electron beam irradiation for structuring of molecular assemblies

6 IEEE TRANSACTIONS ON NANOBIOSCIENCE, VOL. 3, NO. 1, MARCH 2004

Electron Beam Irradiation for Structuring ofMolecular Assemblies

Victor Erokhin*, Tatiana Berzina, and M. P. Fontana

Abstract—Nontraditional applications of electron beam irradi-ation for patterning of molecular assemblies are considered. Theelectron beam can have the following effects on molecular layers:destruction of molecular structure under e-beam irradiation witha successive formation of new molecular system when the irradia-tion is stopped; variation of the properties of the layer after e-beamirradiation; crosslinking of molecules in the layer under irradia-tion; modification of the templates for the successive film growth,providing different growing conditions in irradiated and nonirra-diated areas; and activation of the solid support surface and molec-ular systems in the film resulting in the increased adhesion of thelayer to the substrate in irradiated areas. All these effects were usedfor patterning of thin layers of different compounds. Five classes ofmolecular systems were considered, namely, films of simple surfac-tant molecules, layers of charge-transfer complexes, films of con-ducting polymers, aggregated nanoparticulate layers and films ofnanoengineered polymeric capsules. Characteristic features of pat-terning processes in each particular case are discussed.

Index Terms—Electron beam, Langmuir–Blodgett films, organiclayers, patterning.

I. INTRODUCTION

E LECTRON BEAM LITHOGRAPHY (EBL) is awell-known tool widely used in microelectronics for the

fabrication of electronic elements and circuits with decreasedsizes, as it allows reaching a resolution much higher thanphotolithography [1]–[5]. The main applications of EBL inindustry is the fabrication of masks for photolithography andnonserial production of low-size circuits and elements forstudying processes in systems with decreased dimensionalityfor further development of new types of devices due to itsscanning nature and, therefore, low productivity. Briefly, theprinciple of EBL is based on the polymerization of the organicmolecules in the layer (negative lithography) or on the depoly-merization of the polymer molecules into monomers in thelayer (positive lithography) under the electron beam irradiation.In other words, e-beam action varies the solubility of the layerin exposed zones. Development of the sample after lithographyresults in the removal of nonpolymerized areas from the

Manuscript received March 11, 2003; revised October 31, 2003. This workwas supported by Ministero dell’Istruzione dell’Universitá e della Ricerca underProject Fondo per gli Investimenti della Ricerca di Base. Asterisk indicates cor-responding author.

*V. Erokhin is with the Department of Physics and the National Institutefor the Physics of Matter, University of Parma, Parma 43100, Italy (e-mail:[email protected]).

T. Berzina and M. P. Fontana are with the Department of Physics and theNational Institute for the Physics of Matter, University of Parma, Parma 43100,Italy (e-mail: [email protected]; [email protected]).

Digital Object Identifier 10.1109/TNB.2004.824257

sample. The most simple development process consists in thesample treatment with organic solvents. Different solubility ofthe polymeric and monomeric areas of the layer results in thefabrication of patterns on the sample surface. Wet developmenthas some limitations in resolution due to meniscus problems.Therefore, dry development processes are used when fine(submicrometer) features must be fabricated.

Besides this main stream of the electron beam treatmentfor lithography, there are several applications, resulting in theformation of patterned surfaces, where the mechanism cannotbe explained by polymerization or depolymerization of themolecules in the layer. The following phenomena can occurunder e-beam treatment.

1) Destruction of molecular structure under e-beam irradia-tion with a successive formation of new molecular systemwhen the irradiation is stopped.

2) Variation of the properties of the layer after e-beamirradiation.

3) Crosslinking of molecules in the layer under irradiation.4) Modification of the templates for the successive film

growth, providing different growing conditions in irradi-ated and nonirradiated areas.

5) Activation of the solid support surface and molecular sys-tems in the film resulting in the increased adhesion of thelayer to the substrate in irradiated areas.

This paper reviews results on the utilization of electronbeam treatment in nonconventional manner. It is mainly basedon the experience accumulated by our group. Five types ofmaterials, illustrating five types of phenomena, listed above,will be considered, namely, simple surfactants with saturatedhydrocarbon chains, charge-transfer complexes, conductingpolymers, aggregated layers of inorganic nanoparticles grownin organic matrix, and layers of nanoengineered polymericcapsules. Specific features of the treatment will be discussedfor each group. The first three, and partially the fourth, groupsdeal mainly with Langmuir–Blodgett (LB) films—layersdeposited by the technique allowing to manipulate with filmsof up to single-molecule thickness, providing, therefore, thepossibility to realize molecular structures in one direction. Inorder to make real molecular devices, it is necessary to havethe possibility of the in-plane patterning of these layers. Themethods of the electron beam treatment we developed offer thispossibility. The fifth group, instead, deals with a new type ofmaterials—nanoengineered polymeric capsules. The developedmethod allows to perform patterned fixation of these objectson solid support surfaces.

1536-1241/04$20.00 © 2004 IEEE

EROKHIN et al.: ELECTRON BEAM IRRADIATION FOR STRUCTURING OF MOLECULAR ASSEMBLIES 7

Fig. 1. Dependence of the ordering length of Ba behenate LB film on the dose of electron beam irradiation and schematic representation of the film structureexposed to different irradiation doses.

II. LAYERS OF SURFACTANTS

A. Fatty Acid Salt Films

Fatty acids are widely used surfactant molecules with a polarheadgroup and hydrocarbon chains of different length with ageneral formula

CH CH COOH

where is usually between 14 and 22.Such acids as stearic, arachidic, and behenic are “classic”

objects for the LB method of deposition due to their pronouncedamphiphilic properties [6], [7]. In most cases, LB films offatty acid salts with divalent metals are deposited instead ofpure acids [6]. The metal ions provide a compensation of theelectrical charge in dissociated headgroups of these moleculesspread on subphases of near neutral pH. Without metal ions,the charge can be compensated by pH adjustment (lower than4.0) that results in the decreased stability of the monolayer anddifficulty in its transfer onto solid supports.

Along with the academic interest, these objects are consid-ered as potentially useful ones for realization of functionalmolecular structures. Reproducibility and fixed thickness ofmolecular layers of fatty acids determines their utilization asspacers providing well-defined distances between functionalelements in molecular systems [8].

As the chains of fatty acids contain only saturated bonds,there is no possibility of their polymerization. However, elec-tron beam irradiation modifies the structure and properties offatty acid LB films.

The effect of the electron beam can be illustrated with bariumbehenate LB films.

Behenic acid is a long hydrocarbon chain molecule. Repetitive unit (period) of its LB film is a bilayer with a

thickness of 5.9 nm [8]. Several new features of such layers wererecorded after their exposition to the electron beam [9].

First of all, irradiation of the barium behenate LB film withelectron beam with the acceleration voltage of 1–3 kV andradiation dose more than 10 C/cm results in the completeinsolubility of the layer in organic solvents, such as hexane,benzene, or chloroform. Mechanical and chemical stabilities ofthe layer are also significantly increased. The layer with onebilayer thickness only after its exposition to the electron beamcan be used as a protecting mask for plasma-chemical etchingof metal films.

X-ray reflectivity analysis of treated samples showed the vari-ation of the layer organization with the increase of the irradiationdose [9]. Initially, the well-ordered lamellar structure with sev-eral sharp Bragg reflections in the X-ray curve was transformedinto a quasi-amorphous structure when the dose of the irradi-ation was about 10 C/cm . Approaching this value, the re-flectivity curve reveals continuous broadening of the reflectionhalf-width, indicating the decrease of the long-range orderingand its successive complete disappearance. Dependence of theordering length on the irradiation dose together with a schematicillustration of the film structure variation are shown in Fig. 1.

The other effect of the electron beam treatment is the decreaseof the film thickness. Thickness reduction begins at lower dosesof irradiation (10 C/cm ) and comes to a saturation levelat doses of about 3 10 C/cm (Fig. 2). At saturation, thedecrease in thickness, corresponding to one bilayer of bariumstearate, is about 1.0 nm.

Summarizing, electron beam irradiation of fatty acid salt LBfilms results in the insolubility of the layer in organic solvents,increase of the chemical and mechanical stability of the layer,continuous decrease of the layer ordering up to its complete dis-appearance, and the decrease of the total thickness of the layer.

The observed effect of the electron beam on the bariumbehenate layer was explained by the breaking of molecularstructure. Molecules under the beam are splitted into shortactivated hydrocarbon fragments. When the beam is stopped, the


Fig. 2. Dependence of the bilayer thickness variation of Ba behenate LB filmon the irradiation dose.

activated parts of these molecular fragments tend to bind witheach other, providing a randomly formed hydrocarbon net. Forlower doses, breaking of molecules is not so pronounced and,therefore, the initial layer organization is in general maintained.As a result, Ba behenate structure of the LB film is still dominantwith, however, decreased ordering degree. When the dose isenough for splitting practically all molecules in fragments,their mutual binding after stopping of the radiation results inthe spontaneous formation of uniform amorphous structure ofthe layer.

The layer thickness decrease from its initial value can beexplained in the following way. Bilayer thickness of bariumbehenate is 5.9 nm, which corresponds to the practically verticalarrangement of hydrocarbon chains in the film [8]. The areaper molecule for such film, obtained from -A isotherm ofthe monolayer at the air–water interface is 20.5 [6]. Asthe deposition ratio for this compound is practically alwaysabout 1.0, it is possible to state that the same average areaper molecule is in the film transferred onto solid supports. Onthe other hand, the closest possible packing of hydrocarbonchains gives the value of the area per molecule of 18.4[9]. Thus, there is an increase of the area per molecule in LBfilm with respect to the crystal packing. This increase is dueto the nature of the LB monolayers. The layer is composed ofdomains where fatty acid molecules are packed practically intwo-dimensional (2-D) crystal structures. Sizes of the crystallitesare in the micrometer range. These 2-D crystallites are separatedby domain boundaries. The presence of these boundaries isresponsible for the increased value of the average area permolecule with respect to that in closely packed systems (crystal).Simple calculations allow to suggest that the electron beamtreatment results in the complete removal of these empty zones(boundary regions). The layer becomes amorphous, dense,and practically defectless. This fact determines the increasedmechanical and chemical stability. However, this removal ofempty zones in the film can explain only about 50% of theobserved effect (the thickness of the bilayer must decreasefor about 0.5 nm instead of observed 1.0 nm). The rest ofthe thickness decrease can be due to the decomposition ofhydrocarbon chains under irradiation with successive sputteringof low molecular weight compounds with electron beam.

B. Octadecylphenol Films

Octadecylphenol is another simple amphiphilic molecule, onwhich the effect of electron beam was investigated. However,

Fig. 3. Octadecylphenol molecule.

the obtained results are not completely the same with respect tothose obtained for the fatty acid salt layers [9]. The moleculestructure is shown in Fig. 3.

LB films of 4-n-octadecylphenol were irradiated withelectron beam of 1–3 keV energy and with doses of10 –10 C/cm . When the energy and the dose of theirradiation is adequate, the film becomes insoluble in organicsolvents even in the case of boiling. Its mechanical andchemical stability, as well as a low number of defects allow toperform etching of metal films, such as Al or Cr, through themask, formed with only one bilayer of octadecylphenol [9].

As in the case of the fatty acid salts, the film thicknessdecreased during the irradiation.

Electron diffraction characterization revealed the structurechanges of the film. The layer was transformed into amorphousone after the radiation dose of 3 10 C/cm when theelectron energy is 3 keV. However, the film became insolublemuch earlier, when for an irradiation dose of 8 10 C/cm .

The mechanism of the electron beam effect in the caseof octadecylphenol is different from that for fatty acid salts.The presence of double bonds in the phenol ring makes theprocess more similar to polymerization. Low doses of theirradiation result in the “opening” of double bonds (less energyis required) and in the lateral binding of adjacent moleculesthrough “opened” double bonds after the irradiation is stopped.In fact, IR measurements revealed the disappearance of thedeformation vibrations of the CH groups in the phenyl ring(1260 cm ) and of the stretching vibrations of the C atomring (1518 cm ) [10]. The fact indicates that the formationof the crosslinking between the molecules takes place in thephenol ring area. If the dose is not high enough, the layermaintains its lamellar organization. Increasing the dose, thefilm structure becomes practically amorphous. This can beexplained by the fact that also aliphatic chains begin to beinvolved into the crosslinking, similar to the case of fatty acidsalt layers.

C. Alternating Layers of Amphiphilic Molecules(Heterostructures)

One of the main reasons of the wide spreading of LB tech-nique in nanotechnology research is the possibility to operatewith layers of molecular thickness and to realize heterostruc-tures with the desired alternation of layers of different materials[6], [7]. Several examples of successful realization of moleculararchitectures with the LB technique were demonstrated startingafter the pioneering works of H. Kuhn’s group [10]–[14].

However, in most cases, the actual realization of the desiredheterostructure is not so simple. During the deposition (in themeniscus when passing through the air–water interface) and/orafter aging, the film can organize itself in a structure which is


significantly different from that expected from the performeddeposition procedure. Even for rather simple molecules, suchas fatty acid salts, it was demonstrated that molecules in the lastthree deposited monolayers can be involved into the reorgani-zation as long as the layer thickness increases during horizontallift (Langmuir–Schaefer) deposition [15]. Such molecular mo-tion can result in the mixing of functional molecules in adja-cent monolayers, which were planned to be separated by fixeddistances for performing desirable function of the constructedsystem. Therefore, it is necessary to have the possibility to sta-bilize the realized structure. Otherwise, it will be reorganized,coming to the equilibrium structure that can be absolutely dif-ferent from the desired one.

The possible solution of the mentioned problem can be inthe layer-after-layer crosslinking of the spacers in the realizedstructures by electron beam treatment with low doses of irradi-ation. The applicability of this approach was demonstrated onBa behenate–octadecylphenol heterostructures [9]. Analyzedsamples were composed by the alternation of bilayers of thesecompounds. Irradiation of the samples was performed afterthe deposition of each octadecylphenol bilayer varying thedose. It was demonstrated that low doses (0.2 10 C/cm ,much lower with respect to that necessary for the bulk samplecrosslinking) resulted in the insolubility of the octadecylphenollayers in organic solvents. However, such treatment practicallydoes not vary the organization of the whole heterostructure.There is still long range ordering, and the spacing valuescorrespond to that of the untreated sample. Increase of the dose(0.42 10 C/cm ) results in the changes in the ordering.Ordering length, calculated from the half-width of Braggreflections in X-ray patterns, decrease by about a factor oftwo. The spacing values, however, do not change. When theirradiation dose reaches 0.69 10 C/cm , the spacing valuesof the heterostructure decrease and long-range ordering practi-cally disappears. Such a dose is enough to penetrate the totalthickness of octadecylphenol layer and to perform a significanteffect also on Ba behenate layers under it, which was confirmedalso by electron diffraction data. Further increase of the dosetransforms the whole heterostructure in an amorphous layer.

The above considerations demonstrate the applicability oflayer-after-layer crosslinking by electron beam irradiation forstabilizing the realized molecular structures. It is important thatlow irradiation doses result in the improvement of mechanicaland chemical properties of only the upper layer, leaving un-derlayers unaffected. Therefore, even fragile molecules can beincorporated into the structures with crosslinked interlayers ifthe dose of electron beam irradiation is adequate (Fig. 4). Theapproach gives the possibility of realizing stable heterostruc-tures with reduced risk of having structural changes duringaging of samples.

III. FILMS OF CHARGE-TRANSFER COMPLEXES

Charge-transfer complexes are very important compounds formolecular electronics as they provide rather high electrical con-ductivity in systems with molecular resolution [16]. Some ofthese molecules are shown in Fig. 5. Conducting regions arelocated in the headgroups of these molecules, while aliphaticchains are attached for providing amphiphilic properties, neces-

Fig. 4. Scheme of the molecular superlattice with crosslinked interlayers.

Fig. 5. Charge-transfer complexes.

sary for the applicability of the LB method. Two demands mustbe satisfied for their practical applications, namely, rather highconductivity and processibility. The first demand is clear, whilethe second one means the availability of some external factorsthat can vary properties, such as the conductivity, of the layersof these compounds allowing the realization of desired connec-tions. The values of the conductivity, obtained on LB films ofcharge-transfer complexes are better than 10 S/cm, which allowsto consider them already as promising materials from the con-ductivity point of view [17]. Moreover, research in the field ofsearching new compounds and improvement of the technologyof the layer formation are still in progress and the value of theconductivity gradually increases [18]–[20]. On the other hand,the possibility of the conductivity variation was shown to be re-alizable with electron beam treatment.

The electron beam effect in this case is different from thatconsidered before for simple amphiphilic molecules. The filmstructure after irradiation is not strongly affected, while its prop-erties (conductivity) are significantly changed.

Typical doses of irradiation, used for the conducting layerspatterning of charge-transfer complexes, are about 10 C/cm[21], [22]. Such doses provide conductivity decrease for aboutfour orders of magnitude in irradiated areas. Therefore, the


Fig. 6. Influence of the substrate relief on the structure and properties of layersdeposited onto it. (A) Breaking of the continuity of the layer. (B) Formationof local compressions and expansions resulting in the variation of the layerproperties.

approach allows the realization of rather high contrast betweenconducting and insulating zones. Such conductivity variationswith rather low doses of irradiation can be due to the destructionof double bonds in the conducting areas of molecules with theirsuccessive random rebinding into less conducting systems. Animportant feature of such lithography is the maintenance ofthe layer thickness in the whole layer varying locally only theproperties. The last feature of the process is very useful for theconstruction of functional molecular systems. Fig. 6 illustratesits importance. In the case of traditional lithography, patterningof the layer is performed by etching of the film. The etchingresults in the formation of a relief on the sample surface with athickness steps corresponding to the height of the etched layer.The next layer must be formed on this relieved surface. Whenthe structure is a molecular one, the presence of these steps canbe very critical. As shown in Fig. 6(a), etched feature wallscan break the continuity of the successively deposited layer.It will likely happen when the deposited layer is a rigid one.However, if the layer is rather soft and it is possible to avoidits breaking, it is still not favorable to deal with a surface withrelief for its deposition. Bending of the layer on the surfacefeatures can result in the local compression or expansion ofthe layer, resulting in the noncontrollable local variationsof its properties, decreasing the yield of realized structures[Fig. 6(b)]. In the case of the suggested method of patterning,the process does not perform significant variations of the layermorphology, leaving the sample surface practically atomicallyflat (Fig. 7). Such surface remains ideal one for successivelayer deposition allowing the formation of molecular structureswith predictable properties.

IV. CONDUCTING POLYMER FILMS

Conducting polymers are the other class of materials con-sidered as very promising for molecular or plastic electronicsapplications due to their wide variety of electrical properties,stability, and processability [23]–[25]. Efforts on the patterningof layers of conducting polymers for realization of electronicelements and circuits are concentrated now in the direction ofusing jet-printer formation of the patterned features [26], [27].

Fig. 7. Deposition of extra layer on patterned charge-transfer complex layer.

Fig. 8. Polyaniline molecule.

Of course, the approach is very interesting, as it can allow fab-rication of working circuits with very simple and inexpensivemethod. However, the element sizes realized with the techniqueare rather large, and fundamental limits will not allow to reducethem to a submicrometer range. Consequently, the speed of suchelements cannot be very high. Therefore, also in the case of con-ducting polymer films, it is necessary to have a method for thefine patterning of realized layers. Traditional lithography, wherethe resist layer is affected by the UV or electron beam radiation,and the layer etching is performed through the mask formed bythe exposed resist film, is difficult to apply in this case. Themain reason is that the mask layer is composed also of polymermaterials. Therefore, the etching rates are very similar for theactive and resist layers, making difficult the control of etchingonly in the desirable areas when thickness of the resist and ac-tive layers are comparable. Instead, if the resist layer thicknesswill be high, in order to guarantee complete removal of the ac-tive layer in opened desirable zones, it will be very difficult toremove the residual resist layer without affecting the active one.

The attempt to transfer on the polymer layers the approach,used for patterning charge-transfer salt films, namely, selectivediminishing of the conductivity with an electron beam irradia-tion, was not very successful. The reason was a low sensitivity(conductivity variation under the electron beam irradiation) ofthe conducting polymer films to the electron beam action.

Polyaniline layers can be considered as an example to illus-trate the behavior of conducting polymer films. The structure ofthe polyaniline molecule is shown in Fig. 8. The possibility ofdepositing LB and Langmuir–Schaefer films of polyaniline hasbeen demonstrated [28]–[31]. However, considering the mor-phology and uniformity of the layers, the best films of polyani-line were realized by the deposition of its mixture with severalsurfactants with successive removal of the surfactant moleculesfrom the film by washing it with the solvent in which polyani-line is insoluble [32]. This approach demands the realization ofsolid solution of the polyaniline with the surfactant in the mono-layer without their separation into different phases. Otherwise,


the morphology of the layer after the surfactant removal willbe significantly different with respect to the film just after thedeposition.

For such layers it was demonstrated, that irradiation byelectron beam with a dose as high as 3 mC/cm does notdiminish the conductivity lower than 10 S/cm [32]. Suchrather high conductivity value does not allow to isolate effec-tively electronic compounds which must be formed in the layer.Moreover, such a high dose of the irradiation is practicallyunrealizable for scanning electron beam sources that are usedfor high-resolution patterning.

However, it was found that rather low doses of irradiationresult in the insolubility of polyaniline layers in the solventsin which they were soluble before the electron beam treatment[32]. Thus, the mechanism of the electron beam action in thiscase is the crosslinking of polymer molecules that results in theincrease of their molecular weight and, therefore, in their de-creased solubility in organic solvents. The mechanism is rathersimilar to that responsible for feature formation in negative re-sists in electron beam lithography.

In the case of polyaniline, the solvent used was1-methyl-2-pyrrolidone (NMP). For successful patterning,the polyaniline layer must be irradiated by the electron beamafter the removal of the surfactant molecules and beforedoping. If, for some technological reasons, the layer wasalready doped, it is necessary to immerse it in water for acouple of minutes in order to perform a dedoping procedure.It was found that irradiation doses of 55–75 C/cm resultin the complete insolubility of polyaniline in NMP, while itsconductivity decreases only 40–50%, which is still reasonablefor molecular electronics applications [32]. In some cases,it was necessary to repeat the doping procedure after thepatterning. Patterns with characteristic features of 20 m wereobtained on polyaniline layers with conductivity values of0.15–0.3 S/cm. Moreover, the possibility was demonstrated ofrealizing patterned molecular heterostructures, where fullereneLB films were enclosed between molecular electrodes ofpatterned polyaniline layers (Fig. 9). Special care was paid tothe patterning of top polyaniline electrode—the most fragilestep of the process. The irradiation dose must be enough tomake crosslinking of polyaniline molecules and, on the otherhand, it must be rather low in order to avoid any effect on theC underlayer. The dose was found to be 10 C/cm when theelectron energy was 1.5–2.0 keV. The realized heterostructurewith molecular electrodes allowed the measurement of theelectrical characteristics of the C layer of about 25-nmthickness without a risk of inducing any defect in the layer,providing a very soft contact to the film [33].

V. FILMS OF AGGREGATED NANOPARTICLES

The main method currently used for the formation of ultrathinsemiconductor inorganic layers is molecular beam epitaxy [34],[35]. The technique allows to grow films with nanometer resolu-tion and also to realize complex heterostructures, i.e., superlat-tices, with desirable alternation of layers of different materials.

As an alternative to this technology, a method for theinorganic semiconductor layer formation by aggregation of

Fig. 9. Molecular ultrathin structure with fullerene ultrathin layer betweenpolyaniline molecular electrodes.

nanometer-size particles, grown in LB matrices, was suggestedin [36]. The particles can be grown in LB films of fatty acid saltswith bivalent metals [37]–[39], as well as in other materials,where bivalent metal atoms are available [40]–[42]. In mostcases, for the particle formation, the layers after the depositionare then exposed to the atmosphere of hydrogen sulphide.However, some other reagents are also under consideration[43], [44]. The particle formation takes place in fatty acid saltmatrix according to the following reaction:

CH CH COO M H S

2CH CH COOH MS

The value of is usually between 16 and 20 for the formationof good-quality LB layers. M is a bivalent metal, such as Cd, Pb,Zn, Cu, etc.

Particle size formed by this method is in the nanometer range,as estimated by different methods [37]–[39]. Such sizes allowthe observation of several quantum phenomena, such as blueshift in the absorbance [37] and single-electron conductivity[45]–[47].

An important step forward in technical applications was per-formed when the method of the selective removal of the organicmatrix after the particle formation was developed [36]. Washingof samples after the reaction with organic solvents, such as chlo-roform, resulted in the removal of fatty acid molecules fromthe film, leaving aggregated nanoparticulate layers at the sub-strate surface. Thickness of the aggregated layer obtained fromone bilayer precursor was estimated by ellipsometry and wasfound to be 0.7 nm [36], which is comparable with the res-olution realizable with molecular beam epitaxy. For very thinaggregated layers (several nanometers), the film morphology isnot uniform [48]. Initially (obtained from a couple of precursorbilayers), the film is composed of separated particle aggregatestens of nanometers in width. When the film thickness grows,the morphology becomes fractal-like. Particles are aggregatedin one–dimensional (1-D) strings, providing pathways for elec-tric carriers. Specific conductivity of such layers is four to fiveorders of magnitude higher with respect to layers obtained froma thinner layer (a couple of precursor bilayers). Finally, when thethickness is higher than 25 nm, the film becomes uniform andits conductivity in the case of CuS layers is better than 1 S/cm[49]. Therefore, it is possible to state that the method can beapplied for the formation of uniform layers with the thickness


higher than 25 nm. Further increase of the film thickness canbe performed with 0.7-nm resolution (step corresponding to theresidual aggregated layer thickness obtained from one bilayerprecursor).

A wide variety of properties can be obtained on layers ofdifferent materials, realized with the nanoparticle aggregationmethod. Thin films of wide band-gap semiconductors suchas ZnS, narrow-band semiconductors such as PbS, and evenconductors such as CuS were realized with this technique.The possibility of the deposition of heterostructures withnanometer-thick alternation of layers of different materials,similar to superlattices, formed by molecular beam epitaxy,was also demonstrated [50]. Therefore, the developed methodallows very high 1-D resolution of the film thickness growth,comparable with the best one currently available. The methodhas also other advantages, such as simplicity and the lack ofspecial conditions and equipment. However, before consid-ering possible practical applications of the technique, it wasnecessary to develop the method of in-plane patterning ofthese layers for element and circuit realization. As in previouscases, electron beam treatment was found to be useful for thesereasons [51].

LB films of fatty acid salts were exposed to the electron beamof 1.5–2.0 keV energy and 30 C/cm dose immediately afterthe deposition. The treatment resulted in the crosslinking ofmolecules in the irradiated areas, as described in Section II-A.The reaction of the particle formation was carried out after theelectron beam treatment. The particles were formed in the wholelayer including irradiated and nonirradiated zones. After the re-action, the layer was aggregated by washing in chloroform. Asdescribed in theSection II-A, the fatty acid layers in the irra-diated areas became insoluble. Therefore, the aggregation tookplace only in nonirradiated zones. In treated regions, instead,fatty acid molecules were maintained on the sample surface.Particles formed in these zones remained entrapped in this ma-trix layer. The process is schematically shown in Fig. 10.

The mechanism of the electron beam action in this case is theformation of different conditions for the particulate film growthand aggregation in treated and nontreated areas of the precursorlayer. Properties of the layer in aggregated zones are determinedby the nature and thickness of the composing nanoparticulatematerial, while in the crosslinked zones, the properties are de-termined mainly by the crosslinked aliphatic chains (as particlescan be considered as only small inclusions in the organic ma-trix), i.e., insulating ones, when we consider the electrical be-havior. In fact, conductivity of the CuS aggregated layers morethan 30 nm thick is better than 1 S/cm, while in the crosslinkedzones the conductivity is less than 1 nS/cm, providing good iso-lation of conducting zones, what was demonstrated by electricalmeasurements between stripes of aggregated particulate films.

VI. LAYERS OF NANOENGINEERED POLYMERIC CAPSULES

Recently, a technique was developed for the formation of newobjects, namely, nanoengineered polymeric capsules (NPCs)[52], [53]. The previously developed method of polyelectrolytelayer-by-layer self-assembling on flat surfaces [54]–[56] wasapplied for the cover shell formation on nonplanar (usually

Fig. 10. Scheme of the formation of patterned aggregated layers. Step 1:exposition of the precursor layer to the electron beam through mask or byscanning lithography. Step 2: formation of nanoparticles. Step 3: aggregationof particles into layers in nonirradiated zones.

spherical) surfaces. The method allows the deposit of a layerwith high control of the thickness by successive adsorption ofoppositely charged polymer molecules on specially preparedsolid surfaces. In the case of NPCs, the polymer layer is formedon the nonplanar template surface. The template sizes are usu-ally in the range of 0.1–5 m, while the shell thickness is in thenanometer range (2–100 nm) [53]. After the shell formation,the template can be dissolved by variation of the environmentalconditions (usually, pH variation). After the template removal,the capsules are hollow ones and they can be filled withdifferent materials, such as inorganic crystals, biomoleculessuch as proteins, organic dyes, etc. The possibility of filling thecapsules with different substances, together with smart natureof the shell allowing such functions as specific attachment todesirable zones by inserting special receptors into the shellsurface, reversible pore formation as a result of environmentalcondition variations, etc., marks the objects as very promisingfor applications in such areas as medicine, material science,drug design, and the food and perfume industries. NPCs arealso interesting from the fundamental point of view, allowingthe study of different phenomena, such as crystal growth,material properties, and structure, in space-confined conditions.A scheme of the NPC formation process is shown in Fig. 11.


Fig. 11. Scheme of the capsule formation process. Step 1: covering of thetemplate by polyelectrolyte layer. Step 2: dissolution of the template. Step 3:filling of the capsule volume by desired molecule.

Fig. 12. Optical microscopy image of hollow capsules assembled on glasssubstrate. Image size 200 � 200 �m.

Several application of NPCs, such as biosensors, reactors,arrays of quantum dots, etc., demand their immobilization onsolid surfaces. In most cases, it is also necessary to have the pos-sibility of patterning these immobilized layers. This task is ob-viously more difficult than those discussed in previous sections,as NPCs are much larger and more complicated than previouslyconsidered molecules. However, it turned out that electron beamtreatment can be useful also in this case [57].

Capsules must be deposited onto hydrophilic surfaces evenby such simple techniques as solution casting. Electron beamtreatment was performed with the energy of 1.5–2.0 keV and

Fig. 13. Optical microscopy image of gold-containing capsules assembled onglass substrate. Image size 100 � 100 �m.

with doses of 5 C/cm through contact masks with differentfeatures. Development of the exposed samples was carried outby sample washing in detergent solution in an ultrasonic bath.Weaker development procedures in different solvents did notallow effective patterning of capsule layers. Only such treatmentpermitted the removal of capsules from the surface in nonirradi-ated zones. Capsules in the treated areas, instead, remain at thesample surface. Typical image of the immobilized NPCs layerafter patterning is shown in Fig. 12. The parameters of the de-velopment procedure are rather critical and depend on the typeof capsules. Wrong parameters can result in underdevelopment(some capsules in nontreated zones remain on the substrate sur-face) or in overdevelopment (significant reduction of the capsuleconcentration in the treated areas). Instead, when the developingprocedure was performed in the correct way, the concentrationof the capsules in treated zones remains unchanged, while prac-tically no capsules are present on the sample surface in non-treated areas.

Two possible mechanisms responsible for the increase incapsule adhesion were considered, namely, crosslinking ofthe adjacent capsule shells and activation of the capsule shellsand substrate surface resulting in the NPCs’ attachment to thesupport. However, the last one was found to be more probable.In fact, the first mechanism can work only if the capsules are inclose contact with each other on the sample surface. Instead,even for distantly distributed capsules (Fig. 13), their increasedadhesion to the substrate surface was still observed after theelectron beam irradiation. Therefore, it is possible to claim thatthe treatment results in the activation of the capsule shell and ofthe support surface. Interaction of the activated capsules withsurface results in their attachment to it.

In the case of hollow capsules, the activation mechanism isclear, as the electrons of the used energy (1–3 keV) and dose(5 C/cm ) can penetrate the whole capsule volume and activateall molecules in the capsule shell. In the case of filled capsules,


Fig. 14. Activation of the substrate and capsule shell surfaces by backscattered electrons.

instead, it is not so obvious, as the activation must be performedin the contact areas of capsules and support surface. The usedenergy and doses are not enough to penetrate the whole filledcapsule volume of some micrometers in diameter. Therefore,electrons backscattered from the substrate surface must beresponsible for this activation resulting in better adhesion ofcapsules to the support surface. A scheme of the process isshown in Fig. 14. Electrons backscattered from the supportsurface activate polymer molecules in the capsule shell, resultingin their binding to the substrate surface.

VII. CONCLUSION

Apart from the traditional utilization of electron beam treat-ment for the electron beam lithography, there are other usefulpossibilities of its application allowing patterning of layers ofdifferent materials. In this paper, we have summarized the expe-rience accumulated in our group on nontraditional applicationsof electron beam irradiation for patterning purposes. Several dif-ferent mechanisms can be responsible for the pattern formationprocess depending on the nature of the used materials. It can becrosslinking or even complete destruction of molecules in thelayer, resulting in the insolubility of irradiated areas of the layer.It can be modification of the layer properties, such as electricconductivity, after the irradiation. It can be the variation of thetemplate structure and properties resulting in the difference inthe film growth mechanism. And it can be activation of the sup-port and molecules resulting in the better adhesion of the layerto the surface.

What started as a specific effect of electron beam irradiationturned out to be a much more general phenomenon, whichallows patterning on the electron beam lithography scale ofseveral classes of molecular systems from simple fatty acids tocomplex structures. Thus, our techniques can be quite usefulfor nanoelectronic applications with molecular components,paving the way to new technology for entirely molecularnanoelectronics.

ACKNOWLEDGMENT

This paper is dedicated to the memory of V. Troitsky. The au-thors would like to thank Dr. S. Erokhina for help in preparationof figures and useful discussion.

REFERENCES

[1] M. Isaacson, A. Muray, M. Scheinfein, I. Adesida, and E. Kratschmer,“Nanometer structure fabrication using electron beam lithography,” Mi-croelectron. Eng., vol. 2, pp. 58–64, Oct. 1984.

[2] L. H. Veneklasen, “Electron beam lithography and information transfer,”Microelectron. Eng., vol. 3, pp. 33–42, Dec. 1985.

[3] H. I. Smith, “A review of submicron lithography,” Superlattices Mi-crostruct., vol. 2, pp. 129–142, 1986.

[4] J. Romijn and E. van der Drift, “Nanometer-scale lithography for largelateral structures,” Phys. Rev. B, Condens. Matter, vol. 152, pp. 14–21,Aug. 1988.

[5] T. H. P. Chang, M. Mankos, K. Y. Lee, and L. P. Muray, “Multiple elec-tron-beam lithography,” Microelectron. Eng., vol. 57–58, pp. 117–135,Sept. 2001.

[6] G. Roberts, Ed., Langmuir–Blodgett Films. New York: Plenum, 1990.[7] A. Ulman, An Introduction to Ultrathin Organic Films from Lang-

muir–Blodgett to Self-Assembly. New York: Academic, 1991.[8] V. Erokhin, Yu. Lvov, L. Mogilevski, A. Zozulin, and E. Iljin, “Two types

of hydrocarbon chain packing in Langmuir–Blodgett films: Interdigi-tated and abutted tails,” Thin Solid Films, vol. 178, pp. 153–158, 1989.

[9] L. A. Feigin, Yu. M. Lvov, and V. I. Troitsky, “X-ray and electron-diffraction study of Langmuir–Blodgett films,” Sov. Sci. Rev. A. Phys.,vol. 11, pp. 285–377, 1989.

[10] H. Kuhn, D. Mobius, and H. Bucher, “Spectroscopy of monolayerassemblies,” in Techniques of Chemistry, A. Weissberger and B. W.Rossiter, Eds. New York: Wiley, 1972, vol. 1, pp. 577–702.

[11] H. Kuhn, “Functional monolayer assembly,” Thin Solid Films, vol. 99,pp. 1–16, 1983.

[12] M. Sugi, K. Nembach, D. Mobius, and H. Kuhn, “Quantum mechanicalhopping in one-dimensional superstructure,” Solid State Commun., vol.15, pp. 1867–1870, 1974.

[13] B. Mann and H. Kuhn, “Tunnelling through fatty acid salt monolayers,”J. Appl. Phys., vol. 42, pp. 4398–4405, 1971.

[14] H. Kuhn, “Electron tunnelling effects in monolayer assemblies,” Chem.Phys. Lipids, vol. 8, pp. 401–404, 1972.

[15] T. Kato, K. Ohshima, and K. Suzuki, “Thermally stimulated area relax-ations of insoluble monolayers and densification of Langmuir–Blodgettfilms with isobaric thermal treatment of monolayers,” Thin Solid Films,vol. 178, pp. 37–45, 1989.

[16] L. Henrion, G. Derost, A. Ruaudel-Teixier, and A. Barraud, “Investiga-tion of N-docosyl pyridinium TCNQ Langmuir–Blodgett films as gassensors,” Sens. Actuators, vol. 14, pp. 251–257, 1988.

[17] V. I. Troitsky, T. S. Berzina, E. Dalcanale, and M. P. Fontana, “An ap-proach for fabrication of junctions with Langmuir–Blodgett films incor-porated between molecular electrodes,” Thin Solid Films, vol. 405, pp.276–289, 2002.

[18] K. Ikegami, C. Mingotaud, and M. Lan, “Intermolecular charge transferin merocyanine dye molecules enhanced by formation of J-aggregates,”Thin Solid Films, vol. 393, pp. 193–198, Aug. 2001.

[19] S. Kuroda, “Electron spin resonance characterization of Lang-muir–Blodgett films containing functional molecules,” Colloids Surf.A, vol. 198–200, pp. 735–744, Feb. 2002.

[20] T. P. Majumder and K. Ikegami, “A structural model for Lang-muir–Blodgett films based on charge-transfer salts of alkylTCNQ andCu,” Synthetic Metals, vol. 135–136, pp. 515–516, Apr. 2003.

[21] T. S. Berzina, V. I. Troitsky, and M. P. Fontana, “Electrical propertiesof Langmuir–Blodgett films enclosed between molecular electrodes,”Mater. Sci. Eng. C, vol. 15, pp. 315–317, 2001.


[22] V. I. Troitsky, T. S. Berzina, and M. P. Fontana, “Macroscopic conduc-tance in Langmuir–Blodgett bilayers of charge-transfer salts,” ColloidsSurf. A, vol. 198–200, pp. 689–701, 2002.

[23] D. K. Campbell, “Conducting polymers and relativistic field theories,”Synthetic Metals, vol. 125, pp. 117–128, Nov. 2001.

[24] M. Gerard, A. Chaubey, and B. D. Malhotra, “Application of conductingpolymers to biosensors,” Biosens. Bioelectron., vol. 17, pp. 345–359,May 2002.

[25] V. Saxena and B. D. Malhotra, “Prospects of conducting polymers inmolecular electronics,” Curr. Appl. Phys., vol. 3, pp. 293–305, Apr.2003.

[26] D. Pede, G. Serra, and D. De Rossi, “Microfabrication of conductingpolymer devices by ink-jet stereolithography,” Mater. Sci. Eng. C, vol.5, pp. 289–291, Feb. 1998.

[27] H. Dreuth and C. Heiden, “A method for local application of thin organicadhesive films on micropatterned structures,” Mater. Sci. Eng. C, vol. 5,pp. 227–231, Feb. 1998.

[28] K. Ramanathan, M. K. Ram, B. D. Malhotra, and A. S. N. Murthy,“Application of polyaniline Langmuir–Blodgett films as a glucosebiosensor,” Mater. Sci. Eng. C, vol. 3, pp. 159–163, Dec. 1995.

[29] L. Rebattet, M. Pineri, M. Escoubes, and E. M. Genies, “Gas sorption inpolyaniline powders and gas permeation in polyaniline films,” SyntheticMetals, vol. 71, pp. 2133–2137, Apr. 1995.

[30] C. J. L. Constantino, A. Dhanabalan, A. Riul Jr., and O. N. Oliveira Jr.,“Surface potentials of polyaniline LB films,” Synthetic Metals, vol. 101,pp. 688–689, May 1999.

[31] M. Ferreira, C. J. L. Constantino, A. Riul Jr., K. Wohnrath, R. F. Aroca,J. A. Giacometti, O. N. Oliveira Jr., and L. H. C. Lattoso, “Preparation,characterization, and taste sensing properties of Langmuir–Blodgettfilms from mixtures of polyaniline and a ruthenium complex,” Polymer,vol. 44, pp. 4205–4211, July 2003.

[32] V. I. Troitsky, T. S. Berzina, and M. P. Fontana, “Deposition of uniformconductive polyaniline films and approach for their patterning,” Syn-thetic Metals, vol. 129, pp. 39–46, 2002.

[33] , “Langmuir–Blodgett assemblies with patterned conductivepolyaniline layers,” Mater. Sci. Eng. C, vol. 22, pp. 239–244, Dec.2002.

[34] J. De Boeck, W. Van Roy, V. Motsnyi, Z. Liu, K. Dessein, and G. Borghs,“Hybrid epitaxial structures for spintronics,” Thin Solid Films, vol. 412,pp. 3–13, June 2002.

[35] H. Ohno, “Molecular beam epitaxy and properties of ferromagneticIII-V semiconductors,” J. Cryst. Growth, vol. 251, pp. 285–291, Apr.2003.

[36] P. Facci, V. Erokhin, A. Tronin, and C. Nicolini, “Formation of ultrathinsemiconductor films by CdS nanostructure agregation,” J. Phys. Chem.,vol. 98, pp. 13 323–13 327, 1994.

[37] E. S. Smotkin, C. Lee, A. J. Bard, A. Campion, M. A. Fox, T. E.Mallouk, S. E. Webber, and J. M. White, “Size quantization effects incadmium sulphide layers formed by a Langmuir–Blodgett technique,”Chem. Phys. Lett., vol. 152, pp. 265–268, Nov. 1998.

[38] C. Zylberajch, A. Ruaudel-Teixier, and A. Barraud, “2D monomolec-ular inorganic semi-conductors, inserted in a Langmuir–Blodgett ma-trix,” Synthetic Metals, vol. 27, pp. 609–614, Dec. 1988.

[39] V. Erokhin, L. Feigin, G. Ivakin, V. Klechkovskaya, Yu. Lvov, and N.Stiopina, “Formation and x-ray and electron diffraction study of CdSand PbS particles inside fatty acid matrix,” Macromol. Chem. Macromol.Symp., vol. 46, pp. 359–363, 1991.

[40] A. V. Nabok, T. Richardson, C. McCartney, N. Cowlam, F. Davis, C. J.M. Stirling, and A. K. Ray, “Size-quantization in extremely small CdSclusters formed in calixarene LB films,” Thin Solid Films, vol. 327–329,pp. 510–514, Aug. 1998.

[41] A. V. Nabok, B. Iwantono, A. K. Hassan, A. K. Ray, and T. Wilkop,“Electrical characterization of LB films containing CdS nanoparticles,”Mater. Sci. Eng. C, vol. 22, pp. 387–391, Dec. 2002.

[42] R. Zhu, Y. Wei, C. Yuan, S. Xiao, Z. Lu, and H. J. Schmitt, “Studiesof generation of ultra fine semiconductor particles in polydiacetyleneLangmuir–Blodgett films accompanied with color transition,” SolidState Commun., vol. 84, pp. 449–451, Oct. 1992.

[43] G. K. Zhavnerko, V. E. Agabekov, M. O. Gallyamov, I. V. Yaminsky, andA. L. Rogach, “Composite Langmuir–Blodgett films of behenic acid andCdTe nanoparticles: The structure and reorganization on solid surfaces,”Colloids Surf. A, vol. 202, pp. 233–241, Apr. 2002.

[44] G. B. Khomutov, V. V. Kislov, R. V. Gainutdinov, S. P. Gubin, A. Y.Obydenov, S. A. Pavlov, A. N. Sergeev-Cherenkov, E. S. Soldatov, A.L. Tolstikhina, and A. S. Trifonov, “The design, fabrication, and charac-terization of controlled-morphology nanomaterials and functional planarmolecular nanocluster-based nanostructures,” Surf. Sci., vol. 532–535,pp. 287–297, June 2003.

[45] V. Erokhin, P. Facci, S. Carrara, and C. Nicolini, “Observation of roomtemperature mono-electron phenomena on nanomete-sized CdS parti-cles,” J. Phys. D, Appl. Phys., vol. 28, pp. 2534–2538, 1995.

[46] , “Monoelectron phenomena in nanometer scale particles formedin LB films,” Thin Solid Films, vol. 284–285, pp. 891–893, 1996.

[47] P. Facci, V. Erokhin, S. Carrara, and C. Nicolini, “Room-temperaturesingle-electron junction,” in Proc. Nat. Acad. Sci., vol. 93, 1996, pp.10 556–10 559.

[48] S. Erokhina, V. Erokhin, C. Nicolini, F. Sbrana, D. Ricci, and E. Di Zitti,“Microstructure origin of the conductivity differences in aggregated CuSfilms of different thickness,” Langmuir, vol. 19, pp. 766–771, 2003.

[49] S. Erokhina, V. Erokhin, and C. Nicolini, “Electrical properties of thincopper sulfide films produced b the aggregation of nanoparticles formedin LB presursor,” Colloids Surf. A, vol. 198–200, pp. 645–650, 2002.

[50] V. Erokhin, P. Facci, L. Gobbi, S. Dante, F. Rustichelli, and C. Nicolini,“Preparation of semiconductor superlattices from LB precursor,” ThinSolid Films, vol. 327–329, pp. 503–505, 1998.

[51] V. Erokhin, V. Troitsky, S. Erokhina, G. Mascetti, and C. Nicolini, “In-plane patterning of aggregated nanoparticle layers,” Langmuir, vol. 18,pp. 3185–3190, 2002.

[52] G. B. Sukhorukov, M. Brumen, E. Donath, and H. Mohwald, “Hollowpolyelectrolyte shells: Exclusion of polymers and donnan equilibrium,”J. Phys. Chem. B, vol. 103, pp. 6434–6440, Aug. 1999.

[53] G. B. Sukhorukov, “Designed nano-engineered polymer films on col-loidal particles and capsules,” in Novel Methods to Study InterfacialLayers, D. Mobius and R. Miller, Eds. Amsterdam, The Netherlands:Elsevier, 2001, p. 384.

[54] Yu. Lvov, G. Decher, H. Haas, H. Mohwald, and A. Kalachev, “X-rayanalysis of ultrathin polymer films self-assembled onto substrates,”Physica B, vol. 198, pp. 89–91, Apr. 1994.

[55] G. Decher, Yu. Lvov, and J. Schmitt, “Proof of multilayer structuralorganization in self-assembled polycation-polyanion molecular films,”Thin Solid Films, vol. 244, pp. 772–777, May 1994.

[56] G. B. Sukhorukov, Y. M. Lvov, H. Mohwald, and G. Decher, “As-sembly of polyelectrolyte multilayer films by consecutively alternatingadsorption of polynucleotides and polycations,” Thin Solid Films, vol.284–285, pp. 220–223, Sept. 1996.

[57] T. Berzina, S. Erokhina, D. Shchukin, G. Sukhorukov, and V. Erokhin,“Deposition and patterning of polymeric capsule layers,” Macro-molecules, vol. 36, pp. 6493–6496, 2003.

Victor Erokhin, photograph and biography not available at the time of publi-cation.

Tatiana Berzina, photograph and biography not available at the time of publi-cation.

M. P. Fontana, photograph and biography not available at the time of publica-tion.

electron beam irradiation for structuring of molecular assemblies

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