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Comparative scanning-tunnelling-microscopy investigations of nanostructures prepared by different techniques
applied surface science ELSEVIER
Applied Surface Science 107 (1996) 212-217
Comparative scanning-tunnelling-microscopy investigations of nanostructures prepared by different techniques E. Hartmann
a, *
p. Radojkovic a M. Schwartzkopff a M. E n a c h e s c u a,1 P. Marquardt b
a Physik-Department E16, Technische Universitgtt Miinchen, D-85748 Garching, Germany b Fraunhofer-Institutf~tr Festkgrpertechnologie, Hansastr. 27d, D-80686 Mfmchen, Germany
Received 11 October 1995; accepted 23 December 1995
Abstract Conducting nanoscale structures (dots, particles and wires) are fabricated by means of four different techniques on chemically cleaned, H-terminated Si substrates and investigated using a scanning tunnelling microscope (STM) operated under high-vacuum conditions. We distinguish between STM tip-independent or 'global' techniques (formation of colloids by wet chemical methods; condensation of particles from the vapour phase in a low-pressure background noble gas) and tip-assisted or 'local' techniques (field-induced transfer of tip material; electron-stimulated decomposition of organometallic compounds). These fabrication techniques are compared and evaluated, e.g., with respect to the particle stability on the substrate surface at room temperature and the capability of manipulating individual particles, in order to address the issue of designing prototype hybrid structures by the controlled assembling of nanoparticles and electrically connecting these structures to the 'outside world' by means of an STM.
1. Introduction In the past eight years, tremendous activities have been devoted to make use of the unique techniques of scanning probe microscopy in order to manipulate matter at the nanometre or atomic scale. For instance, individual atoms [1,2] have been removed from and redeposited onto the substrate surface or arranged with the tip of a scanning tunnelling microscope (STM) in order to form desired patterns [3,4].
* Corresponding author. Tel.: +49-89-28912345; fax: +49-8928912317; e-mail: [email protected]. 1 Present address: Laboratory for Surface Science and Technology, University of Maine, 5764 Sawyer Research Center, Orono, ME 04469.
The feasibility of manipulating even larger objects at room temperature with nanometre precision has been demonstrated using a scanning force microscope (SFM) operated in ambient air [5-7]. Our efforts are dedicated to the detection, imaging, and manipulation of nanoscale structures (dots, particles and wires) on atomically flat S i ( l l l ) substrate surfaces using an STM operated at room temperature. For the preparation of these nanostructures, four different techniques are employed which, in turn, are distinguished by S T M tip-assisted or 'local' and tip-independent or 'global' deposition methods. The two local (tip-assisted) techniques include fieldinduced transfer of tip material and electron-stimulated decomposition of organometallic precursors. In particular, both are suitable to manufacture only a
0169-4332/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII SO 169-4332(96)00508-9
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very small number of particles on predetermined locations of the substrate surface. In contrast, the utilization of global (tip-independent) techniques, as the ex-situ wet chemical composition of particles (colloidal solutions) and the in-situ condensation of substances from the vapour phase in the presence of a low-pressure inert background gas (noble gas technique), allows the formation of individual particles, particle agglomerations, or even completely covering granular films. We are aiming at the elaboration and evaluation of these techniques for future technological applications in the light of controllably assembling artificial prototype hybrid structures and electrically connecting them to the 'outside world' [8].
2. Sample and tip preparation As substrate material, commercially available ptype Si(111) wafers are employed which are usually covered with a native oxide layer. Atomically flat, H-terminated substrate surfaces are prepared by a wet chemical treatment in a weakly alkaline 40% NH4F solution, resulting in a strongly anisotropic corrosion of di- and tri-hydride Si atoms ('step-flow' etch mechanism) [9]. STM imaging clearly resolves a step-terrace structure caused by a slight misalignmerit of the wafer with respect to the [111] direction. Tungsten tunnelling probes are prepared from a polycrystalline 0.7-mm-diameter wire by dc-electrochemical etching in a 2 M NaOH solution. The W tunnelling probes are used for the two global techniques and for the electron-stimulated decomposition of organometallic compounds. For the field-induced transfer of tip material, a 0.25-mm-diameter Au wire is electrochemically etched in a 2 : 1 mixture of double-deionized, filtered H 2 0 ( p = 15 M12 cm) and concentrated HC1 at a dc bias of 0.8 V. The samples are rapidly transferred via a fast load-lock system to the STM chamber. All measurements are conducted under high-vacuum conditions ( p < 5 × 10 -8 mbar) at room temperature. Constant-current ( I w = 80 pA) images are generated by applying a voltage of + 2.7 to + 3 V to the sample. In the following we content ourselves with a short description of the respective technique used for preparing the nanometre-sized structures and report
on selected results which, however, remain valid for a much greater variety of substances.
3. Results and discussion 3.1. Wet chemical ('global type')
methods:
colloidal
solutions
Semiconducting CdS particles have been composed by wet chemical methods as detailed in the literature [10]. This preparation method is distinguished to yield crystalline particles with a very sharp size distribution (monodisperse). In the present case, the CdS particles are ca. 5 nm in diameter and encapsulated with a stabilizing layer of sodium polyphosphate impeding bunching in solution. However, transmission electron microscopy (TEM) micrographs reveal the tendency of the particles to agglomerate as soon as they are deposited on suitable substrates, forming irregular two-dimensional structures with some 10 to 100 constituents. For our studies, a tiny droplet of the colloidal solution is deposited on the Si substrate surface, and the particle stabilizing fluid is immediately removed by a tissue. The analytical concentration of CdS particles amounts to 2 X 10 -4 M and determines the particle density covering the substrate surface. Fig. la displays a quasi-3D STM image showing protruding features of 5 nm in height, according to the average particle size. This provides strong evidence that the detected elevations can be attributed to single CdS particles or agglomerations which are exclusively arranged two-dimensionally, as expected from the TEM micrographs. Interestingly, no bilayer steps on the S i ( l l l ) substrate surface [9,11] could ever be resolved on specimens where Colloidal solutions have been deposited. Presumably, the stabilizing sodium polyphosphate solution is still covering the Si surface a n d / o r leads to a formation of a thin oxide layer. In addressing the issue of manipulation, we have examined the capability of structurally modifying isolated particulates of CdS particles with the STM. For these experiments, the probing tip is positioned at the edge of the two prominent protrusions of Fig. la, having lateral dimensions of roughly 30 nm in
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E. Hartmann et aL /Applied Surface Science 107 (1996) 212-217
for fabricating compound semiconductors but can also be refined to the preparation of metallic particles.
3.2. Preparation of Ag particles using the noble gas technique ('global type')
Fig. 1. (a) Qnasi-3D STM representation showing agglomerations (ca. 30 nm in diameter) of CdS nanoparticles deposited on a Si substrate. The image covers an area of 230 nm X 230 rim. The height of these protruding features amounts to 5 nm. (b) Same surface area as in (a) after traversing the tip through both agglomerations at constant height which results in the formation of V-shaped structures.
diameter (some 10 CdS particles): Subsequently, the feedback loop is interrupted and the probe is laterally traversed through the respective particulate at constant height. Afterwards, the feedback loop is activated and the same surface area is imaged again. The resulting changes are documented in Fig. lb. Evidently, in each case, a notch is formed in the central part of both agglomerations, implying that individual particles can literally be removed with the tip (their coupling to the substrate is very weak). More importantly, the removed particles could never be traced out on the substrate surface with the STM, leading to the assumption that they preferably stick to the tip material as soon as a mechanical contact between tip and particles has been established. The CdS particles as well as the V-shaped structures formed by the tip-traversing experiments resist thermal agitation and can be imaged repeatedly with the STM. Exciting technological applications may arise if, in effect, CdS particles adhere to the probing tip: Upon applying appropriate voltage pulses, sticking particles might be redeposited onto the substrate surface in a very controlled manner in order to design artificial structures. The wet chemical method is distinguished
For preparing Ag nanoparticles using the noble gas technique, the material is evaporated in a separate preparation chamber which is filled with high purity He gas (pressure 0.4 to 1.4 mbar). In the presence of the He background, the Ag vapour cools down into a supersaturated state, entailing nucleation and growth of nanometre-sized particles that are deposited on a substrate via convection at their sticking place. Subsequently, after restoring high vacuum, the particle-carrying substrate is transferred to the STM chamber via a valve for further in-situ investigation without breaking the vacuum. By varying the evaporation rate and He pressure, the average diameter of the particles and their size distribution can be adjusted. In addition, the coverage density can conveniently be modified in a wide range upon varying the exposure time of the substrate to the Ag particle beam. Evidence for the presence of Ag has been confirmed by Auger electron spectroscopy. A detailed description of this technique is found in Ref. [121. An example for a completely covering granular film of Ag particles on the S i ( l l l ) surface is depicted in Fig. 2. Obviously, the size distribution is widened compared to particles fabricated via wet chemical methods and ranges in the present case between 3 and 20 nm (He pressure ca. 0.6 mbar, exposure time ca. 4 s). Similar to colloids, the particles are only weakly bound to the substrate. Actually, these weak (Van der Waals) binding forces allow the tip-assisted positioning of selected particles to preselected locations on the substrate surface [13]. On the other hand, the interaction between particle and substrate as well as between adjacent particles is sufficiently pronounced to resist extended STM investigations under room-temperature conditions. Technologically, the noble gas technique can be applied to fabricate sufficiently mobile particles being manipulated on substrate surfaces with nanometre precision by means of STM or SFM. In contrast to colloids which are usually coated by a stabilizing
E. Hartmann et al. / A p p l i e d Surface Science 107 (1996) 212-217
Fig. 2. Top-view STM image of a completely covering granular film on a Si substrate composed of individual Ag nanoparticles which are fabricated by means of the noble gas technique. The image covers an area of 300 n m × 300 nm and the particle sizes range between 3 and 20 nm.
layer, the noble gas technique yields nanoparticles of high purity. In addition, the evaporation procedure is not restricted to metals but can also be used to fabricate nanoparticles from semiconductors [14], semimetals and insulators, and, thus, opening a great variety of opportunities to tune the properties of nanosystems composed of individual building blocks.
3.3. Fabrication of Au dots by field-induced transfer of tip material ('local type,) For the fabrication of local deposits by field-induced transfer of tip material, Au tips are used [15]. The tunnelling probe is positioned above the Si substrate and the distance to the sample surface is controlled by choosing appropriate values for tunnel current and voltage. Subsequently, a short voltage pulse (ca. 10 V) is applied for 100-500 ns, resulting in the formation of a tiny metallic deposit. The appreciable reliability and reproducibility in creating structures by this technique is demonstrated in Fig. 3. Four almost equal Au dots are deposited on the atomically flat Si(111) surface having lateral dimensions of approximately 50 nm. However, as a result of the finite tip size, the geometric dimensions of the Au deposits fall in the 20-30 nm range (convolution effect). The height of the protrusions is assessed at ca. 9 nm. Within the 1 / x m × 1 /xm image area, the step-terrace structure of the Si(111)
215
surface is clearly resolved. Although tip material has been transferred, there is no evidence for a worsening of the imaging quality of the tunnelling probe. In the course of our studies we succeeded in the fabrication of Au dots as small as 5-10 urn in width and ca. 2 nm in height [16]. The number of Au atoms forming the deposits is of the order of 105 to 10 6 , transferred during the pulse duration of 100 ns according to a transfer rate of 1012 to 1013 at/s. This value is in striking contrast with the field-evaporation process as postulated by Mamin et al. [15]. Since, theoretical calculations predict an upper limit for the transfer rate of 10 6 a t / s [17]. Proceeding on the experiments made by Pascual et al. [18] who assume the formation of a point contact between an Au tip and an Au substrate, we systematically studied the influence of the field strength prevailing the voltage pulse by varying the distance between Au tip and the Si surface. Our observations reveal that right before deposition of Au material, triangular shaped holes are created in the substrate surface (electroetching of Si), while the current exceeds 100 nA in any case. Quite simultaneously, a nanometre-sized protrusion on the Au tip might be formed. If the tip is sufficiently close to the substrate surface, contact formation will occur. The latter effect is inferred from the fact that the current remains still high (ca. 150 nA, saturation value of the current-voltage converter) for ca. 5ms, although the duration of the voltage pulse is much less ( = 100 ns) [19]. As the experiments were carried out under feedback control, the tip is withdrawn entailing the disruption of the contact. Upon our experimental findings, we state that the dominant parameter for the material transfer process is the electric field strength between tip and substrate. The formation of
Fig. 3. Perspective representation of four almost equal Au dots (ca. 50 nm wide and ca. 9 nm high) fabricated by field-induced transfer of tip material. The step-terrace structure of the S i ( l l l ) surface is clearly resolved. The STM image covers an area of 1 /~m X 1 /Lm.
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E. Hartmann et al. /Applied Surface Science 107 (1996) 212-217
the nanometre-sized protrusion on the tip is assisted by the high current density flowing along this tiny conducting path (current-heating effect). In contrast to the two global techniques mentioned above, nanoscale structures deposited by the field-induced process can be created on preselected locations of the sample surface and the material's purity accords with that of the tip substance employed. However, exclusively dotted patterns can be generated and the deposits are strongly coupled to the substrate surface, impeding the displacement of dots by means of the STM tip. Additionally, Au deposits remain stable even up to temperatures of ca. 80°C and can repeatedly imaged without observing any changes. 3.4. E l e c t r o n - s t i m u l a t e d decomposition organometallic compounds ('local type')
of
In order to deposit metallic nanostructures on a substrate surface, an appropriate precursor gas is admitted into the STM chamber. Operating the STM in the field-emission mode (VT = 10 to 25 V), the precursor gas is decomposed by the electron flow between tip and substrate, leaving predominantly the metallic fragments on the substrate. The prevailing energy of the tip-emitted electrons is well above the typical binding energy of most gases ( < 10 eV). The technique of STM-CVD (chemical vapour deposition) was pioneered by the group around de Lozanne almost eight years ago [20]. For creating the W containing deposits shown in Fig. 4, W(CO) 6 is admitted into the STM chamber rising the pressure up to 2 × 10 . 4 mbar. The solid
Fig. 4. Creation of two W containing nanowires by electronstimulated decompositionof W(CO)6 using a voltage pulse technique (see text for details). The dimensions are 10 nm in height, 40 nm in width, and 700 nm in length. The scan range is 1 ~.mx 0.8 /xm.
source and the entire chamber are kept at ca. 80°C well above the sublimation point of W(CO) 6 of ca. 45°C. Within successive time intervals of 10 ms, voltage pulses of 15 V for 0.2 ms are applied, while the tip is moved with a constant speed of 16 n m / s . Two different stabilization currents are employed: the line on the left hand side is created at 330 pA and the line on the fight hand side at 65 pA. The dimensions of both lines are quite identical: 10 nm high, 40 nm wide and 700 nm long, implying that the 10 nm resolution is accessible. However, the nanowire manufactured with the higher current shows a more homogeneous structure. Contamination from the surroundings can be a major problem during the synthesis of nanostructures, indicating that these experiments should be performed under ultra-high vacuum conditions. In case of W(CO) 6, chemical analyses have shown that the deposits consist of a mixture of W, C and O [21]. However, under conditions of gas-flux limited reaction, which we observed as the relevant process of STM-CVD, the resultant deposition is characterized by a lower C content and is therefore more suitable for the production of low-resistance interconnects [22]. It has been demonstrated that the metal content can be as high as ca. 95% [20]. The most important fact of this technique arises from the opportunity to create continuous lines (nanowires) as compared with the above mentioned techniques, allowing us to electrically connect individual particles or artificially assembled nanosystems to the 'outside world'.
--
4. Summary
We have demonstrated the feasibility of an STM to detect, repeatedly image, and manipulate nanostructures on atomically flat Si(111) surfaces. The nanostrnctures have been fabricated using four different techniques which we classify into two local (tip-assisted) and two global (tip-independent) types. Individual deposits by means of the two 'local types', field-induced transfer of tip material and electronstimulated decomposition of organometallic compounds, can be performed on preselected locations of the surface and they couple to the substrate very strongly. In contrast, weakly bound nanostructures covering more or less the entire substrate surface can
E. Hartmann et al./Applied Surface Science 107 (1996) 212-217
be produced by applying wet chemical and noble gas t e c h n i q u e s , r e f e r r e d to as ' g l o b a l t y p e s ' . T h e elect r o n - s t i m u l a t e d d i s s o c i a t i o n o f p r e c u r s o r g a s e s is particularly distinguished in the fabrication of continuous c o n d u c t i n g n a n o w i r e s .
Acknowledgements T h e w o r k is p a r t i a l l y s u p p o r t e d b y t h e G e r m a n Bundesministerium ftir B i l d u n g , W i s s e n s c h a f t , Forschung und Technologie (BMBF), under contracts 1 3 N 6 3 2 8 a n d 1 3 N 6 3 5 3 .
References [1] R.S. Becker, J.A. Golovchenko and B.S. Schwartzentruber, Nature (London) 325 (1987) 419. [2] I.-W. Lyo and Ph. Avouris, Science 253 (1991) 173. [3] D.M. Eigler and E.K. Schweizer, Nature (London) 344 (1990) 524. [4] P. Zeppenfeld, C.P. Lutz and D.M. Eigler, Ultramicroscopy 42-44 (1992) 128. [5] J. Hu, X.-D. Xiao, R. Carpick and M. Salmeron, J. Vac. Sci. Technol. B 14, in press. [6] D.M. Schaefer, R. Reifenberger, A. Patil and R.P. Andres, Appl. Phys. Lett. 66 (1995) 1012. [7] T. Junno, K. Deppert, L. Montelius and L. Samuelson, Appl. Phys. Lett. 66 (1995) 3627.
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[8] E. Hartmann, P. Marquardt, J. Ditterich and H. Steinberger, Adv. Coll. Interf. Sci. 46 (1993) 221. [9] H.E. Hessel, A. Feltz, M. Reiter, U. Memmert and R.J. Behm, Chem. Phys. Lett. 186 (1991) 275. [10] L. Spanhel, M. Haase, H. Weller and A. Henglein, J. Am. Chem. Soc. 109 (1987) 5649. [11] M. Schwartzkopff, P. Radojkovic, M. Enachescu, E. Hartmann and F. Koch, J. Vac. Sci. Technol. B 14, in press. [12] P. Marquardt and J. Schilz, Adv. Coll. Interf. Sci. 46 (1993) 185. [13] E. Hartmann, P. Radojkovic, M. Schwartzkopff, P. Marquardt, J. Ditterich and H. Steinberger, in preparation. [14] E. Hartmann, P. Radojkovic, M. Schwartzkopff, P. Marquardt, J. Ditterich and H. Steinberger, in preparation. [15] H.J. Mamin, P.H. Guethner and D. Rugar, Phys. Rev. Lett. 65 (1990) 2418. [16] P. Radojkovic, E. Hartmann, M. Schwartzkopff, M. Enachescu, E. Stefanov and F. Koch, Surf. Sci., in press. [17] V.-T. Binh and N. Garc~a, Ultramicroscopy 42-44 (1992) 80. [18] J.I. Pascual, J. M6ndez, J. G6mez-Herrero, A.M. Bar6, N. Garcla and V.T. Binh, Phys. Rev. Lett. 71 (1993) 1852. [19] P. Radojkovic, M. Schwartzkopff and E. Hartmann, in preparation. [20] R.M. Silver, E.E. Ehrichs and A.L. de Lozanne, Appl. Phys. Lett. 51 (1987) 247; E.E. Ehrichs, W.F. Smith and A.L. de Lozanne, Ultramicroscopy 42-44 (1992) 1438. [21] H.W.P. Koops, R. Weiel, D.P. Kern and T.H. Baum, J. Vac. Sci. Technol. B 6 (1988) 477; M.A. McCord, D.P. Kern and T.H.P. Chang, J. Vac. Sci. Technol. B 6 (1988) 1877. [22] P.C. Hoyle, M. Ogasawara, J.R.A. Cleaver and H. Ahmed, Appl. Phys. Lett. 62 (1993) 3043.
Applied Surface Science 107 (1996) 212-217
Comparative scanning-tunnelling-microscopy investigations of nanostructures prepared by different techniques E. Hartmann
a, *
p. Radojkovic a M. Schwartzkopff a M. E n a c h e s c u a,1 P. Marquardt b
a Physik-Department E16, Technische Universitgtt Miinchen, D-85748 Garching, Germany b Fraunhofer-Institutf~tr Festkgrpertechnologie, Hansastr. 27d, D-80686 Mfmchen, Germany
Received 11 October 1995; accepted 23 December 1995
Abstract Conducting nanoscale structures (dots, particles and wires) are fabricated by means of four different techniques on chemically cleaned, H-terminated Si substrates and investigated using a scanning tunnelling microscope (STM) operated under high-vacuum conditions. We distinguish between STM tip-independent or 'global' techniques (formation of colloids by wet chemical methods; condensation of particles from the vapour phase in a low-pressure background noble gas) and tip-assisted or 'local' techniques (field-induced transfer of tip material; electron-stimulated decomposition of organometallic compounds). These fabrication techniques are compared and evaluated, e.g., with respect to the particle stability on the substrate surface at room temperature and the capability of manipulating individual particles, in order to address the issue of designing prototype hybrid structures by the controlled assembling of nanoparticles and electrically connecting these structures to the 'outside world' by means of an STM.
1. Introduction In the past eight years, tremendous activities have been devoted to make use of the unique techniques of scanning probe microscopy in order to manipulate matter at the nanometre or atomic scale. For instance, individual atoms [1,2] have been removed from and redeposited onto the substrate surface or arranged with the tip of a scanning tunnelling microscope (STM) in order to form desired patterns [3,4].
* Corresponding author. Tel.: +49-89-28912345; fax: +49-8928912317; e-mail: [email protected]. 1 Present address: Laboratory for Surface Science and Technology, University of Maine, 5764 Sawyer Research Center, Orono, ME 04469.
The feasibility of manipulating even larger objects at room temperature with nanometre precision has been demonstrated using a scanning force microscope (SFM) operated in ambient air [5-7]. Our efforts are dedicated to the detection, imaging, and manipulation of nanoscale structures (dots, particles and wires) on atomically flat S i ( l l l ) substrate surfaces using an STM operated at room temperature. For the preparation of these nanostructures, four different techniques are employed which, in turn, are distinguished by S T M tip-assisted or 'local' and tip-independent or 'global' deposition methods. The two local (tip-assisted) techniques include fieldinduced transfer of tip material and electron-stimulated decomposition of organometallic precursors. In particular, both are suitable to manufacture only a
0169-4332/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII SO 169-4332(96)00508-9
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E. Hartmann et al. /Applied Surface Science 107 (1996) 212-217
very small number of particles on predetermined locations of the substrate surface. In contrast, the utilization of global (tip-independent) techniques, as the ex-situ wet chemical composition of particles (colloidal solutions) and the in-situ condensation of substances from the vapour phase in the presence of a low-pressure inert background gas (noble gas technique), allows the formation of individual particles, particle agglomerations, or even completely covering granular films. We are aiming at the elaboration and evaluation of these techniques for future technological applications in the light of controllably assembling artificial prototype hybrid structures and electrically connecting them to the 'outside world' [8].
2. Sample and tip preparation As substrate material, commercially available ptype Si(111) wafers are employed which are usually covered with a native oxide layer. Atomically flat, H-terminated substrate surfaces are prepared by a wet chemical treatment in a weakly alkaline 40% NH4F solution, resulting in a strongly anisotropic corrosion of di- and tri-hydride Si atoms ('step-flow' etch mechanism) [9]. STM imaging clearly resolves a step-terrace structure caused by a slight misalignmerit of the wafer with respect to the [111] direction. Tungsten tunnelling probes are prepared from a polycrystalline 0.7-mm-diameter wire by dc-electrochemical etching in a 2 M NaOH solution. The W tunnelling probes are used for the two global techniques and for the electron-stimulated decomposition of organometallic compounds. For the field-induced transfer of tip material, a 0.25-mm-diameter Au wire is electrochemically etched in a 2 : 1 mixture of double-deionized, filtered H 2 0 ( p = 15 M12 cm) and concentrated HC1 at a dc bias of 0.8 V. The samples are rapidly transferred via a fast load-lock system to the STM chamber. All measurements are conducted under high-vacuum conditions ( p < 5 × 10 -8 mbar) at room temperature. Constant-current ( I w = 80 pA) images are generated by applying a voltage of + 2.7 to + 3 V to the sample. In the following we content ourselves with a short description of the respective technique used for preparing the nanometre-sized structures and report
on selected results which, however, remain valid for a much greater variety of substances.
3. Results and discussion 3.1. Wet chemical ('global type')
methods:
colloidal
solutions
Semiconducting CdS particles have been composed by wet chemical methods as detailed in the literature [10]. This preparation method is distinguished to yield crystalline particles with a very sharp size distribution (monodisperse). In the present case, the CdS particles are ca. 5 nm in diameter and encapsulated with a stabilizing layer of sodium polyphosphate impeding bunching in solution. However, transmission electron microscopy (TEM) micrographs reveal the tendency of the particles to agglomerate as soon as they are deposited on suitable substrates, forming irregular two-dimensional structures with some 10 to 100 constituents. For our studies, a tiny droplet of the colloidal solution is deposited on the Si substrate surface, and the particle stabilizing fluid is immediately removed by a tissue. The analytical concentration of CdS particles amounts to 2 X 10 -4 M and determines the particle density covering the substrate surface. Fig. la displays a quasi-3D STM image showing protruding features of 5 nm in height, according to the average particle size. This provides strong evidence that the detected elevations can be attributed to single CdS particles or agglomerations which are exclusively arranged two-dimensionally, as expected from the TEM micrographs. Interestingly, no bilayer steps on the S i ( l l l ) substrate surface [9,11] could ever be resolved on specimens where Colloidal solutions have been deposited. Presumably, the stabilizing sodium polyphosphate solution is still covering the Si surface a n d / o r leads to a formation of a thin oxide layer. In addressing the issue of manipulation, we have examined the capability of structurally modifying isolated particulates of CdS particles with the STM. For these experiments, the probing tip is positioned at the edge of the two prominent protrusions of Fig. la, having lateral dimensions of roughly 30 nm in
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E. Hartmann et aL /Applied Surface Science 107 (1996) 212-217
for fabricating compound semiconductors but can also be refined to the preparation of metallic particles.
3.2. Preparation of Ag particles using the noble gas technique ('global type')
Fig. 1. (a) Qnasi-3D STM representation showing agglomerations (ca. 30 nm in diameter) of CdS nanoparticles deposited on a Si substrate. The image covers an area of 230 nm X 230 rim. The height of these protruding features amounts to 5 nm. (b) Same surface area as in (a) after traversing the tip through both agglomerations at constant height which results in the formation of V-shaped structures.
diameter (some 10 CdS particles): Subsequently, the feedback loop is interrupted and the probe is laterally traversed through the respective particulate at constant height. Afterwards, the feedback loop is activated and the same surface area is imaged again. The resulting changes are documented in Fig. lb. Evidently, in each case, a notch is formed in the central part of both agglomerations, implying that individual particles can literally be removed with the tip (their coupling to the substrate is very weak). More importantly, the removed particles could never be traced out on the substrate surface with the STM, leading to the assumption that they preferably stick to the tip material as soon as a mechanical contact between tip and particles has been established. The CdS particles as well as the V-shaped structures formed by the tip-traversing experiments resist thermal agitation and can be imaged repeatedly with the STM. Exciting technological applications may arise if, in effect, CdS particles adhere to the probing tip: Upon applying appropriate voltage pulses, sticking particles might be redeposited onto the substrate surface in a very controlled manner in order to design artificial structures. The wet chemical method is distinguished
For preparing Ag nanoparticles using the noble gas technique, the material is evaporated in a separate preparation chamber which is filled with high purity He gas (pressure 0.4 to 1.4 mbar). In the presence of the He background, the Ag vapour cools down into a supersaturated state, entailing nucleation and growth of nanometre-sized particles that are deposited on a substrate via convection at their sticking place. Subsequently, after restoring high vacuum, the particle-carrying substrate is transferred to the STM chamber via a valve for further in-situ investigation without breaking the vacuum. By varying the evaporation rate and He pressure, the average diameter of the particles and their size distribution can be adjusted. In addition, the coverage density can conveniently be modified in a wide range upon varying the exposure time of the substrate to the Ag particle beam. Evidence for the presence of Ag has been confirmed by Auger electron spectroscopy. A detailed description of this technique is found in Ref. [121. An example for a completely covering granular film of Ag particles on the S i ( l l l ) surface is depicted in Fig. 2. Obviously, the size distribution is widened compared to particles fabricated via wet chemical methods and ranges in the present case between 3 and 20 nm (He pressure ca. 0.6 mbar, exposure time ca. 4 s). Similar to colloids, the particles are only weakly bound to the substrate. Actually, these weak (Van der Waals) binding forces allow the tip-assisted positioning of selected particles to preselected locations on the substrate surface [13]. On the other hand, the interaction between particle and substrate as well as between adjacent particles is sufficiently pronounced to resist extended STM investigations under room-temperature conditions. Technologically, the noble gas technique can be applied to fabricate sufficiently mobile particles being manipulated on substrate surfaces with nanometre precision by means of STM or SFM. In contrast to colloids which are usually coated by a stabilizing
E. Hartmann et al. / A p p l i e d Surface Science 107 (1996) 212-217
Fig. 2. Top-view STM image of a completely covering granular film on a Si substrate composed of individual Ag nanoparticles which are fabricated by means of the noble gas technique. The image covers an area of 300 n m × 300 nm and the particle sizes range between 3 and 20 nm.
layer, the noble gas technique yields nanoparticles of high purity. In addition, the evaporation procedure is not restricted to metals but can also be used to fabricate nanoparticles from semiconductors [14], semimetals and insulators, and, thus, opening a great variety of opportunities to tune the properties of nanosystems composed of individual building blocks.
3.3. Fabrication of Au dots by field-induced transfer of tip material ('local type,) For the fabrication of local deposits by field-induced transfer of tip material, Au tips are used [15]. The tunnelling probe is positioned above the Si substrate and the distance to the sample surface is controlled by choosing appropriate values for tunnel current and voltage. Subsequently, a short voltage pulse (ca. 10 V) is applied for 100-500 ns, resulting in the formation of a tiny metallic deposit. The appreciable reliability and reproducibility in creating structures by this technique is demonstrated in Fig. 3. Four almost equal Au dots are deposited on the atomically flat Si(111) surface having lateral dimensions of approximately 50 nm. However, as a result of the finite tip size, the geometric dimensions of the Au deposits fall in the 20-30 nm range (convolution effect). The height of the protrusions is assessed at ca. 9 nm. Within the 1 / x m × 1 /xm image area, the step-terrace structure of the Si(111)
215
surface is clearly resolved. Although tip material has been transferred, there is no evidence for a worsening of the imaging quality of the tunnelling probe. In the course of our studies we succeeded in the fabrication of Au dots as small as 5-10 urn in width and ca. 2 nm in height [16]. The number of Au atoms forming the deposits is of the order of 105 to 10 6 , transferred during the pulse duration of 100 ns according to a transfer rate of 1012 to 1013 at/s. This value is in striking contrast with the field-evaporation process as postulated by Mamin et al. [15]. Since, theoretical calculations predict an upper limit for the transfer rate of 10 6 a t / s [17]. Proceeding on the experiments made by Pascual et al. [18] who assume the formation of a point contact between an Au tip and an Au substrate, we systematically studied the influence of the field strength prevailing the voltage pulse by varying the distance between Au tip and the Si surface. Our observations reveal that right before deposition of Au material, triangular shaped holes are created in the substrate surface (electroetching of Si), while the current exceeds 100 nA in any case. Quite simultaneously, a nanometre-sized protrusion on the Au tip might be formed. If the tip is sufficiently close to the substrate surface, contact formation will occur. The latter effect is inferred from the fact that the current remains still high (ca. 150 nA, saturation value of the current-voltage converter) for ca. 5ms, although the duration of the voltage pulse is much less ( = 100 ns) [19]. As the experiments were carried out under feedback control, the tip is withdrawn entailing the disruption of the contact. Upon our experimental findings, we state that the dominant parameter for the material transfer process is the electric field strength between tip and substrate. The formation of
Fig. 3. Perspective representation of four almost equal Au dots (ca. 50 nm wide and ca. 9 nm high) fabricated by field-induced transfer of tip material. The step-terrace structure of the S i ( l l l ) surface is clearly resolved. The STM image covers an area of 1 /~m X 1 /Lm.
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E. Hartmann et al. /Applied Surface Science 107 (1996) 212-217
the nanometre-sized protrusion on the tip is assisted by the high current density flowing along this tiny conducting path (current-heating effect). In contrast to the two global techniques mentioned above, nanoscale structures deposited by the field-induced process can be created on preselected locations of the sample surface and the material's purity accords with that of the tip substance employed. However, exclusively dotted patterns can be generated and the deposits are strongly coupled to the substrate surface, impeding the displacement of dots by means of the STM tip. Additionally, Au deposits remain stable even up to temperatures of ca. 80°C and can repeatedly imaged without observing any changes. 3.4. E l e c t r o n - s t i m u l a t e d decomposition organometallic compounds ('local type')
of
In order to deposit metallic nanostructures on a substrate surface, an appropriate precursor gas is admitted into the STM chamber. Operating the STM in the field-emission mode (VT = 10 to 25 V), the precursor gas is decomposed by the electron flow between tip and substrate, leaving predominantly the metallic fragments on the substrate. The prevailing energy of the tip-emitted electrons is well above the typical binding energy of most gases ( < 10 eV). The technique of STM-CVD (chemical vapour deposition) was pioneered by the group around de Lozanne almost eight years ago [20]. For creating the W containing deposits shown in Fig. 4, W(CO) 6 is admitted into the STM chamber rising the pressure up to 2 × 10 . 4 mbar. The solid
Fig. 4. Creation of two W containing nanowires by electronstimulated decompositionof W(CO)6 using a voltage pulse technique (see text for details). The dimensions are 10 nm in height, 40 nm in width, and 700 nm in length. The scan range is 1 ~.mx 0.8 /xm.
source and the entire chamber are kept at ca. 80°C well above the sublimation point of W(CO) 6 of ca. 45°C. Within successive time intervals of 10 ms, voltage pulses of 15 V for 0.2 ms are applied, while the tip is moved with a constant speed of 16 n m / s . Two different stabilization currents are employed: the line on the left hand side is created at 330 pA and the line on the fight hand side at 65 pA. The dimensions of both lines are quite identical: 10 nm high, 40 nm wide and 700 nm long, implying that the 10 nm resolution is accessible. However, the nanowire manufactured with the higher current shows a more homogeneous structure. Contamination from the surroundings can be a major problem during the synthesis of nanostructures, indicating that these experiments should be performed under ultra-high vacuum conditions. In case of W(CO) 6, chemical analyses have shown that the deposits consist of a mixture of W, C and O [21]. However, under conditions of gas-flux limited reaction, which we observed as the relevant process of STM-CVD, the resultant deposition is characterized by a lower C content and is therefore more suitable for the production of low-resistance interconnects [22]. It has been demonstrated that the metal content can be as high as ca. 95% [20]. The most important fact of this technique arises from the opportunity to create continuous lines (nanowires) as compared with the above mentioned techniques, allowing us to electrically connect individual particles or artificially assembled nanosystems to the 'outside world'.
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4. Summary
We have demonstrated the feasibility of an STM to detect, repeatedly image, and manipulate nanostructures on atomically flat Si(111) surfaces. The nanostrnctures have been fabricated using four different techniques which we classify into two local (tip-assisted) and two global (tip-independent) types. Individual deposits by means of the two 'local types', field-induced transfer of tip material and electronstimulated decomposition of organometallic compounds, can be performed on preselected locations of the surface and they couple to the substrate very strongly. In contrast, weakly bound nanostructures covering more or less the entire substrate surface can
E. Hartmann et al./Applied Surface Science 107 (1996) 212-217
be produced by applying wet chemical and noble gas t e c h n i q u e s , r e f e r r e d to as ' g l o b a l t y p e s ' . T h e elect r o n - s t i m u l a t e d d i s s o c i a t i o n o f p r e c u r s o r g a s e s is particularly distinguished in the fabrication of continuous c o n d u c t i n g n a n o w i r e s .
Acknowledgements T h e w o r k is p a r t i a l l y s u p p o r t e d b y t h e G e r m a n Bundesministerium ftir B i l d u n g , W i s s e n s c h a f t , Forschung und Technologie (BMBF), under contracts 1 3 N 6 3 2 8 a n d 1 3 N 6 3 5 3 .
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