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Tip loading effects on AFM-based transport measurements of metal–oxide interfaces
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2013 Nanotechnology 24 395703 (http://iopscience.iop.org/0957-4484/24/39/395703) View the table of contents for this issue, or go to the journal homepage for more
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IOP PUBLISHING
NANOTECHNOLOGY
Nanotechnology 24 (2013) 395703 (5pp)
doi:10.1088/0957-4484/24/39/395703
Tip loading effects on AFM-based transport measurements of metal–oxide interfaces Jiechang Hou1 , Baptiste Rouxel2 , Wei Qin1 , Stephen S Nonnenmann1 and Dawn A Bonnell1 1 2
Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia PA, USA Department of Materials Science and Engineering, INP Grenoble, Grenoble, France
E-mail:
[email protected]
Received 8 May 2013, in final form 7 August 2013 Published 5 September 2013 Online at stacks.iop.org/Nano/24/395703 Abstract Here we demonstrate the effects of tip loading force on the contact quality and local current–voltage character between conductive AFM tips and individual noble metal nanoparticle–strontium titanate (NP–STO) interfaces. These results show that though contact quality may improve with increased loading force, nanoparticle deformation remains negligible for loading forces in the nN–µN range. Maintaining a moderate loading force in the tens to hundreds of nN therefore enables size-dependent transport of individual NP–STO interfaces to be determined. (Some figures may appear in colour only in the online journal)
1. Introduction
contact poses significant implications for size-dependent characterization of nanostructured electronic devices, and is vital in evaluating next-generation miniaturization methodologies. Here we demonstrate the systematic effects of AFM tip loading on transport properties of individual metal–oxide interfaces. We take noble metal nanoparticle (NP)–niobium-doped SrTiO3 (NSTO) interfaces often used as a model system for the study of metal–oxide (semiconductor) contacts [15–19]. We quantify the threshold force necessary to assure an adequate electrical contact for reliable transport measurements, as well as the nanoparticle deformation with applied mechanical loads ranging from 10−9 to 10−6 N.
Oxide electronics garner intense research interest due to their ability to exhibit a broad range of nanoscale phenomena ranging from resistive switching [1, 2] to the formation of two-dimensional electron gases [3], which leads to increased functionality. The origin of these complex processes typically involves strongly correlated interactions between mechanical and electrical degrees of freedom along some critical interface [4]. Improving device functionality requires an understanding of these processes at the length scales on which they occur. Due to its spatial resolution and extreme force sensitivity in the z-direction, atomic force microscopy (AFM) has proven to be an invaluable tool in examining physical phenomena with nanometer resolution [5–7]. At nanometer length scales elastic/plastic [8], adhesive [9, 10], and frictional/wear [11] properties influence AFM measurements, presenting challenges in the collection and analysis of quantitative information [12]. Implementing contact-based techniques such as conductive atomic force microscopy (c-AFM) presents additional complications, in that electrical continuity and response signals depend on contact quality and area [13, 14]. Therefore the local electrode 0957-4484/13/395703+05$33.00
2. Experimental details To produce an atomically flat, clean substrate individual 5 mm × 5 mm × 0.5 mm 0.02 at.% Nb-doped SrTiO3 (100) single crystals (Princeton Scientific) were first cleaned in acetone and ethanol consecutively, then annealed in air at 1000 ◦ C for 1 h [19, 20]. Deposition of both gold nanoparticles (AuNP) and platinum nanoparticles (PtNP) involved drop casting commercial citrate-stabilized colloidal solutions (British Biocell) of various diameters 1
c 2013 IOP Publishing Ltd Printed in the UK & the USA
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Figure 1. (a) A high-resolution TEM image of an individual AuNP–STO interface showing a clean, discrete interface after annealing; (b) a schematic illustration of the current–voltage (I–V) measurement configuration. The tip is placed above individual noble metal nanoparticles using piezopositioners and held in constant contact under continuous feedback. The setpoint voltage is then varied to alter the tip loading force.
(20/40/60/100/150 nm) onto the substrates at 110 ◦ C using a hot plate. Samples were subsequently annealed in air at 950 ◦ C for 1 h to remove organics and form an intimate contact between the noble metal nanoparticle and NSTO, as shown by the high-resolution TEM image (figure 1(a)). Note that no extraneous phases are observed at the interface and the metal and oxide meet with atomic dimensions. The STO lattice planes extend to the metal interface within a unit cell dimension. To ensure an Ohmic back contact, a 100 nm aluminum thin film was deposited on the reverse side via thermal evaporation. Samples were then finally silver-pasted (Ted Pella LeitsilberTM ) onto a metallic AFM mount puck. All AFM measurements were performed under ambient conditions within an enclosure equipped to isolate the sample from acoustic, vibrational, and thermal drift effects. Sample topography was collected using non-contact, AC tapping mode (Asylum MFP-3D AFM) with Ti/Pt-coated tips (Olympus Electrilever AC240TM). The topographic images were then used to characterize sample morphology, and the topographic height was used to estimate the nanoparticle size by assuming a truncated octahedral particle geometry. The error in this analysis is ≈10% due to uncertainty in the metal–substrate interface energy. Figure 1(b) illustrates the experimental configuration. Current–voltage (I–V) measurements were collected using a transimpedance amplifier module (ORCATM , Asylum Research), with the conductive tip placed in direct contact with individual nanoparticles under constant force feedback (figure 1(b)). Here a triangular bias (±10 V, f = 0.2 Hz) was applied to the metal nanoparticles while the substrate was held at virtual ground. All I–V traces were obtained over at least six cycles each. After local electrical characterization each particle was again imaged to ensure mechanical damage did not occur. The transimpedance amplifier utilizes a 2 nA current limit set point during collection of the I–V traces to protect the tip and nanoparticle from melting. As a result, the observed saturation at high currents is due to this set point, while lower currents fall under the detection limits (0.1 pA) of the amplifier. From Hall measurements the resistivity of 0.02 at.% Nb-doped STO is on the order of 100 m, consistent with previous observations.
3. Results and discussion A common characteristic of I–V traces collected via c-AFM is a near-monotonic dependence of current with applied tip loading force. When performing measurements with commercially coated conductive tips, such as the Olympus Electrilever AC240TM cantilevers used in this study, tip loading force greatly influences the overall signal quality, as the platinum coating degrades over time with continued use [14]. Therefore, a significant loading force is typically required to produce an Ohmic contact between the tip and nanoparticle. This effect is demonstrated in a sequence of I–V traces shown in figure 2, which begins (figure 2(a)) with an open-circuit response under a minimal low tip loading force of 7 nN. As the tip loading force is increased to 20 nN (figure 2(b)), a current is detected with a poor signal-to-noise ratio, with an onset voltage of approximately 5 V. A further increase in tip loading force to 33 nN (figure 2(c)) marks an improvement in overall signal quality, and a decrease in onset voltage to approximately 1.5 V. It is not until an applied loading force of 67 nN (figure 2(d)) that a clean, stable signal is obtained; here the onset voltage stabilizes at approximately 1.5 V. At this point the conductive character of the substrate is observed within the third quadrant, indicative of resistive switching at the NP–NSTO interface. Strontium titanate is well known to exhibit resistive switching behavior [1, 2] which results in hysteretic transport properties. This behavior is not observed at lower loads, indicating that below some critical tip loading force the tip–nanoparticle interface resistance dominates the measurement. Therefore, in order to probe the properties of the nanoparticle–substrate interface itself, a load sufficient to eliminate the metal(tip)–metal (nanoparticle) interface resistance is required. This raises the question of whether the required load would deform the nanoparticle and/or the interface. To characterize possible deformation, the vertical dimensions, i.e. heights, of the nanoparticles before and after load application were compared. These size differences in the AuNPs are shown in figure 3, both as a function of (a) nanoparticle size under a constant applied load of 48 nN, and (b) under various applied tip loading forces ranging from 10−9 to 10−6 N for a constant particle size of 113 nm. 2
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Figure 2. I–V traces of an individual (d = 113 nm) AuNP–STO interface under an applied tip loading force of (a) 7 nN, (b) 20 nN, (c) 33 nN, and (d) 67 nN, respectively.
Figure 3. Size variations in Au nanoparticles for (a) various diameter nanoparticles under an applied 48 nN load and (b) applied tip loading forces ranging from 10−9 to 10−6 N on a (d = 113 nm) NP; (c) the current versus applied tip loading force for a Ti/Pt-coated AFM tip in direct contact with a 0.02 at.% Nb-doped STO substrate.
There is no trend in the size differences with either particle size or load over the range of conditions tested. Here 80% of size differences were less than 2 nm and 90% of the size differences with load were less than 2 nm. In most cases deviations from zero size difference were positive, which implies that some material from the tip was added to the particle on tip–particle separation, rather than compressive plastic deformation of the nanoparticle. These results indicate that an appropriate protocol for characterizing
nanoparticle–substrate properties would be to accept only data from particles that exhibited 40 nm would result in a pressure determined solely by the tip–metal nanoparticle contact area. As such, any nanoparticles larger than the tip/metal contact area will yield nearly identical usable loading force ranges. The additional effects of wear on a conductive tip may also influence the overall loading pressure. Previously reported combined theoretical/experimental contact AFM studies [28] showed that radii of new ultra-sharp tips wear down to values within approximately one order of magnitude of the original value (r > 10 nm). Extracting precise applied tip pressures from repeated approach–retract cycles therefore requires careful, exhaustive observation of tip size, shape, and geometry after each contact measurement via high-resolution SEM. Taking the results of figures 2 and 3 into consideration in order to set the load to exceed the threshold the interface transport properties between Pt nanoparticles and STO substrates were characterized. Figure 4 shows the I–V traces of interfaces (PtNP–STO) with (a) 31 nm,
Tip diameter (nm)
Loading force (nN)
20 100
5–200 5–10 000
(b) 49 nm, and (c) 112 nm in diameter collected under loading forces of 101 nN, 79 nN, and 104 nN, respectively. The Schottky-like rectification and the hysteretic behavior are evident in all three interfaces. The onset voltage within the third quadrant (negative bias, negative current) distinctly shifts to more negative values with increasing size. This increased conductivity with decreasing size has also been observed in Au–STO interfaces [1, 2], and this study confirms that the interface properties dominate the measurement under the appropriate loading conditions.
4. Summary To conclude, we have demonstrated that the contact between individual metallic nanoparticle–strontium titanate interfaces and commercial AFM tips are sensitive to the effects of loading force. Though minimal forces are necessary to ensure sufficient contact (F ≥ 5 nN), larger loading forces ranging from nN to µN have minimal impact on the deformation of the NPs, as they fall below the plastic deformation limit. Therefore, tip loads on the order of tens to hundreds of nN appear to provide a sufficient range to experimentally investigate localized electrical phenomena. We envision that these results may assist the experimental design of those interested in the electrical characterization of individual nanostructures using atomic force microscopy.
Acknowledgments We thank Professor Daniel Gianola for the discussion of mechanical properties. This research was supported by the Department of Energy, Office of Basic Science on grant DE-FG02-00ER45813. Use of facilities in the Nano/Bio Interface Center under grant DMR08-32802 is acknowledged. 4
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