Scanning probe microscopy for energy applications

Resume : Quantitative Scanning Force Microscopy in ambient conditions Understanding tip-sample interaction in a Scanning Force Microscopy (SFM) setup is fundamental for optimum data acquisition as well as data interpretation. To minimize noise and simplify the interpretation of experiments, data should be acquired in the (true) non-contact regime at low oscillation amplitude. Two methods will be described to characterize nanoscale systems: data I(U,z) can be acquired either as a function of tip-sample voltage U and tip-sample distance z allowing to separate the Van der Waals force from the electrostatic force. Alternately, normal images U(x,y) can be acquired by minimizing the electrostatic tip-sample interaction (Kelvin Force Microscopy [1]) in order to determine the local Contact Potential. Experiments will be presented to demonstrate the capability of nanoscale characterization of samples in air. We will show, on the one hand, how the spatial and time evolution of the surface potential can be studied. We find strong (time) fluctuations of the surface potential and a static lateral corrugation of this potential. On the other hand, we will discuss how multidimensional SFM spectroscopy (?interaction images?, [2]) allows separation of Van der Waals and electrostatic forces. From these forces, the Hamaker Constant and the Contact Potential can be determined, allowing for precise characterization of material properties on the nanoscale. 1. S. Sadewasser, Sascha, Th. Glatzel, Kelvin Probe Force Microscopy. Springer Series in Surface Sciences. 2012. 2. E. Palacios-Lidón and J. Colchero, Nanotechnology 2006, 17 (21), 5491-5500.

Resume : Since its emergence, piezo-response force microscopy (PFM) is the prevailing method to examine fundamental ferro/piezoelectric materials. However, a fundamental limitation lies in its contact mode operation, leading research to focus on flat and durable samples. Recently, a great deal of attention has been dedicated to ferro/piezoelectric nanomaterials, and to soft piezoelectric biological or bio-compatible materials. Considering the limitations of contact PFM, a significant barrier for quantitative analysis of these materials arises. We present an intermittent contact PFM mode (IC-PFM), overcoming some of the barriers for the study of ferro/piezoelectric nanomaterials. The IC-PFM mode stems from a scanning nanoindentation mode (peakforce by Bruker), where the tip is oscillated into intermittent contact with the sample, and resulting force-curves are analysed to yield mechanical information about the sample. We superimpose an electrical signal atop this mode, and extract the piezo-response related deflection signal for each indentation point of a scanning line; notably, only data obtained when the tip contacts the sample is analyzed. We present measurements performed on poly-L-lactic-acid nanowires (NWs), III-V semiconductor NWs, and on a ceramic NW network, where all samples proved nearly impossible to examine in contact mode. We expect this operation mode would become invaluable for ferro/piezoelectric nanomaterials research and induce further advances in this filed.

Resume : The combination of optical spectroscopy and atomic force microscopy (AFM) is very promising, because it enables the chemical mapping far below the diffraction limit of light. Amongst the available techniques are the scanning scattering near-field microscopy (sSNOM), photothermal induced resonance (PTIR), and Photo-induced Force Microscopy (PiFM): three techniques that are based on physically distinct sensing mechanisms. While sSNOM relies on collection of scattered light, PTIR and PiFM detect the light-induced changes in the sample mechanically. Both illuminate the tip-sample interface with a pulsed tunable laser source and utilize the resonance enhancement of AFM cantilevers. PTIR, which operates in contact mode, relies on the detection of the thermal expansion of the sample and requires a metal, often gold, substrate for best sensitivity. PiFM, presented here, senses the induced polarization of the sample via an attractive image force acting on a metalized tip and is thus applicable in a much wider wavelength range. In PiFM, the tip-sample interaction is highly localized and provides spectrographic mapping with < 10 nm lateral resolution and a penetration depth in the same order of magnitude. Operated with a tunable IR light source, PiFM routinely achieves low noise IR absorption spectra from samples as thin as a few nm. Due to the fact that PiFM relies on the detection of polarizability, it easily operates on insulating substrates such as mica or glass, on very thin layers and on samples with low thermal expansion, such as graphene. This presentation will focus on the application of PiFM at various examples. Results on a wide variety of organic and inorganic samples, including self-assembled block copolymers, polymer blends, perovskites, pharmaceutical compounds, biological materials, microelectronic devices, boron nitride nanotubes, and many others will be presented. Finally, its combination with Kelvin-Probe Force Microscopy (KPFM) and the resulting benefits will be discussed.

Resume : We present physical principles of Apertureless Scanning Near-field Optical Microscopy (ASNOM) [1,2,3] and report on its application to a mapping of the nanoscale objects optical properties with nm-resolution. Normally, the dipole oscillations in a point-like object (like a single molecule or nano-particle) can hardly be emitted into an environmental space as a running electromagnetic wave. Its dipole momentum determined by a few oscillating electrons (typically single one) and angstrom-range oscillation span of charge density yields in vanishingly low efficiency in a light emission. The situation changes dramatically if a rod-like antenna of some micrometer length is attacted ?electrically? to that object (see Figure.1). A huge amount of free electrons in a metal of a tip being involved in the charge density oscillations under an influence of the object being investigated, as well as tip large dimensions lead to 104-105 increase of radiation efficiency [4]. An external electromagnetic field can, vice versa, be applied to a surface with a help of an ASNOM tip. The dimensions of area where the field is applied are determined just by mechanical dimensions of a tip (5-20 nm) [5,6,7], regardless to the wavelength of a light being used (up to THz range). The amplitude and phase of the wave re-emitted by a tip antenna depend on its ?grounding conditions?, and therefore an ASNOM signal being collected contains information on the sample local dielectric function as a complex value [8]. We present ASNOM images of semiconductor and polymer (e.g. Figure 2) structures of nanometer scale to demonstrate an ability of an ASNOM to map a clear material contrast, with a lateral resolution of 10-30 nm. Figure 1. Polystyrene embedded in a PVAC matrix. (a) AFM-like topography map of the sample. ASNOM signal (?=10.6 ?m) with (b) an optical phase corresponding to Au (known to perfectly reflect 10.6 ?m radiation) and (c) collected with an optical phase 90° with respect to (b). We also present the maps of running surface polariton waves collected with an ASNOM on different samples. A nanometer lateral resolution of ASNOM allows for mapping the surface waves with a lateral resolution much better than a wavelength and, therefore, to investigate the polariton optics phenomena in such kind of structures with a rather good precision. References. [1] H.K. Wickramasinghe, C.C. Williams, Apertureless near field optical microscope. //US Pat. 4,947,034, (1990) [2] F. Zenhausern; Y. Martin & H.K. Wickramasinghe, Scanning Interferometric Apertureless Microscopy: Optical Imaging at 10 Angstrom Resolution. //Science, 269, 1083-1085 (1995). [3] Y. Inouye & S. Kawata, Near-field scanning optical microscope with a metallic probetip. //Opt. Lett., OSA, 19, 159-161 (1994). [4] L. Novotny; R. X. Bian & X. S. Xie, Theory of Nanometric Optical Tweezers. //Phys. Rev. Lett., American Physical Society, 79, 645-648 (1997). [5] Y. Martin; F. Zenhausern & H. K. Wickramasinghe, Scattering spectroscopy of molecules at nanometer resolution. //Applied Physics Letters, AIP, 68, 2475-2477 (1996). [6] A. Bek; R. Vogelgesang & K. Kern, Apertureless scanning near field optical microscope with sub-10 nm resolution. //Review of Scientific Instruments, AIP, 77, 043703 (2006). [7] A. Huber; D. Kazantsev; F. Keilmann; J. Wittborn & R. Hillenbrand, Simultaneous IR Material Recognition and Conductivity Mapping by Nanoscale Near-Field Microscopy. //Advanced Materials, WILEY-VCH Verlag, 19, 2209-2212 (2007). [8] F. Keilmann & R. Hillenbrand, Near-Field Microscopy by Elastic Light Scattering from a Tip. //Philosophical Transactions: Mathematical, Physical and Engineering Sciences, The Royal Society, 2004, 362, 787-805.

Resume : The electrical behavior of materials for photovoltaics (PV) at the mesoscale strongly determines device performance. Thus, spatially resolving the electrical and optical response of emerging materials, such as perovskites and polycrystalline owns, is crucial to advance our understanding of charge carrier generation, recombination and collection at the nanoscale. We use scanning probe microscopy methods to realize a novel platform to image the functionality of materials for PV. Through time-dependent Kelvin-probe force microscopy (KPFM) we quantify the transient behavior of perovskite solar cells. Upon and post illumination we measure ion accumulation and migration, respectively, resulting from the occupation of trap states and in a residual Voc even when the material is under dark conditions. In the realm of polycrystalline materials, we implement KPFM to quantify the local Voc of CdTe and CIGS, where we find spatial variations >20%, most likely caused by the material?s structural properties. We anticipate our functional imaging approach to impact the rational design of the next generation of high-efficiency and low-cost PV devices, by establishing a correlation between the materials electrical, chemical and structural properties. Further, the time-dependent KPFM could be extended to probe the stability of lead-free perovskites.

Resume : Organic?inorganic hybrid perovskite solar cells, e.g. CH3NH3PbI3, with perovskite structure have been received extensive attention for photovoltaic applications owing to their superior properties. However, anomalous current?voltage (I?V) hysteresis has hindered the attainment of an even higher efficiency performance. Although recent experiemtnal studies based on piezoresponse force microscopy (PFM) have shown that the perovskite solar cells have ferroelectricity that causes hysteresis, there still remain an ambiguity because PFM response can originate from the surface volume change by ion migration as well as piezoresponse. In this presentation, we examine the origin of the I-V hysteresis in CH3NH3PbI3 thin films. We first demonstrate the existence of ferroelectricity by observing collapse of the PFM hysteresis loop above the Curie temperature of CH3NH3PbI3 thin films through temperature dependent PFM. Furthermore, by measuring the voltage scan-rate-dependent nano/macroscopic I?V curves, we find that grain boundaries assisted ion migration can be a dominant origin of I?V hysteresis at the macroscopic scale, while ferroelectricity contributes to the I-V hysteresis at the grains. Our findings suggest that, although there is ferroelectricity in the CH3NH3PbI3 thin films, ion migration mainly contributes to the macroscopic I?V hysteresis. The presented results could provide basic guidance to the resolution of I-V hysteresis issues in perovskite materials.

Resume : The charge carrier transport behavior and solar cells efficiency of perovskite solar cells were shown to be dramatically influenced by the existence of PbI2 phase in perovskite films. In present work, the original combination of light-modulated scanning tunneling microscopy (LT-STM) and spectroscopy (STS) reveals the interfacial electronic configuration at the PbI2/perovskite hetero-interface of polycrystalline CH3NH3PbI3 perovskite grain under light illumination. The unique advantage of the LM-STM/STS technique enable directly visualizing the spatially-resolved mapping images of electron and hole carriers photogeneration and photoinduced interfacial band bending of both the valence bands and conduction bands at the PbI2/perovskite interface of perovskite crystals. According to the investigation at the interfacial electronic configuration of individual perovskite grains under illumination, the enhanced photogenerated carrier separation and reduced back recombination were observed as an optimal interfacial PbI2 passivation layers with a thickness less than 20 nm at perovskite crystal grains was applied.

Resume : A range of emerging materials, from perovskite solar cells, to polymer batteries, and even bioelectronic transistors, show strong variations in performance associated with the nanoscale structure of the active material. In this talk, I will discuss our work using a combination of multimodal in situ probes to unravel the often complex interplay of electronic, ionic, and even ferroelectric, behaviors that connect processing, structure, and function in these materials. For instance, by classifying local current-voltage curves according to the corresponding local photoluminescence, we show that local variations in electronic coupling between the semiconductor and electrode are present across a wide range of common hybrid perovskite semiconductor photovoltaic device architectures, pointing a clear path forward in attempts to improve Voc in these materials. In addition, we disentangle the competing effects of electronic carrier injection with voltage bias stress, providing a clearer picture of the role of ion migration vs. trap filling in device aging. Finally, we discuss nanoscale measurements of ion transport in a range of soft polymer systems, using both electrochemical strain microscopy (ESM) as well as time-resolved electrostatic force microscopy (trEFM), showing how local polymer crystallinity influences ion uptake and motion in organic semiconductors that serve as a model for materials being studied for battery binders as well as bioelectronic transducers.

Resume : Although hybrid lead halide perovskites have shown great promise as a new cheap and flexible alternative to silicon, these so-called perovskite solar cells exhibit unusual behaviour such as current-voltage hysteresis. This effect is also manifested in an unstable power output over the course of milliseconds to minutes and hours following the exposure to light. A stable and reliable power output, however, is a prerequisite for the commercialization of new photovoltaic materials. Depending on the choice of materials for the selective electrodes, hysteresis could recently be mostly suppressed, indicating that internal interfaces play a decisive role. Nevertheless, the exact mechanism behind the hysteresis could not yet be clarified. Space charge layers formed due to migrating ionic species and non-radiative recombination at the interfaces are thought to play a dominating role. Here, Kelvin probe force microscopy (KPFM) offers the unique opportunity to map charge accumulation at the interfaces of operating devices [1]. However, the conventional scanning approach of KPFM limits the time resolution to ~1 s due to the scan rate of the SFM tip. Here, we present a pointwise KPFM spectroscopy approach that allows recording complete potential maps of a device cross section with a time resolution of up to 500 µs. Thereby, the formation and dissipation of the internal electric field caused by the migrating ions in the perovskite layer can be measured and visualized. [1] Bergmann et al., Nat. Comm., 5, 5001 (2014); ACS AM&I, 8, 19402 (2016).

Resume : Light-Assisted Scanning Tunneling Microscopy (LA-STM) experiments have been performed on very thin molecular assemblies on Au(111), which have been elaborated by a solution-based process. D/A assemblies have been achieved by adsorbing a submomolayer of fullerene (PC71BM) on a monolayer of PTB7/Au(111), the pair (PTB7/PC71BM) being known to exhibit a high power conversion efficiency in organic photovoltaic cells.[1] Photo-induced current images are expected to provide a signature of exciton formation-dissociation. Nevertheless, the tip and sample thermal expansion caused by the power modulation of the laser source introduces an important « thermic » component.[2] By working simultaneously at two opposite potentials and by studying the signals obtained on non-photoactive systems, such as self-assembled monolayers of alkanethiols, we succeed to isolate the real molecular photocurrent and to image it. A model of the photochemical process with location of the expected electronic levels is proposed and discussed. References [1] Y. Liang, Z. Xu, J. Xia, S.-T. Tsai, Y. Wu, G. Li, C. Ray, L. Yu, Adv. Mater. 2010, 22, E135. [2] S. Grafström, J Appl. Phys. 2002, 91, 1717.

Resume : Quantitative surface potential measurements are crucial to understand electronic processes on functional nanostructures in solar cells. Here, Kelvin probe force microscopy (KPFM) on device cross sections has become a popular method to map the potential distribution across the functional layers of solar cell devices. Cross sectional KPFM can both visualize and quantify effects such as an unbalanced charge transport [1] or slow migration of ions and charge trapping [2]. To provide quantitative and local information, the KPFM methods have to be free of cross talk from topography and electrostatic interactions with adjacent layers. In particular in thin film solar cells with active layers thinner than one micrometer, the proper choice of KPFM method is crucial. Here we study the influence of the KPFM method on the measured potential distribution as well as on crosstalk on a model electrode geometry. We directly compare the lateral resolution and the measured contact potential difference for different Amplitude Modulation (AM) and Frequency Modulation (FM) KPFM methods in air. We then suggest a ?best practice? for quantitative measurements on nanoscale device cross sections to understand the limiting factors of the solar cell performance. [1] Bergmann et al., Nat. Comm. 2014, 5 : 5001. [2] Bergmann et al., ACS Appl. Mater. Interfaces, 2016, 8 (30):19402?19409.

Resume : Conjugate polymers are of great interest because of their ability to control the energy gap and electronegativity through molecular design that has made possible synthesis of conducting polymers with a range of ionization potentials and electron affinities. The excellent properties of ?-electron delocalization at the backbone of conjugate polymer can be initiator of charge transfer mechanism with the incorporated nanoparticles, resulting tunibility of electronics dynamics within the composites, can have exciting opportunities to nano-electronics. Aim of our work is to develop novel strategies for synthesis of PANI and its composite via simple cost effective techniques towards the smart applications of ?electro-nanoimprinting? memory devices based on controlled and localized charge trapping phenomenon. As the size of electronic device is scaled down towards the nanometres dimensions architecture, AFM can be used as a technique of choice to study the influence of controlled charge injection phenomenon into the materials at the time of scanning. Our experimental findings open up a way for ?writing-reading? of memory bits on polymer data sheet using mechanic/electrical bias by AFM probe. For pristine PANI, core idea of ?charge pattern? was by transmuting mechanical stress in to electrical outputs by contact electrification mechanism, to the concept of non-destructive self-powered data sheet. Secondly, carbon nanoparticles (CNPs) have been deployed into PANI as a charge trapping sites, where data has been recorded on the device layer by electrical pulse (±1V to ± 7V) through AFM by localized charge injection method. Time evolution of ?read-out? image indicates well retention effect of stored charge for a significant time up to 6 hrs. kkkk

Resume : Blending small conjugated molecules as organic semiconductor with an amorphous insulating polymer has been probed to improve the device processability, reproducibility and stability1-2. The key to understand the superior performance of organic field effect transistors (OFETs) fabricated with blended films seems to be the vertical phase separation of the two material components. Thus, deciphering the microstructural details of the vertical stratification of the two organic materials in the blend and its influence on the macroscopic electrical performance is an issue of fundamental importance. This important question is addressed here for blends of 2,7-Dioctyl[1]benzothieno[3,2?b][1]benzothio-phene (BTBT-C8), promising organic semiconductor with some of the highest mobilities reported to date, and polystyrene (PS) processed by a solution-shearing technique [1,2]. We show that Friction Force Microscopy (FFM) can be used to distinguish between regions of pure BTBT-C8 and PS thanks to their different frictional behavior, allowing us to obtain a complete nanoscale characterization of their lateral and vertical distribution. In addition, a Kelvin Probe Force Microscopy (KPFM) study was performed to understand the influence of the morphology on the macroscopic device performance and elucidate charge trapping and contacts-related effects. (1) Temiño, I.; Del Pozo, F. G.; Ajayakumar, M. R.; Galindo, S.; Puigdollers, J.; Mas-Torrent, M. A Rapid, Low-Cost, and Scalable Technique for Printing State-of-the-Art Organic Field-Effect Transistors. Adv. Mater. Technol. 2016, 1600090. (2) Del Pozo, F. G.; Fabiano, S.; Pfattner, R.; Georgakopoulos, S.; Galindo, S.; Liu, X.; Braun, S.; Fahlman, M.; Veciana, J.; Rovira, C.; et al. Single Crystal-like Performance in Solution-Coated Thin-Film Organic Field-Effect Transistors. Adv. Funct. Mater. 2016, 26, 2379?2386.

Resume : Scanning Microwave Microscopy (SMM) is a recently developed nanoscale imaging technique that combines the lateral resolution of Atomic Force Microscopy (AFM) with the high measurement precision of microwave analysis at GHz frequencies, provided by Vector Network Analyzers (VNA). SMM enables measuring complex materials properties for nano-electronics, materials science, and life science applications. SMM operates at broadband frequencies between 1 MHz and 20 GHz. We developed novel calibration workflows for complex impedance imaging [1-2] and dielectric quantification [3]. Various nanodevices are studied including dopant profiling layers [4], high voltage transistors [5], buried oxide structures, and 2D materials like graphene and MoS2. Owing to the high frequency of the measurements, the laborious fabrication of back electrode contacts is not required, making it an easily applicable tool for electrical characterization of nanodevices. The capability of the electromagnetic waves to penetrate the surface of the sample under study allows the technique to be used to selectively sense sub-surface features [6,7]. The sub-surface and quantitative resistivity measurement capabilities are demonstrated for silicon back-wafer imaging and semiconductor failure analysis. The SMM was also applied to study magnetic materials including YIG films and permalloy samples [8]. In summary, we present an extended SMM for advanced voltage and impedance spectroscopy, along with novel RF calibration workflows that can be applied to nanoscale semiconductor devices and advanced materials at high frequency. References: [1] G. Gramse et al, ?Calibrated complex impedance and permittivity measurements with scanning microwave microscopy?, Nanotechnology, 25, 145703 (2014) [2] M. Kasper et al, ?An advanced impedance calibration method for nanoscale microwave imaging at broad frequency range,? IEEE Transactions on MTT, (2017), in press [3] M.C. Biagi et al, ?Nanoscale Electric Permittivity of Single Bacterial Cells at Gigahertz Frequencies by Scanning Microwave Microscopy,? ACS Nano, 10, 280 (2016) [4] E. Brinciotti et al., ?Probing resistivity and doping concentration of semiconductors at the nanoscale using SMM,? Nanoscale 7, 14715 (2015) [5] E. Brinciotti et al, ?Calibrated nanoscale dopant profiling and capacitance of a high-voltage lateral MOS transistor at 20 GHz using Scanning Microwave Microscopy,? IEEE Transactions on Nanotechnology, vol.16, no.2, pp.245-252 (2017) [6] G. Gramse and E. Brinciotti et al, ?Quantitative sub-surface and non-contact imaging by scanning microwave microscopy,? Nanotechnology 26, 135701 (2015) [7] E. Brinciotti et al, ?Frequency Analysis of Dopant Profiling and Capacitance Spectroscopy Using Scanning Microwave Microscopy,? IEEE Transactions on Nanotechnology, vol.16, no.1, pp.75-82 (2017) [8] C. Joseph et al., ?SMM for nanoscale characterization of magnetic materials,? J. Magnetism and Magnetic Materials 420, 62 (2016)

Resume : Graphene nanoscroll (GNS),1-3 a wrapped graphene sheet with tubular structure, has been receiving increasing interest due to its potential applications in the fields of hydrogen storage, supercapacitors, batteries, and nanodevices. Several methods have been reported to fabricate the GNS, such as scrolling of the mechanically exfoliated graphene sheets with the aid of isopropyl alcohol in water, sonication of graphite intercalation compounds, microwave-assisted scrolling in ethanol or liquid nitrogen, and Langmuir-Blodgett compression of functionalized graphene oxide single sheets. In this work, well-aligned functionalized graphene oxide (GO) scrolls are prepared through the controlled folding/scrolling of single-layer GO and functionalized GO sheets by using molecular combing on hydrophobic substrates,4-5 such as aged gold substrate, polydimethylsiloxane film, and octadecyltrimethoxysilane-modified silicon dioxide. As a proof of concept, as-prepared functionalized GO scrolls have been used to detect NO2 gas and humidity.6 This simple method shows the potential ability to control the shape, orientation, position and functionalization of GO sheets over large area. References (1) S. F. Braga, V. R. Coluci, S. B. Legoas, R. Giro, D. S. Galvao, R. H. Baughman, ?Structure and dynamics of carbon nanoscrolls? Nano Lett. 2004, 4, 881-884 (2) L. M. Viculis, J. J. Mack, R. B. Kaner, ?A chemical route to carbon nanoscrolls? Science 2003, 299, 1361 (3) X. Xie, L. Ju, X. F. Feng, Y. H. Sun, R. F. Zhou, K. Liu, S. S. Fan, Q. L. Li, K. L. Jiang, ?Controlled Fabrication of High-Quality Carbon Nanoscrolls from Monolayer Graphene? Nano Lett. 2009, 9, 2565-2570 (4) H. Li, J. Wu, X. Qi, Q. He, C. Liusman, G. Lu, X. Zhou, H. Zhang. ?Graphene Oxide Scrolls on Hydrophobic Substrates Fabricated by Molecular Combing and Their Application in Gas Sensing?, Small 2013, 9, 382-386 (5) J. Wu, H. Li, X. Qi, Q. He, B. Xu, and H. Zhang, ?Graphene Oxide Architectures Prepared by Molecular Combing on Hydrophilic-Hydrophobic Micropatterns?, Small 2014, 10, 2239-2244 (6) L. Wang, P. Yang, Y. Liu, X. Fang, X. Shi, S. Wu, L. Huang, H. Li,* X. Huang, W. Huang, ?Scrolling up graphene oxide nanosheets assisted by selfassembled monolayers of alkanethiol?, Nanoscale, under revision.

Resume : The physical nature of solvent vapor annealing (SVA) treatment is quite straightforward, and its application is ideal in small molecule-based bulk heterojunction solar cells. It has been suggested to rapidly achieve high-performance small molecule photovoltaics by alternating the blends to ideally connect the crystallite morphology. However, most previous reports on SVA have shown only influences of the degree of donor/acceptor phase separation within part of active textures. Here, we investigated solution-processed small molecule solar cells consisting of the previously developed DR3TSBDT and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) after SVA in terms of the vertical gradient crystalline phase in three-dimensional (3D) active layers. These systematic studies of the vertically phase-separated morphology for 3D heterojunction structures clarified in more detail the fundamental mechanisms and dynamics underlying SVA and showed a clear structure-property relationship in related device performance. This not only provided a clear understanding of precise effect of SVA treatments on varied 3D morphological structures, but also a path towards the rational optimization of device performance.

Resume : The organic-inorganic hybrid material methylammonium lead iodide (MAPI) is successfully used as active layer in a new class of photovoltaics called perovskite solar cells. Crystallizing in a perovskite structure it has long been discussed if MAPI is ferroic, a common property in many perovskite minerals. In a recent piezoresponse force microscopy (PFM) study, we imaged a previously unobserved pattern of nanoscale domains in solvent annealed MAPI thin films[1]. We concluded that the domain pattern is connected to ferroelastic domains in MAPI that form due to sample stress during the cubic-tetragonal phase transition after the solvent annealing. This hypothesis was recently supported by others [2, 3]. Here, we used combined variable temperature PFM and local photoluminescence measurements as well as time-resolved Kelvin probe force microscopy to investigate the influence of the ferroelastic domain walls on the charge carrier dynamics within µm-sized MAPI crystals. 1. Hermes, I.M., et al., The Journal of Physical Chemistry C, 2016. 120(10). 2. Strelcov, E., et al., Science Advances, 2017. 3(4). 3. Rothmann, M.U., et al., Nature Communications, 2017. 8.

Resume : Polycrystalline Cr-doped hematite thin films have been successfully deposited on fluorine-doped tin oxide coated (FTO) glass substrates using the facile hydrothermal method. The hydrothermal bath consists of an aqueous solution containing a mixture of FeCl3.6H2O and NaNO3 adjusted to a pH = 1.5. The samples were introduced in an autoclave and heated for a fixed duration and temperature. Afterward, the hematite coated samples were annealed for a 4 h at 550°C. The Cr doping fractions were varied from 2 to 20 %. All samples were submitted to structural and morphological studies using X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM) and High-resolution transmission electron microscopy (HRTEM). Cr doping induces a slight shift of the main diffraction peaks (012) and (104) towards lower angles. On the other hand, chronoamperometry technique showed that Cr-doped films exhibited higher photoelectrochemical activity relatively to un-doped α-Fe2O3 thin films. Maximum photocurrent densities as well as incident photon conversion efficiencies (IPCE) have been obtained for 16% Cr-doped films in a normal alcaline solution and under standard illumination conditions. We attributed this high photoactivity to the high active surface area of the nanostructured hematite and to the increasing donor density caused by Cr doping.

Resume : Measurement methods to quantitatively determine the doping in semiconductor nanowires (NW) are strongly requested for understanding the doping incorporation in such one-dimensional structures and so for developing technology which would use them. In the last two decades, scanning capacitance microscopy (SCM) and scanning spreading resistance microscopy (SSRM) based on atomic force microscopy, has emerged as promising tools for two-dimensional high resolution carrier/dopant profiling. These two techniques need of an accurate calibration method for a quantitative doping analysis. A calibration method allowing the quantitative measurement of n-type ZnO doping by SCM and SSRM has been established, based on cross-sectional scanning of multilayers samples with different Ga doping concentration. Then, in order to study ZnO NWs, we have developed a sample preparation method using dip-coating filling of vertical NWs field. The non-intentionally n-type doping (nid) of the ZnO nanowires have been measured using SCM and SSRM, estimated at 2.E18cm-3, explaining the difficulty to turn these NWs into p-type during p-type doping experiments, a crucial problematic in ZnO. Then we have successfully determined the decrease of carrier concentration with respect to the nid core ZnO in antimony (Sb) doping in nid ZnO core/ Sb ZnO shell NW structures. This carrier concentration lowering can be ascribed to the formation of Sb-related acceptors compensating the native donors in ZnO NW. The understanding of this electric compensation mechanism is a clear signature of p-type Sb doping in ZnO NW. This important result opens the way to a possible development of p-type doping in ZnO.

Resume : The solar cells using the electrode consisted of titania nanoparticles (NPs), such as dye-sensitized solar cells or perovskite solar cells, sometimes have very low power conversion efficiencies due to the lack of homogeneity of the layers comparing with that prepared by chemical or physical vaper deposition methods. It is important for improving power conversion efficiencies of solar cells to develop the method to prepare more homogenous titania NPs layer, and also to propose the technique to analyze the homogeneity of the layer, for example, Kelvin probe force microscopy (KPFM). In this study, we used KPFM with the frequency modulation system and measured the local working function of the thin layer of titania NPs prepared by the electrophoretic deposition method. When UV-light was irradiated on the titania NPs layer, the local surface potential of the layer increased immediately. After turn off the light, the local surface potential generally decreased and settled on the original potential at dark state. We found the local surface potential of this layer was more homogeneous than that prepared by the ordinal squeegee method.

Resume : Over the past 30 years, Atomic Force Microscopy has evolved from a microscope to measure just the surface topography to a wide variety of measurement modes that provides a way to characterize other atomic interactions or physical properties like magnetic field, electric field, nanoscale dissipation processes, thermal conductivity, electrical conductivity, resistance, surface potential, piezoresponse, Young modulus,… Electrical nanocharacterization with AFM has emerged as a powerful tool to map electrical properties at the nanoscale, like surface potential (work function) and conductivity. However, traditional setups in AFM make difficult to obtain accurate and repeteable results over several types of samples. In this contribution we will show the capabilities new developed AFM modes: High Definition Kelvin Force Microscopy (HD-KFM), ResiScope, Soft-Resiscope and Scaning Microwave that overcome the intrinsic difficulties of electrical nanocharacterization with AFM. This two techniques have been applied on a wide variety of substrates: bidimensional materials, like graphene or molibdene disulfide, organic and peruvskite solar cells or nanoparticles providing high stability, sensitivity and lateral resolution.

Resume : Over the last 30 years, scanning probe microscopy has emerged as a powerful tool for imaging structure and functionality of matter on nano- and atomic scales. In the most general terms, SPM images represent the convolution of dynamic interactions at the tip-surface junction as interrogated through chosen measurement protocol. Correspondingly, much of the attention of SPM practitioners was aimed at the development of progressively more complex protocols for excitation and detection, starting from original static modes in contact AFM, to lock-in and phase-locked loops in single frequency SPMs, to multifrequency methods of the last decade. The increase in sensitivity these developments have afforded have enabled access to capture new material functionalities. Along with this added capability is the requirement that the myriad of possible nanoscale phenomena be differentiated. Part of this talk will discuss the development of contact mode kelvin force probe microscopy (cKPFM) used to measure the induced charge on surfaces by a biased tip and differentiate this from piezo electric effects and ferroelectric switching thereby enabling more reliable measurements of nanoscale material behavior especially those related to charge migration. However, even such complex measurements invariably rely on a predefined model for processing the information flow, as limited by data analysis electronics. In this this presentation, I will also discuss existing approaches for information processing in SPM and introduce an approach for full information capture in SPM based on recording and more complete analysis of the data stream from photodetector. This general-mode (G-Mode) SPM is illustrated for classical SPM modes such as intermittent contact mode SPM and Kelvin probe microscopy. The analysis of the information allows deducing in which cases classical signal processing allows unbiased representation of the tip-surface interactions and which it incurs significant information loss. These approaches for full mapping of the frequency response provides a complete view of tip-surface interactions and also enables far higher temporal resolution than existing methods. Finally, perspective and critical needs for the full information analysis imaging in SPM are presented, and some possible solutions based on crowd sourcing data analytic platforms are discussed. This research is supported by and performed at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, BES DOE.

Resume : The elaboration of hybrid molecule/nanoparticle self-assemblies is a first step towards nanoelectronic systems, where the charging energy of the nanoparticles is tuned by the nature of the molecules. We recently developed simple chemical tools to tune Coulomb blockade in dense three-dimension self-assemblies of metallic nanoparticles. Several techniques give the possibility to measure charge transport in such systems (interdigitated combs, evaporated electrodes, dielectric spectroscopy, etc.) and to follow the response of a given sample to variation of external parameters (temperature, light, magnetic field). However, the experimental variability of such techniques is often too high to reliably compare different samples, and even more samples of different nature. To circumvent this drawback, we developed an original approach combining conductive AFM measurements and statistical analysis. Thanks to it, we were able to relate information between charge transport at the nanoscale and the structural geometry of the molecules, in the Coulomb blockade regime. This presentation will describe the advantage of this approach using a scanning probe technique, and its complementarity to other charge transport measurements at larger scales. Towards energy applications, the electrical behavior of self-assembled systems responsive to light will be described under irradiation.

Resume : Near-field microwave microscopy is an emerging tool exploiting reflection of GHz-frequency microwaves from a sharp scanning probe in contact with or in the vicinity of an object. Dependence of the microwave reflection from material properties allows non-destructive mapping and quantitative characterization of dielectric permittivity and conductivity with a spatial resolution as high as ~50 nm. The technique is versatile and can be straightforwardly applied to investigations of solids as well as liquids. In this talk, we illustrate the power of this technique in applications to ferroelectric thin films and in-situ imaging of processes in electrolytes. Ferroelectric as well as multiferroic materials and structures are currently considered for electrocaloric and photovoltaic applications. Using the technique, we were able to show that spontaneous and recorded domain walls in thin films of Pb(Zr0.2Ti0.8)O3 and BiFeO3 exhibit large conductance at GHz frequencies being insulating at dc, which points to the existence of free-carrier clouds at the walls. Simultaneous detection of piezo- and microwave responses separates relaxations of polarization and injected charges. We also demonstrate in situ imaging of dendrite formation near electrodes in a liquid electrolyte. This work was sponsored in part by DMSE, BES, US DOE. SPM was conducted at ORNL?s CNMS, sponsored by SUFD, BES, US DOE. 1. A. Tselev, et al., ACS Nano 10, 3562 (2016) 2. A. Tselev, et al., Nat. Comm. 7, 11630 (2016)

Resume : Understanding thermal transport in thin-films on the nanometer scale is essential for solar cells, thermoelectric generators, batteries, etc.. Consequently, advanced Scanning Probe Microscopy methods and novel approaches are mandatory. Within this contribution, the transition from diffusive in-plane heat flux to a ballistic Stefan?Boltzmann like out-of-plane heat transport will be studied in thin, amorphous TiO2 films prepared by atomic layer deposition. We employ Scanning Near-field Thermal Microscopy (SThM), which as we will demonstrate is perfectly suited to assess thermal transport in ultra-thin layers as the width of the heater/thermometer is very small. With the support of equivalent thermal circuit modelling we show, that coating of a substrate with thin films of higher thermal conductivity than that of the substrate does not immediately lead to an increase of the heat flux from a nanoscale heat source. On the contrary, the heat transport can be significantly reduced at certain layer thicknesses. Besides, already at film thicknesses, which are much larger than the phonon mean-free path, the description of thermal transport in this assembly has to take phonon scattering as well as ballistic thermal phonon transport phenomena into account. Our results are expected to significantly influence the thermal management and reliability investigation using thin film technologies in energy applications.

Resume : Intermodulation Electrostatic Force Microscopy (ImEFM) has been used for imaging the contact potential difference Vcpd on organic photovoltaic materials, as well as nanocomposite insulators and 2D electron gases. We now propose a time resolved variant of ImEFM that is able to image the rise and fall time of Vcpd, in response to an external illumination coherent with the tip oscillation. The technique uses the intermodulation of the cantilever mechanical oscillation frequency with the modulation frequency of a luminous excitation of the sample to extract the dynamics of the charge carriers. The method relies on a calibrated measurement of force and the intermodulation products are measured near the mechanical resonance, where the force sensitivity is close to the thermal limit. We describe the theory of the technique, its validation with simulations, and we show the first experimental results.

Resume : Proton-Exchange Membrane Fuel Cells (PEMFC) has emerged as a promising emission-free energy conversion device. We work on a new generation of hybrid membrane for fuel cell based on commercial sPEEK membrane modified with a Sol-Gel (SG) phase (mechanical and chemical stabilization). To study the process-structure-properties relationship of our alternative membranes we had to develop an innovative approach of characterization that couples Atomic Force Microscopy (AFM) and Raman microspectroscopy with an unusual sample preparation technique for polymer membranes: cryo-ultramicrotomy without epoxy embedding. Here we demonstrate the powerfulness of colocalized AFM-Raman analysis, revealing the inner structure of hybrid polymer membranes by probing the cross-section (accessing both bulk and surfaces). We obtained quantitative data on the diffusion of SG precursors through the host sPEEK membrane, and complementary AFM and Electron Microscopy (EM) measurements on the morphology of the SG phase. Co-localized analyses revealed the formation of a depleted skin layer (lower SG concentration) about one micrometer thick on both sides of the membrane (SG being homogeneously distributed otherwise). The diffusion of water-soluble species from the SG phase up to the cross-section surface, and the resulting crystallization of these species (morphologically, mechanically and chemically characterized by co-localized AFM-Raman), only observed for the membrane at the first step of fabrication, gave us an insight on the SG condensation process through the fabrication steps. Finally, the nano-mechanical data collected by AFM revealed a densification of the SG phase through the fabrication process.

Resume : The performance of polymer electrolyte fuel cells and electrolysers, depends on the nanostructure of its components. In particular, the nanostructured electrodes have an important impact on cell performance and degradation. In working fuel cell electrodes, the ionomer layers are in the range of 4-20 nm and fall within the range of ultrathin films. In this contribution, the analysis of cross-sections of fuel cell electrodes investigated by material-sensitive AFM will be presented. The high contrast between the ionomer- and the Pt/C phase in adhesion force mapping allowed studying the distribution and thickness of the ionomer layers that cover the Pt/C agglomerates. After operation, significant thinning of the ionomer layers, depending on the location within the MEA and the preparation, was found. Differences of the swelling behavior prior and after operation were assigned to ionomer degradation. The average ionomer layer thickness was found to correlate with the macroscopic degradation of fuel cells. As the thickness of the ionomer layer that covers the catalyst agglomerates is crucial for fuel cells performance, ultra-thin model layers of different commonly used ionomers (i.e. Nafion®, Aquivion® PFSA) and alternative materials were examined on their morphology and phase-separation on various substrates of different surface energies. Conductive AFM allowed investigations of through- and in-plane conductivity in dependence of the film thickness of these ultrathin films.

Resume : The reduced form of graphene oxide (GO) is an attractive alternative to graphene for producing large-scale flexible conductors and for creating devices that require an electronic gap. By controlling the reduction degree of GO, varied electronic properties of graphene which can be applied in the construction of electronics can be obtained. Herein, scanning polarization force microscopy (SPFM) is used to distinguish the one-atom-thick GO and reduced GO (rGO) sheets and monitor the thermal reduction process in-situ. The reduction degree related electrostatic properties such as the charge storage character of nanostructured and partially reduced GO sheets are further studied by using the scanning probe microscopy (SPM) based charging and discharging technique and the sample-charged mode SPFM (SC-SPFM). Furthermore, charged nanostructures with controllable geometry and charge density are achieved with the thermal nanolithography method. And such charged nanostructures could be used to construct charge gated graphene nanoelectronics. In the last part, we will briefly report the effects of injected charges to adjacent rGO sheets, such as the inhomogeneous charges distribution in rGO sheet induced by adjacent charged ones and the charges transfer behavior between physically separated rGO sheets on insulating substrates. These effects are of great value for rational design of graphene nanoelectronics with desired functionality.