Applied Scanning Probe Methods VIII: Scanning Probe Microscopy Techniques

2. Scanning Probe Microscopy—Principle of Operation, Instrumentation, and Probes
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In order to fulfil these requirements, specific functional materials are being developed, investigated and optimized. Energy-related materials often include electrochemical reactions and opto- electronic transport phenomena at their interfaces. In particular, material properties on the nanometer scale play a major role.

Applied Scanning Probe Methods VIII : Scanning Probe Microscopy Techniques

The understanding of these nanoscale phenomena occurring at material interfaces is therefore essential. Furthermore, these interface phenomena are strongly linked to material properties such as grain size, roughness, mechanical properties and work function. In an attempt to address the diversity of phenomena on the nanoscale, scanning probe microscopy SPM methods play an significant role for the in-operando characterization.

SPM methods offer a plethora of operation modes beyond topography imaging, which is well reflected in the articles of this thematic issue. The majority of contributions stem from research on photovoltaic materials. Here, electrical conductive atomic force microscopy cAFM and Kelvin probe force microscopy KPFM are the major methods that enable the study of the movement of charge carriers and their pathways [1].

We note that the KPFM method is rapidly becoming a tool capable of time-resolved studies. In this context, Yann Almadori and co-workers discuss the time-dependent changes of the surface potential occurring under illumination.

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This work also unravels lattice expansion phenomena under illumination in perovskite structure forming photo-absorbing materials [2]. Pablo A. This contribution focuses on a mathematical model to calculate time constants [3].

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Such a model is critical for understanding the photophysics at the nanometer scale. Amelie Axt and co-workers discuss the applicability and reliability of different ways of performing KPFM measurements on nanoscale electrical devices [4]. In particular, the knowledge of the true potential of surfaces is required for the analysis of cross-sections of solar cell devices [5,6]. Thus, this work is the basis for future quantitative analysis of nanoscale devices even beyond the scope of solar cells. Katherine Atamanuk and co-workers impressively demonstrate that SPM methods can also be used to perform tomography [7].

They apply photoconducting scanning force microscopy for mapping the open-circuit voltage of cadmium telluride CdTe polycrystalline thin film solar cells. Tomography is achieved by gradually removing surface material during continuous high-load topographic imaging. For photovoltaic materials, the interface between materials accepting electrons or holes is of crucial importance.

Laurie Letertre and co-workers study a nanocolumnar TiO 2 surface covalently grafted with a monolayer of poly 3-hexylthiophene functionalized with carboxylic groups [8]. Their study unravels the physical mechanisms taking place locally during the photovoltaic process and its correlation to the nanoscale morphology.

Electrochemical energy storage i. Jonathan Op de Beeck and co-workers identify the ionic processes occurring inside Li-ion composites in order to understand the impact on the entire battery cell [9]. In particular, the authors combine cAFM and secondary-ion mass spectrometry to correlate the presence of nanometer-sized conductive paths with the Li concentration. This study exemplifies that SPM combined with complementary methods providing information on the chemistry or atomic composition of materials is very beneficial for understanding the performance of devices.

Furthermore, dielectric properties play a role for the storage of electrochemical energy. Ying Wang and co-workers report on a novel method for the characterization of the local dielectric distribution based on surface adhesion mapping by SPM [11]. This method is evidently easy in terms of operation and thus has the potential to be widely used.

Electrostatic force spectroscopy EFS is used here to characterize the degree of reduction of uniformly reduced one-atom-thick graphene oxide GO sheets at the nanoscale. The identification and chemical control of the degree of reduction of GO sheets is highly desired to realize nanoscale electronic devices in the future. We thank all authors for participating with their contribution to this thematic issue. The published manuscripts will be a significant contribution to the advancement of the field of understanding energy materials on the nanometer scale.

In addition, we acknowledge the expertise of the reviewers who provided helpful reports to us and the authors.

Consistent permittivity values were also deduced by comparison of experimental data and numerical simulations, as well as the interface state of silicone dioxide layers. Keywords: atomic force microscopy; building-block materials; dielectric permittivity; electrostatic force microscopy; finite element simulation; interphases; nanocomposites. Although the mechanical and thermal behavior of the base insulating polymer can be enhanced by microcomposites, its electrical performance is usually degraded [4,5].

The interphase region can range from a few angstroms to a few nanometers, and usually has properties that differ from those of the polymer and filler. For instance, although the inorganic filler usually displays higher permittivity values than the polymer, the resulting nanofiller mixture in the matrix presents, at low nanofiller concentrations, a surprisingly lower dielectric constant than that of the two mixture components []. It is commonly agreed that the surface interaction of the nanoinclusions with the host polymer acts to rearrange the polymer chains and reduces their mobility at the interface [].

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The mobility reduction can lead to a decrease of the permittivity of the polar polymer, and consequently, also of the interphase. Moreover, it has been reported that water can be absorbed at the interface []. Water molecules within the interphase increase its effective dielectric constant, and this can explain the unexpectedly higher nanocomposite permittivity. Therefore, the local characterization of the interphase is of utmost importance to explain most of the nanocomposite macroscopic behavior, particularly their electrical polarization properties.

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This local characterization requires high-resolution techniques that are sensitive to the dielectric properties of the material. These two conditions are fulfilled by electrostatic force microscopy EFM [18,19]. EFM is an atomic force microscopy AFM -based electrostatic method in which a conductive tip and a metallic sample holder are used.

The probe-to-stage system is electrically polarized for the detection of electrostatic forces or force gradients. A proper interpretation of EFM results allows for the determination of the dielectric permittivity and dimensions of the sample components. Importantly, EFM is particularly suitable for electrical insulators, as opposed to electron microscopy where the rough specimen preparation procedures and electron beam bombardment can irreversibly electrically charge the material [20,21].

Moreover, while in the case of composites with 1D or 2D inclusions, the interphase can be directly accessed after cross-sectioning [20,21] ; interphase characterization is more difficult for nanoparticle-filled materials. Indeed, the interphase is likely to be confined between the particles and the matrix below the probed region. Therefore, as the probing field is electrical in EFM, and this technology is expected to offer higher subsurface sensitivity compared with other scanning probe microscopy methods [].

Subsurface imaging and 3D-multilayered structure studies with EFM have resulted in advances in our knowledge of specific types of materials. For instance, EFM comparison of the nanofiller diameter before and after insertion into the polymer matrix [26] showed an increase of the apparent particle diameter in the matrix that was attributed to the interphase.

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However, some experimental conditions during the comparison were not similar. Moreover, the highly probable presence of a matrix layer over the particles near the surface in the nanocomposite was ignored, although a matrix covering the nanoparticles would increase their apparent size even without an interphase. In a more rigorous study [27] , Peng et al. To explain this EFM signal reduction, the authors hypothesized that an interphase with lower permittivity than that of particles and fillers surrounds the particles.

However, the authors did not compare this remarkable change in local dielectric permittivity with macroscopic dielectric spectroscopy measurements. Nevertheless, the interphase characterization in nanocomposites is still inadequate. One common issue in these EFM-based works is that the exact source of the measured signal was not completely identified. Therefore, the measurement of complex materials remains a big challenge, mainly due to the complex geometry of the probe that scatters the electric field, and the long range nature of the electrostatic forces that complicate the identification of the actual probed region.

Therefore, the objectives of this study were to determine whether EFM can identify an interface region, and most importantly, to identify the appropriate experimental methods to extract the artifact-free EFM signal of the interphase. These were made of a stack of three dielectric constituents that represent a simplified configuration of the particle—interphase—matrix assembly found in nanodielectrics. Specifically, polystyrene PS nanoparticles of nm diameter were prepared by self-assembly on metallized substrates.

Then, two shells of variable thicknesses 50 and nm were deposited or grown over the whole sample surface.

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The signature of each dielectric constituent was correlated to its dielectric permittivity. EFM measurements were performed using the double-pass method, while extracting the frequency shifts due to the acting electrostatic force gradients over the probe. During the second scan, the system was polarized at an AC voltage and the tip was retracted from the surface at a known distance, called the lift distance. At this stage, the component of the frequency shift that varies at the double frequency of the applied electrical voltage was extracted because it represents the purely capacitive response of the material.

In our previous work, we verified that EFM can distinguish homogeneous from heterogeneous stacked materials and provide information on the permittivity of their constituents relative to each other [28]. Particularly, at a constant tip—sample distance, the signal increases with the thickness of the added material, and this can only be explained by an increase of the effective permittivity of the global material. This confirms the higher permittivity of the additional component compared with the initial one.

In all the tested model samples, the particle topography could be determined by a line scan because the covering layers were prepared in such a way to keep the curvature associated with the spheres. Completely embedded particles could also be used. Moreover, discriminating the particles from the topography is important in the case of nanocomposites that include particles and matrix with low dielectric permittivity difference.

These samples mimicked a nanodielectric that has interphase Al 2 O 3 without and with a matrix surface layer, respectively. Figure 1: Interphase detection, approach 1: schematic representation of the samples to be compared, and methodology. Figure 1: Interphase detection, approach 1: schematic representation of the samples to be compared, and metho Conversely, in the presence of the Al 2 O 3 interphase, alumina is supposed to enhance the effective permittivity of the covered particles and, consequently, PVAc addition should decrease the signal i.

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The topography height was lower in samples with than in those without the matrix surface layer. Additional nanomechanical measurements indicated that this decrease was due to the non-uniformly thick spin-coated PVAc film [38]. In fact, as our samples were highly rough owing to the sub-micrometer PS spheres, spin-coating could not produce uniformly thin films [37].

However, this did not affect our study because the EFM results were compared at the center of the particles. Concerning the EFM signals, the average values calculated around the center were This result confirmed the sensitivity of the first approach within the correct geometry and permittivity ratios in order to detect the region at the interface between PS particles and PVAc matrix. However, this methodology is limited by its concept based on comparing nanodielectrics with and without an upper matrix layer. Indeed, it is difficult to produce samples that lack a surface matrix layer without influencing the initial properties of the interphase.

Therefore, it would be more realistic to compare samples that model nanodielectrics with and without an interphase. To this aim, matrix deposition required a technique that allows the precise control of the thickness. This method can precisely and homogeneously spread SiO 2 molecules over the whole sample surface, quite similar to ALD, as explained in the Experimental section [40].

Figure 3: Interphase detection, approach 2: schematic representation of the samples to be compared, and methodology. Figure 3: Interphase detection, approach 2: schematic representation of the samples to be compared, and metho Although in our comparisons the added material was not at the surface but in the middle, the principle was the same [38]. The maximum corresponded to the response from the bottom of the sample, and the lowest peak was the response from the particle center. This indicates that EFM can deduce the presence of an embedded Al 2 O 3 interphase layer underneath the SiO 2 matrix, and corroborates the hypothesis underlying this approach.