NANO LETTERS
Ferroelectric Lithography: Bottom-up Assembly and Electrical Performance of a Single Metallic Nanowire
2009 Vol. 9, No. 2 763-768
Alexander Haussmann,† Peter Milde,† Christiane Erler,‡ and Lukas M. Eng*,† Institut fu¨r Angewandte Photophysik, TU Dresden, Dresden, Germany, Max-Bergmann-Zentrum fu¨r Biomaterialien, Dresden, Germany Received November 7, 2008; Revised Manuscript Received December 15, 2008
ABSTRACT We report on both the assembly of noble-metal nanowires by means of the nanotechnological and large-scale integrable approach of ferroelectric lithography and their performance testing upon electrical transport. Our results on LiNbO3 single crystal templates show that the deposition of different elemental metals from ionic solutions by photochemical reduction is confined to the ferroelectric 180° domain walls. Current-voltagecharacteristics recorded from such nanowires of typically 30-300 µm in length revealed an Ohmic behavior that even improved with time. Additionally, we also examined the local topographic and potentiostatic properties of such wires using dynamic scanning force microscopy in combination with Kelvin probe force microscopy.
The potential to assemble dissimilar atomic and molecular structures on a template incorporating a high degree of functionality has triggered huge activities both in top-down and bottom-up technologies. On the search of novel technological approaches for integrating nanosystems with prospective (opto)electronic, optical, and/or chemical properties, we witness progress in nanotube1 and graphene2 research, as well as activities on tuning metallic nanojunctions for single molecular electronic applications and other fields with a gap control on the sub-10 nm range.3 Assembling nanowires so far suffers from the fact that the spontaneous and controlled growth, for instance, as known from carbon, tungsten, or silicon nanotubes, is limited to certain materials. Moreover, the main focus on nanowire electronics is concentrated on attempts in assembling predefined and presynthesized tubes and wires with theoretically predicted properties into a technologically feasible environment where the main issue lies in keeping the electronic injection losses to the contact leads low. In most such cases, the theoretical expectations in assembling a multifunctional device have not been met.4 Also, little has been reported on using template structures such as vicinal surfaces5 or other discontinuities serving as rails for the real in situ bottom-up assembling of different nanotechnological structures. Ferroelectric lithography (FE-litho) constitutes a novel lithographic method that bears the potential of controlling * To whom correspondence should be
[email protected]. † Institut fu¨r Angewandte Photophysik. ‡ Max-Bergmann-Zentrum fu¨r Biomaterialien. 10.1021/nl8033784 CCC: $40.75 Published on Web 01/21/2009
addressed.
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2009 American Chemical Society
surface assembly processes with nanoscale precision, but moreover being applicable over practically infinite lateral sample sizes, for example, millimeter to centimeter. The presence of different surface charges in ferroelectrics and polar materials, which results in different surface reactivities through the variation of the surface density of states (such as band bending), offers the possibility of exploiting domainstructured ferroelectrics as templates for the assembly of various functional nanostructures. Here, we demonstrate that FE-litho has this potential and versatility by assembling and testing a functional nanoelectronic device. While previous work using FE-litho focused on the proof-of-principle by testing which materials could possibly be incorporated in order to be adsorbed to ferroelectric templates (such as noble-metal nanoparticles on barium-titanate (BTO) and lead-zirconate-titanate (PZT) serving as pin-points for attaching specific molecules,6,7 or the direct adsorption of biological viruses on PZT,8 or metallic nanowires on lithium-niobate (LNO)9), we will show here that FE-litho really bears the huge potential for assembling and testing functional nanoelectronic devices. In our experiments, defined domain wall (DW) structures were written into highly Mg-doped (5 mol%) z-cut congruent LiNbO3 (LNO) single crystals with a thickness of 300 µm by UV-assisted poling. Using a HeCd laser beam at λ ) 325 nm focused (NA ) 0.3) through liquid electrodes, we were able to write domain structures with a size in the range of few micrometers (see Figure 1a). Because of the effect of nucleation and coercive field reduction in LNO under UV illumination,10,11 domain nucleation and growth occurs only
Figure 1. (a) Domain structure writing into LiNbO3 single crystals by UV-assisted poling. A high voltage well below the nucleation field is applied between two liquid electrodes, while simultaneously focusing a UV laser beam onto one face of the LNO sample. Domain structures of the desired shape could be written by scanning the sample along the x,y-plane. (b) Photochemical reduction of noble-metal ions from aqueous saline solutions by illumination with a Hg-lamp. Elemental Ag, Au, or Pt was deposited almost exclusively at the domain wall positions due to the strong electrical stray field and defect concentration.
Figure 2. (a) PFM image of a single domain structure obtained by UV-assisted poling. The shape of the poled c– areas is influenced by the hexagonal symmetry constraint in LNO. (b) Sample topography (dynamic mode) after photochemical deposition of a Pt nanowire along the domain wall separating the c+ and c- domain areas. Height scaling ranges from 0 nm (black) to 50 nm (white).
in the vicinity of the focused laser spot when the applied electric field between the electrodes is adjusted suitably. By scanning the sample laterally, this effect can be exploited in order to establish a serial domain writing method. Subsequent to the above poling process, wirelike metallic nanostructures were deposited on the sample surface directly to the 180° DW positions by photoinduced chemical reduction of appropriate metal or metal complex ions from aqueous saline solutions (see Figure 1b) using a Hg lamp illumination. We tested aqueous solutions made up from AgNO3, HAuCl4, and Pt(NO3)2. As a result, nanowires of pure Ag, Au, and Pt were grown along the predefined 180° DWs in Mg-doped LNO. In order to investigate possible temperature influences such as a reduced activation barrier on the nanowire growth, the cuvette containing sample and solution can be heated from below (see Figure 1b). For the case of platinum deposition, Figure 2 illustrates the correlation between the previously generated domain structure and the subsequently grown metallic nanostructures. We first operated our scanning force microscope (SFM) in 764
contact mode making use of the Piezoresponse Force Microscopy (PFM) technique that is able to image the effective domain distribution in ferroelectrics.12 As shown in Figure 2a, c- domains of approximately 4 µm in diameter were written by the above-described process, clearly contrasting from the surrounding domain with c+ polarity in the z-cut LNO crystal. As shown, the domain shape is influenced to some degree by the hexagonal symmetry constraint of LNO. After photochemical reduction, a Pt nanowire was adsorbed to the domain wall that separates c- from c+ poled areas as seen in Figure 2b. The dynamic mode SFM topography (Figure 2b) clearly indicates that DW decoration is dominating, whereas only a very small amount of material was deposited within the c+ and c- domain areas. This behavior clearly contrasts the observations made for BTO and PZT,7,13 where always the whole c+ domain surface was decorated. In BTO and PZT, ferroelectric surface charges lead to an appreciable band bending within c domain areas, being bent either up or down with respect to the Fermi energy for c- and c+ domains, respectively. Illumination with Nano Lett., Vol. 9, No. 2, 2009
Figure 3. (a) Sample topography (dynamic mode) of a deposited Pt nanowire after 5 min exposure (scaling from 0 to 60 nm). (b) Sample topography (dynamic mode) of a deposited Pt nanowire after 90 min exposure (scaling from 0 to 160 nm). (c) Comparison of nanowire peak height profiles (data taken between AA and BB in a and b, respectively). (d) Comparison of nanowire mean cross section (4 µm longitudinal average over data from between AA and BB). Broadening effects from the SFM tip were not corrected in our data.
superbandgap light results in the generation of charge carrier pairs that consequently will be spatially separated close to the surface due to the strong electric field gradient made up from band bending. As a result, in c+ domains electrons will accumulate at the surface hence dramatically promoting photochemical reduction processes. For LNO the high defect concentration within the DWs will lead to higher separating fields within the UV absorption volume, since the band bending decay length is reduced due to internal screening.9 Accordingly, nanowires are expected to adsorb close to the domain wall position only; a more detailed inspection of the local electric field distribution even suggests the maximum field to be slightly shifted away from the DW into the c+ domain area. In fact, such subtle details are accessible with SFM but will be reported elsewhere. Also the complete decoration of the entire c+ domain area similar to BTO and PZT is possible for LNO from AgNO3 solution, although under a much more intense UV illumination.14 In our experiments, preference for metal deposition to the c+ area compared to the c- surface was found especially for depositions from AgNO3 or HAuCl4 solutions, though the domain wall decoration always remained dominant in these cases. Moreover, when using Pt(NO3)2 solution, only a very Nano Lett., Vol. 9, No. 2, 2009
low area decoration was observed, leading to wire structures of very high quality. Highly resolved dynamic mode SFM viewgraphs of asgrown Pt nanowires are illustrated in Figure 3. As seen, metal wires adsorb in a clustered fashion indicating that electronic transport along the wire has to proceed in a quasi-hopping type mechanism. In fact, this clustered arrangement seems to be a general outcome of FE-litho to DWs, being independent from the time of metal adsorption; as seen, the general morphology of nanowires does not change when the incubation time increases from 5 to 90 min, as shown in Figure 3a,b, respectively. However, as displayed in Figure 3c,d, the overall lateral nanowire size increases from approximately 30 to 120 nm in height. Since in noble metal photochemistry autocatalytic processes are expected,3 it is very likely that in our case only the formation of initial seed clusters is driven by the LNO surface reactivity. After this process, further accumulation of material (cluster growth) might possibly be mediated through the previously deposited metal nanoparticles serving as seeds. In this case the subbandgap illumination does contribute to the growth process as well. 765
Figure 4. Current vs time measurement of a 330 µm long Pt nanowire contacted with silver paint. Triangular voltage cycles (see inset) were applied for testing the conductivity properties. Almost ideal ohmic behavior is observed especially for slopes returning to 0 V. Conductivity stabilization and increase arise with increasing number of applied voltage cycles due to material flow in-between metallic nanoparticles initiated through electromigration.
We proceeded by connecting longer nanowires to macroscopic contact leads either using silver paint or evaporated gold film structures in order to investigate their transport properties through I-V curve and impedance mapping. Hence, we were able for the first time to assemble an electrically functional device using the method of ferroelectric lithography. As an example, Figure 4 depicts the measured current response of a 330 µm long Pt nanowire of 120 nm in height and 250 nm in width contacted with silver paint. As shown, a slow (5 mHz frequency) triangular voltage of (10 V was applied between the two contacts resulting in a wire resistivity of 250-290 kΩ. While initial potential ramping indicated a slight non-ohmic behavior due to migration and material transport in between nanoclusters within the nanowire (see first cycle), this behavior became stabilized and purely ohmic after some ten cycles, indicating even an improved conductivity. Note however, that the burn-in process did not trigger any morphological or topological changes of our nanowires, as was routinely controlled with high resolution SFM inspection. Furthermore, several nanowires show an appreciable spread in conductivity depending on subtle conditions such as the incubation time (nanowire size), ion concentration in the growth solution, temperature, spectral modifications of the illuminating setup, and material inhomogeneities. As mentioned above, the conductivity along such nanowires is dominated by electron hopping in-between clusters. To illustrate that the measured resistivity in fact stems from the nanowire itself, we performed a detailed dynamic mode SFM study by simultaneously recording both topographic and potentiostatic information on the nanometer length scale. For the latter, dynamic mode SFM was combined with Kelvin-force probe microscopy (KPFM)15 in order to read quantitative potential values from submicrometer areas. Figure 5a,c depicts the topographic and potentiostatic information for an operating 30 µm long Pt-nanowire contacted with 80 nm gold electrodes on both sides and being 766
biased with +8 V. Note that the potential drops uniformly along the nanowire (Figure 5c) with the contact leads left and right acting as electrostatic reservoirs thus serving as equi-potential surfaces. We then used the SFM tip in contact to the sample surface in order to destroy the Pt nanowire on purpose, that is, the tip was scanned almost perpendicular to the nanowire approximately 5.5 µm from the right contact while still recording the current response with the two macroscopic leads. The goal of this experiment was (a) to prove that current transport is confined to the nanowire, and (b) to quantify the spurious dark current potentially occurring between the contact leads (surface or bulk conductivity in LNO, or adsorbate conductivity). Figure 5f depicts the time-resolved resistivity measurement of such a Pt nanowire monitored during such an erosion process. While the intact nanowire shows a reproducible resistance of 12,3 MΩ, the resistance increases over more than 3 decades to beyond 8 GΩ (calculated from the noise level of the current measurement) after destroying the conductive Pt wire with the SFM tip. Several regeneration stages of the conductivity could be observed as seen in the fluctuating (flip-floplike) current response in Figure 5f, likely due to electromigration over the previously generated narrow gap and guided by the high field between the open ends. Finally, the wire becomes open circuited, no longer allowing for electron transport, and thus forming a Coulomb blockade at the local interruption. Figure 5b,d depicts this open-loop case where we mapped the new situation after erosion again using dynamic mode SFM and KPFM for topographic and potentiostatic mapping, respectively. As seen, the intersection induced by the SFM tip lies approximately 5.5 µm from the right electrode, as clearly remarkable from the KPFM image. Also note the >3 decade current drop, which shows that in fact current transport is confined to the metallic nanowire and is not due to any spurious conductivity neither arising from surface charges nor from defects within the LNO domain wall. Figure 5e finally reveals the difference plot of the two KPFM images, that is, Figure 5d minus Figure 5c. By doing so we are able to highlight those areas along the Pt nanowire only where changes in the conductivity occurred. This is very clearly seen in Figure 5e, not only supporting the current blocking fact 5.5 µm left to the right Au electrode, but furthermore also demonstrating how the electric field effectively drops along such a Pt nanowire. Note that the slope along the interesting part of the Pt nanowire in Figure 5e stems from our image processing procedure taking the difference of the two data sets (Figure 5d minus Figure 5c). Hence, a black color represents areas along the Pt nanowire where changes in the conductivity occur upon destroying the 1D-conductor (i.e., a more negative potential), while the gray shaded areas display no differences before and after manipulation. As a result, we were able to show for the first time that nanostructures assembled by FE-litho really can bear nanoelectronic functionality. This was demonstrated by measuring the conductivity of a Pt nanowire assembled through the Nano Lett., Vol. 9, No. 2, 2009
Figure 5. (a) Topography image of an ohmic Pt nanowire contacted by 80 nm thick gold electrodes evaporated through a shadow mask (wire length across the insulating groove approximately 30 µm). (b) Topography image after achieving a permanently insulating state of the wire by scratching with the SFM tip in contact mode. Some material fragments either from the wire or from the tip have been shifted toward the edges of the contact mode scan range. (c) Lateral potential distribution imaged by KPFM (data recorded simultaneously with Figure 5a after application of +8 V at the right electrode (left electrode grounded), showing a smooth potential gradient between the electrodes as well as along the wire. (d) Potential distribution (data recorded simultaneously with b) showing a sudden potential drop along the wire at the break position. After current interruption, the intact sections on both sides of the break position exhibit almost the same potential as their corresponding electrode areas. The smooth gradient inside the groove remained unchanged. (e) Differential potential image (c subtracted from d), highlighting the changes in potential distribution resulting from current interruption. (f) Current vs time measurement during contact mode scans across the wire. The previously stable current drops drastically after a substantial damage is achieved. Scanning was repeated until no more recovery processes of the conductivity were observed; thereafter, measurements b and d were carried out. The applied voltage between the two macroscopic Au contacts was kept constant at +8 V during all measurements. In panels a-e, electrode edges are marked by dotted lines for better orientation. In panels b, d, and e, the break position approximately 5.5 µm left from the right electrode is marked as well. Topographical scaling in a and b reaches from 0 to 200 nm, voltage scaling in c and d from -1 (black) to 9 V (white), in e from -4 (black) to +2 V (white). Nano Lett., Vol. 9, No. 2, 2009
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method of FE-litho at the LNO domain wall, as well as imaging potentiostatic information with submicrometer resolution by means of KPFM. Attention has been paid to demonstrate that the observed transport characteristics are indeed inherent properties of the nanowire, which could be proven by breaking up the wire on the submicrometer scale by contact mode SFM scanning. During our research, several advantages of FE litho on LNO single crystals could be worked out: (i) First, ferroelectric LNO is mechanically stable and relatively rigid, which allows the reuse of the same substrate for many deposition test series. This suitability is further supported by the fact that ferroelectric domain structures in LNO generated by UV-assisted poling show a very good long-term stability (in our samples no noticeable changes were observed over more than 2 years!). (ii) Second, photochemical reduction at the sample surface is confined to regions very close to the DWs in contrast to BTO and PZT. This implies that in LNO no nanodomains are necessary for the assembly of (at least one-dimensional, i.e., wirelike) nanostructures. Future research efforts are needed and will concentrate on the detailed understanding of photochemical reactivity at the LNO surface, especially the confinement to the DWs, as well as the nanoscale comparison between reactive zone and DW widths and positions. Acknowledgment. Financial support by the Deutsche Forschungsgemeinschaft (DFG) within the research training
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group 1401/1: “Nano- and Biotechniques for Electronic Device Packaging” and the DFG Forschergruppe FOR 520: “Ferroic functional elements” is gratefully acknowledged. Supporting Information Available: Experimental details for UV-assisted poling, photochemical reduction, piezoresponse force microscopy, and KPFM. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Korneva, G.; Haihui, Y.; Gogotsi, Y.; Halverson, D.; Friedman, G.; Bradley, J.-C.; Kornev, K. G. Nano Lett. 2005, 5, 879. (2) Stampfer, C.; Schurtenberger, E.; Molitor, F.; Gu¨ttinger, J.; Ihn, T.; Ensslin, K. Nano Lett. 2008, 8, 2378. (3) Ha¨rtling, T.; Alaverdyan, Y.; Wenzel, M. T.; Kullock, R.; Ka¨ll, M.; Eng, L. M. J. Phys. Chem. C 2008, 112, 4920. (4) Law, M.; Goldberger, J.; Yang, P. Ann. ReV. Mater. Res. 2004, 34, 83. (5) Jung, T. A.; Schlittler, R. R.; Gimzewski, J. K.; Tang, H.; Joachim, C. Science 1996, 271, 181. (6) Giocondi, J. L.; Rohrer, G. S. J. Phys. Chem. B 2001, 105, 8275. (7) Kalinin, S. V.; Bonnell, D. A.; Alvarez, T.; Lei, X.; Hu, Z.; Ferris, J. H.; Zhang, Q.; Dunn, S. Nano Lett. 2002, 2, 589. (8) Dunn, S.; Cullen, D.; Abad-Garcia, E.; Bertoni, C.; Carter, R.; Howorth, D.; Whatmore, R. W. Appl. Phys. Lett. 2004, 85, 3537. (9) Hanson, J. N.; Rodriguez, B. J.; Nemanich, R. J.; Gruverman, A. Nanotechnology 2006, 17, 4946. (10) Mu¨ller, M.; Soergel, E.; Buse, K. Appl. Phys. Lett. 2003, 83, 1824. (11) Wengler, M. C.; Heinemeyer, U.; Soergel, E.; Buse, K. J. Appl. Phys. 2005, 98, 064104. (12) Abplanalp, M.; Eng, L. M.; Gu¨nter, P. Appl. Phys. A 1998, 66, 231. (13) Dunn, S.; Tiwari, D.; Jones, P. M.; Gallardo, D. E. J. Mater. Chem. 2007, 17, 4460. (14) Dunn, S.; Tiwari, D. Appl. Phys. Lett. 2008, 93, 092905. (15) Zerweck, U.; Loppacher, C.; Otto, T.; Grafstro¨m, S.; Eng, L. M. Phys. ReV. B 2005, 71, 125424.
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Nano Lett., Vol. 9, No. 2, 2009
