Foundation, Via Giotto 2, Genova 16153 Italy
2 Polo Nazionale Bioelettronica, Via A. Moro 17,
57030 Marciana Marina (Li) Italy
3 Institute of Biophysics, University of Genova, Via Giotto 2,
Genova 16153 Italy
4 Small-Angle station of Elettra Synchrotrone, Trieste, Italy
This is a draft paper
for a talk at the
Fifth Foresight Conference on Molecular Nanotechnology.
The final version has been submitted
for publication in the special Conference issue of Nanotechnology.
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Semiconductor nanoparticles were synthetized by exposing fatty acid salt Langmuir-Blodgett films to the atmosphere of H2S. The particle sizes were characterized by small-angle X-ray scattering of their solutions using synchrotron radiation source at higher resolution, as it was impossible previously to study it with usual laboratory X-ray sources. The particle sizes were found to correspond to the demands of single-electron and quantum junctions. Semiconductor heterostructures were grown by self aggregation of these particles of different types. Electrical properties of these nanostructures were studied by using STM. Voltage-current characteristics revealed the presence of differential negative resistance. Measurements confirmed the formation of semiconductor superlattices directed towards a development of new nanodevices, like tunneling diods and semiconductor lasers.
The development of the electronics demands continuos decrease of the element sizes. The aim of this trend is not only to increase the integration level, but, mainly, to increase the operation speed. Moreover, several specific phenomena can be observed only when the elements have very small sizes. As one of the examples of such phenomena we can refer to single-electron junctions [1-3]. These interesting phenomenon takes place when a small granule is placed between two electrodes being separated from them by tunnelling gaps. Voltage-current characteristics of such junction has a depression near zero bias voltage (Coulomb blockade) or a stairway behaviour (Coulomb staircase) due to quantization of the charge present in the granule. The phenomenon can be observed when the electrostatic energy is higher then thermal excitation. Therefore, the granule must be smaller than 3 nm in order to provide the possibility of observing single electron characteristics at room temperature .
Even toward this aim the main stream of the development of the electronics technology remains the same and is connected with the continuos reduction of the element sizes by using electron beam and X-ray lithography, dry etching processes and molecular beam epitaxy [5-7]. Nevertheless, recently it appear several alternative approaches, allowing to reach small sizes in a more simple way [8-10].
In 1988 it was suggested a method of the formation of CdS particles in Langmuir-Blodgett (LB) film of cadmium arachidate by exposing it to the atmosphere of H2S . Later the same approach was applied to synthetize also some different materials, such as PbS, HgS etc [12-15]. Sizes of the particles, estimated by some not directed techniques, were found to be in the range of 2-10 nm.
The particles were characterized also with STM and single electron conductivity was observed realizing the structure: conductive substrate - gap- particle - gap - STM tip . Staircase characteristics with different voltage step, depending on the particle size, were obtained. A step forward the device application was done, when the particle was synthetized directly on the tip of the sharp metal stylus . It allowed to avoid the localization of the particle position by STM and demanded only to have 1D mover, for bringing the tip to the vicinity of the conductive surface.
In parallel, it was shown the possibility to form thin polycrystal films by self-aggregation of these particle, performing a selective removal of the fatty acid molecules . The films were characterized by different techniques. It turned out that the average increase of the film thickness obtained from each bilayer in a precursor is about 6 Å. Such resolution in the thickness can be obtained only with molecular beam epitaxy. Therefore, the suggested technique, being applied for the heterostructure formation, can be interesting for realizing the elements for the devices based on resonant tunnelling and light emitting elements working without carrier recombination. The possibility of the formation of heterostructures, containing alternating layers of two different compounds, namely CdS and MgS, was already shown with scanning electron microscopy .
Nevertheless, several questions are still open. One of them is the statistical distribution of the particle sizes. The other question is the properties of such prepared heterostructures. In fact, it is not clear, whether it will be possible to obtain the properties similar to that prepared with molecular beam epitaxy, as in this case we are dealing with polycrystals.
The aim of this work is to characterize the statistical distribution of the particle sizes and to realize heterostructures by this technique.
For solving the first problem, small-angle X-ray scattering was applied. Small-angle X-ray scattering is a well known methodology suitable for characterizing the structure of rather small objects in the solution. However, in the case of small particles dispersed in a very dilute solution the intensity provided by usual X-ray sources is not enough to obtain reliable scattering curves and, thus, to resolve the particle structure. Therefore, the synchrotron radiation X-ray source, providing the intensities of several order of magnitude higher than laboratory sources, became very important for small-angle scattering characterization of diluted nanoparticle solutions.
Arachidic acid and CdCl2 and CuSO4 5H2O were from Sigma.
LB films were deposited with Langmuir trough (MDT, Russia) . Water used was purified with Milli-Q system till 18.2 M[Omega] cm. Monolayers were formed on the subphase, containing 10-4 M of CuSO4 or CdCl2.
Reaction of the particle formation was performed by exposing the deposited films to the atmosphere of H2S for the time enough to complete the reaction, depending on the film thickness determined previously by quartz crystal balance measurements .
Particle aggregation was performed by washing the films with chloroform for selective removal of arachidic acid molecules .
Particle solution was prepared by washing the aggregated film in small amount of water. Therefore, it was impossible to estimate precisely the concentration of the particles in the solution.
First, the solution was investigated with small-angle diffractometer with linear position sensitive detector (AMUR-K)  equipped by a usual X-ray tube and generator (Phillips).
Second, the X-ray measurements were performed in synchrotrone Elettra (Trieste, Italy). Elettra laboratory is a synchrotron radiation source having more then ten beamlines dedicated to gas phase photoemission, X-ray microscopy, diffraction and small-angle scattering, characterisation with circularly polarized light, VUV emission, spectroscopy, lithography (under construction). One of this beamline is the SAXS (small-angle X-ray scattering) beamline building up some year ago by the Institute of Biophysics and X-ray scattering of the Graz University under the supervision of Prof. Peter Laggner .
The photon source is a 57-pole wiggler, producing a 8 keV radiation with beam size of 3.9 x 0.26 mm2 and a maximum flux equal to 3.5 1014 photons/s/mrad (at 400 mA). The optic is constituted by a flat double mirror monochromator and a double focusing toroidal mirror. The detector is a one-dimensional position sensitive one (delay-line type) and it is mounted over an movable slide in order to adjust the sample-detector distance for the better resolution required.
Heterostructures were prepared by successive formation of different sulphides.
The reported heterostructure was realized in the following way. 10 bilayers of cadmium arachidate were deposited onto the freshly cleaved graphite substrate. The film was exposed to the atmosphere of H2S for 5 hours. Self-aggregated films were obtained by washing the sample in chloroform. In the next step 10 bilayers of cupper arachidate were deposited. The film was exposed to the atmosphere of H2S for 5 hours. Self-aggregated films were obtained by washing the sample in chloroform. In the next step 10 bilayers of cadmium arachidate were deposited again. The film was exposed to the atmosphere of H2S for 5 hours. Self-aggregated films were obtained by washing the sample in chloroform.
Voltage-current characteristics were measured through such heterostructures, using STM for these reasons. Effectivness of such approach for measuring lockal V/I characteristics was already shown on epitaxially grown heterostructures . Graphite substrate was used as one electrode and STM tip as the second one, realizing the structure as shown in Fig.1.
Scheme of the realized heterostructure
STM used was one by (ASSE-Z Italy). The measuring scheme, thus, was rather similar to that, already used for electrical characterization of structures, grown by molecular beam epitaxy .
In the case of the present work, it was impossible to register the scattering curve of the CdS nanoparticle solution (it practically coincided with that of the solvent) with conventional X-ray sources. Conversely, the problem was successfully solved at Elettra synchrotron in Trieste (Italy). Scattering curves different from that of the solvent were obtained allowing to resolve nanoparticle sizes.
X-ray scattering curves of CdS nanoparticles in a solution of pure water were measured in SAXS beamline. The results of these measurements are presented in figure 2.
X-ray scattering spectrum by a diluted solution of a monodisperse nanopartilcles
In order to obtain the nanoparticle size distribution function it is possible to refer to the well known theory of the light scattering by particles  and, in particular, to consider a simplified version of the Debay equation to write the scattered intensity by a system of n spherical nanoparticles having radius R :
Comparison of the theoretical scattering curve obtained from the equation (1) and the experimental curves as one presented in figure 2 allows to estimate the size distribution of the nanoparticles in our solutions.
Size distribution of the nanoparticles radii estimated by the comparison of scattering spectra as one present in figure 1 with theoretical X-ray scattering intensity by the simplified Debay equation (1)
Figure 3 shows such size distribution function presenting a maxima of probability at about 30 Å. The obtained dependence corresponds well to all results of previous indirect characterizations. Moreover, it provides one more evidence of the possibility of observing single electron conductivity through such objects, as their sizes, with high probability, correspond to that, suitable for room temperature observation.
The V/I characteristics, obtained by the described technique over the heterostructures, had a shape shown in fig 4 a and b. 95% of all characteristics were similar to them, namely, they had a regions with negative resistance, even if the position of the peak was varied. The appearance of the negative resistance can be connected to the fact, that the realized structure can be represented as the quantum well between two tunnelling barriers (Fig.5).
Different examples of V/I characteristics obtained from the junction shown in Fig. 1.
As it was already reported, a resonant level appears in such structures, the position of which determines the peak voltage in I/V characteristics . The position of this level (or levels) depends upon the geometry of the junction. Therefore, it is not strange that we have different positions of the peak for different measurements, as our structure is formed from polycrystal layers. Polycrystal nature of the layers results in variation of the junction parameters, and, therefore, in position of the peak in V/I characteristics.
Band profile of the quantum well
The performed measurements enabled to estimate the average sizes distribution of the nanoparticles formed in LB precursor at nanoscale level. The found sizes confirmed once more the possibility of observing single-electron and quantum phenomena on them.
V/I characteristics of heterostructures revealed the presence of differential negative resistance, indicating the tunnelling through the resonant levels. Not fixed position of the peaks in V/I characteristics was attributed to the polycrystal nature of the formed layers.
Measurements confirmed the formation of semiconductor superlattices directed towards a development of new nanodevices, like tunneling diods and semiconductor lasers .
Authors want to thank Paolo Faraci for the assistance in providing electrical characterization with STM.
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