Metal and Ceramic

29 May 2023

AFM methodologies for quality assessment of lithium-ion battery electrodes

Introduction Lithium ion-based batteries (LIBs) are the state-of-the-art portable energy storage to power our daily life. Ever progressing advances in both miniaturization and storage capacities made it the system of choice to be applied in smartphones, laptops or even cars. However, further advances and quality control require a deep understanding of the interplay of morphology and electronic properties on a nanometer scale. This application note briefly introduces the basic principles of batteries and the analysis of lithium-ion battery materials using atomic force microscopy (AFM). A typical LIB consists of two electrodes, a separator, and an electrolyte solution, as depicted in Fig. 1 [1]. When electrochemical oxidation and reduction reactions occur in the electrode, ions move between the oxidation and reduction electrodes through the electrolyte, and electrons move between the two electrodes through the conducting wire at the same time. In this case, electrons move along an external conductor connecting the two electrodes, and thus a closed circuit is configured as a whole. In the anode, an electrochemical oxidation reaction of an electrode material occurs during discharge of a battery, and the electrode is called an anode. Conversely, discharge refers to the process of converting chemical energy in a battery into electrical energy. During the discharge of the battery, a reduction reaction of the electrode material occurs in the cathode by electrons transferred from the anode through an external circuit, which is called a cathode. A separator is positioned between the two electrodes to prevent an electrical short between the cathode and the anode, and electrolyte is an ion conductor that transfers ions between the two electrodes, and has no conductivity to electrons but only ion conductivity. Fig. 1. Schematic of lithium-ion battery. Electrodes for LIB typically contain active materials, conductive materials, and binders as shown in Fig. 2. The active material is responsible for generating electrical energy by reacting chemically with the anode material of the battery. An active material in a cathode material is called a 'cathode active material', and an active material in an anode material is called an 'anode active material'. The cathode active material in the cathode material has lithium ions and serves to provide lithium ions to the anode when charging the battery. The cathode active material affects battery capacity and power, and a combination of characteristics of each material such as lithium cobalt oxide (LCO), lithium manganese oxide (LMO), lithium cobalt aluminum (NCA), and lithium cobalt manganese (NCM) is used as necessary. In addition, the anode active material in the anode material generates electrical energy by storing and releasing lithium ions moving from the anode when the battery is discharged. In the anode active material, graphite, which is stable for the movement of lithium, is mainly used, and silicon, which can increase energy density, is added to improve the performance of the anode material. The conductive material is a material that promotes the movement of electrons between the cathode active material and the anode active material. It mainly uses carbon black (Super-p), and it plays an important role in connecting active materials to ensure electrical properties even in small amounts and improving battery performance. Finally, the binder is an adhesive material that helps the active material and the conductive material adhere well to the current collector. If a battery is repeatedly charged and discharged, its performance may be degraded due to volume change of anode material, such as battery life and charging time, and this problem can be supplemented by a binder that improves the binding strength of electrode materials.   Fig. 2. Cross-sectioned pristine cathode sample (a) CCD vision image, (b) a schematic diagram As mentioned earlier, the voltage and capacity of the battery are determined by the type of active material, and the cycling stability of LIB is directly related to the integrity of the electrode. For example, silicon was introduced as a promising anode material that can provide 10 times higher capacity than a conventional graphite anode [2]. However, silicon anodes exhibit huge volume changes during the charge-discharge processes of LIBs [3]. The stress induced by volume change can cause anode fracture and delamination. In addition, cathode materials were found to experience structural reconstruction and chemical evolution at the surface which directly influences the functionalities of cathode materials [4,5]. As a result, the formation of microcracks, porosity, and fragmented pieces leads to voltage decay and capacity loss during cycling. Therefore, it is crucial to investigate local electronic properties as well as changes to the morphology to gain a better understanding of the failure mechanism of LIBs. In this application note, we demonstrate how AFM can serve as a versatile platform to provide correlative information of the topography as well as key parameters like the work function locally. In the past few years, various analytic methods such as scanning electron microscope (SEM), transmission electron microscope (TEM), and AFM have been intensively employed in research and development of LIBs. Although SEM and TEM measurements can provide quick results, sample analysis has limitations when using conventional electron beam-based imaging methods. Depending on the properties of interest, experiments should be carried out under a variety of conditions and parameters. Hence, a flexible and comprehensive analytical tool is required. AFM has increasingly been used for studying electrode materials for LIBs [6–9] as it allows measuring various material properties at micro-and nano-meter spatial resolutions. In particular, cathode and anode materials for LIBs can be investigated and evaluated using the AFM to give an overview of pristine material characteristics and to help control electrode fabrication. AFM is also useful in probing mechanical and electrical degradations caused by cycling of LIBs to give an insight into the degradation mechanism of electrode materials. Surface potential measurements of electrode materials have been used to detect the degradation of electrodes. It was found that the surface potential of the cathode decreased with an increasing number of charge/discharge cycles [8]. Also, a study on the electrical degradation of cathode materials has reported a reduction in electrical conductivity along with microcracks formation within particles of the active material as the number of cycles increased [6]. Our previous study introduced Park Systems’ PinPoint™ scanning spreading resistance microscopy (SSRM) measurement using a high-vacuum AFM as an effective way to simultaneously characterize the surface, mechanical, and electrical properties of LIB electrodes [9]. In-vacuum measurements are typically used for materials that are easily oxidized or hydrolyzed in ambient conditions like cycled electrodes. However, for simplicity in instrumental and experimental setups, pristine samples can be measured under ambient conditions using conventional AFMs. This application note introduces an example of measurements on a pristine cathode as a first step into the characterization and quality assessment of LIBs. Materials and Methods A Ni-based layered material LiNi0.8Co0.15Al0.05O2 (NCA) is a promising cathode material for LIBs and was investigated for this application note. The cathode slurry including NCA, conductive agent, and binder was deposited onto an Al current collector. For microscopic measurements, a cross-sectional sample was prepared using an argon-ion cross-section polisher. The Al current collect part of the sample was electrically connected to the AFM’s sample bias line using silver paste. SEM measurements were conducted prior to AFM scanning to provide an overview of the structure and microscopic uniformity of the sample. It was measured using automatic AFM [10] (FX40, Park Systems) and is equipped with advanced technology that automatically enables superluminescent diode (SLD) beam alignment and position sensitive photo diode (PSPD) alignment on the top of the cantilever. The beam was focused within the cantilever to minimize optical interference that can cause artifacts in measurement results. The automatic tip exchanger and laser aligner help reduce labor work and human error when handling the AFM tips. By employing StepScan™ function of SmartScan software, which is data acquisition software of Park Systems’ AFM, an easy setup for automated measurements, areas of interest as well as imaging modes, and scan parameters can be programmed with a simple mouse click, freeing up the operator’s time for other tasks. A conductive material coated tip (PPP-EFM, Nanosensors) was applied to measure the surface potential across the sample surface using sideband kelvin probe force microscopy (KPFM) mode. For a quantitative KPFM measurement, the work function of the AFM tip was calibrated on a freshly-cleaved HOPG surface prior to data collection on the sample. Then, SSRM measurements were conducted to measure the conductivity and resistance of the sample locally. Here, a conductive diamond-coated tip (AD-40-AS, Adama Innovations) with a nominal spring constant of 40 N/m was used to enhance the electrical signal stability and to reduce tip wear caused by lateral shear forces. Measurements were conducted at three randomly selected positions on the sample to check the uniformity of the cathode fabrication method. In addition, all AFM experiments were measured in the environmental control glove box because the uniformity could be reduced due to deformation of the sample when exposed to air. Fig. 3. SEM images obtained on cross-sectioned pristine cathode sample. Fig. 3 presents SEM images taken on the cross-sectioned cathode sample. The cathode thickness was determined to be about 80 ± 10 μm. (The thickness of the slurry may vary depending on the loading level.) It can be clearly seen from the result that the cathode film was well-attached to the Al current collector. As shown in the data, the active material particles exhibit a relatively uniform spherical shape and high distribution density, which is expected to help enhance the electrical contact and avoid damage to the current collector during electrode fabrication. In addition, a small amount of porosity was observed (as denoted by yellow arrows), likely associated with the method of sample fabrication. However, it is expected that this small degree of porosity does not affect the overall cathode performance. Based on SEM measurements, the microscopic structure and uniformity of the electrode can be observed to provide an initial evaluation and quality control of the electrode fabrication process. Fig. 4. Topographies of the pristine and unqualified cathode. Fig. 5. KPFM results obtained at three different positions on the sample. Surface potential measurements at different locations were summarized in Fig. 5. Since KPFM is a non-contact-based technique, a KPFM measurement yields both topography and surface potential simultaneously in a single scan. Considering that different AFM tips exhibit different work function values, a potential offset between tip and sample will vary accordingly. Therefore, to compare the surface potential between different batches of samples or between samples that are prepared and characterized at different facilities, all measurements should be conducted using the same single tip, which is sometimes not possible. For comparison, it is better to quantify the measured surface potential into a work function unit. The calibration can be performed using reference materials with a known work function such as HOPG, gold, etc. From the data in Fig. 4, the work function of the NCA particle was determined to be 5.0 ± 0.2 eV, which is in good agreement with the reported theoretical value of 4.9 eV. The results of these KPFM measurements suggest that the pristine electrode sample is in good condition and is ready to go to a battery assembly line. In addition, the comparative data in Fig. 4 shows that KPFM can reveal more details on the sample characteristics which are difficult to observe by solely inspecting the surface topography. Fig. 6. SSRM results obtained at three different positions on the sample. A DC bias of 1V was applied to the current collector. Fig. 6 shows SSRM measurements obtained on the sample. A similar level of current was observed regardless of measurement positions indicating a uniformly distributed conductivity across the cathode film. Although the applied bias to the current collector was relatively small, a sufficient amount of current was measured at the cathode film. This suggests that the prepared cathode sample exhibit high electrical conductivity which is advantageous in improving the performance of LIBs [6]. From SEM and AFM measurements, it can be concluded that the pristine cathode sample has a uniform structure and good electrical characteristics which are preferred for LIBs with prolonged lifetime. Additionally, due to the characteristics of battery samples, it is recommended to measure with PinPoint SSRM mode because the active material may be removed or broken if measured with general SSRM mode. PinPoint electrical measurements can simultaneously measure terrain, electrical and mechanical properties and prevent tip wear and sample damage by lateral forces after contact. Conclusion In this application note, a pristine cathode for LIBs was prepared and characterized. In particular, the structure, topographical, and electrical properties of the sample were measured and interpreted to provide a better understanding of the relationship between electrode characteristics and battery performance. The approach used in this work suggests an example of how to utilize the AFM to inspect and assess the quality of the electrodes for LIBs and therefore provide a pathway for future battery research and development. In addition to the techniques introduced in this work, various AFM modes are also available for intensive battery characterization, establishing AFM powerful tool for battery research. Such modes include: - Conductive AFM mode to measure current distribution and conductance change of the electrode during battery cycling. - PinPoint electrical modes to simultaneously measure the mechanical and electrical properties of the electrode. - Electrochemical AFM for in-situ measurement of the electrode surface changes due to electrochemical processes in the battery. - Environment control options: AFM measurements are available in different conditions such as high vacuum, in gas and liquid, and in controlled temperature and humidity conditions. Accordingly, AFM provides a versatile platform to provide correlative information on the nanometer scale. Reference 1 . B. Goodenough, K.-S. Park, The Li-Ion Rechargeable Battery: A Perspective, J. Am. Chem. Soc. 135 (2013) 1167–1176. 2. U. Kasavajjula, C. Wang, A.J. Appleby, Nano- and bulk-silicon-based insertion anodes for lithium-ion secondary cells, J. Power Sources. 163 ( 2007) 1003–1039. 3. L.Y. Beaulieu, T.D. Hatchard, A. Bonakdarpour, M.D. Fleischauer, J.R. Dahn, Reaction of Li with Alloy Thin Films Studied by In Situ AFM, J . Electrochem. Soc. 150 (2003) A1457. 4. F. Lin, I.M. Markus, D. Nordlund, T.-C. Weng, M.D. Asta, H.L. Xin, M.M. Doeff, Surface reconstruction and chemical evolution o f stoichiometric layered cathode materials for lithium-ion batteries, Nat. Commun. 5 (2014) 3529. 5. J. Zheng, M. Gu, J. Xiao, P. Zuo, C. Wang, J.-G. Zhang, Corrosion/Fragmentation of Layered Composite Cathode and Related C apacity/Voltage Fading during Cycling Process, Nano Lett. 13 (2013) 3824–3830. 6. S.Y. Park, W.J. Baek, S.Y. Lee, J.A. Seo, Y.-S. Kang, M. Koh, S.H. Kim, Probing electrical degradation of cathode materials for lithium-ion b atteries with nanoscale resolution, Nano Energy. 49 (2018) 1–6. 7. W. Zhao, W. Song, L.-Z. Cheong, D. Wang, H. Li, F. Besenbacher, F. Huang, C. Shen, Beyond imaging: Applications of atomic force m icroscopy for the study of Lithium-ion batteries, Ultramicroscopy. 204 (2019) 34–48. 8. J. Wu, S. Yang, W. Cai, Z. Bi, G. Shang, J. Yao, Multi-characterization of LiCoO2 cathode films using advanced AFM-based techniques w ith high resolution, Sci. Rep. 7 (2017) 11164. 9. J.P. Pineda, C. Lee, B. Kim, K. Lee, Electrical and Mechanical Characterization of Li Ion Battery Electrode using PinPoint™ SSRM, M icros. Today. 28 (2020) 48–53. 10. Park Systems Corporation, A New Class of Atomic Force Microscope: FX40, the Automatic AFM

18 Mar 2022

Study of dental implant using Atomic force microscopy

Introduction The occurrence of tooth loss can be related to population ageing and accidents associated with road traffic, workplace, or sport activities. It is not only a cosmetic issue but also oral health and bite function of patients are affected. To help solving this issue, implant treatment has been widely adopted to repair missing teeth. Dental implants exhibit high success and survival rates over a long period of use [1]. However, there are potential risk factors remaining that may lead to implant failure. For instance, bone density, implant design, surgical technique and protocol were found to be responsible for the implant’s primary stability [2,3], and hence affect the implant recovery process. Surface characteristics of the implant screw play an important role in bone-to-implant contact (BIC) and structural remodeling of bone to accommodate the presence of an implant. A roughened implant helps enhancing the BIC rate through friction and mechanical anchorage by bone growth into the implant surface [4]. In this regard, different techniques including physical approaches (e.g., sandblasting, plasma spraying, thermal treatment) and chemical approaches (e.g., acid etching, anodizing) were introduced to control the implant surface roughness. Previous studies have found that chemical-based techniques provide higher surface roughness and friction coefficients than the physical ones [2,4], which is more advantageous in attaining an implant system with improved stability. In this study, the surface characteristics of chemically etched dental implant screws were studied using atomic force microscopy (AFM). In detail, effects of acid mixture components and etching temperature on the implant surface roughness were experimentally quantified. The outcome is expected to add more information on surface modification of dental implants and provide a general approach on how to characterize the surface roughness of a sample with AFM, especially for samples with complicated structures such as implant screws. Experimental approach and results Conventional titanium (Ti) dental implant screws were used as samples. The implant surfaces were modified based on acid etching processes. Mixtures of hydrochloric acid: nitric acid: deionized water (HCl:HNO3:H2O) with ratios of 3:3:4 and 3:5:2 (in wt. %) were prepared. Given that Ti metal exhibits different corrosion resistance in hydrochloric and nitric acids [5], the use of different mixing ratios was expected to give a better understanding on the effect of acid components on the surface roughness of the Ti implant screw. In addition, to investigate the effect of temperature on the etching result, the etching was conducted at ambient temperature and 80 °C. Etching time was set to 5 minutes for each sample. A total number of five samples including one pristine reference sample and four chemically etched samples were then employed for AFM measurement. A Park SYSTEMS FX40 was used to investigate the effects of acid ratio and etching temperature on the surface roughness of the samples. A conventional silicon probe (OMCL-AC160TS, Olympus) was used to collect topography data in True Non-ContactTM mode. Prior to the measurements, the AFM scanners were calibrated using a reference grating. Considering that the structure of dental implant screws is relatively complicated, an optical microscope integrated into the AFM was used to initially inspect the sample surface to find appropriate positions for AFM measurements. Fig. 1 shows the optical images captured at different locations including top, bottom, thread, and cut groove areas of the implant screw. The green arrows indicate good focal areas while the red ones indicate out-of-focus areas. The camera cannot fully focus on the overall surface of the top, thread, and cut groove locations. This indicates that these locations have a large variation of height, which can be an issue for AFM scanning as the Z scanner range is limited to roughly 15 μm. In contrast, the bottom location of the sample exhibits a relatively smooth surface and is an appropriate location for AFM measurements. Fig. 1. Optical images obtained at different locations on an implant screw. The screw has a length of about 13 mm and a diameter of about 4-5 mm. All images were obtained using a 10x optical microscope integrated into the AFM system Considering that the surface roughness is a function of scan size, the scan size was varied to find optimal conditions for AFM scanning. Firstly, a maximum scan size of 100 x 100 μm² was used to collect topography data over a large area. Fig. 2 presents the height image obtained on the reference sample. As shown in the height image and cross-sectional profile in Fig. 2(a), the sample surface has a wave structure with waviness spacing up to approximately 40 μm. This structure may originate from the implant manufacturing process. To measure the impact of the wave structure on the surface roughness calculation, the root-mean-square roughness (Rq) was calculated at different scan sizes, ranging from 10 x 10 μm² to 100 x 100 μm² as denoted by the masked blue boxes in the height image of Fig. 2(b). Rq was found to increase with increasing scan sizes up to 50 μm² and saturates beyond 50 μm². The results suggest that cut-off length plays an important role in roughness calculation results, depending on specific structures of the sample surface. To appropriately characterize the sample surface in this case, a minimum scan size of 50 x 50 μm² must be selected. It should be noted that in AFM, to optimize the scan time and preserve the tip apex, the scan size should be controlled at a minimum extent necessary to adequately provide required information on sample, as it plays a huge role in the effective speed of the tip movement and in some cases the tip wear. Hence, optimization of scan size is important. Fig. 2. Height image obtained on reference sample. In (a), black dotted line represents polynomial fit to cross-sectional profile. From 100 x 100 μm² scan image in (a), masked regions with different scan length from 10 to 100 μm are selected to deduce surface roughness using Park data analysis software. As examples, masked regions of 30 x 30 μm², 40 x 40 μm², and 50 x 50 μm² are denoted by blue boxes in (b). Fig. 3. 3D height images obtained on all samples. X:Y:Z ratio was set to 1:1:15. All images were plotted at same scale with matching color palette range. In Rq plot, error bar represents standard deviation of Rq obtained at three randomly chosen locations. Effects of etching temperature and acid ratio on roughness were denoted using red and green arrows, respectively   Fig. 3 summarizes the topography measurement results on the samples. For each sample, three locations on the surface were randomly selected and measured to analyze the surface roughness distribution and surface homogeneity. The scan size was set to 50 x 50 μm² for all samples. From the results in Fig. 3, a large height variation on the surface due to acid etching was observed. This effect is quantified in table 1. In general, variation of surface roughness was found to be smaller than 10 % for the samples used in this study, suggesting a relatively uniform surface and well-controlled etching process. Compared to the reference sample, surface roughness was found to increase by 11-48 % as a result of acid etching process. Also, when temperature increased from ambient to 80 °C, Rq increased by 14 % for 3:3:4 acid ratio (sample #3 compared to #1), and 11 % for 3:5:2 ratio (sample #4 compared to #2). As expected, chemical interaction between implant sample and acid mixture can be enhanced at elevated temperature, leading to increased corrosion of Ti surface [5]. In terms of the effect of acid ratio, samples with acid ratio of 3:3:4 exhibited a rougher surface than 3:5:2 ratio. In particular, Rq values of sample #1 were about 18 % larger than sample #2, and sample #3 was about 21 % larger than sample #4. Considering that Ti is resistant to oxidizing acids such as HNO3, and is corroded by reducing acids such as HCl, it is plausible that the addition of more nitric acid component into the mixture reduces the corrosion of sample surface [6], as observed on samples #2 and #4. This result suggests that Ti etching reaction can be controlled by varying the ratio of acid components in the reaction agent. Among the treated samples, sample #3 shows the most advantageous roughness to attain high BIC rate for a more stable implant. Table 1. Summary of surface roughness of all samples. Uncertainty of Rq represents 1 standard deviation. Summary In this study, the effects of acid etching conditions on surface characteristics of Ti implants were investigated at the nanoscale using AFM. The results suggest that one can tailor the surface roughness of Ti implants by controlling the etching conditions such as temperature and acid mixture. Additionally, a general guideline for surface characterization of samples with complicated structure using AFM was introduced and discussed. References 1. O. Geckili, H. Bilhan, E. Geckili, A. Cilingir, E. Mumcu, C. Bural, Evaluation of Possible Prognostic Factors for the Success, Survival, and Failure of Dental Implants, Implant Dent. 23 (2014) 44–50. 2. M.V. Dos Santos, C.N. Elias, J.H. Cavalcanti Lima, The effects of superficial roughness and design on the primary stability of dental implants., Clin. Implant Dent. Relat. Res. 13 (2011) 215–223. 3. X. Gao, M. Fraulob, G. Haïat, Biomechanical behaviours of the bone–implant interface: a review, J. R. Soc. Interface. 16 (2019) 20190259. 4. F. Butz, H. Aita, C.J. Wang, T. Ogawa, Harder and stiffer bone osseointegrated to roughened titanium., J. Dent. Res. 85 (2006) 560–565. 5. D. Prando, A. Brenna, M.V. Diamanti, S. Beretta, F. Bolzoni, M. Ormellese, M. Pedeferri, Corrosion of titanium: Part 1: aggressive environments and main forms of degradation., J. Appl. Biomater. Funct. Mater. 15 (2017) e291–e302. 6. Titanium Metals Corporation (TIMET) technical manual (https://www.timet.com/assets/local/documents/technicalmanuals/corrosion.pdf).

24 Jan 2022

Correlating the electrochemical response of 2D materials to their thickness

Marc Brunet Cabréa, Aislan Esmeraldo Paivaa, Matej Velickýb, c, d, Paula E. Colavitaa, Kim McKelveya, e a Trinity College Dublin (Ireland) b University of Manchester (United Kingdom) c Cornell University (United States) d J. Heyrovský Institute of Physical Chemistry (Czech Republic) e Victoria University of Wellington (New Zealand) This Application Note is based on a publication by Marc Brunet Cabré et al. “Electrochemical kinetics as a function of transition metal dichalcogenide thickness”, Electrochimica Acta 393, 139027 (2021), https://doi.org/10.1016/j.electacta.2021.139027 Introduction: Two-dimensional (2D) transition metal dichalcogenides (TMDCs) display unique electronic, chemical, and mechanical properties [1]. These properties facilitate the use of 2D TMDCs as active materials for electrochemical and -catalytic applications [2 – 4], where they can serve as electrodes for Li-ion batteries [5] or as electrocatalysts for the hydrogen evolution reaction (HER) [3, 6 – 9]. However, the preparation of 2D TMDCs often results in heterogeneous sample morphologies that feature flakes of various sizes and thicknesses. Here, particularly variations in the flake thickness i.e., the number of TMDC layers, can change the material’s electronic structure and electrochemistry substantially [2, 10, 11]. The correlation of certain TMDC thicknesses to their respective electrochemical activity requires real-space characterization techniques with sub-nanometer height resolution to distinguish the number of layers and highly localized electrochemical measurements. Scanning probe microscopy (SPM) can meet these requirements by combining the sub-nanometer height sensitivity of the atomic force microscopy (AFM) topography with local electrochemistry measurements available in scanning electrochemical cell microscopy (SECCM). SECCM can map the electrochemical activity with a spatial resolution of tens to hundreds of nanometers by limiting the measurement area to a nanodroplet at the end of a nanopipette, which serves as electrochemical cell [12–14]. Therefore, SECCM facilitates localized electrochemistry measurement at the area of interest, without exposing the rest of the surface to the electrolyte. The combination of SECCM data with the complementary AFM topography allows correlating the electrochemical activity with the thickness of the TMDC flakes. In our publication, we used Park Systems’ SECCM (Figure 1 A) and AFM capabilities to compare the electrochemistry of monolayers, bilayers, and trilayers of the four different 2D TMDCs MoS2, MoSe2, WS2, and WSe2 on gold. Therefore, we identified suitable imaging areas via the optical microscope of the Park NX10, analysed the electrochemical response from a [Ru(NH3)6]3+/2+ redox couple on the samples via SECCM (Figure 1 B and C) and resolved the layer numbers of the measurement spots in subsequent AFM topography measurements. This allowed the direct correlation of sample thickness with the electrochemical activity. Fig. 1. (A) Schematic of the Park NX10 experimental setup, featuring a SECCM probe and an exchangeable AFM probe head (B) Schematic of the SECCM nanopipette probe mapping the electrochemical response of the TMDC surface. (C) Schematic of the nanodroplet-based electrochemical cell (located at the end of a SECCM probe), showing the electrochemical reaction taking place at TMDCs surface, the TMDC structure, and the electrical contact with the Au substrate. Images are reproduced with permission.[16] Copyright 2021, Electrochimica Acta   Experimental: TMCD layers (MX2, M = Mo or W and X = S or Se) were mechanically exfoliated from bulk crystals and deposited onto Au substrates serving as bottom-contact for electrical connection. Optical, AFM, and SECCM measurements were performed on the NX10 by Park Systems, South Korea. AFM images were obtained in non-contact mode (NCM) with a PPP-NCHR cantilever type (force constant = 42 N/m, resonance frequency = 330 kHz, Nanosensors). For the SECCM measurements, nanopipettes from single-barrel borosilicate capillaries with 300 – 600 nm aperture radius were filled with 1 mM [Ru(NH3)6]3+ solution in 10 mM aqueous KCl. An Ag/AgCl reference electrode was inserted into the nanopipette. The nanopipette was positioned on the surface using the NX10 optics. Linear sweep voltammograms (LSV) were recorded on a predefined grid with a distance of 1.5 μm between individual measurement spots. To avoid damage to the nanopipette probe, a spectroscopy mode was used (Figure 1 B). Here, the probe retracts prior to the lateral movement between measurement spots and approaches each spot until contact between the droplet and the surface is established at a threshold current of 5 pA. In contact, LSVs were recorded by sweeping the potential between +0.2 V to -0.8 V at a speed of 1 V/s.   Results and Discussion: The optical microscopy function of the NX10 AFM was used to identify suitable measurement areas that feature TMDCs flakes with different layer numbers (1L-3L) prior to SECCM characterization, as shown in Fig. 2 A. To resolve the exact layer number of the TMDC at the SECCM measurement spots, AFM step height profiles were extracted from topography images obtained after the SECCM experiments (Fig. 2 B). The profiles of 1L/2L and 2L/3L MoSe2 are displayed in Fig. 2 C and 2 D. In agreement with previous AFM measurements on MoSe2 [15], we determined monolayer thickness of 0.81 nm and 0.78 nm. Salt residues from the SECCM allowed correlating the electrochemical response of each measurement spot to the exact layer thickness. Moreover, the diameter of the salt residue measured via AFM facilitated an estimate of the droplet/surface contact area during the SECCM measurements. Fig. 2. (A) Optical image of MoSe2 crystals with differing number of layers. (B) AFM image of a region depicted by the red rectangle in part (A). The AFM was taken after the SECCM measurements and therefore revealed the electrolyte residues at the areas of contact between the droplet and surface. (C) and (D) line profiles of 1L/2L and 2L/3L boundaries, respectively, taken from areas shown by black lines in (B). Step height was calculated as the difference of the average terrace height (dashed coloured lines). Images are reproduced with permission.[16] Copyright 2021, Electrochimica Acta LSVs on 1L, 2L, and 3L WS2 in Fig. 3 A, B, and C clearly show that the electrochemical behaviour depends on the number of TMDC layers. The [Ru(NH3)6]3+ reduction is more facile on monolayer WS2 and the onset potential increases as the number of layers increases. To quantify the observed electrochemical response, we modelled the diffusional transport of [Ru(NH3)6]3+/2+ and the electron transfer kinetics at the sample surface. Finite element method allowed to generate a simulated electrochemical response. By adjusting the kinetic parameter kmax, the simulation was matched to each individual experimental LSV, which facilitates an estimation of the apparent kmax for each layer number (kmax.nL for n L), as shown in Fig. 3 A - C for WS2 based on the corresponding AFM measurement in Fig. 3 D. We found that an increase in layer number leads to a decrease in the reaction rate, as highlighted in Fig. 3 E. In our publication, we furthermore performed the same analysis on 1L, 2L, and 3L of MoS2, MoSe2, and WSe2 to find that all investigated TMDC materials show a slowing down of the kinetics with the increasing number of layers. [16] Figure 3: (A), (B), and (C) LSVs of [Ru(NH3)6]3+ reduction on 1L, 2L, and 3L respectively from the points shown in Figure 3 B (black traces). Simulated response corresponding to the median fitted in red and simulated response corresponding to 25th and 75th percentiles of kmax fitting in blue. (D) Color-coded surface type assignment over the AFM image showing SECCM point grid on WS2. (E) Values determined from fitting simulation to experimental data for different layer thickness of WS2. Solid points correspond to each value and the dashed line is displayed to guide the eye. Images are reproduced with permission.[16] Copyright 2021, Electrochimica Acta   Conclusion: In this application note, we summarized the findings of our recent publication “Electrochemical kinetics as a function of transition metal dichalcogenide thickness”, Electrochimica Acta 393, 139027 (2021), https://doi.org/10.1016/j.electacta.2021.139027. To investigate the electrochemical response of 2D TMDC materials in dependence of their layer number we correlated the AFM sample height to a SECCM grid measurement with the outer sphere redox couple [Ru(NH3)6]3+/2+ performed on a Park NX10 AFM. We found that the electrochemical response on the different numbers of stacked layers changes significantly, with increasing reaction rates on decreasing layer numbers on all tested TMDCs.[16]   References: [1] S. Manzeli, D. Ovchinnikov, D. Pasquier, O. V. Yazyev, A. Kis, 2D transition metal dichalcogenides, Nat. Rev. Mater. 2 (2017) 17033–17048, DOI: https://doi.org/10.1038/natrevmats.2017.33 [2] M. Velický, P. S. Toth, From two-dimensional materials to their heterostructures: An electrochemist's perspective, Appl. Mater. Today 8 (2017) 68–103, DOI: https://doi.org/10.1016/j.apmt.2017.05.003 [3] X. Chia, A. Y. S. Eng, A. Ambrosi, S. M. Tan, M. Pumera, Electrochemistry of Nanostructured Layered Transition-Metal Dichalcogenides, Chem. Rev. 115 (2015) 11941–11966, DOI: https://doi.org/10.1021/acs.chemrev.5b00287 [4] M. Chhowalla, H. S. Shin, G. Eda, L. J. Li, K. P. Loh, H. Zhang, The chemistry of two-dimensional layered transition metal dichalcogenide nanosheets, Nat. Chem. 5 (2013) 263–275, DOI: https://doi.org/10.1038/nchem.1589 [5] T. Stephenson, Z. Li, B. Olsen, D. Mitlin, Lithium ion battery applications of molybdenum disulfide (MoS2) nanocomposites, Energy Environ. Sci. 7 (2014) 209–231, DOI: https://doi.org/10.1039/C3EE42591F [6] Z. Zhu, H. Yin, C. T. He, M. Al-Mamun, P. Liu, L. Jiang, Y. Zhao, Y. Wang, H. G. Yang, Z. Tang, D. Wang, X. M. Chen, H. Zhao, Ultrathin Transition Metal Dichalcogenide/3d Metal Hydroxide Hybridized Nanosheets to Enhance Hydrogen Evolution Activity, Adv. Mater. 30 (2018) 1–7, DOI: https://doi.org/10.1002/adma.201801171 [7] Q. Fu, J. Han, X. Wang, P. Xu, T. Yao, J. Zhong, W. Zhong, S. Liu, T. Gao, Z. Zhang, L. Xu, B. Song, 2D Transition Metal Dichalcogenides: Design, Modulation, and Challenges in Electrocatalysis, Adv. Mater. 33 (2020) 1907818, DOI: https://doi.org/10.1002/adma.201907818 [8] L. Lin, P. Sherrell, Y. Liu, W. Lei, S. Zhang, H. Zhang, G. G. Wallace, J. Chen, Engineered 2D Transition Metal Dichalcogenides—A Vision of Viable Hydrogen Evolution Reaction Catalysis, Adv. Energy Mater. 10 (2020) 1903870, DOI: https://doi.org/10.1002/aenm.201903870 [9] Q. Lu, Y. Yu, Q. Ma, B. Chen, H. Zhang, 2D Transition‐Metal‐Dichalcogenide‐Nanosheet‐Based Composites for Photocatalytic and Electrocatalytic Hydrogen Evolution Reactions, Adv. Mater. 28 (2016) 1917–1933, DOI: https://doi.org/10.1002/adma.201503270 [10] M. Velický, G. E. Donnelly, W. R. Hendren, S. McFarland, D. Scullion, W. J. I. Debenedetti, G. C. Correa, Y. Han, A. J. Wain, M. A. Hines, D. A. Muller, K. S. Novoselov, H. D. Abruna, R. M. Bowman, E. J. G. Santos, F. Huang, Mechanism of Gold-Assisted Exfoliation of Centimeter-Sized Transition-Metal Dichalcogenide Monolayers, ACS Nano 12 (2018) 10463–10472, DOI: https://doi.org/10.1021/acsnano.8b06101 [11] Y. Yu, S. Y. Huang, Y. Li, S. N. Steinmann, W. Yang, L. Cao, Layer-Dependent Electrocatalysis of MoS2 for Hydrogen Evolution, Nano Lett. 14 (2014) 553–558, DOI: https://doi.org/10.1021/nl403620g [12] M. E. Snowden, A. G. Güell, S. C. S. Lai, K. McKelvey, N. Ebejer, M. A. O’Connell, A. W. Colburn, P. R. Unwin, Scanning Electrochemical Cell Microscopy: Theory and Experiment for Quantitative High Resolution Spatially-Resolved Voltammetry and Simultaneous Ion-Conductance Measurements, Anal. Chem. 84 (2012) 2483–2491, DOI: https://doi.org/10.1021/ac203195h [13] N. Ebejer, A. G. Güell, S. C. S. Lai, K. McKelvey, M. E. Snowden, P. R. Unwin, Scanning Electrochemical Cell Microscopy: A Versatile Technique for Nanoscale Electrochemistry and Functional Imaging, Ann. Rev. Anal. Chem. 6 (2013) 329–351, DOI: https://doi.org/10.1146/annurev-anchem-062012-092650 [14] N. Ebejer, M. Schnippering, A. W. Colburn, M. A. Edwards, P. R. Unwin, Localized High Resolution Electrochemistry and Multifunctional Imaging: Scanning Electrochemical Cell Microscopy, Anal. Chem. 82 (2010) 9141–9145, DOI: https://doi.org/10.1021/ac102191u [15] J. C. Shaw, H. Zhou, Y. Chen, N. O. Weiss, Y. Liu, Y. Huang, X. Duan, Chemical vapor deposition growth of monolayer MoSe2 nanosheets, Nano Res. 7 (2014) 511–517, DOI: https://doi.org/10.1007/s12274-014-0417-z [16] M. Brunet Cabré, A. Esmeraldo Paiva, M. Velický, P. E. Colavita, K. McKelvey, Electrochemical kinetics as function of transition metal dichalcogenides thickness, Electrochimica Acta. 393 (2021) 139027, DOI: https://doi.org/10.1016/j.electacta.2021.139027

17 Dec 2021

MFM on Mn1.4PtSn: Investigating magnetic hosts for Anti-Skyrmions

B. E. Zuniga Cespedes1,2, A. S. Sukhanov1,3, P. Milde1, L. M. Eng1,4, I. Hermes5, A. Klasen5 1 Institute of Applied Physics, Technische Universität Dresden, D-01062 Dresden, Germany 2 Max Planck Institute for Chemical Physics of Solids, D-01187 Dresden, Germany 3 Institut für Festkörper- und Materialphysik, TU Dresden, D-01062 Dresden, Germany 4 ct.qmat: Dresden-Würzburg Cluster of Excellence – EXC 2147, TU Dresden, 01062 Dresden, Germany 5 Park Systems Europe GmbH Abstract: Skyrmions and antiskyrmions are quasiparticles based on topological spin arrangements. Due to their small size and high stability, they are studied for applications in the field of spintronics, including high density data storage and transfer, as well as for neuromorphic computing.1,2 This Application Note bases on the publications of Sukhanov et al. “Anisotropic fractal magnetic domain pattern in bulk Mn1.4PtSn”, Phys. Rev. B 102, 174447 (2020), https://doi.org/10.1103/PhysRevB.102.174447, and Zuniga Cespedes et al. “Critical sample aspect ratio and magnetic field dependence for antiskyrmion formation in Mn1.4PtSn single crystals”, Phys. Rev. B 103, 184411 (2021), https://doi.org/10.1103/PhysRevB.103.184411, that focus on the effect of geometrical confinement on the antiskyrmion formation in the tetragonal Heusler compound Mn1.4PtSn.3,4 Via magnetic force microscopy (MFM), the researchers were able to resolve a thickness dependent formation of either a fractal ferromagnetic domain pattern or a spin helix pattern serving as the template for the nucleation of antiskyrmions. Introduction: In an ever-growing digitalized world, recent estimations suggest that we already produce 2.5⋅1018 bytes of data per single day.5 This staggering amount of data calls for new data storage technologies to stem the tide of expanding information. Here, skyrmions and antiskyrmions might provide novel solutions for smaller and faster data storage with improved energy efficiency.6 Skyrmions and antiskyrmions are topological spin structures arranged in small vortices, that act as quasiparticles in the magnetic texture of certain host materials.7 Their size may vary between 100 µm and 1 nm.8 Due to their size, stability and response to external electric or magnetic fields, the application of skyrmions and antiskyrmions in spintronics is studied for high density data storage and transfer, as well as for neuromorphic computing.1,2 Antiskyrmions have been observed in some tetragonal Heusler compounds, including also Mn1.4PtSn. To locally resolve magnetic patterns on these compounds and to investigate their dependence on the sample geometry, an imaging method with nanoscale spatial resolution and high magnetic sensitivity is required. Here, magnetic force microscopy (MFM) available on all Park Systems NX research Atomic Force microscopes (AFM), enables the simultaneous real-space imaging of both surface topography and magnetic structure. MFM utilizes magnetized AFM cantilevers that allow imaging of magnetic nanostructures due to their discrimination and sensitivity towards van der Waals (vdW) and magnetic tip-sample interactions. Both vdW and magnetic forces are measured via changes in the oscillation properties of the cantilever, being reflected either in changes of the oscillation amplitude, resonance frequency or phase. To disentangle vdW from magnetic contributions, MFM uses the different distance dependence of van der Waals and magnetic force. At close proximity, van der Waals interactions dominate the tip-sample force, which allows to image sample topography with high accuracy. At larger distances of 50-500 nm, the short-ranged van der Waals interactions become minimal and magnetic forces govern the tip-sample interaction. Accordingly, MFM traces each scan line twice: the first run is used to image the sample topography, while in the second run the magnetic texture information of the sample is recorded (Figure 1). To keep the tip-sample distance constant during the second pass, the cantilever follows the previously recorded topographic profile, thus always recording magnetic interactions at the same nominal distance. The resulting phase shift of the oscillation provides information on the local magnetic polarization of the magnetic moment, and thereby resolves ferromagnetic domain patterns such as the antiskyrmion lattice that is present in the sample investigated here.   Figure 1: Dual pass MFM mode with the detection of the sample topography in the first pass and the detection of the magnetic structure in the second pass.9 Experimental: The magnetic properties of a single crystal and thin lamellas of Mn1.4PtSn were investigated using a Park Systems NX10 AFM at ambient conditions. The sample was mounted on a non-magnetic sample holder to avoid magnetic crosstalk. For MFM, a cantilever with a magnetic coating (PPP-MFMR, Nanosensors) was magnetized prior to operation and used for all experiments. MFM measurements were conducted in dual-pass scanning mode (Figure 1). In the first pass, the topography was imaged in Park’s True Non-ContactTM Mode. Subsequently, the cantilever was lifted by 100 – 150 nm and the magnetic stray field was detected via cantilever oscillation demodulation, while retracing the topographic profile (see Figure 1). Single crystals of Mn1.4PtSn were investigated that were grown by the flux method.10 Properly-grown crystals were polished along the ab plane using a focused xenon ion beam in order to create smooth surfaces, and then imaged in Figure 2.4 For investigating the thickness dependence on magnetization, suspended lamellae of different thicknesses that range from 400 nm to 10 µm were created by cutting a 100 × 100 µm² groove into the side of the single crystal (see Figure 3a).3 Results and Discussion: Previous studies have proven the existence of antiskyrmions in Mn1.4PtSn, at least in ultrathin layers thereof. 10,11 However, the room temperature MFM measurements on the polished ab plane of a bulk Mn1.4PtSn single crystal reveal no antiskyrmion lattice; instead, the formation of a fractal ferromagnetic domain pattern made up of lamellar stripes with widths up to 3 µm is witnessed (Figure 2b). Further magnifications in Figure 2c and 2d display additional smaller nested domains of an arrowhead shape and opposite orientation around domain walls, thus, introducing chirality. This chirality around domain walls is indicative of the presence of the Dzyaloshinski-Moriya interaction (DMI) – an anisotropic magnetic exchange interaction, fundamental for the formation of skyrmions and antiskyrmions. The study by Sukhanov et al. thus shows that bulk Mn1.4PtSn single crystals favor a ferromagnetic ground state that is dominated by lamellar stripe pattern, rather than antiskyrmion lattice formation.4   Figure 2: a) Sample topography b)-d) Fractal ferromagnetic domain pattern in the MFM phase of bulk crystals of Mn1.4PtSn.4 The subsequent study by Zuniga Cespedes et al. used the MFM to resolve the thickness-dependent transition from the fractal ferromagnetic domain structure in bulk Mn1.4PtSn, to a simple pattern with single rows of nested domains, as the thickness decreased towards 4.4 µm (Figure 3b-e). Below the threshold thickness at 4.4 µm, the nested domains disappeared to form uniform sinusoidal stripes that are the building blocks for nucleating spin helices (Figure 3b-e). This magnetic pattern based on spin helices may act as the prototype for antiskyrmion lattice formation when magnetic fields are applied.3   Figure 3: a) Mn1.4PtSn single crystal with groove cut along the ab plane to create a suspended thin plate with defined thickness. b- e) Magnetic patterns at varying plate thicknesses with the appearance of a sinusoidal spin helices pattern.3 Conclusion: In conclusion, room temperature MFM measurements on the Park Systems NX10 AFM revealed that, instead of antiskyrmions, bulk Mn1.4PtSn features anisotropic fractal ferromagnetic domains, indicating the presence of DMI as the prerequisite for antiskyrmion formation.4 By decreasing the layer thickness of Mn1.4PtSn to below 4.4 µm, a sinusoidal stripe pattern appears that originates from the nucleation of spin helices caused by DMI. Under applied magnetic fields this pattern then acts as the template structure for antiskyrmion nucleation.3 References: 1. Zhang, X. et al. Skyrmion-electronics: writing, deleting, reading and processing magnetic skyrmions toward spintronic applications. J. Phys. Condens. Matter 32, 143001 (2020). 2. Büttner, F., Lemesh, I. & Beach, G. S. D. Theory of isolated magnetic skyrmions: From fundamentals to room temperature applications. Sci. Rep. 8, 4464 (2018). 3. Zuniga Cespedes, B. E., Vir, P., Milde, P., Felser, C. & Eng, L. M. Critical sample aspect ratio and magnetic field dependence for antiskyrmion formation in Mn1.4PtSn single crystals. Phys. Rev. B 103, 1–7 (2021). 4. Sukhanov, A. S. et al. Anisotropic fractal magnetic domain pattern in bulk Mn1.4PtSn. Phys. Rev. B 102, 1–9 (2020). 5. Price, D. How much Data is produced every day? https://cloudtweaks.com/2015/03/how-much-data-is-produced-every-day/. 6. Karube, K. et al. Room-temperature antiskyrmions and sawtooth surface textures in a non-centrosymmetric magnet with S4 symmetry. Nat. Mater. 20, (2021). 7. Koshibae, W. & Nagaosa, N. Theory of antiskyrmions in magnets. Nat. Commun. 7, (2016). 8. Nagaosa, N. & Tokura, Y. Topological properties and dynamics of. Nat. Publ. Gr. 8, 899–911 (2013). 9. Pineda, J. P., Newcomb, C., Pascual, G., Kim, B. & Lee, K. Detection of Magnetization Reversal in Magnetic Patterned Array using Magnetic Force Microscopy. Park Syst. Appl. Note 17 (2017). 10. Vir, P. et al. Tetragonal superstructure of the antiskyrmion hosting Heusler compound Mn1. 4PtSn. Chem. Mater. 31, 5876–5880 (2019). 11. Saha, R. et al. Intrinsic stability of magnetic anti-skyrmions in the tetragonal inverse Heusler compound Mn 1.4 Pt 0.9 Pd 0.1 Sn. Nat. Commun. 10, 1–7 (2019).

12 Jul 2021

Intrinsic electrical characterization of two-dimensional transition metal dichalcogenides via scanning probe microscopy

Yuanyuan Shi1, Jill Serron1, Benjamin Groven1, Albert Minj1, Pierre Morin1 and Thomas Hantschel1 1IMEC, Kapeldreef 75, 3001 Leuven, Belgium *The following application note is based on ACS Nano publication, 2021 15, 6, 9482–9494. Publication Date: May 27, 2021.   Introduction In a conventional planar silicon field effect transistor (FET), the gate controllability becomes weaker when its lateral dimension becomes smaller than the transistor thickness, which results in adverse short-channel effects, including leakage current, saturation of the carrier mobility in the channel, channel hotcarrier degradation, and time-dependent dielectric breakdown. Hence, the transistor body thickness needs to be reduced to ensure efficient electrostatic control from the gate. Due to the atomic thickness and dangling bond-free surface of twodimensional (2D) materials, theoretical studies have shown that particularly 2D transition metal dichalcogenides (TMDs) can outperform Si as the channel material, enable the atomic-level scaling, excellent electrostatic gate control, decrease off-state power consumption and further extend Moore’s Law.[1-6] Suitable techniques to characterize the intrinsic physical and electrical properties of as-deposited 2D materials, are a key link between the quality of as-deposited 2D materials and the performance of 2D materials-based electronic devices. This link can help us to better understand, control and improve the performance of 2D materials-based devices. However, techniques for analyzing the intrinsic electrical properties of as-deposited 2D materials on the nanoscale without any transfer and patterning process are limited. In this application note, scanning probe microscopy (SPM) is used to investigate the intrinsic electrical properties of as-deposited 2D TMDs. Conductive atomic force microscope (C-AFM) is performed directly on the surface of as-grown 2D materials without any patterning. C-AFM allows correlating the electrical conductivity of as-grown 2D materials to their topography, thereby linking the electrical properties of 2D materials to their physical properties such as layer thickness, etc. With all of this, C-AFM gives us comprehensive information of as-deposited 2D materials and helps us to evaluate the impact of these intrinsic properties on 2D materials-based nanoelectronics. Experimental Details The intrinsic electrical properties of the as-grown MoS2 and WS2 layers on sapphire are evaluated by C-AFM using Pt/Ir coated Si probes (spring constant k~3 N/m, resonance frequency f0~ 75 kHz, PPP-EFM) on a Park NX-Hivac AFM under high vacuum (~10-5 Torr). The high vacuum environment can help to reduce the water layer which always exists on the sample.[4,6] The bias of the C-AFM measurement is applied to the sample chuck and the resulting current is measured through a linear current amplifier. The applied bias to collect all the C-AFM current maps is all at 1 V. Electrical contact is ensured by applying silver paint to the top and side of the samples. Results and Discussion The as-deposited MoS2 layer on on-axis cut sapphire displays a non-uniform conductance over the whole surface in the C-AFM current maps (Figure 1b), even though the topography maps in Figure 1a show a fully coalesced monolayer MoS2 with ~37% superficial crystals on top of it (named as 1.3 ML). By introducing the off-axis 1º cut sapphire as substrate, the electrical conductivity of the MoS2 layer becomes more homogeneous in line with their more uniform surface structure (Figure 1c and d). Overall, a fraction of ~83 % of monolayer MoS2 on off-axis 1º cut sapphire possesses a higher electrical conductivity, as compared to only 51% using on-axis cut sapphire.[7] The regions with lower conductivity are marked in pink in Figure 1b, d with a threshold current of ~ 0.3 μA. Figure 1. (a, c) C-AFM topography of 1.3 ML MoS2 grown on on-axis and off-axis 1º cut sapphire, respectively. (b, d) The simultaneously acquired C-AFM current maps of (a, c), respectively. Non-homogenous and poorly conductive regions in the first single layer of MoS2 are highlighted in pink by current thresholding (~ 0.3 μA). Images are reproduced with permission.[7] Copyright 2021, American Chemical Society.   Thus, the density of poorly conductive regions is reduced by introducing the off-axis 1º cut sapphire (from 49% to 17% in Figure 1b, d). This density can be further reduced to ~6.5% by skipping the pre-epi treatment process of the sapphire wafers (Figure 2a-b). The shapes of the MoS2 regions with lower conductivity are not random, but correspond to the specific underlying sapphire terraces. The regions with lower MoS2 conductivity on off-axis 1º cut sapphire correspond to the terraces that have bunched together. Figure 2. Inhomogeneous electrical conductivity of MoS2 grown on sapphire. (a-b) C-AFM topography and simultaneously obtained current map of 1.3 ML MoS2 on as-received off-axis 1º cut sapphire. (c) Corresponding cross-sectional height (red) and current (blue) profiles at the positions in (a-b), respectively. (d-e) Topography and simultaneously obtained current map of 3.5 ML MoS2 on on-axis cut sapphire. Images are adapted with permission from Ref. [7]. Copyright 2021, American Chemical Society.    During the pre-epi treatment and MOCVD process, steps decompose and agglomerate. Step (de-)formation is primarily driven by the high temperatures used in both pre-epi treatment and MOCVD process. Step bunching becomes more probable as Wterrace narrows, as expected for off-axis 1º cut sapphire. When the monolayer MoS2 is deposited on off-axis 1º cut sapphire without any pre-epi treatment, the density of highly conductive regions increases further from 83% (Figure 1d) to 93.5% (Figure 2b). A clear correlation can be observed between the bunched steps (with a higher Hterrace, 5.8% in Figure 2a) and poorly conductive regions (6.5% dark regions in Figure 2b). The extracted cross-sectional profiles from the topography and current maps in Figure 2c further support this observation. However, the poorly conductive regions are not completely removed in Figure 2b. This should be related to the growth temperature (1000 °C in our work), which is high enough to introduce step bunching on the sapphire surface during the deposition.[8-10] Regarding the observed inhomogeneity of the MoS2 conductivity distribution, we found that the presence of MoS2 crystals in the non-closed top layers does not influence the conductivity. Indeed, MoS2 regions with lower conductivity remain nearly constant with MoS2 layer thickness, since they also exist in the 3.5 ML MoS2 (Figure 2d-e): A comparison of the dashed yellow outlined regions in topography and current image shows that MoS2 crystals with a misoriented basal plane in the non-closed top layers do not influence the conductivity in that region. Moreover, it is worth to note that the existence of regions with different conductance not only occurs in the MoS2 epitaxial layer, but also in the MOCVD WS2 layer grown on on sapphire, as shown in Figure 3.   Figure 3. (a-b) Topography and simultaneously obtained current map of 1.7 ML WS2 on on-axis cut sapphire. Images are adapted with permission from Ref. [7]. Copyright 2021, American Chemical Society.   Thus, the lower conductivity relates primarily to the fully closed first MoS2 single layer rather than the nonclosed top layers. Figure 4a-b further supports this by showing two 2nd layer MoS2 crystals with some regions of higher conductivity and some regions of lower conductivity. Figure 4. Figure 4. (a-b) Electrical conductivity of 2nd-3rd layer MoS2 islands on the 1.3 ML MoS2 grown on on-axis cut sapphire. (a) Topography and its corresponding (b) current map of the MoS2 grown on on-axis cut sapphire. The white color outlines crystals show partial regions with higher conductivity and partial regions with lower conductivity, indicating that the superficial crystals do not contribute much to the inhomogeneous conductivity of the MoS2 on sapphire. (c-f) Degradation of 1.3 ML MoS2 grown on on-axis cut sapphire. (c-d) Topography and its corresponding current map at 1 V of the 1.3 ML MoS2 collected immediately after the MOCVD growth. (e-f) Topography and current map at 1 V of the same sample after 6 months storage in a N2 cabinet. No oxidized regions in (c), but the MoS2 are partially oxidized in (e), which always correlates to the poorly conductive regions in (f). Images are adapted with permission from Ref. [7]. Copyright 2021, American Chemical Society. The results indicate that the state of the sapphire starting surface is one of the key parameters determining the physical and electrical properties of the first MoS2 single layer. Conclusions The intrinsic electrical properties of 2D TMDs are evaluated via C-AFM and linked to the sample topography. We found a nonhomogenous conductivity in the as-deposited 2D TMDs single layer, which possibly originates from: (i) TMDs surface roughness from TMDs layer thickness variation; (ii) TMDs strain induced by sapphire surface topography; (iii) TMDs intra-grain defectivity due to dependence of TMDs nucleation rate per sapphire terrace; or (iv) TMDs interface defectivity induced by sapphire surface structure and termination, which could result in differently local doping effects. Further investigation is in process to combine C-AFM with advanced spectroscopy techniques (such as Raman, PL and TOFSIMS) to further explore the intrinsic properties of the epitaxial 2D materials. References (1) Liu, Y.; Duan, X.; Shin H.-J.; Park, S.; Huang, Y.; Duan, X. Promises and Prospects of Two-Dimensional Transistors. Nature 2021, 591, 43–53. (2) Su, S.-K.; Chuu, C.-P.; Li, M.-Y.; Cheng, C.-C.; Wong, H.-S. P.; Li, L.-J. Layered Semiconducting 2D Materials for Future Transistor Applications. Small Struct. 2021, 2, 2000103. (3) Akinwande, D.; Huyghebaert, C.; Wang, C.-H.; Serna, M. I.; Goossens, S.; Li, L.-J.; Wong, H.-S. P.; Koppens, F. H. L. Graphene and Two-Dimensional Materials for Silicon Technology. Nature 2019, 573, 507–518. (4) Agarwal, T.; Szabo, A.; Bardon, M. G.; Soree, B.; Radu, I.; Raghavan, P.; Luisier, M.; Dehaene, W.; Heyns, M. Benchmarking of Monolithic 3D Integrated MX2 FETs with Si FinFETs. In 2017 IEEE International Electron Devices Meeting (IEDM); 2017; p 5.7.1–5.7.4. (5) Smets, Q.; Arutchelvan, G.; Jussot, J.; Verreck, D.; Asselberghs, I.; Nalin Mehta, A.; Gaur, A.; Lin, D.; Kazzi, S. E.; Groven, B.; Caymax, M.; Radu, I. Ultra-Scaled MOCVD MoS2 MOSFETs with 42nm Contact Pitch and 250μA/Mm Drain Current. In 2019 IEEE International Electron Devices Meeting (IEDM); 2019; p 23.2.1–23.2.4. (6) Smets, Q.; Verreck, D.; Shi, Y.; Arutchelvan, G.; Groven, B.; Wu, X.; Sutar, S.; Banerjee, S.; Nalin Mehta, A.; Lin, D.; Asselberghs, I.; Radu, I. Sources of variability in scaled MoS2 FETs. In 2020 IEEE International Electron Devices Meeting (IEDM); 2020; p 3.1.1–3.1.4. (7) Shi, Y.; Groven, B.; Serron, J.; Wu, X.; Nalin Mehta, A.; Minj, A.; Sergeant, S.; Han, H.; Asselberghs, I.; Lin, D.; Brems, S.; Huyghebaert, C.; Morin, P.; Radu, I.; Caymax, M. Engineering Wafer-Scale Epitaxial Two-Dimensional Materials through Sapphire Template Screening for Advanced High- Performance Nanoelectronics. ACS Nano 2020, DOI: 10.1021/ acsnano.0c07761. (8) Cuccureddu, F.; Murphy, S.; Shvets, I. V.; Porcu, M.; Zandbergen, H. W.; Sidorov, N. S.; Bozhko, S. I. Surface Morphology of C-Plane Sapphire (α-Alumina) Produced by High Temperature Anneal. Surf. Sci. 2010, 604, 12941299. (9) Curiotto, S.; Chatain, D. Surface Morphology and Composition of C-, a- and m-Sapphire Surfaces in O2 and H2 Environments. Surf. Sci. 2009, 603, 2688–2697. (10) Ribič, P. R.; Bratina, G. Behavior of the (0001) Surface of Sapphire upon High-Temperature Annealing. Surf. Sci. 2007, 601, 44–49.

9 Jan 2020

Electric Force Microscopy of samples having an appreciable impedance

Perovskites responding to light (Credit Greg Stewart/SLAC National Accelerator Laboratory) Ryan P. Dwyer1,2, John A. Marohn1   Summary KPFM experiments are used to image capacitance and surface potential in a wide variety of samples. The widely used KPFM frequency-shift equation rests on assumptions that are questionable in samples having an appreciable impedance or properties that evolve on a fast timescale. We present new equations describing the cantilever frequency and dissipation in a KPFM experiment carried out on a sample with an appreciable stationary or time-dependent impedance, such as a photovoltaic film, a battery material, or mixed electronic-ionic conductors. Article Scanned probe microscopy (SPM) techniques that measure electrostatic forces have revealed the fate of charges in a broad array of semiconductor electronic and photovoltaic devices. The equation commonly used to interpret frequency modulated scanning Kelvin probe force microscopy (KPFM) measurements of electric forces is  where Δf is the time-dependent cantilever frequency shift , f0 is the cantilever resonance frequency, k0 is the cantilever spring constant, C″ is the second derivative of the tip-sample capacitance with respect to the vertical direction, Vts is the applied tip-sample voltage, and Φ is the sample surface potential. Recent experiments have called the validity of this equation into question. Figure 1 shows one striking example.1 The sample was a thin film of the perovskite semiconductor CsPbBr3 prepared on a conductive indium tin oxide substrate, illuminated from above with visible light (Fig. 1(a)). The cantilever frequency shift and amplitude were measured at different applied tip-sample voltages to determine the curvature of the frequency- and dissipation-versus-voltage parabolas (Fig. 1(b)). Figure 1(c) and (d) show how these parabola curvatures depend on the light intensity. According to Eq. 1, the curvature of the frequency-shift -versus-voltage parabola is proportional to the tip-sample capacitance derivative C″, which in turn should be proportional to the free carrier density. In a semiconductor, this free carrier density has a power-law-dependence on the light intensity. The capacitance data, in contrast, increases rapidly but then flattens out as the light intensity is increased. Likewise, the dissipation should increase linearly with the carrier density. The experimental data contradicts this prediction even more dramatically, showing a sharp peak in dissipation at moderate light intensity, and decreasing dissipation at high light intensities, even as the free carrier density surely continues to increase. The source of these apparent contradictions is not the sample or the microscope, but Eq. 1. The problem lies in two hidden, seldom stated assumptions needed to derive Eq. 1. The two assumptions are 1. the tip charge oscillates exactly in phase with the cantilever motion to keep the tip voltage constant, and 2. any changes in the tip-sample interaction happen slowly. In the Fig. 1 experiment, the problem is that the increased dissipation implies a tip charge oscillating out of phase with the cantilever motion — a clear violation of assumption 1. To move forward, we needed a model that describes the cantilever frequency shift and dissipation even when these assumptions are violated. We use Lagrangian mechanics to describe the coupled motion of cantilever tip displacement, tip charge, and sample charge. Based on its usefulness in electrochemical impedance spectroscopy, we describe the sample using a complex impedance. Using this new model, we derived two equations that replace Eq. 1. The frequency shift is  and the sample-induced dissipation is  with two  distinct capacitance derivatives, and  the transfer function between the applied tip-sample voltage and the tip voltage drop, which depends on the tip capacitance C and the complex sample impedance Z(ω). These two equations should replace Eq. 1 whenever the tip charge does not oscillate exactly in phase with the cantilever motion. To explain the data of Fig. 1, we model the sample as a capacitor Cs and light dependent resistor Rs operating in parallel (Fig. 2(a)). The central new quantity is H (ωc ), the transfer function evaluated at the cantilever resonance frequency. Figure 2(b) shows that the transfer function has a roll-off frequency determined by the time constant τ=Rs (C+Cs )≈Rs Cs. The real part of the transfer function H' causes a force F' oscillating in phase with the cantilever motion which results in a cantilever frequency shift Δf (Fig. 2(c)). The imaginary part of the transfer function H″ causes a force F″ oscillating out of phase with the cantilever motion which results in sample-induced dissipation — this is the component of the tip-sample interaction completely neglected in Eq. 1 (Fig. 2(d)). We can explain the dramatic peak in dissipation by assuming Rs is large in the dark and small under illumination. The peak in dissipation occurs at an illumination intensity where τ-1 equals the cantilever frequency ω0.   Figure 1. Apparent violation of Eq. 1 in an illuminated thin-film semiconductor, CsPbBr3. Modified from Tirmzi et al., 2017, ACS Energy Lett.; copyright American Chemical Society.1 Figure 2. A Lagrangian-impedance treatment of the tip-sample interaction and a parallel-RC model of the sample explains the seemingly anomalous results of Fig. 1. Modified from Tirmzi et al., 2017, ACS Energy Lett.; copyright American Chemical Society.1 Figure 3. The Lagrangian-impedance model provides a framework for interpreting (a–c) broadband local dielectric spectroscopy, and (d–e) cantilever ring down dissipation measurements. Modified from Tirmzi et al., 2017, ACS Energy Lett.; copyright American Chemical Society.1 Figure 4. Phasekick electric force microscopy accurately measures light induced changes in the tip-sample interaction even when the light or voltage induces abrupt changes in the tip-sample interaction, in violation of assumption 2. Modified from Dwyer et al., 2017, Sci. Adv.; copyright American Association for the Advancement of Science, CC BY-NC 4.0.[9] The lines in Fig. 1(c,d) are a fit of the data to Eqs. 2 and 3. The joint fit nicely captures the nonlinear behavior of both the frequency and the dissipation versus illumination intensity. This new model of the tip-sample interaction connects and integrates the results of a wide variety of scanned probe experiments. Figure 3(a–c) shows a broadband local dielectric spectroscopy measurement2 carried out on the CsPbBr3 film of Fig. 1. The measured quantity is the difference in cantilever frequency shift caused by modulating the tip-voltage at a frequency fm. According to our Lagrangian impedance model, this experiment mainly measures the real part of the transfer function at the modulation frequency H'(fm). The change in the spectrum’s knee with increasing light intensity is in qualitative agreement with the light dependent Rs used to explain the Fig. 1 data, validating the use of a relatively simple RC sample impedance model to explain a wide range of experiments carried out on the photovoltaic CsPbBr3 sample. To gain a better understanding of the microscopic origin of this light dependent resistance Rs in CsPbBr3, we examined the kinetics of the resistance recovery in the dark at different temperatures. Figure 3(e) shows measurements of the sample-induced dissipation inferred from repeated cantilever ring down measurements acquired just aft er turning off the light. The increased dissipation, caused by a reduction in the sample resistance Rs, decreases slowly with a time constant ranging from 2 seconds at 313 K to over 30 seconds at 275 K. An Arrhenius plot shows that the process by which the resistance goes back to its dark value is highly activated, with an activation energy Ea=0.52±0.03 eV. This slow, highly activated process rules out many simple explanations of the sample induced resistance3,4 and supports the suggestion that light creates mobile ionic vacancies in CsPbBr3.5 The Lagrangian-impedance model links each experiment shown in Figs. 1 and 3 back to the sample impedance Z(ω), allowing the experiments to be interpreted in a common framework. Figures 1–3 show that the Lagrangian impedance model helps us interpret results when the assumption that the tip charges oscillates exactly in phase with the cantilever is violated (assumption 1). What about the other assumption, that any changes in the tip-sample interaction occur slowly? This assumption can be violated in electric force microscopy (EFM) experiments that follow the time evolution of photo capacitance in response to illumination.6-8 These experiments have pushed the limits of time resolution in electric force microscopy, with claimed time resolutions down to less than 1 percent of the cantilever period.8 Our analysis shows that the standard equation for frequency shift in KPFM cannot be used to analyze these single-shot, transient EFM experiments because the abrupt changes in the tip-sample force shift the cantilever’s amplitude and phase. The Lagrangian formalism, however, can be used to interpret these results and design experiments that accurately measure abrupt changes in the tip-sample interaction. The phase kick electric force microscopy experiment sketched in Fig. 4 shows how. The light is turned on at time t=0, inducing a possibly abrupt change in the capacitance. The voltage is turned to zero abruptly at a time tp (Fig. 4(a)). We infer the light-induced phase shift as a function of the stepped pulse time tp (Fig. 4(b)). Representative data at two pulse times is shown in Fig. 4(c). A photo capacitance induced phase shift is computed by comparing relative phase-shift data in the dark (dashed line) and under illumination (solid line). Figure 4(d) shows representative phase-shift versus pulse-time data acquired for 768 pulse times at two different light intensities over a polymeric solar-cell film. In Ref. 9, analyzing the Fig. 4(d) data revealed that the photovoltaic sample’s photo capacitance had biexponential dynamics. The Lagrangian-impedance approach to understanding electric force microscopy highlighted here has a number of advantages. It accounts for dissipation of energy in both the sample and the cantilever; treats both steady-state and transient phenomena in a unified way; and captures the effects of sub-cycle changes in sample capacitance, conductivity, and tip charge that are missing from previous treatments of the cantilever sample interaction in EFM and KPFM. The Lagrangian-impedance point of view has inspired new measurements and has forced us to rethink established measurements. The phase kick EFM measurement of Fig. 4 enables the measurement of sample charge motion on a timescale much shorter than even a single cantilever period. The assumptions underlying Eq. 1 are questionable in many interesting samples. Equations 2 through 4 should be used in place of Eq. 1 in any frequency-modulated KPFM experiment carried out on a sample with appreciable impedance, including photovoltaic films, battery materials, and mixed electronicionic conductors. Methods The data in Figs. 1, 3, and 4 were acquired on a custom-built vacuum/variable-temperature scanned probe microscope in Marohn’s laboratory at Cornell. Acknowledgements The authors acknowledge financial support from the U.S. National Science Foundation (Grant DMR-1709879). References [1] A. M. Tirmzi, R. P. Dwyer, T. Hanrath, and J. A. Marohn, Coupled slow and fast charge dynamics in cesium lead bromide perovskite, ACS Energy Lett., 2017, 2, 488 – 496, URL http://dx.doi.org/10.1021/ acsenergylett.6b00722. [2] M. Labardi, M. Lucchesi, D. Prevosto, and S. Capaccioli, Broadband local dielectric spectroscopy, Appl. Phys. Lett., 2016, 108, 182906, URL http://dx.doi. org/10.1063/1.4948767. [3] A. M. Tirmzi, J. A. Christians, R. P. Dwyer, D. T. Moore, and J. A. Marohn, Substratedependent photoconductivity dynamics in a high-efficiency hybrid perovskite alloy, J. Phys. Chem. C, 2019, 123, 3402 – 3415, URL http://dx.doi.org/10.1021/acs. jpcc.8b117832019. [4] R. P. Dwyer, L. E. Harrell, and J. A. Marohn, Lagrangian and impedance spectroscopy treatments of electric force microscopy, Phys. Rev. Appl. (in press), 2019, URL https://arxiv.org/abs/1807.01219. [5] G. Y. Kim, A. Senocrate, T.-Y. Yang, G. Gregori, M. Grätzel, and J. Maier, Large tunable photoeffect on ion conduction in halide perovskites and implications for photodecomposition, Nat. Mater., 2018, 17, 445 – 449, URL http://dx.doi. org/10.1038/s41563-018-0038-0. [6] D. C. Coffey and D. S. Ginger, Timeresolved electrostatic force microscopy of polymer solar cells, Nat. Mater., 2006, 5, 735 – 740, URL http://dx.doi.org/10.1038/ nmat1712. [7] R. Giridharagopal, G. E. Rayermann, G. Shao, D. T. Moore, O. G. Reid, A. F. Tillack, D. J. Masiello, and D. S. Ginger, Submicrosecond time resolution atomic force microscopy for probing nanoscale dynamics, Nano Lett., 2012, 12, 893 – 898, URL http://dx.doi.org/10.1021/nl203956q. [8] D. U. Karatay, J. S. Harrison, M. S. Glaz, R. Giridharagopal, and D. S. Ginger, Fast time-resolved electrostatic force microscopy: Achieving sub-cycle time resolution, Rev. Sci. Instrum., 2016, 87, 053702, URL http://dx.doi. org/10.1063/1.4948396. [9] R. P. Dwyer, S. R. Nathan, and J. A. Marohn, Microsecond photocapacitance transients observed using a charged microcantilever as a gated mechanical integrator, Sci. Adv., 2017, 3, e1602951, URL http://dx.doi.org/10.1126/ sciadv.1602951.  

28 Aug 2019

Electrical and Mechanical Characterization of Li Ion Battery Electrode using Pinpoint SSRM

John Paul Pineda, Cathy Lee, Byong Kim, and Keibock Lee Park Systems Inc., Santa Clara, CA USA    Introduction Lithium ion batteries (LIBs) are key components of modern electronics and are regularly utilized as their primary power source [1-3].LIBs have become ubiquitous in a variety of applications ranging from portable devices to electric vehicles because of their high energy density, flexible and lightweight design, lower self-discharge, low cost and longer lifespan compared to other battery technologies [4,11]. Despite these advantages, the reliability and performance of LIBs still need to be upgraded to meet the requirements of applications such as large-scale energy storage and hybrid electric vehicles (HEV) [2,5,11]. Extensive research has focused on the development of four cell materials in particular to achieve better performance: positive and negative electrode active materials (AM) as well as separators and electrolytes [2]. Understanding the electrical and mechanical properties of electrode materials plays a major role in the performance improvement of LIBs. It has been shown that improved adhesion between electrode particles and between electrode films and current collectors leads to better retention of discharge capacity during cycling, especially when electrode materials exhibit faster and/or larger volume expansion [2]. Moreover, enhanced electronic conductivities and ionic diffusivities in electrodes also lead to LIB capacity improvements [6]. As devices are becoming more compact, optimizing electrical and mechanical properties on a nanometer scale becomes more relevant and lead to improved interfaces. There are several methods which can measure these local properties; the more common methods include impedance spectroscopy and nanoindentation [3,7,8]. However, even when using both methods, one cannot get the full local information about the above mentioned properties. Impedance spectroscopy needs an exact model and can only distinguish between interfaces and therefore does not have local information for each interface. Nanoindentation, is destructive and does not provide any electrical information. One of the more effective tools used to overcome the problems encountered in electrical property measurement is Scanning Spreading Resistance Microscopy (SSRM). This technique simultaneously measures both electrical properties and topography. However, SSRM has some disadvantages such as rapid wearing of the tip, degradation of image resolution and low signal-to-noise ratio. These disadvantages stem from the high frictional force arising from continuous tip-sample contact during scanning. Recently, a new operational AFM mode developed by Park Systems called PinPoint™ [10] can be coupled with SSRM to offer scientists and engineers an innovative solution to avoid the troublesome frictional forces during a scan. This method operates in an approach-retract manner, ensuring a frictionless operation which eliminates the lateral force from continuous tip-sample contact. In addition, the integration of these methods with AFM enables it to acquire topography as well as electrical/mechanical property data simultaneously without changing the sample or tip. Here, we demonstrate that PinPoint™ SSRM AFM effectively measures both electrical and mechanical properties of LIBs electrode surfaces at a much higher quality using a Park NX-Hivac AFM system. Experimental ALIB electrode was investigated using a Park NX-Hivac AFM system[9]. The topographical, electrical and mechanical data of the sample were measured under high-vacuum to perform a 20 μm x 12 μm scan with a resolution of 256 x 150 pixels. Conducting this experiment under high-vacuum led to sensitivity and resolution improvements [9].High-vacuum also led to accuracy and repeatability enhancements while minimizing damage sufferend by the tip and/or sample. The applied force between the tip and sample under high-vacuum can be chosen substantially lower relative to experiments conducted in ambient air or N2as the liquid film, present under ambient conditions, reduces the quality of the electrical contact[9]. A conductive diamond coated probe (NANOSENSORS™ CDT-NCHR) with a nominal force constant of 80 N/m was used in this experiment. In PinPoint SSRM, the sample’s topography, electrical properties and mechanical properties can be acquired simultaneously. The conductive tip maps the topography by monitoring its feedback signal, approaching the sample until it reaches a predefined force threshold point, measuring Z scanner height and rapidly retracting. The current distribution of the samples are mapped each time the tip engages the sample surface. The current flow, produced by the applied bias voltage between the conductive tip and sample, is measured at each landing point to acquire the electrical data. Generally, the current flow is very small and needs to be augmented by a current amplifier before it can be processed into an image. The Park NX-Hivac is equipped with an internal current amplifier with a variable gain of 106~1012 V/A that is applicable to most measurements. The applied external sample bias used in this experiment was 3V. Additionally, at each point of the image, the force-distance curve is acquired and used to calculate the mechanical properties of the sample being measured. During data acquisition, the XY scanner stops and the contact time is controlled to give enough time for the scanner to acquire precise and accurate data. Results and Discussion The qualitative and quantitative results acquired during the experiment are shown in Figures 1 and 2. All images were analyzed using Park XEI software developed by Park Systems; the software generated a corresponding line profile as well as image analysis. The Park XEI software maps the acquired signals into a color table. Figure 1 presents the topography and electrical data measured in the sample. The topography image (top-left) shows that the sample surface consists of circular shaped plateaus with diameters ranging from 2 to 8μm. The circular plateaus with brighter colors represent areas with greater height while the areas with darker color represent low lying areas. The resistance image (top-right), feature brighter colored areas that represent higher surface resistivity while areas with darker colors represent lower surface resistivity. The darker colors on the conductance image (middle-left) represent areas with lower surface conductivity while brighter colors represent higher conductivity. By comparing the topography and resistance image, one can easily observed that circular plateaus with higher height have lower resistance compared to low laying areas, while comparing the topography and conductance image will show that circular plateaus with higher height have higher conductance compared to low laying areas. This can also be observed in the 3D-overlay image of topography and conductance (bottom) acquired in XEI. Both resistance and conductance image also show a clear picture of the small grains surrounding the circular plateaus. It was observed that these small grains have lower resistivity and higher conductivity compared to the circular plateaus. Analysis of the corresponding line profile (bottom-right) of the topography, conductance and resistance images confirms that the circular plateaus are regions with highest height (approximately 50-60 nm). On the other hand, low lying areas possess highest resistivity (approximately 8 GΩ). The line profile of the conductance image shows that the tiny grains around the circular plateaus are regions with the highest conductivity (approximately 1.3 μS). Fig. 1. Topography (top-left) resistance (top-right) and conductance (middle-left) images acquired from an LIB electrode sample. Line profile (middle-right): Topography line profile (red line, y-axis on left),resistance line profile (green, y-axis on right), and conductance line profile (blue line, y-axis on right). 3D-overlay image of topography and conductance (bottom). Figure 2 represents the mechanical data measured in the sample. In the stiffness images, the brighter areas represent a surface with higher stiffness and modulus while the darker areas correspond to the surface with lower stiffness and modulus. The adhesion image does not show significant variations between the plateaus. A tendency of higher adhesion is observed between the plateaus. The average adhesion force was measured to be 80 nN. The stiffness image shows noticeable stiffness variations in the sample surface. The measured stiffness value of the circular features represented by the dark color map is approximately–2.8 μV (note that the stiffness unit given here is for qualitative data comparative purposes and does not represent an absolute physical unit value of stiffness), while the areas represented by the bright color map were in a range from –1.5 to –2.0 μV. Fig. 2. Adhesion (top-left) and stiffness images (top-right) acquired from an LIB electrode sample. Line profile (bottom): Adhesion line profile (red line, y-axis on left) and stiffness line profile (green, y-axis on right). Conclusion A lithium ion battery electrode was successfully characterized by the Park NX-Hivac system using Pinpoint SSRM. The data collected in this experiment demonstrate that this technique is an effective means for measuring the quantitative and qualitative topographical, electrical and mechanical data of advanced materials with improved image quality and data accuracy. Overall, the PinPoint technique described in this study will greatly help researchers and device engineers in understanding the electrical and mechanical behavior of certain materials with unique properties such as LIB electrodes. References 1. Tarasco M, et al., Issues and challenges facing rechargeable lithium batteries. Nature. 2001 Nov 15;414(6861):359-67. 2. J. Chen, et al., Unveiling the Roles of Binder in the Mechanical Integrity of Electrodes for Lithium-Ion Batteries. Journal of The Electrochemical Society, 160 (9) A1502-A1509 (2013). 3. L. Vasconcelos, et al., Grid indentation analysis of mechanical properties of composite electrodes in Li-ion batteries. Extreme Mechanics LettersVolume 9, Part 3, December 2016, Pages 4. 4. G. Kermani, et al., Review: Characterization and Modeling of the Mechanical Properties of Lithium-Ion Batteries. Received: 30 September 2017; Accepted: 24 October 2017; Published: 30 October 2017. 5. A. Mishra, et al., Electrode materials for lithium-ion batteries. Materials Science for Energy TechnologiesVolume 1, Issue 2, December 2018, Pages 182-187. 6. M. Park, et al., A review of conduction phenomena in Li-ion batteries. Journal of Power Sources, Received 12 May 2010 Received in revised form 16 June 2010 Accepted 17 June 2010. 7. Y. Gao, et al., Mechanical reliability of alloy-based electrode materials for rechargeable Li-ion batteries. Journal of Mechanical Science and Technology 27 (5) (2013) 1205~1224. 8. E. Reinholz, et al., Composition and Manufacturing Effects on Electrical Conductivity of Li/FeS2Thermal Battery Cathodes. doi: 10.1149/2.1191608jesJ. Electrochem. Soc. 2016 volume 163, issue 8, A1723-A1729 9. Park Systems Introduces Park NX-Hivac, a High Vacuum SSRM AFM System for Optimal Results in Semiconductor Manufacturing Failure Analysis. 10. J.Pineda, et al.,Optimum Current Distribution Measurement of Zinc Oxide Nanorods via PinPoint™ Conductive AFM 11. F. Susai, et al., Horizons for Li-Ion Batteries Relevant to Electro-Mobility: High-Specific-Energy Cathodes and Chemically Active Separators. doi: 10.1002/adma.201801348. Adv Mater. 2018 Oct;30(41):e1801348.

11 Jan 2018

Electrochemical Atomic Force Microscopy: In Situ Monitoring of Copper Electrodeposition on Gold Surface

Conductive AFM (C-AFM) is uniquely suited to perform electrical measurements such as current distribution with nanoscale precision and can be performed in contact mode, tapping mode, or PinPoint™ mode. Here we perform C-AFM on ZnO nanorods, which have been widely studied due to their remarkable performance in electronics, optics, photonics, and photocatalysis.

10 Jul 2017

Optimum Current Distribution Measurement of Zinc Oxide Nanorods via PinPoint™ Conductive AFM

Conductive AFM (C-AFM) is uniquely suited to perform electrical measurements such as current distribution with nanoscale precision and can be performed in contact mode, tapping mode, or PinPoint™ mode. Here we perform C-AFM on ZnO nanorods, which have been widely studied due to their remarkable performance in electronics, optics, photonics, and photocatalysis.

10 May 2017

Piezoelectric Force Microscopy: Multilayered Ceramic Capacitor

The coupling between an electrical and mechanical response in a material is a fundamental property that provides functionality to a variety of applications ranging from sensors and actuators to energy harvesting and biology. Most materials exhibit electromechanical coupling in nanometer-sized domains.