC nanosystems well matched couple with coherent boundary (lattice were introduced and a strong mathematical methods of mismatch less than 1.5%) was presented in [54]. Ultra- modern theory of interfaces ware applied to experimental thin layers with coherent boundaries were prepared in [55] results. where in the UHV-conditions @10 -7 Pa) by the hetero- Methodological base for a nano-design of ASICs epitaxy method at the grow rate about one monolayer/min should include the synergetic principles: the effective were prepared the films (1-5 monolayer) of LiC1 (001) management of nonequilibrium system can be realized onto the Cu (001) plates (mutual rotation of crystals was under the condition of adequacy (the resonance) of external about 45 degrees for better lattice matching). Local controlling influences and internal collective properties of electronic structure at the heteroboundaries can be defined the system (the result of self-organization) [48]. The in- by means of the EELS method [55]. According to the ternal parameters of investigated ASIC nanosystems are: EELS data, at the presence of coherentness, first layers crystallochemistry of heteroboundaries, lattice con- dielectric (which was epitaxially grown onto an oriented jugating, boundary polarization of chemical bonds, zone metal support) may have a bulk electron structure. Thus, and electronic characteristic heterojunctions and so on. the topical tasks of nanoionics are a searching of con- The external parameters are the change of chemical com- ditions for the formation of coherent heterojunctions and position, and symmetry of external (deformation, electric, investigation of the properties of ASIC-based nano- magnetic and others) fields. objects. Theoretical and experimental investigations of the influence of boundary design factors on the synthesis of 4. FIT Conservation Parameter and Leveling nano-ASICs and the analysis of controlling external of Fermi Levels in Nanoionics of ASICs effects matching with the anisotropic internal properties As stated above, the characteristic parameters were absent and processes of system self-organization should allow in nanoionics of ASIC and in the present work for nano- one to produce a model generation and experimental se- ASICs we are introducing the appropriate parameter. Let lection of the nanosystems with FIT and unique pro- d be the thickness of transition layer with a defect crystal perties (thin-film ASIC//electrode heterostructures, multi- structure at the ASIC//IE heteroboundary, and ~.Q - the layer and powder electrode compositions on the base screening length of charge for mobile ions for a bulk ASIC and others). 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- If the concentration of electronic carriers in ASIC has RbAg4Is) is less than ~0.5 rim. Then the relationship P = the values similar to the wide zone semiconductor mate- d/3.Q is the parameter for which a range of values with the rials, i.e. n~ ~ 10 21 - 10 22 m -3, then for the creation of practical significance is limited by only a few. For P = 1, surface charge with the density 6 on IE, the sample ASIC a crystal structure of ASIC is violated only in the first should have the thickness l which satisfies the equation: monolayer at the boundary. The formation of hetero- junctions with P = 1 is a complex challenge including 6 = l n e e. (4) theoretical and experimental tasks. This case is a most interesting for application (conservation of FIT) since at From eqs. (3) and (4), it follows more large values the defect domain stretches over several lattice parameter a. Note, that in general case an elastic l ~ eeo (491e - q)AS~C) / n,. e 2 )~o" (5) strain (which is always present on the epitaxial raised heteroboundary) should influence the FIT in nano-ASICs. If contact potential V= (~t, El- CI)Astc)/e is about 0.3 V, However, if a strain is not great (that determine by then 1 ~ 1-10 mm. It is absolutely incredible for micro matching of lattice parameters) and does not significantly and . change the size and distribution of FIT channels in the Thus, the main statement of heterojunction physics boundary structure of ASIC then the influence of strain (the levelling of Fermi levels at equilibrium state) trans- will be less as compared with the ones from local surface forms for the case of nano-ASIC where such a levelling defects, phase precipitates and so on. occurs without an appearance of noticeable contact po- After the creation of ASIC//IE heterojunction, the tentials (eV ~ k~T) and bend of electronic zones. It is a levelling of Fermi levels of both materials and transfer of consequence of small values of l (nanosizes) and ~.Q (very electrons through an interface take place. For bulk ASICs high concentration of mobile ions). the ion-electron processes at the heteroboundaries were If in thin-film structure (l ~< 1-10 mm) q)lE < ~AS~C considered in [56]. Here, we show for the first time that then a transfer of electrons from IE to ASIC forms a in nanoionics of ASICs (contrary to bulk ASICs) the surface positive charge on IE. The transferred electrons levelling of Fermi levels is not accompanied by an distribute uniformly in the thin-film ASIC and rise Fermi appearance of the noticeable contact potentials (noticeable level of ASIC to that of IE (again without an appearance bends of conduction and valence zones). Let us first of noticeable contact potentials). Penetration of electric consider the contact between an IE electronic conductor field into the ASIC screens by a withdrawal of mobile and the RbAgaIs ASIC. If the work function of IE exceeds ions from the AS1C surface (appearance of DEL with the the one of ASIC (4~ m > ~astc), then the electrons of thickness )vQ < ~0.5 nm for RbAg4Is). Here the change of ASIC transfer on IE and create the contact potential with electron concentration in the ASIC thin-film Ane is maximum values [56] compensated by increasing of mobile ion concentration Ani. It does not noticeably change the ion-transport pro- V = (~1~:- q)aslc)/e. (2) perties of nano-ASIC because, for example, AnAg = Ane ~ nag ~- 10 28 m 3 (RbAg4is)" The characteristic time r for the establishment of Simultaneously, at the interface of ASIC the charge of equilibrium states in nano-ASICs can be evaluated by mobile ions with the opposite signs is induced. The depth using the data [53] for the diffusion coefficient of self- of electric field penetration in ASIC and the thickness of trapped electrons De in RbAg4I5 (D e ~ 10 -8 cm2/s at 300 arising DEL are ~Zo" This value is determined by the K). For nano-sized ASIC (1 ~ 10 5 cm) the value of T is of concentration of mobile ions n~ at the boundary of ASIC the order of 12/D,, ~ 10 2 c.

(for the RbAg4IJIE coherent interface nAg ~ 10 28 m 3). Electrical capacitance per unit area of DEL is C ~ e e(/3. o. 5. Creation of New Types of Nanoionic Using the formula C = 6/V and (2), it can be found that Devices the surface density of ionic charge on a facing of DEL It has been shown above that: (1) screening lengths ZQ in is ASICs (ct-RbAg4Is, 300 K, nag ~ 10 28 m -3) may be less than ~ 0.5 nm; (2) FIT in DEL should be conserved at the 6 ~ ee o (Pro - cI)astc) / eZo. (3) ASIC//IE coherent interfaces because of the concentration Ionics 11 (2005) 311 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 Fig. 2. Electron-microscopic image of matrix cells (100 • C ~ e/~,o ~ 20 gF/cm 2 (e is dielectric constant, 1 for 100 nm) which have been formed by the direct electron vacuum) in the time At ~ 10 -1~ s. Total concentration of beam lithography method in the RbAg415 ASIC thin-films on the carbon support (V.I. Nikolaichik, A.L. Despotuli). 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 thousands autonomous nodes. Each node should contain heterojunctions allow one to create the high frequency efficient source of energy and power. The creation of such DEL capacitors (supercapacitors) with specific capacity sources is now recognized as challenge. We expect that higher than ~ 20 gF/cm 2. The specific capacity ~1000 the nanoionic sources should find a wide application in ~tF/cm 2 can be reached if a system of the nano-sized this problem area. crystallographic steps and (or) facets on the IE surface Figure 2 shows electron-microscopic image of matrix (unbroken coherent structure of heteroboundary) is created. cells (100 x 100 nm) which have been formed by the Maximum energy which can be stored in such a thin-film direct electron beam lithography method in the RbAg415

at the voltage 0.5 V is C'V2/2 ~ 10 -3 F ASIC thin-films (on the carbon supports) with thickness (0.5 V)2/2 = 10~* J/cm 2. If the thickness of device about 40 nm [5]. On the base of such nanostructures the structure is ~ 10 -5 cm and the average density of 5 g/cm 3, high-density matrix of nanoionic supercapacitors can be then the specific energy is about ~2 J/g. This value is made. It is well known [59], that for a normal operation about 20 times less than the specific energy of advanced of matrix capacitor memory, the capacity of separate cell bulk supercapacitors [8,9,57] based on the distributed should be no less than 25 fF. In the 5 Gbit matrix nanostructured carbon electrode materials, liquid electro- DRAM the area of separate cell should be less than 0.1 lyte and 3 V work voltage. But in the tablet type devices, ~tm2. Therefore, a specific capacity of cells in such high specific characteristics (F/g, J/g, W/g) are con- DRAM should be higher than 250 fF/~tm 2 (25 gF/cm2). ditioned by the rational using of device volume. However, As a result of long years R&D of the large-scale in micro(nano)devices the "surface/volume" ratio is specialized firms and world electronic corporations (NEC, 103-105 times greater than in the tablet ones. Therefore, SAMSUNG, MITSUBISHI, TOSHIBA and others), the for thin-film devices the rational using of interfaces is the attained maximum values of specific capacity in ferro- only way to increase the specific characteristics. Fabrica- electric memory are about 150 fF/gm 2 (15 ~tF/cm2). The tion of heterojunctions with the coherent boundaries is ASIC/IE coherent heterojunctions can provide more high the key point for the creation of new types of nanoionic values. The data of reviews [59,60] and press-releases [61] devices (sensors, effective microsources of energy and of the hi-tech companies show the absence of noticeable power). Such devices are urgent for the development of progress in the creation of high-density ferroelectric micro(nano)electronics and autonomous micro(nano)- DRAM within 1995-2003. In the area of matrix ferro- electro-mechnical-systems, i.e. MEMS and NEMS [6,8- electric memory the Moore's law was broken a long time 10,31,46,47]. In next decade microsystem technology ago as the area of a separate capacitor cell exceeded 0.5 will be among main breakthrough factors in the trans- ~un 2. In a sub-micron nanoionic cell, the thickness of the forming of civilization. Principal way of microsystem ASIC layer may be easily done about ~10 -5 cm. Such a technology is the wireless integrated microsensor and layer of the RbAg4I5 ASIC (o, ~- 0.3 ff2-tcm-l at 300 K) microrobot networks [58]. Such networks may include with 1 cm 2 area has the resistance about 3x104 Ohm. 312 Ionics 11 (2005) 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 Fig. 3. Specific energy and power of different types of layouts, etc. m value may be more than 6. It implies sources and projected nanoionic supercapacitors. strongly the diameter of network - power used of node (distance-power) relationship and high power micrsources are much required. Then at the specific capacity of 100 ~F/ cm 2 the time Thus, we are sure that nanoionics of AS1Cs and constant of cell is of the order of 3 • 10 -9 s. It corresponds interface engineering of coherent heteroboundaries in to maximal operation frequency of-300 MHz. The only ASICs are the way towards new discoveries and appli- possibility to decrease the time constant of nanoionic cell cations. and to reach the operation frequency of -2 GHz is to reduce the thickness of ASIC film to -10 nm in a sand- 6. Summary wich capacitive structure. Thus, the ASIC//IE coherent 1. New scientific direction - the nanoionics of advanced heterojunctions with specific capacity ~10 -4 F/cm 2 and superionic conductors (ASIC) was proposed (it operational frequency ~10~-109 Hz are promising struc- implies the existence of two fundamentally different tures for the creation of devices suited to high power and nanoionics). energy applications and to capacitor DRAM with the 2. Nanosystems of solid state ionic conductors were density larger than 5 Gbit. divided into two classes: (i) nanosystems on the base Due to the record high operation frequencies, the of compounds with small ionic conductivity ~ and nanoionic supercapacitors with coherent heterojunctions parameter P = d/L ~ 1 (d is the thickness of boundary will provide mach more specific power than tablet-type domain with specific properties, and L is the current supercapacitors [8]. The "specific power - specific characteristic size of nanostructure). For nanosystems energy" diagram (Fig. 3) shows the possibilities of diffe- with double electric layers (DEL) d ~ AD, where Z D is rent sources and projected thin-film nanoionic super- the Debye's length, and P = )~D/L; (ii) nanosystems capacitors to provide different requirements of practice. In on the base of ASICs. Fig. 3, the domain of projected nanoionic supercapacitors 3. For nanoionics of AS1Cs were introduced: (a) nano- is shown as ellipse (1: the specific capacity is 300 ASIC characteristic parameter P = d/AQ (d is the gF/cm 2, Vwork ~- 0.5 V, and maximal operation frequency thickness of layer with defect crystal structure at the is 1 MHz; 2: the specific capacity is 300 ~tF/cm 2, Vwork boundary of ASIC, and ~.Q is the screening length for 0.5 V, and maximal operation frequency is 10 MHz; 3: mobile ions in the volume of ASIC; and (b) criterion the specific capacity is 300 gF/cm 2, V~o,-k ~" 3 V, and of conservation of fast ion transport (FIT) at the maximal operation frequency is 1 MHz). On the diagram, ASIC//electrode boundary (P ~- 1). the nanoionic supercapacitor ellipse overlays the unassi- 4. Fundamental task of nanoionics of ASICs was milated area of the parameters (extending on several de- formulated as theoretical and experimental interface cimal orders) that indicates the existence of large potential engineering of new materials, structures and devices needs in the devices of this new class. on the base ASICs with conservation of FIT. Ionics 11 (2005) 313 5. For the solution of fundamental task (conservation of [10] A.L. Despotuli, A.V. Andreeva, e-publication, FIT in the nano-ASICs), the idea to form structure- http://preprint.chemweb.com/physchern/0306011 ordered (coherent) ASIC//indifferent electrode (IE) (2003) heterojunctions was proposed. [11] K.J. Lehovec, J. Chem. Phys. 21, 1123 (1953). 6. It was shown that at the equilibrium conditions [12] S. Chandra, Superionic Solids, North-Holland in nano-ASICs the contact potentials V at the Publishing Company, 1981, p. 404. ASIC//IE coherent heterojunctions should be V ~ [13] A.L. Despotuli, L.A. Despotuli, Phys. Solid State kBT/e. (Rus) 39, 1544 (1997). 7. It was shown that the ASIC//IE coherent hetero- [14] A.L. Despotuli, in: New Trends in Intercalation junctions should provide high specific capacity and Compound for Energy Storage. NATO-SCIENCE record small relaxation time on the change of SERIES. Volume 61 (C. Julien et al., Eds.) electrode potential (record high operation frequencies). Kluwer Academic Publishers, Dordrecht-Boston- This opens possibilities for the creation of new types London, 2002, p. 455. nanoionic devices such as the cells memory for 5 [15] A.L. Despotuli, V.I. Levashov, e-publication, Gbit DRAM, supercapacitors for hybrid power & http://preprint.chemweb.com/inorgchem/0208001 energy sources and thin-film sensors. (2002). 8. Interface engineering approach "from advanced mate- [16] A.L. Despotuli, V.I. Levashov, L.A. Matveeva, rials to advanced devices" was proposed as funda- Electrochemistry (Rus) 39,526 (2003). mentals for the development of applied nanoionics. [17] P. 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