- Email: [email protected]
Preparation and characterization of low-dimensional nanostructures
Applied Surface Science 141 Ž1999. 219–227
Preparation and characterization of low-dimensional nanostructures L. Augustin, L.F. Chi, H. Fuchs ) , S. Hoppner, S. Rakers, C. Rothig, T. Schwaack, ¨ ¨ F. Starrberg Physikalisches Institut, Westfalische Wilhelms-UniÕersitat, Germany ¨ ¨ D-48149 Munster, ¨ Received 20 July 1998; accepted 1 August 1998
Abstract Low-dimensional nanostructures usable under ambient conditions may provide a way to fulfil increasing demands for ultra-high density storage media, for novel electronic and optoelectronic devices and for miniaturized mechanical structures. In this paper, we present four different methods for producing low-dimensional structures prepared under ambient conditions. Firstly, we show how to produce flat-topped evenly-spaced gold particles on glass. Secondly, we discuss the production and filling of nanopores in mica. Thirdly, we show how to create atom-size electronic nanostructures on the ternary telluride TaNi 2Te 2 and, lastly, we demonstrate the production of nanometer-size holes on the blue bronze Rb 0.3 MoO 3. All of these nano-scale structures are characterized with either STM Žscanning tunneling microscopy. or SFM Žscanning force microscopy.. q 1999 Elsevier Science B.V. All rights reserved. PACS: 07.79.C2; 42.82.Cr; 61.16.Ch; 71.45.y d; 79.60.Ju; 81.65.Cf; 82.70.Dd; 87.64.D2; 85.40.Hq Keywords: Nanostructures; Scanning tunneling microscopy ŽSTM.; Scanning force microscopy ŽSFM.
1. Introduction Due to the ongoing miniaturization of electrical components a large interest exists for preparation and characterization methods of nanometer-sized structures. Reliable production of such structures is expected to lead to the development of ‘nanoelectronic’ devices, i.e., functional units with linear dimensions in the order of some nanometers. As the behavior of such devices is mainly governed by quantum mechanical principles, new phenomena can be observed like loss-free conduction via ballistic )
Corresponding author. Tel.: q49-251-8333621; Fax: q49251-833602; E-mail: [email protected]
electrons or single electron tunneling. These phenomena will help to make electronic devices faster, less dissipative and more reliable. However, to achieve such a goal, reliable preparation methods for such small structures and instruments for their characterization have to be developed. Scanning probe methods like STM Žscanning tunneling microscopy. or AFM Žatomic force microscopy. have the potential to be both in one. With the same method surfaces can be modified and characterized. Although nano-structuring of surfaces is fairly reliable, it is often slow due to the serial process involved. Therefore, parallel processes working on the same scale are of extreme importance for technical purposes.
0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 5 0 8 - X
220
L. Augustin et al.r Applied Surface Science 141 (1999) 219–227
In Section 2, we will present a method based on natural lithography for the preparation of regular gold particles on glass. In Section 3, we will discuss a parallel structuring method for the production of nanopores in mica. These pores traverse the whole thickness of a mica slab Žup to 30 mm. and are open on both sides. They can be filled with a variety of substances by electroplating or electrophoresis resulting in filling factors up to 100%. We studied the electronic properties of the filled pores by investigating the face sides of the pores with a conductive SFM. In Section 4, we will present a method to reliably create atomic-size structures on the surface of the ternary layered telluride TaNi 2Te 2 . These structures are created by applying voltage pulses to the tip. We believe the resulting structures to be mainly electronic in nature. In Section 5, we will show the results of the STM structuring experiments on the blue bronzes, especially Rb 0.3 MoO 3 . The same method was used for structuring, but the application of voltage pulses resulted in this case in the removal of material from the surface.
2. Latex projection patterns One of the simplest ways to produce nanometersized structures in a parallel way is to use natural
Fig. 1. Projection pattern of untempered gold on a latex spheremask imaged with AFM. Islands with triangular shape are formed.
Fig. 2. Projection pattern as in Fig. 1, but with the sample tempered. Shape, lateral and vertical extent of the dots change considerably.
lithography. The surface on top of which the structures are to be created is covered by an appropriate mask, foreign material is evaporated through the mask onto the surface and the mask is removed. This can easily be achieved by using polystyrol latices as first described by Fischer and Zingsheim w1x. These dispersions are commercially available with Žuniform. latex sphere diameters ranging from 50 nm up to several hundred micrometers w2x. We used aqueous dispersions for our investigations. The preparation on hydrophilic substrates consists of the application of a drop of dispersion onto the tilted hydrophilic surface and the controlled evaporation of the solvent. Especially on the upwards pointing part of the surface, a well-ordered monolayer of latex spheres is observed after the evaporation. The preparation on hydrophobic surfaces requires one additional step. After preparing the latex sphere monolayer on some sort of hydrophilic surface as mentioned before, the layer is transferred onto a water surface by slow immersion of the hydrophilic surface. The latex sphere monolayer will usually break up into a number of smaller islands floating on the water surface. These islands can be subsequently collected by a thin platinum wire loop and transferred onto the hydrophobic surface floating on a small drop of water trapped in the loop. After the removal of the water with a little piece of paper, the
L. Augustin et al.r Applied Surface Science 141 (1999) 219–227
221
Table 1 Geometrical data of various islands: s s length of side of a triangular island, h s height of an island, d s diameter of an hexagonal island, A s base area, Õ s volume. The volumes of the tempered and untempered islands stay the same Mask
S944 S453 BA221
Untempered
Tempered
s Žnm.
A Žnm2 .
h Žnm.
Õ Žnm3 .
d Žnm.
A Žnm2 .
h Žnm.
Õ Žnm3 .
330 150 72
47,155 9742 2244
12 12 12
565,860 116,913 26,936
70 40 20
15,400 5024 1256
38 30 16
585,200 150,720 20,096
islands bind firmly to the surface and form wellordered monolayers of latex spheres. We used this last method for the preparation of latex spheres patterns on top of cleaned Tempax glass surfaces. On top of this latex pattern gold was evaporated Žabout 10% of the sphere diameter. with a 1–2 nm chromium layer acting as a surfactant for the gold. After the evaporation, the latex spheres were removed by tetrahydrofuran ŽTHF, HPLC-quality.. The samples were immersed in fresh THF three times for 15 min at 508C inside an ultrasonic cleaner and afterwards dried in the THF vapor. This procedure yielded structures like the ones depicted in Fig. 1. The latex spheres in this case had a diameter of 453 nm, the resulting triangular islands showed a cornerto-corner distance of about 120 nm. After heating the prepared glass sample to about 7008C for 60 s we observed a recrystallization of the gold islands forming small hexagonal islands ŽFig. 2.. These islands are smaller in diameter but are much higher than the original triangular islands, i.e., the volume of the islands was conserved. Furthermore, the surface topography of the triangular islands changed from hyperbolic to atomically flat on top of the hexagonal islands. In Table 1 are some typical island parameters summarized before and after heating for different latex sphere diameters. The presented method is a simple way to produce small evenly-spaced regular, probably monocrystalline, gold islands with diameters down to 20 nm. It seems feasible to further reduce the size of the recrystallized gold islands by reducing the thickness of the evaporated gold layer. The technique can also be applied to other materials such as ferromagnetic metals.
3. Nanopores in mica Standard lithography methods often suffer from a very low height-to-width ratio of the created structures Žaspect ratio.. Even with very sophisticated techniques, the highest achievable ratios are in the order of 1000 with a maximum height of about 100 nm. If one would like to have tube-like structures with aspect ratios of 10 5 and depths of several hundred micrometers one has to look for different methods. One way to produce such tube-like structures consists of the irradiation of electrically insulating materials with swift heavy ions Žseveral MeVru.. Such ions deposit their kinetic energy almost continuously along their straight trajectory through the insulating material. The deposition of the energy
Fig. 3. AFM image of latent swift ion tracks on mica.
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L. Augustin et al.r Applied Surface Science 141 (1999) 219–227
Fig. 4. Current curves used to determine the size of the pores dependent on the etching time in 40% HF. The upper curve shows the long, the lower curve the small diagonal of the rhombic pores.
leads to an amorphized crystal region roughly 10 nm in diameter around the ion trajectory, often referred to as a latent ion track. Such a disturbed region is much more susceptible to attacks of etching chemicals than the undisturbed crystal. We have used thin mica sheets of up to 30 mm thick which were irradiated by 238 Uq and 197Au24q ions with a kinetic energy of 11.4 MeVru w3x. The ions hit the target at normal incidence. The resulting disturbed regions were investigated by AFM ŽFig. 3. where they showed up as little mounds. The irradi-
ated crystal was etched in hydro-fluoridic acid Ž40%.. After a very fast etch of the disturbed regions, a slow etching of the undisturbed regions is set which leads to a linear dependence between pore diameter and etching time ŽFig. 4.. The smallest diameter of the pores which we achieve reliably by this method is about 20 nm. As the fast ions are able to penetrate mica sheets up to 100 mm in thickness it is guaranteed that our pores are open on both sides. For pores with diameters of more than 50 nm diamond-shaped pores can be observed which reflects the crystalline structure of the mica ŽFig. 5.. Although such a porous material is very interesting in itself, e.g., for filtering applications, one would like to fill these pores to create very long and narrow wires or to use these pores as small containments for chemical reactions. We have used an electroplating process to fill the mica nanopores with various metals. On the back side of the mica sheet, a thin silver layer Ž20 nm. was evaporated acting as the working electrode. Despite the problem of rapid oxidation silver is advantageous compared to gold as no additional surfactant is required. With the help of a speciallyconstructed frame the front side is pressed against the bottom of a Plexiglas vessel guaranteeing that contact between the electroplating solution and the working electrode is only possible through the pores.
Fig. 5. Etched ion tracks in mica: left, overview image; right, single pores in mica.
L. Augustin et al.r Applied Surface Science 141 (1999) 219–227
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Table 2 Various electrolytes electroplating conditions for several metals on a silver electrode Filling
Composition of the electrolyte
Electroplating potential
Reference electrode
Cu
45 grl H 3 BO 3 1.76 grl CuSo 4 P 5H 2 O 252 grl CoSO4 P H 2 O 50 grl H 3 BO 3 7 grl NaCl commercially-available Au-electrolyte; 99.9% Au, 24 carat
540 mV
AgqrAgCl
1.4 V
AgqrAgCl
1.25 V
platinum electrode
Co
Au
The metal deposition takes place near the working electrode thereby slowly filling up the pores. It was helpful to agitate the electroplating solution with the help of a loudspeaker w4x on which the mica sheet was mounted. The composition of the electroplating solutions for different metals are tabulated in Table 2. Fig. 6 shows a typical plot of the electroplating current as a function of time. At the beginning, the current is constant as the integral area of all pores is constant. As soon as the first pore is completely filled, metal deposition on top of the mica sheet is
Fig. 6. Voltage drop at a 1 k V resistance while filling the nanopores with Co ŽUs1.4 V vs. AgqrAgCl-electrode.. The voltage drop is proportional to the current. The curve can be divided into three parts: part I: low current plateau, the nanopores are partially filled; part II: steep increase of the current, the pores become completely filled up; part III: high current plateau, all pores are filled up, only the thickness of the upper electrode increases.
preferred which leads to a rapid increase in the area of the working electrode and therefore a rapid increase in the electroplating current. After some time, the whole area of the mica sheet is covered with metal and the electroplating current saturates. The filling factor of the pores is very difficult to determine from integral measurements. We have therefore applied electrostatic force microscopy ŽEFM. and electron-conductivity scanning force microscopy ŽEC-SFM. measurements, the details of which are given elsewhere w3x. From these measurements, we can deduce that after cleaving off a few tens of nanometers from the front side, almost all of the pores are electrically connected to the backside of the mica sheet as seen in Fig. 7, i.e., we can deduce a filling factor of almost 100%. The presented method is suited for the reliable production of metallic wires with a minimum diame-
Fig. 7. AFM phase-image Žtapping mode. of a cleaved mica substrate with gold-filled nanopores. Bright dots represent the top of the metallic nanowires.
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L. Augustin et al.r Applied Surface Science 141 (1999) 219–227
ter of 20 nm and a length of 30 mm inside an insulating mica matrix. Such arrays of metallic wires are interesting for a number of applications including magnetic storage and new conduction phenomena.
4. TaNi 2 Te 2 The highest two-dimensional storage density one can hope to achieve is by manipulating single atoms on a surface. This can be done either by moving single atoms around with the tip of an STM as shown by Eigler and Schweizer w5x or by influencing the electronic properties of individual atoms or groups of atoms as shown by Fuchs et al. w6x. The second method especially holds a great potential for future storage applications as the change of electronic structure is often reversible and no lateral transport of surface material takes place. On the other hand, only a few materials are susceptible to this type of locally tip-induced surface modifications. Fuchs et al. w6x, as well as Asenjo et al. w7x and Boneberg w8x, investigated mainly dichalcogenides like WSe 2 , MoSe 2 or MoS 2 . A major advantage of these materials in comparison to metals, for example,
Fig. 8. High resolution STM image of TaNi 2Te 2 of the Ž001.surface, UT sy50 mV, I T s1 nA. The lower inlet shows a calculated charge density distribution in the energy range Ef . . . Ef y0.2 eV.
Fig. 9. A sequence of six voltage pulses ŽUpulse sy3 V, t pulse s 30 ms. arranged in as a triangle on a TaNi 2Te 2 -Ž001.-surface. Imaging parameters: UT sy100 mV, I T s1 nA.
is their layered crystal structure and their chemical inertness. The first property allows one to prepare large, defect-free and atomically flat surfaces, the second allows to work with these materials under ambient conditions. Here, we present the results of our surface modification studies of the ternary telluride TaNi 2Te 2 . Its crystal lattice is orthorhombic, space group Pmna, ˚ bs with unit cell dimensions of a s 6.488Ž3. A, ˚ and c s 17.014 A˚ w9,10x. Like the 3.566Ž1. A dichalcogenides, TaNi 2Te 2 has a layered crystal structure. In it the metal atoms are sandwiched between Te layers. The Te atoms of the top and bottom sheet form a slightly distorted close-packed arrangement. The two crystallographically-independent Te atoms are suited in four-fold Žm 4 Te. and five-fold Žm 5 Te. bridging positions above and below the TaNi 2 slab. They form alternating rows running parallel to the b-axis. These rows show up clearly in high resolution STM pictures of the Ž001.-surface ŽFig. 8. forming a track-like pattern on the surface. The rows consists of m 4 Te which can be deducted from Extended Hueckel calculations w9,10x. We could induce local modifications on top of these rows by applying voltage pulses between y2 and y3 V and 20 to 30 ms duration ŽFig. 9.. These local modifications appear without a perturbation of the surrounding surface. However, it often occurred that even when using these parameters, either no effect could be observed or the local order of the surrounding surface area was destroyed in addition. This was very likely due to the strong influence of the shape of the tip apex on the varying transient electrical fields between the tip and the surface
L. Augustin et al.r Applied Surface Science 141 (1999) 219–227
Fig. 10. ‘Holes’ created by voltage pulses ŽUpulse sy3 V, t pulse s 20 ms. on a TaNi 2Te 2 -Ž001.-surface. Imaging parameters: UT s y81 mV, I T s 2.4 nA.
225
instance, the surface is locally heated up by the structuring process. To conclude, we have shown that local modifications on the Ž001.-surface of TaNi 2Te 2 can be induced under ambient conditions by applying voltage pulses to the tip of an STM, that the modifications in most cases do not disturb the surrounding atomic lattice and that the modifications always appear on top of the naturally occurring rows of Ž001.-surface which help to guide the readrwrite probe.
5. Nanostructures on Rb 0.3 MoO 3 during pulsing. In such cases, a transport of material to or from the tip cannot be excluded. The tip-induced surface modifications could be used to produce larger structures on the surface. In Figs. 10 and 11, equilateral triangles consisting of six points each are drawn with y3 V, 20 ms and y3 V, 30 ms pulses, respectively. Although slightly distorted one can easily see the six modifications forming the triangles. The striking difference between the two pictures is the nature of the modification. In Fig. 10, the modifications appear as holes while in Fig. 11 the modifications appear as mounds. The reason for this behavior might be an overlap of local topographic and local electronic change if, for
Fig. 11. The p-states of the surface m 4 Te atoms form Želectronic. dimer row structures in w010x direction with bilobal shape near Ef on a TaNi 2Te 2-Ž001.-surface. Imaging parameters: UT sy100 mV, I T s1 nA.
Although electronic nanostructures seem to be the most natural way to store information on surfaces they suffer a major drawback in the achievable writing speeds. As shown in Section 4, structures on TaNi 2Te 2 can be written with voltage pulses of 20to 30-ms duration. This corresponds to about 50 bitsrs if we neglect additional movements of the tip from bit to bit and storage administration overhead. In comparison to the transfer rates of tens of several megabitsrs that modern hard disks can easily reach, such low writing speeds are certainly unacceptable. On the other hand, the storage capacity of a surface is huge, thus, it might not be necessary to delete anything written on the surface, i.e., the usage
Fig. 12. The topography STM image shows Rb 0.3 MoO 3 with molecular resolution under ambient conditions and room temperature. The imaging parameters are UT sy400 mV and I T s 0.65 nA. Each spot represents three MoO6 octahedra. The directions of the surface lattice vectors are indicated by arrows.
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L. Augustin et al.r Applied Surface Science 141 (1999) 219–227
of reversible structuring mechanisms is not an absolute necessity Žstrategy of affluence.. In general, non-reversible processes can reach much higher writing speeds than non-destructive processes as one is able to use processes which are much more violent as we will show for the blue bronze Rb 0.3 MoO 3 . Rb 0.3 MoO 3 is a one-dimensionally conductive layered oxide. Its unit cell consists of 20 MoO6 octaeders and six Rb atoms located between sheets formed by groups of octaeders. The lattice is monoclinic, space group C2rm. The lattice vectors are ˚ b s 7.556Ž1. A, ˚ c s 10.035Ž5. A, ˚ a s 18.536Ž2. A, bŽ a,c . s 118.52Ž1.8 w11x. Modifications of the bronze were reported by Garfunkel et al. w12x, but with very slow etching techniques, i.e., 10 min for a single structure. The crystal can easily be cleaved perpendicular to the w001x-direction leaving behind large terraces with rows of groups of MoO6 octaeders. Fig. 12 shows an STM image with molecular resolution of this surface. One bright dot in the STM image represents a group of three linked MoO6 octaeders. If one applies voltage pulses of q4.0 V and 15 ms duration to the STM tip one can readily remove material from the surface revealing the layered structure of the material ŽFig. 13.. To determine the
Fig. 13. Topographic STM image of Rb 0.3 MoO 3 , UT sy400 mV, I T s1.1 nA. This image reveals the layered structure of the blue bronze. In the upper part, there is a row of three pulsed nanostructures of less than 5 nm in diameter. The structuring parameters are Upulse sq4.0 V and t pulse s15 ms.
Fig. 14. Topographic STM image of Rb 0.3 MoO 3 , UT sy300 mV, I T s 0.9 nA. The image shows three rows with five pulsed modifications. The pulse duration for all structures is the same, t pulse s15 ms, but the amplitude of the rectangular-shaped pulse varies: first row, Upulse s 4.2 V; second row, Upulse s 4.5 V; third row, Upulse s 4.2 V.
optimal pulse parameters, we created test structures like the ones in Fig. 14 and plotted the apparent volume of the holes against the height of the voltage pulse and the pulse duration. We found that the volume of the holes remain constant between 200 and 500 nm3 over a wide range of parameters as seen in Fig. 15. The size and shape of the created holes does not change with voltage over a range of
Fig. 15. Plot of the apparent volume of the created holes vs. time for a given pulse voltage. The removed volume does not change considerably over a wide range of time and over a voltage range of more than half a volt. The shortest pulse duration is 80 ns.
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227
more than half a volt and does not change either with pulse duration over five orders of magnitude. The details of our analysis are presented elsewhere w13x. The shortest pulse so far with which we could create our test structures had a duration of only 80 ns. If we again neglect the movement of the tip between pulses this leads to a writing speed of 10 Mbitrs, comparable to data rates of today’s hard disks. Still, despite the open questions concerning the exact nature of this structuring process, we can conclude that a destructive structuring of the Rb 0.3 MoO 3 Ž001. surface can be performed very reliably and that the structuring process is comparable with the writing speed of modern hard disks.
bly more important aspect is nano-electronics and nanomechanics, i.e., the production and testing of nanoscale electronic and mechanical devices. With reliable structuring methods, four of them presented in this paper, remarkable progress can be expected in the near future.
6. Conclusion
References
In this paper, we presented four methods for the reliable production of low-dimensional nano-scale structures under ambient conditions using selforganizing techniques and local probes. From these experiments, it seems more likely that a combination of techniques rather than a single technique is promising, i.e., a pre-structuring of the surface via parallel, self-organizing methods and local modifications via scanning probe methods will lead to nanotechnological applications. For storage applications, however, major obstacles still remain. The reliability of the presented serial Žtip probe. structuring methods is at about 95% in the case of Rb 0.3 MoO 3 , fairly high but still unacceptably low compared to the 10y7 to 10y8 writing errors per writing operation in modern hard disks. Another challenge is the preparation of large flat surfaces usable for structuring. One would like to have, at least, square centimeters of suitable surfaces while at the moment only surface measuring 250 = 250 nm2 are available. However, data storage is not the only application for nanoscale technology. Another major and proba-
w1x U.Ch. Fischer, H.P. Zingsheim, J. Vac. Sci. Technol. 19 Ž4. Ž1981. 881. w2x S944, Boehringer Ingelheim; S453, Boehringer Ingelheim; BA221, BASF, Ludwigshafen. w3x S. Hoppener, Diploma thesis: Entwicklung und Charakter¨ isierung von nanostrukturierten Bauteilen und deren potentielle Anwendungen, Physikalisches Institut der Westfalischen ¨ Wilhelms-Universitat 1997. ¨ Munster, ¨ w4x W.D. Williams, N. Giordano, Rev. Sci. Instrum. 55 Ž3. Ž1984. 410. w5x D.M. Eigler, E.K. Schweizer, Nature 344 Ž1990. 524. w6x H. Fuchs, Th. Schimmel, M. Lux-Steiner, E. Bucher, Ultramicroscopy 42 Ž1992. 1295. w7x A. Asenjo, T. Schwaack, P. de Pablo, J. Gomez-Herrero, ´ E.K. Schweizer, C. Pettenkofer, H. Fuchs, A.M. Baro, Surf. Sci. 398 Ž1998. 231. w8x J. Boneberg, Habilitationsschrift: Methoden zur Erzeugung und Charakterisierung von Nanostrukturen, Universitat ¨ Konstanz, 1997. w9x W. Tremel, Angew. Chem. 104 Ž1992. 230. w10x W. Tremel, Angew. Chem., Int. Ed. Engl. 31 Ž1992. 217. w11x W.J. Schutte, J.L. de Boer, Acta Cryst. B 49 Ž1993. 579. w12x E. Garfunkel, R. Rudd, D. Novak, S. Wang, G. Ebert, M. Greenblatt, T. Gustafsson, S.H. Garofalini, Science Ž1989. 246. w13x F. Starrberg, Diploma thesis: Modifikation von Molybdan¨ bronzen mit dem Rastertunnelmikroskop, Physikalisches Institut der Westfalischen Wilhelms-Universitat 1998. ¨ ¨ Munster, ¨
Acknowledgements We thank Prof. W. Tremel in Mainz for providing us with the blue bronzes and Prof. Neumann for structuring of the mica samples at the GSI Darmstadt. This work was supported by the BMBF 03N1023E7.
Preparation and characterization of low-dimensional nanostructures L. Augustin, L.F. Chi, H. Fuchs ) , S. Hoppner, S. Rakers, C. Rothig, T. Schwaack, ¨ ¨ F. Starrberg Physikalisches Institut, Westfalische Wilhelms-UniÕersitat, Germany ¨ ¨ D-48149 Munster, ¨ Received 20 July 1998; accepted 1 August 1998
Abstract Low-dimensional nanostructures usable under ambient conditions may provide a way to fulfil increasing demands for ultra-high density storage media, for novel electronic and optoelectronic devices and for miniaturized mechanical structures. In this paper, we present four different methods for producing low-dimensional structures prepared under ambient conditions. Firstly, we show how to produce flat-topped evenly-spaced gold particles on glass. Secondly, we discuss the production and filling of nanopores in mica. Thirdly, we show how to create atom-size electronic nanostructures on the ternary telluride TaNi 2Te 2 and, lastly, we demonstrate the production of nanometer-size holes on the blue bronze Rb 0.3 MoO 3. All of these nano-scale structures are characterized with either STM Žscanning tunneling microscopy. or SFM Žscanning force microscopy.. q 1999 Elsevier Science B.V. All rights reserved. PACS: 07.79.C2; 42.82.Cr; 61.16.Ch; 71.45.y d; 79.60.Ju; 81.65.Cf; 82.70.Dd; 87.64.D2; 85.40.Hq Keywords: Nanostructures; Scanning tunneling microscopy ŽSTM.; Scanning force microscopy ŽSFM.
1. Introduction Due to the ongoing miniaturization of electrical components a large interest exists for preparation and characterization methods of nanometer-sized structures. Reliable production of such structures is expected to lead to the development of ‘nanoelectronic’ devices, i.e., functional units with linear dimensions in the order of some nanometers. As the behavior of such devices is mainly governed by quantum mechanical principles, new phenomena can be observed like loss-free conduction via ballistic )
Corresponding author. Tel.: q49-251-8333621; Fax: q49251-833602; E-mail: [email protected]
electrons or single electron tunneling. These phenomena will help to make electronic devices faster, less dissipative and more reliable. However, to achieve such a goal, reliable preparation methods for such small structures and instruments for their characterization have to be developed. Scanning probe methods like STM Žscanning tunneling microscopy. or AFM Žatomic force microscopy. have the potential to be both in one. With the same method surfaces can be modified and characterized. Although nano-structuring of surfaces is fairly reliable, it is often slow due to the serial process involved. Therefore, parallel processes working on the same scale are of extreme importance for technical purposes.
0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 5 0 8 - X
220
L. Augustin et al.r Applied Surface Science 141 (1999) 219–227
In Section 2, we will present a method based on natural lithography for the preparation of regular gold particles on glass. In Section 3, we will discuss a parallel structuring method for the production of nanopores in mica. These pores traverse the whole thickness of a mica slab Žup to 30 mm. and are open on both sides. They can be filled with a variety of substances by electroplating or electrophoresis resulting in filling factors up to 100%. We studied the electronic properties of the filled pores by investigating the face sides of the pores with a conductive SFM. In Section 4, we will present a method to reliably create atomic-size structures on the surface of the ternary layered telluride TaNi 2Te 2 . These structures are created by applying voltage pulses to the tip. We believe the resulting structures to be mainly electronic in nature. In Section 5, we will show the results of the STM structuring experiments on the blue bronzes, especially Rb 0.3 MoO 3 . The same method was used for structuring, but the application of voltage pulses resulted in this case in the removal of material from the surface.
2. Latex projection patterns One of the simplest ways to produce nanometersized structures in a parallel way is to use natural
Fig. 1. Projection pattern of untempered gold on a latex spheremask imaged with AFM. Islands with triangular shape are formed.
Fig. 2. Projection pattern as in Fig. 1, but with the sample tempered. Shape, lateral and vertical extent of the dots change considerably.
lithography. The surface on top of which the structures are to be created is covered by an appropriate mask, foreign material is evaporated through the mask onto the surface and the mask is removed. This can easily be achieved by using polystyrol latices as first described by Fischer and Zingsheim w1x. These dispersions are commercially available with Žuniform. latex sphere diameters ranging from 50 nm up to several hundred micrometers w2x. We used aqueous dispersions for our investigations. The preparation on hydrophilic substrates consists of the application of a drop of dispersion onto the tilted hydrophilic surface and the controlled evaporation of the solvent. Especially on the upwards pointing part of the surface, a well-ordered monolayer of latex spheres is observed after the evaporation. The preparation on hydrophobic surfaces requires one additional step. After preparing the latex sphere monolayer on some sort of hydrophilic surface as mentioned before, the layer is transferred onto a water surface by slow immersion of the hydrophilic surface. The latex sphere monolayer will usually break up into a number of smaller islands floating on the water surface. These islands can be subsequently collected by a thin platinum wire loop and transferred onto the hydrophobic surface floating on a small drop of water trapped in the loop. After the removal of the water with a little piece of paper, the
L. Augustin et al.r Applied Surface Science 141 (1999) 219–227
221
Table 1 Geometrical data of various islands: s s length of side of a triangular island, h s height of an island, d s diameter of an hexagonal island, A s base area, Õ s volume. The volumes of the tempered and untempered islands stay the same Mask
S944 S453 BA221
Untempered
Tempered
s Žnm.
A Žnm2 .
h Žnm.
Õ Žnm3 .
d Žnm.
A Žnm2 .
h Žnm.
Õ Žnm3 .
330 150 72
47,155 9742 2244
12 12 12
565,860 116,913 26,936
70 40 20
15,400 5024 1256
38 30 16
585,200 150,720 20,096
islands bind firmly to the surface and form wellordered monolayers of latex spheres. We used this last method for the preparation of latex spheres patterns on top of cleaned Tempax glass surfaces. On top of this latex pattern gold was evaporated Žabout 10% of the sphere diameter. with a 1–2 nm chromium layer acting as a surfactant for the gold. After the evaporation, the latex spheres were removed by tetrahydrofuran ŽTHF, HPLC-quality.. The samples were immersed in fresh THF three times for 15 min at 508C inside an ultrasonic cleaner and afterwards dried in the THF vapor. This procedure yielded structures like the ones depicted in Fig. 1. The latex spheres in this case had a diameter of 453 nm, the resulting triangular islands showed a cornerto-corner distance of about 120 nm. After heating the prepared glass sample to about 7008C for 60 s we observed a recrystallization of the gold islands forming small hexagonal islands ŽFig. 2.. These islands are smaller in diameter but are much higher than the original triangular islands, i.e., the volume of the islands was conserved. Furthermore, the surface topography of the triangular islands changed from hyperbolic to atomically flat on top of the hexagonal islands. In Table 1 are some typical island parameters summarized before and after heating for different latex sphere diameters. The presented method is a simple way to produce small evenly-spaced regular, probably monocrystalline, gold islands with diameters down to 20 nm. It seems feasible to further reduce the size of the recrystallized gold islands by reducing the thickness of the evaporated gold layer. The technique can also be applied to other materials such as ferromagnetic metals.
3. Nanopores in mica Standard lithography methods often suffer from a very low height-to-width ratio of the created structures Žaspect ratio.. Even with very sophisticated techniques, the highest achievable ratios are in the order of 1000 with a maximum height of about 100 nm. If one would like to have tube-like structures with aspect ratios of 10 5 and depths of several hundred micrometers one has to look for different methods. One way to produce such tube-like structures consists of the irradiation of electrically insulating materials with swift heavy ions Žseveral MeVru.. Such ions deposit their kinetic energy almost continuously along their straight trajectory through the insulating material. The deposition of the energy
Fig. 3. AFM image of latent swift ion tracks on mica.
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Fig. 4. Current curves used to determine the size of the pores dependent on the etching time in 40% HF. The upper curve shows the long, the lower curve the small diagonal of the rhombic pores.
leads to an amorphized crystal region roughly 10 nm in diameter around the ion trajectory, often referred to as a latent ion track. Such a disturbed region is much more susceptible to attacks of etching chemicals than the undisturbed crystal. We have used thin mica sheets of up to 30 mm thick which were irradiated by 238 Uq and 197Au24q ions with a kinetic energy of 11.4 MeVru w3x. The ions hit the target at normal incidence. The resulting disturbed regions were investigated by AFM ŽFig. 3. where they showed up as little mounds. The irradi-
ated crystal was etched in hydro-fluoridic acid Ž40%.. After a very fast etch of the disturbed regions, a slow etching of the undisturbed regions is set which leads to a linear dependence between pore diameter and etching time ŽFig. 4.. The smallest diameter of the pores which we achieve reliably by this method is about 20 nm. As the fast ions are able to penetrate mica sheets up to 100 mm in thickness it is guaranteed that our pores are open on both sides. For pores with diameters of more than 50 nm diamond-shaped pores can be observed which reflects the crystalline structure of the mica ŽFig. 5.. Although such a porous material is very interesting in itself, e.g., for filtering applications, one would like to fill these pores to create very long and narrow wires or to use these pores as small containments for chemical reactions. We have used an electroplating process to fill the mica nanopores with various metals. On the back side of the mica sheet, a thin silver layer Ž20 nm. was evaporated acting as the working electrode. Despite the problem of rapid oxidation silver is advantageous compared to gold as no additional surfactant is required. With the help of a speciallyconstructed frame the front side is pressed against the bottom of a Plexiglas vessel guaranteeing that contact between the electroplating solution and the working electrode is only possible through the pores.
Fig. 5. Etched ion tracks in mica: left, overview image; right, single pores in mica.
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Table 2 Various electrolytes electroplating conditions for several metals on a silver electrode Filling
Composition of the electrolyte
Electroplating potential
Reference electrode
Cu
45 grl H 3 BO 3 1.76 grl CuSo 4 P 5H 2 O 252 grl CoSO4 P H 2 O 50 grl H 3 BO 3 7 grl NaCl commercially-available Au-electrolyte; 99.9% Au, 24 carat
540 mV
AgqrAgCl
1.4 V
AgqrAgCl
1.25 V
platinum electrode
Co
Au
The metal deposition takes place near the working electrode thereby slowly filling up the pores. It was helpful to agitate the electroplating solution with the help of a loudspeaker w4x on which the mica sheet was mounted. The composition of the electroplating solutions for different metals are tabulated in Table 2. Fig. 6 shows a typical plot of the electroplating current as a function of time. At the beginning, the current is constant as the integral area of all pores is constant. As soon as the first pore is completely filled, metal deposition on top of the mica sheet is
Fig. 6. Voltage drop at a 1 k V resistance while filling the nanopores with Co ŽUs1.4 V vs. AgqrAgCl-electrode.. The voltage drop is proportional to the current. The curve can be divided into three parts: part I: low current plateau, the nanopores are partially filled; part II: steep increase of the current, the pores become completely filled up; part III: high current plateau, all pores are filled up, only the thickness of the upper electrode increases.
preferred which leads to a rapid increase in the area of the working electrode and therefore a rapid increase in the electroplating current. After some time, the whole area of the mica sheet is covered with metal and the electroplating current saturates. The filling factor of the pores is very difficult to determine from integral measurements. We have therefore applied electrostatic force microscopy ŽEFM. and electron-conductivity scanning force microscopy ŽEC-SFM. measurements, the details of which are given elsewhere w3x. From these measurements, we can deduce that after cleaving off a few tens of nanometers from the front side, almost all of the pores are electrically connected to the backside of the mica sheet as seen in Fig. 7, i.e., we can deduce a filling factor of almost 100%. The presented method is suited for the reliable production of metallic wires with a minimum diame-
Fig. 7. AFM phase-image Žtapping mode. of a cleaved mica substrate with gold-filled nanopores. Bright dots represent the top of the metallic nanowires.
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ter of 20 nm and a length of 30 mm inside an insulating mica matrix. Such arrays of metallic wires are interesting for a number of applications including magnetic storage and new conduction phenomena.
4. TaNi 2 Te 2 The highest two-dimensional storage density one can hope to achieve is by manipulating single atoms on a surface. This can be done either by moving single atoms around with the tip of an STM as shown by Eigler and Schweizer w5x or by influencing the electronic properties of individual atoms or groups of atoms as shown by Fuchs et al. w6x. The second method especially holds a great potential for future storage applications as the change of electronic structure is often reversible and no lateral transport of surface material takes place. On the other hand, only a few materials are susceptible to this type of locally tip-induced surface modifications. Fuchs et al. w6x, as well as Asenjo et al. w7x and Boneberg w8x, investigated mainly dichalcogenides like WSe 2 , MoSe 2 or MoS 2 . A major advantage of these materials in comparison to metals, for example,
Fig. 8. High resolution STM image of TaNi 2Te 2 of the Ž001.surface, UT sy50 mV, I T s1 nA. The lower inlet shows a calculated charge density distribution in the energy range Ef . . . Ef y0.2 eV.
Fig. 9. A sequence of six voltage pulses ŽUpulse sy3 V, t pulse s 30 ms. arranged in as a triangle on a TaNi 2Te 2 -Ž001.-surface. Imaging parameters: UT sy100 mV, I T s1 nA.
is their layered crystal structure and their chemical inertness. The first property allows one to prepare large, defect-free and atomically flat surfaces, the second allows to work with these materials under ambient conditions. Here, we present the results of our surface modification studies of the ternary telluride TaNi 2Te 2 . Its crystal lattice is orthorhombic, space group Pmna, ˚ bs with unit cell dimensions of a s 6.488Ž3. A, ˚ and c s 17.014 A˚ w9,10x. Like the 3.566Ž1. A dichalcogenides, TaNi 2Te 2 has a layered crystal structure. In it the metal atoms are sandwiched between Te layers. The Te atoms of the top and bottom sheet form a slightly distorted close-packed arrangement. The two crystallographically-independent Te atoms are suited in four-fold Žm 4 Te. and five-fold Žm 5 Te. bridging positions above and below the TaNi 2 slab. They form alternating rows running parallel to the b-axis. These rows show up clearly in high resolution STM pictures of the Ž001.-surface ŽFig. 8. forming a track-like pattern on the surface. The rows consists of m 4 Te which can be deducted from Extended Hueckel calculations w9,10x. We could induce local modifications on top of these rows by applying voltage pulses between y2 and y3 V and 20 to 30 ms duration ŽFig. 9.. These local modifications appear without a perturbation of the surrounding surface. However, it often occurred that even when using these parameters, either no effect could be observed or the local order of the surrounding surface area was destroyed in addition. This was very likely due to the strong influence of the shape of the tip apex on the varying transient electrical fields between the tip and the surface
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Fig. 10. ‘Holes’ created by voltage pulses ŽUpulse sy3 V, t pulse s 20 ms. on a TaNi 2Te 2 -Ž001.-surface. Imaging parameters: UT s y81 mV, I T s 2.4 nA.
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instance, the surface is locally heated up by the structuring process. To conclude, we have shown that local modifications on the Ž001.-surface of TaNi 2Te 2 can be induced under ambient conditions by applying voltage pulses to the tip of an STM, that the modifications in most cases do not disturb the surrounding atomic lattice and that the modifications always appear on top of the naturally occurring rows of Ž001.-surface which help to guide the readrwrite probe.
5. Nanostructures on Rb 0.3 MoO 3 during pulsing. In such cases, a transport of material to or from the tip cannot be excluded. The tip-induced surface modifications could be used to produce larger structures on the surface. In Figs. 10 and 11, equilateral triangles consisting of six points each are drawn with y3 V, 20 ms and y3 V, 30 ms pulses, respectively. Although slightly distorted one can easily see the six modifications forming the triangles. The striking difference between the two pictures is the nature of the modification. In Fig. 10, the modifications appear as holes while in Fig. 11 the modifications appear as mounds. The reason for this behavior might be an overlap of local topographic and local electronic change if, for
Fig. 11. The p-states of the surface m 4 Te atoms form Želectronic. dimer row structures in w010x direction with bilobal shape near Ef on a TaNi 2Te 2-Ž001.-surface. Imaging parameters: UT sy100 mV, I T s1 nA.
Although electronic nanostructures seem to be the most natural way to store information on surfaces they suffer a major drawback in the achievable writing speeds. As shown in Section 4, structures on TaNi 2Te 2 can be written with voltage pulses of 20to 30-ms duration. This corresponds to about 50 bitsrs if we neglect additional movements of the tip from bit to bit and storage administration overhead. In comparison to the transfer rates of tens of several megabitsrs that modern hard disks can easily reach, such low writing speeds are certainly unacceptable. On the other hand, the storage capacity of a surface is huge, thus, it might not be necessary to delete anything written on the surface, i.e., the usage
Fig. 12. The topography STM image shows Rb 0.3 MoO 3 with molecular resolution under ambient conditions and room temperature. The imaging parameters are UT sy400 mV and I T s 0.65 nA. Each spot represents three MoO6 octahedra. The directions of the surface lattice vectors are indicated by arrows.
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of reversible structuring mechanisms is not an absolute necessity Žstrategy of affluence.. In general, non-reversible processes can reach much higher writing speeds than non-destructive processes as one is able to use processes which are much more violent as we will show for the blue bronze Rb 0.3 MoO 3 . Rb 0.3 MoO 3 is a one-dimensionally conductive layered oxide. Its unit cell consists of 20 MoO6 octaeders and six Rb atoms located between sheets formed by groups of octaeders. The lattice is monoclinic, space group C2rm. The lattice vectors are ˚ b s 7.556Ž1. A, ˚ c s 10.035Ž5. A, ˚ a s 18.536Ž2. A, bŽ a,c . s 118.52Ž1.8 w11x. Modifications of the bronze were reported by Garfunkel et al. w12x, but with very slow etching techniques, i.e., 10 min for a single structure. The crystal can easily be cleaved perpendicular to the w001x-direction leaving behind large terraces with rows of groups of MoO6 octaeders. Fig. 12 shows an STM image with molecular resolution of this surface. One bright dot in the STM image represents a group of three linked MoO6 octaeders. If one applies voltage pulses of q4.0 V and 15 ms duration to the STM tip one can readily remove material from the surface revealing the layered structure of the material ŽFig. 13.. To determine the
Fig. 13. Topographic STM image of Rb 0.3 MoO 3 , UT sy400 mV, I T s1.1 nA. This image reveals the layered structure of the blue bronze. In the upper part, there is a row of three pulsed nanostructures of less than 5 nm in diameter. The structuring parameters are Upulse sq4.0 V and t pulse s15 ms.
Fig. 14. Topographic STM image of Rb 0.3 MoO 3 , UT sy300 mV, I T s 0.9 nA. The image shows three rows with five pulsed modifications. The pulse duration for all structures is the same, t pulse s15 ms, but the amplitude of the rectangular-shaped pulse varies: first row, Upulse s 4.2 V; second row, Upulse s 4.5 V; third row, Upulse s 4.2 V.
optimal pulse parameters, we created test structures like the ones in Fig. 14 and plotted the apparent volume of the holes against the height of the voltage pulse and the pulse duration. We found that the volume of the holes remain constant between 200 and 500 nm3 over a wide range of parameters as seen in Fig. 15. The size and shape of the created holes does not change with voltage over a range of
Fig. 15. Plot of the apparent volume of the created holes vs. time for a given pulse voltage. The removed volume does not change considerably over a wide range of time and over a voltage range of more than half a volt. The shortest pulse duration is 80 ns.
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more than half a volt and does not change either with pulse duration over five orders of magnitude. The details of our analysis are presented elsewhere w13x. The shortest pulse so far with which we could create our test structures had a duration of only 80 ns. If we again neglect the movement of the tip between pulses this leads to a writing speed of 10 Mbitrs, comparable to data rates of today’s hard disks. Still, despite the open questions concerning the exact nature of this structuring process, we can conclude that a destructive structuring of the Rb 0.3 MoO 3 Ž001. surface can be performed very reliably and that the structuring process is comparable with the writing speed of modern hard disks.
bly more important aspect is nano-electronics and nanomechanics, i.e., the production and testing of nanoscale electronic and mechanical devices. With reliable structuring methods, four of them presented in this paper, remarkable progress can be expected in the near future.
6. Conclusion
References
In this paper, we presented four methods for the reliable production of low-dimensional nano-scale structures under ambient conditions using selforganizing techniques and local probes. From these experiments, it seems more likely that a combination of techniques rather than a single technique is promising, i.e., a pre-structuring of the surface via parallel, self-organizing methods and local modifications via scanning probe methods will lead to nanotechnological applications. For storage applications, however, major obstacles still remain. The reliability of the presented serial Žtip probe. structuring methods is at about 95% in the case of Rb 0.3 MoO 3 , fairly high but still unacceptably low compared to the 10y7 to 10y8 writing errors per writing operation in modern hard disks. Another challenge is the preparation of large flat surfaces usable for structuring. One would like to have, at least, square centimeters of suitable surfaces while at the moment only surface measuring 250 = 250 nm2 are available. However, data storage is not the only application for nanoscale technology. Another major and proba-
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Acknowledgements We thank Prof. W. Tremel in Mainz for providing us with the blue bronzes and Prof. Neumann for structuring of the mica samples at the GSI Darmstadt. This work was supported by the BMBF 03N1023E7.