Bruker Nano Confidential 1 Simultaneous Electrical and Mechanical Property Mapping at the Nanoscale with PeakForce TUNA Chunzeng Li, Ph.D. Applications Scientist Nano Surface Business, Bruker Webinar, Feb 23, 2011 Title
Transcript
Bruker Nano Confidential 1
Simultaneous Electrical and Mechanical Property Mapping at the Nanoscale with
PeakForce TUNA
Chunzeng Li, Ph.D. Applications ScientistNano Surface Business, Bruker
Webinar, Feb 23, 2011
Title
Presenter
Presentation Notes
Thank you for joining this webinar. I’m Chunzeng Li, applications scientist at Bruker, specialized on AFM. Today, I want to share with you a new technique - PeakForce TUNA that allows Simultaneous Electrical and Mechanical Property Mapping at the Nanoscale
Let me illustrate the topic with some images. These are images simultaneously taken on a lithium battery cathode material. But, for now, let’s put aside the details about the sample; and focus on what information we have got here. At the left is the height, the topography; this we can get with any AFM mode. In the middle we have Young’s modulus and adhesion, these are the mechanical properties of the sample. And at the right is a current map, one measure of the electrical property of the sample. Also, note, in this blended sample, there are soft/delicate components - polymer and nanoparticles. We are going to talk about the technique-PeakForce TUNA- that enables us to simultaneously take topography, quantitative mechanical and electrical properties measurements at the nanoscale; a technique that enables us to do all of these on challenging soft / delicate samples. I will walk you through the underlying principles, I will give you some examples to showcase its unique capabilities, and along the way compare with other alternatives. In the end, we shall see its strength.
What is TUNA anyway? I’m not talking about the Yellow-fin TUNA that some of you would like to have in your sandwich; so don’t be disappointed. What I am talking about is the AFM-based conductivity measurement technique that is sensitive enough to detect tunneling current, We call it Tunneling AFM; and TUNA in short. Keep in mind, it also measures big current (uA). Let’s do an anatomy. TUNA, as a technique, has 3 key elements: the underlying operation mode of AFM; the conductive AFM probe; the current sensor, also known as the TUNA module. Each of these elements contributes to the technique’s capabilities, but also its limitations. I think you would agree with me: Any improvement to any of these areas has the potential to improve the technique’s overall performance and applications. PeakForce TUNA (PF-TUNA) makes significant improvements to all three of these elements.
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Atomic Force Microscope Modes
Laser Diode
Photodetector
Sample
Scanner
AFM evolution centers around force control.
•1986 Contact Mode, Binnig (IBM), Quate, and Gerber
•1993 Tapping mode, Digital Instruments
•2009 PeakForce Tapping Mode, Veeco/Bruker
Presenter
Presentation Notes
Let me ask this question, which mode will you use to image a soft samples, for instance polymer? I believe most of you would choose Tapping mode. Reason is simple, it does not damage your sample. And if you have had some experience with PeakForce Tapping mode, it may be your preference. It works even better. Since it has not been around for very long, I will briefly touch how it works.
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PeakForce Tapping Mode
A force-distance curve is obtained in each tapping cycle with controlled peak force.
Presenter
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As in Tapping Mode, the tip position is modulated up and down, and the probe is intermittently tapping the surface. The difference is it operates at a much lower frequency than the cantilever resonance, 1-2kHZ. Tip movement vs time Force vs time During imaging, the feedback loop controls the maximum force on the tip (Peak Force) for each individual cycle (at point “C”), thus the name PeakForce Tapping. This direct force control allows a very small force to be applied 10s pN.
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Adhesion Image
1.8nm
High Resolution Imaging on Liquid Crystal
Star-shaped mesogen (liquid crystal) Courtesy of Domitri Ivanov @ ICSI
Presenter
Presentation Notes
Molecular resolution material property mapping for semiflexible star-shaped mesogen (liquid crystal) Domitri Ivanov, Institut de Chimie des Surfaces et Interfaces (ICSI) Ultra low force and direct force controlled imaging of liquid crystal which has 1.8 periodicity along the molecules chains and 4 nm between molecular chains. Even with a sharp tip, force control is critical for such liquid-like sample in order to achieve high resolution.
Quantitative nanomechanical information is derived from the individual force curves in the peak force tapping controlled imaging process. The precise peak force control and the flexibility to use broad range of cantilevers extend the range of the materials that are measurable quantitatively. To achieve a good and consistent data the indentation depth is usually controlled to be a few nm to 10 nm control. This is achieved by selecting cantilevers with proper stiffness. The cantilever stiffness should be comparable to the contact stiffness, defined as multiplication of the modulus and square root of the contact area. With a given cantilever, softer sample will demand larger contact area to achieve good force control for the indentation depth. The above curve illustrate how the quantitative data is calculated: Modulus: using the unloading curve with DMT model fitting. It should be noted that the modulus range, unlike scanner size or z range, is NOT a fixed limit of the system. The ranged stated here reflects the range of material Veeco has benchmark sample and verified quantitative measurement. The range can be expanded for soft materials if the tip radius is increased or on the harder side if the lever stiffness is higher than commercial available diamond probe (~200 N/m) Adhesion: the minimum force occurred at the pulling off point, this range is determined by the force detection sensitivity as a function of the cantilever stiffness. It can also be a function of the system. It should be used as a guidance rather than hard specifications. Energy dissipation: calculated by the work done in the loading-unloading hysteresis area. It should be noted that the dissipation includes material dissipation in plastic-viscoelastic deformation and work of adhesion. To separate the two components, user need to check setpoint dependence of the dissipation by smaller peak force so that work of adhesion become the dominant dissipative component. Deformation; calculated by subtracting the loading contact point and the largest penetration depth z position. Deformation is directly proportional to hardness. With known contact area, one can convert deformation into hardness. The reduced Young’s Modulus, E*, is obtained by fitting the retract curve (green line in Figure 2.5a) using the Derjaguin, Muller, Toropov (DMT) model1 given by Where Ftip is the force on the tip, Fadh is the adhesion force, R is the tip end radius and d is the tipsample separation.
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QNM QNM -- Approach to Material Approach to Material AssignmentsAssignments
Multi-component polymer blend (7 µm scan )
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PeakForce QNM, unlike phase imaging, quantitatively measures sample modulus. This allows one to identify materials at the nanoscale by comparison to the bulk moduli of the materials The real purpose of the AFM mapping is for inhomogeneous materials. This material is the same as the Cryo-polished sample of halogen free formulation for cable insulation, cited as the challenge in the previous slide. AFM was able to identify quantitative data of each component without ambiguity. Multi-component polymer blend imaged on a MultiMode 8 using PeakForce QNM. A 7 µm scan of the sample modulus is shown above. There are three different components clearly present, the light blue component (A), the darker blue component (B), and the red/black component (C). The image was then analyzed using bearing analysis to find the average modulus of each component and its proportion of the total area. This allowed the customer (undisclosed) to easily identify the exact materials in this proprietary polymer blend.
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Which mode will you use to image the conductivity of an conducting polymer?
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We asked the question which mode we will use to image polymer. And we chose Tapping mode or PeakForce Tapping mode. Now, let me ask a similar question. Which mode will you use to image the conductivity of an conducting polymer? C-AFM / TUNA that is based on Contact Mode. Do we like Contact Mode for conducting polymer. Of course not, but we have not had much choice. (TR-TUNA).
How about combining TUNA with Tapping Mode or PeakForce Tapping mode? That would overcome the problem. Let’s see what we can do; and what we can not. Of course, but would it simply work by putting the two together? Like you would not put a turtle and rabbit to work together, putting tapping mode and a TUNA module together has not worked out well. Yes, people have tried, but not been successful in getting a current image simultaneously with topography. After taking a topography image with tapping mode, the tip needs to park at select spots and to take current measurements in contact mode, for instance current-voltage curves. PeakForce Tapping Mode uses an off-resonance frequency, fro instance, 1-2 kHz, much smaller than the resonant frequency used in Tapping Mode (>50 kHz). This is important. But his does not automatically make the coupling to a TUNA module to work. It is still a challenge. But it becomes attainable. I want to give people a sense that this is a mode that is only possible with Bruker (moat): Exclusive PeakForce Tapping Mode Innovative design of PF-TUNA module
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PF-TUNA Module -why a new current amplifier design
High Bandwidth• ~15 kHz
High Gain• 6 gain settings from 100nA/V to
20pA/V
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Presentation Notes
PEAKFORCE TUNA MODULE It is important to note that the Peak Force Tapping, oscillation frequency (1 kHz-2 kHz) falls nicely between the TappingMode (<50 kHz), and Contact Mode (DC) interaction cycles. In fact, this mid-band operation is the single most important element for TUNA to work in an intermittent contact mode. In each tapping cycle, the tip is in contact with the sample only for a fraction of the cycle (10s to 100s of microseconds). The TUNA module must be able to pick up a current signal during this time period with acceptable signal-to-noise ratio. A rule of thumb is that the bandwidth of the TUNA module must be 10x greater than the tapping frequency at the chosen gain. At TappingMode frequencies this is far beyond the reach of current technology; at Peak Force Tapping speeds it is an attainable challenge. The released PF-TUNA module is engineered to have a bandwidth around 15 kHz across a range of gains from 107 V/A to 1010 V/A, with the noise below 100 fA on Cycle-Averaged Current. The PF-TUNA module has 6 gain settings (107 V/A, 108 V/A, 5 x 108 V/A, 109 V/A, 1010 V/A, 5 x 1010 V/A), adjustable through a combination of hardware and software switches. The integration of a wide range of gains on one single module eliminates the need to change modules while searching for the optimal gain to match the conductivity level of a sample, or when different gains are needed to reveal all different conductivity levels present in one single sample. It is noteworthy that the offset at each different gain setting is automatically zeroed out upon each engage, or gain change, to assure measurement accuracy. The PF-TUNA module, while designed to work with Peak Force Tapping mode, is compatible with Contact Mode, and provides equal or better noise performance in Contact Mode compared to existing TUNA modules.
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PF-TUNA Quantities
1) Peak Current C2) Contact-Averaged Current B<-->D3) Cycle-Averaged Current A<------>E
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Presentation Notes
From the current-time plot, the PeakForce TUNA algorithm extracts three measurements: 1) Peak Current, 2) Cycle-Averaged Current, and 3) Contact-Averaged Current. Peak Current is the instantaneous current at point C, coinciding with Peak Force. This corresponds to the current measured at a defined force. Peak Current may be, but is not necessarily, the maximum current, since the limited rise time (imposed by the bandwidth of the TUNA module or the resistance-capacitance of the sample) may cause a lag in the current response. Cycle-Averaged Current is the average current over one full tapping cycle, from point A to point E. This includes both the current measured while tip is in contact with the surface and while it is off the surface. Contact-Averaged Current is the average current only when tip is in contact with the surface, from the snap-in at point B to the pull-off at point D. Here are several tips for using PeakForce TUNA in the imaging mode: 1) use smaller PeakForce setpoints for soft or delicate samples; 2) PeakForce setpoint will affect all 3 reported current quantities (Peak Current, Cycle-averaged Current and Contact-averaged Current), 3) Decreasing Peak Force Tapping amplitude will increase contact time within each tapping cycle, resulting in higher Cycle-averaged Current and higher Contact-averaged Current. 4) If simultaneous mechanical properties are desired, a PeakForce setpoint sufficient to attain a few nanometers of Deformation is necessary for an accurate DMT Modulus reading.
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Lithium Ion Battery Composite Cathode
Gao Liu, LBL
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Simultaneous Morphology, Mechanical and Electrical Mapping (L333-Glue)
Height DMT Modulus Adhesion Current
+Li[Ni1/3 Mn1/3 Co1/3 ]O2
Presenter
Presentation Notes
Lithium-ion battery cathode materials often come in a composite form. Li[Ni1/3Mn1/3Co1/3]O2 (L333) is one of the most used cathode materials. Besides this active material, polyvinylidene difluoride (PVDF) was added as the binder polymer to hold L333 particles together; and acetylene black (AB) as the conductive additive to enhance electronic conductivity. PF-TUNA was used to visualize the distribution of each component, and to characterize the elastic modulus and the formation of a conductive network that is intended to connect all the L333 particles together and connect them to the current collector (in this case Al foil). From the topography (Figure 7), particle sizes measuring 3~15 µm are seen, which represent the size of L333 particles as PVDF+AB contribute little given their smaller sizes (AB: ~50 nm) and percentage. In the current image, two distinct conductivity levels can be readily seen. The less conductive regions (shown in dark purple color, encircled within the dotted green line) can be assigned to L333 particles that are not covered with AB+PVDF; this assignment is further supported by the higher modulus seen in the same location. The more conductive regions (light pink color) suggest where the top surface is covered with PVDF+AB. PVDF itself is not conductive, but becomes so when mixed with a sufficient amount of AB nanoparticles (i.e. when the nanoparticles connect with each other to form conductive networks). Those regions also showed smaller elastic modulus and smaller adhesion. The overlay of the current map (denoted by color) atop the topography (denoted by height) clearly shows which L333 particles are covered with PVDF+AB and which are not. Those uncovered particles are electrically isolated from the collecting electrode, thus do not contribute to the battery capacity and become a dead weight. On material assignments using physical properties: Analogy: to tell apart different people in a room, you can not, or you do not need the DNA of each person. You may use some of their features: Hair color Height Weight The more features you use, the more certain you are…
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Conductivity Distribution and Material Assignments (L333- Glue)
Conductivity Distribution and Material Assignments (L333- Glue)
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Conductive coverage 56%
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L333PVDF+AB
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Two distinct conductivity peaks can be assigned to the less conductive Li[Ni1/3 Mn1/3 Co1/3 ]O2 and the more conductive PVDF+AB blend. A conductive network is formed within the PVDF+AB region; with a overall coverage of 56%.
Presenter
Presentation Notes
Offline bearing analysis (Figure 8) of the current data map shows two distinct peaks corresponding to L333 (left peak) and PVDF+AB (right peak), the coverage of the conductive network is 56%.
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Conductivity vs Additive Content in Li-Ni-Co-Al Cathode
0 20 40 60 80 100
Log(Current) (a.u.)
3.2% (AB+PVDF)12.8% (AB+PVDF)24% (AB+PVDF)
His
togr
am %
The higher conductivity peak increases as more (AB+PVDF) is added, conductive network approaches full coverage with 12.8% additives. More additives (24%) likely just increase the thickness of PVDF+AB coating.
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Weight Percent of (PVDF+AB) (AB:PVDF=0.6:1)
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.u.)
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As the surface is covered with more additives, the higher conductivity peak increases, while the lower conductivity peak decreases. Error bar on average conductivity is conductivity Rq. The higher conductivity peak increases as more (AB+PVDF) is added, conductive network approaches full coverage with 12.8% PVDF+AB. Average conductivity over 50um x 50um measured by conductive AFM (Log(I)) is in good agreement with 4-point probe measurement on mm scale.
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Young’s Modulus vs Additive Content in Li-Ni-Co-Al Cathode
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Weight Percent of (PVDF+AB) (AB:PVDF=0.6:1)
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Young’s modulus decreases as more (AB+PVDF) is added, offering more tolerance for volume change during cycling. The trend change agrees with expectation.
J.-M. Tarascon and M. Armand, Nature, 414(2001):359-367.
Presenter
Presentation Notes
Young’s modulus decreases as more (AB+PVDF) is added, offering more tolerance for volume change upon cycling. The change trend agrees with expectation.
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Organic Solar Cell Donor/Acceptor Pair and Bulk Heterojunction (BHJ)
A common donor/acceptor pair used in organic solar cells: P3HT:PCBM).
The stacking of an organic bulk heterojunction solar cell
Presenter
Presentation Notes
Applications in Organic Solar Cells Organic solar cells have been regarded as a promising candidate in harvesting solar energy due to their great potential of low-cost production, light weight, and mechanical flexibility. However, their widespread adoption is hindered by the efficiency of such organic photovoltaic (OPV) devices, which are now below the threshold for commercial viability. The key component of an organic solar cell is a blend of a donor and an acceptor materials to form bi-continuous networks, named a bulk heterojunction (BHJ).4 The donor/acceptor pair can comprise of two different conjugated polymers, but more often a conjugated polymer such as poly(2-methoxy-5-(3’,7’-dimethyl-octyloxy))-p-phenylene vinylene (MDMO-PPV) or poly-3(hexylthiophene) (P3HT) as the donor and a soluble fullerene derivative such as [6,6]-phenyl C61 - butyric acid methyl ester (PCBM, a C60-derivative) as the acceptor. To fabricate a device, powders of the donor and the acceptor are dissolved in an organic solvent followed by spin casting this solution onto an indium tin oxide (ITO) coated glass substrate (a semitransparent conductive substrate). Subsequently, aluminum electrodes are deposited atop the active layer using a shadow mask and a thermal evaporator. When light is shined on the device through the ITO side, the active layer absorbs light creating excitons (bound electron-hole pairs) which are typically separated into free charges at the donor/acceptor interface (the junction). Although the active layer needs to be 100~200 nm thick to capture most of the incident light, the diffusion length of an exciton is only 10~20 nm. To yield an efficient energy conversion device, the donor and acceptor domains must be around 20 nm. The charge generation and charge transport in organic solar cells depend strongly on the nanoscale morphology and the degree of the donor and acceptor phase separation. The efficiency of the solar cell is largely determined by the morphology of the heterojunction, thus, it is crucial to probe these nanoscale properties.
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P3HT on PEDOT/ITO/Glass
2um scan, Peakforce 1nN, Bias 3V. Au-coated ScanAsyst-Air. P3HT spin coated on PEDOT/ITO/Glass, annealed at 120°C. Sample courtesy: Dr. Ngyuen of UCSB.
Height (10 nm) PeakCurrent
(300 pA) DMT Modulus (10-15 MPa)
Presenter
Presentation Notes
#F ITO-PEDOT-P3HT 120C 1nN 3V.027 EXAMPLE 1: THERMAL ANNEALING EFFECT ON P3HT THIN FILM PF-TUNA data taken on a P3HT deposited atop poly(3,4-ethylenedioxythiophene) (PEDOT) and ITO. In this example, the effect of thermal annealing on P3HT (a common donor) thin film was examined. The substrate was a glass slide coated with transparent ITO and modified by spin-coating a PEDOT layer. A thin film of P3HT was spin-cast on top in a N2-filled glove box (< 1 ppm O2 and H2O) and annealed at 120 °C. Various annealing approaches have been reported to affect the polymer ordering, resulting in changes in morphology and charge transport behavior. The taller features in topography, with associated higher modulus, may be an indication of areas with more order. It is not surprising that most of those ordered areas show higher conductivity. Some of them, however, are poorly conductive, implying there may be a poorly ordered layer underneath that act as traps. Some “hot” spots from flat areas on the surface may have some conductivity enhancing ordered structure lying underneath. Note the conductive spots have similar round shapes, suggesting ordered aggregates tend to be cylindrical.
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P3HT:PCBM on PEDOT/ITO/Glass
2um scan, Peakforce -1.5nN, Bias 2.5V. Au-coated ScanAsyst-Air.Sample courtesy: Dr. Ngyuen of UCSB.
#D PEDOT-P3HT PCBM 2500mV -1.5nN.118 Crop Historically, Tapping Mode is used to obtain topography; and to examine phase separation of the donor and acceptor with PhaseImaging; then TUNA is used to examine conductivity. This often involves mode change (from Tapping Mode to Contact Mode) and tip change, as a result, Tapping mode is looking at one location; and TUNA is looking at another location. Direct correlation between the 2 sets of data is at risk. EXAMPLE 2: P3HT:PCBM ORGANIC SOLAR CELL PF-TUNA data taken on a P3HT:PCBM bulk heterojunction solar cell with the AFM tip in place of the cathode. P3HT:PCBM thin films (~100 nm thick) were prepared by spin coating from a toluene solution of the polymers onto ITO-coated glass substrates modified with a thin PEDOT layer. PF-TUNA was used to image the P3HT:PCBM BHJ networks, and their respective domains. Variations in conductivity (Figure 6b) can be clearly seen in Cycle-averaged Current. The closer match of the workfunction of Au (tip coating) and ITO to the HOMO of P3HT (p-type) determines that the majority of the current comes from hole transport along the P3HT phase. Thus it is postulated the higher conductivity regions are P3HT rich, whereas the poorer conductive regions are PCBM rich. The conductivity through the active layer indicates the formation of vertical conductive networks. A close look of the current image also reveals fiber-like features, evidence that BHJ networks also exist laterally. Nguyen6 using conductive AFM (Contact Mode based) to image both the top surface and the cross section of the same device, revealed the nanoscale three-dimensional interpenetrating networks of P3HT and PCBM. The topography (Figure 6a) shows some granular structures that can be polymer aggregates. The adhesion map (Figure 6d) shows features measuring 10~50 nm that are quite uniformly dispersed across the whole surface, the length scale falls closely to the hypothesized exciton diffusion length of 6~20 nm. This can be a useful criterion in the further optimization of active layer formulation processes. Average conductivity over a certain scan area, and hole and electron motilities extracted from I-V curves were reported to agree with the conversion efficiency of organic solar cells in the general trend. 6 It is worth noting that, as a net-negative PeakForce was used for the imaging, the same tip could last for more than 6 hours without perceivable degradation in resolution and conductivity signal, a stark contrast to Contact Mode based conductive-AFM.
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A Reflection
“Perhaps one of the most significant practical challenges to using pcAFM is obtaining a good electrical image without causing significant damage to the sample. Patience and a willingness to sacrifice many AFM cantilevers in the name of science, are often necessary.”
- David Ginger, Materials Today, 13(2010):50-56.
Presenter
Presentation Notes
Conductive AFM has proven to be a useful tool in revealing the morphology on the nanoscale and detecting the conductivity on the same location for direct correlation. It lends unique insight into the underlying heterogeneity of organic solar cell materials or devices, and provides a nanoscale basis for understanding the interplay between its morphology and performance. However, Conductive AFM has been largely based on Contact Mode, which is not well suited for imaging polymer samples. The vertical and lateral forces involved in Contact Mode imaging inevitably cause damage to the sample and jeopardize data integrity. Conductive tips with small spring constant (~0.2 N/m) and small setpoint value are often used to minimize destruction of the polymer layer while keeping the conductive coating free from surface contamination. Even with all these measures taken, reliable traditional Contact-TUNA remains a challenge. As pointed out by Ginger, who coined photo-current AFM (pcAFM) which is derived from Contact Mode-based Conductive AFM, “Perhaps one of the most significant practical challenges to using pcAFM is obtaining a good electrical image without causing significant damage to the sample. Patience and a willingness to sacrifice many AFM cantilevers in the name of science, are often necessary.”5
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Carbon Nanotubes
Current map clearly identifies electrical connectivity of individual carbon nanotubes .
Sample courtesy of Prof. Hague, Rice University
Presenter
Presentation Notes
the topography and current map simultaneously taken with PF-TUNA of carbon nanotubes connected to conductive pads laid on an insulating SiO2/Si substrate. All the nanotubes present in the topography image are clearly seen in the current map, suggesting they are all conductive and all connected to the conductive pads. The densely packed nanoparticles are residues produced during the sample preparation. Their conductivity can not be assessed as they are not electrically connected to the conductive pads; this is shown as they are clearly not represented in the current map. However, the variation in the conductivity along the tubes may be due to their presence on, or along the tubes. It is worth noting that while the nanotubes are fragile and can be pushed about with an AFM tip (if in Contact Mode), the substrate is hard. With PF-TUNA, the SCM-PIT tip (platinum-iridium coating) endured for hours without the coating being worn off by the substrate. For comparison, the same sample was imaged with TR-TUNA. With TR-TUNA the conductivity trace was wider, and it is hypnotized that this increase is due to the lateral dithering of the probe when operating in this mode.
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Vertical Carbon Nanotube Mat
Topography (left) and current maps (right) of carbon nanotube pillar array. Image size 1μm.
Presenter
Presentation Notes
Impossible in contact mode, easily imaged in PeakForce Tapping. Reveals strong conductivity variation, possibly due to differences in nanotube capping. PF-TUNA images of a vertical multi-walled carbon nanotube mat on a conductive substrate. Seen are the end caps of the nanotubes. Initially, it was expected that all of the multi-walled nanotubes would be conductive. However, as observed in the Peak Current map (Figure 12b), this is not the case, but instead the different bundles exhibit different levels of conductivity. Two possible interpretations are that there is variability in how the nanotubes are connected to the underlying substrate, or that the capping of the tubes is affecting the measured conductivity even if the cylindrical part of the tube is conductive and base attached. The same sample was tried with Contact Mode based TUNA, however no stable images could be attained. TR-TUNA (Figure 12c, d) gave a current image that looked quite different. Several discontinued conductive spots appeared on single tubes, and this is against intuition as we expect more uniformity on each individual tube. The lateral twisting motion of the probe in the Torsional Resonance Mode is likely to be causing an intermittent electrical contact to the surface.
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Point&Shoot I-V Curves on V-CNT
Presenter
Presentation Notes
In addition to the imaging mode, PF-TUNA also measures local current-voltage (I-V) spectra using the spectroscopy mode. In order to obtain I-V spectra, the imaging scan is stopped and the tip is held in a fixed location while the sample bias is ramped up or down. In spectroscopy mode, the feedback is switched to Contact Mode, a constant deflection is maintained by the feedback loop while the sample bias is ramped. This assures tip-sample contact is fixed while I-V curve is taken. The resulting current through the sample is plotted versus the applied bias. The software can either record a single spectrum or average over multiple spectra. The higher bandwidth of the PF-TUNA module allows I-V curves to be taken at higher speeds; and it expands the bandwidth of ac based dI/dV measurements, for instance, using the “Generic Lock-in” feature offered with the Nanoscope V controller. I-V curves can also be taken using the “Point & Shoot” feature. The “Point & Shoot” feature offers the option of drawing a line or a box on an image, defining a number of points, and then the AFM tip will automatically move to those locations to capture one or multiple I-V curves at each point. While this is a powerful automation feature, it often can be more useful to “manually” choose a few spots of interest at specific regions on the sample.
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PF-TUNA vs TR-TUNA
Topography
100nm 100nm
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Current: PF TUNA shows more consistent conductivity across individual nanotubes
Presenter
Presentation Notes
While impossible in contact mode, this sample can be imaged in Torsional Resonance and PeakForce Tapping. Here is a comparison. PF TUNA is seen to have advantages in this measurement (in addition to the advantages of PeakForce Tapping in general such as auto-optimization and correlated mechanical information).
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Sturdy Support
Vibration Isolation
Table
<1ppmO2 /H2 O
Bruker AFM + MBraun Glovebox -Integrated, Turnkey
Presenter
Presentation Notes
Extremely rigid supporting structure, custom designed specifically to address the needs of AFM and mechanical properties of the glove box Integrated active vibration isolation decouples AFM mechanically from glove box Shown here: Icon with TS-140 Other available configurations: MM8 with Accurion, Edge with TS-140 or Minus-k 1-ppm Environmental Control �for Atomic Force Microscopy
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New turnkey environmental control
Presenter
Presentation Notes
The most stringent 1-ppm O2 and H2O environmental control for Atomic Force Microscopy Available for MultiMode 8, Dimension Icon, and Dimension Edge
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Comparison of AFM-based conductivity measurement techniques
PF-TUNA Contact-TUNA Tapping-TUNA TR-TUNA
Conductivity mapping Yes Yes No Yes
Minimum peak force <100 pN <10 nN <3 nN __
Quantitative Mechanical Property Mapping Yes No No No
Ease of use Yes Yes Yes No
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Concluding Remarks
Enable reliable nano-electrical imaging on soft delicate samplessuch as loosely bound nanostructures, conductive polymers
Improve imaging resolution and tip lifetime while making conductive AFM measurements easier
Enhance material assignments on the nanoscaleby making use of both quantitative nano-mechanical and nano-electrical properties
ppm environmental controlPreserve sample and measurement integrity in lithium battery and organic optoelectronic applications (solar, LED)
Presenter
Presentation Notes
Conclusion Through an innovative high-bandwidth, high-gain, low-noise current amplifier design, PeakForce TUNA couples AFM conductivity measurements with Bruker’s exclusive Peak Force Tapping™ technology. Using the unparalleled force control of PeakForce tapping, PF-TUNA enables, for the first time, current imaging on extremely soft and delicate samples, as well as, superior tip lifetime for current imaging on hard samples. In both cases, the enhanced force control improves the repeatability and resolution of conductive AFM imaging. The ScanAsyst™ algorithm, which is included with PeakForce TUNA, dramatically improves ease of operation by automatically optimizing the AFM’s scan parameters (including feedback gain settings). PF-TUNA also includes the quantitative nanomechanical property mapping suite of PeakForce QNM™, thereby providing electrical information simultaneous with topography and mechanical property information (Deformation, Adhesion, DMT Modulus, and Dissipation). Having all of these orthogonal data channels available in a single scan, brings out the PeakForce TUNA’s unique ability to correlate the different properties of the sample at the nanometer scale. This technique is complemented by Bruker’s AFM specific glove-box offering, enabling proper handling of air-sensitive materials. Both individually and combined, PeakForce TUNA and the ppm capable glove-boxes, will be useful tools in the characterization of fragile samples such as loosely bound nanostructures, organic solar cells, lithium ion batteries, fuel cells, and many others. PF-TUNA is available only on Bruker’s Dimension® Icon® and Multimode® 8 Atomic Force Microscopes. When it comes to the conductivity measurements of soft delicate samples, now, you have a different answer – PF-TUNA. And it is a better answer in a number of ways: Exquisite force control lends its gentleness, critical for soft samples; Simultaneous integrated information: Phase separation Material assignments Direct correlation: topography-mechanical-electrical, more certainty, more insights
