Nitrous oxide as an effective AFM tip functionalization: a comparative study

• Taras Chutora,
• Bruno de la Torre,
• Pingo Mutombo,
• Jack Hellerstedt,
• Jaromír Kopeček,
• Pavel Jelínek and
• Martin Švec

Beilstein J. Nanotechnol. 2019, 10, 315–321, doi:10.3762/bjnano.10.30

• spectroscopy measurements, i.e., the interaction energy toward different atomic species in force spectroscopy, the contact potential difference in Kelvin probe force microscopy (KPFM) [9][29] and vibrational levels of inelastic tunneling spectroscopy (IETS) [30][31]. A particular termination of the tip may be

Scanning probe microscopy for energy-related materials

• Rüdiger Berger,
• Benjamin Grévin,
• Philippe Leclère and
• Yi Zhang

Beilstein J. Nanotechnol. 2019, 10, 132–134, doi:10.3762/bjnano.10.12

• 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

• 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

A scanning probe microscopy study of nanostructured TiO₂/poly(3-hexylthiophene) hybrid heterojunctions for photovoltaic applications

• Laurie Letertre,
• Roland Roche,
• Olivier Douhéret,
• Hailu G. Kassa,
• Denis Mariolle,
• Nicolas Chevalier,
• Łukasz Borowik,
• Philippe Dumas,
• Benjamin Grévin,
• Roberto Lazzaroni and
• Philippe Leclère

Beilstein J. Nanotechnol. 2018, 9, 2087–2096, doi:10.3762/bjnano.9.197

• photovoltaic process and the correlation to the nanoscale morphology. A down-shift of the vacuum level of the TiO2 surface upon grafting was measured by Kelvin probe force microscopy (KPFM), evidencing the formation of a dipole at the TiO2/P3HT-COOH interface. Upon in situ illumination, a positive photovoltage

• was observed as a result of the accumulation of photogenerated holes in the P3HT layer. A positive photocurrent was recorded in PC-AFM measurements, whose spatial mapping was interpreted consistently with the corresponding KPFM analysis, offering a correlated analysis of interest from both a

• theoretical and material design perspective. Keywords: hybrid heterojunctions; hybrid photovoltaic; Kelvin probe force microscopy; photoconductive-AFM; photo-KPFM; poly(3-hexylthiophene); TiO2; Introduction Over the past decades, a large range of photovoltaic (PV) technologies have been developed for the

Numerical analysis of single-point spectroscopy curves used in photo-carrier dynamics measurements by Kelvin probe force microscopy under frequency-modulated excitation

• Pablo A. Fernández Garrillo,
• Benjamin Grévin and
• Łukasz Borowik

Beilstein J. Nanotechnol. 2018, 9, 1834–1843, doi:10.3762/bjnano.9.175

• this context, few teams around the world have recently began to develop time-resolved scanning probe microscopies (SPM) techniques, aimed at addressing the photo-carrier dynamics at the local scale in photoactive materials and devices. At this point, Kelvin probe force microscopy (KPFM) emerged as a

• useful technique that, when implemented under frequency-modulated excitation, can be used to investigate the surface photovoltage decay, thus providing access to the photo-carrier dynamics [3][4][5][6][7][8][9][10][11]. A common aspect among all KPFM frequency-modulated spectroscopy techniques is that in

• describe the general aspects of this simulation routine, and we compare it against experimental results from a previous work were single-point Kelvin probe force microscopy under frequency-modulated illumination (FMI-KPFM) was implemented over a silicon nanocrystal solar cell [3]. Analogously, we compare

Know your full potential: Quantitative Kelvin probe force microscopy on nanoscale electrical devices

• Amelie Axt,
• Ilka M. Hermes,
• Victor W. Bergmann,
• Niklas Tausendpfund and
• Stefan A. L. Weber

Beilstein J. Nanotechnol. 2018, 9, 1809–1819, doi:10.3762/bjnano.9.172

• investigate the influence of the operation method in Kelvin probe force microscopy (KPFM) on the measured potential distribution. KPFM is widely used to map the nanoscale potential distribution in operating devices, e.g., in thin film transistors or on cross sections of functional solar cells. Quantitative

• surface potential measurements are crucial for understanding the operation principles of functional nanostructures in these electronic devices. Nevertheless, KPFM is prone to certain imaging artifacts, such as crosstalk from topography or stray electric fields. Here, we compare different amplitude

• modulation (AM) and frequency modulation (FM) KPFM methods on a reference structure consisting of an interdigitated electrode array. This structure mimics the sample geometry in device measurements, e.g., on thin film transistors or on solar cell cross sections. In particular, we investigate how quantitative

Multimodal noncontact atomic force microscopy and Kelvin probe force microscopy investigations of organolead tribromide perovskite single crystals

• Yann Almadori,
• David Moerman,
• Jaume Llacer Martinez,
• Philippe Leclère and
• Benjamin Grévin

Beilstein J. Nanotechnol. 2018, 9, 1695–1704, doi:10.3762/bjnano.9.161

• du Parc 20, B7000 Mons, Belgium 10.3762/bjnano.9.161 Abstract In this work, methylammonium lead tribromide (MAPbBr3) single crystals are studied by noncontact atomic force microscopy (nc-AFM) and Kelvin probe force microscopy (KPFM). We demonstrate that the surface photovoltage and crystal

• photocarrier lifetime is quantified by performing KPFM measurements under frequency-modulated illumination. Our multimodal approach provides a unique way to investigate the interplay between the charges and ionic species, the photocarrier-lattice coupling and the photocarrier dynamics in hybrid perovskites

• . Keywords: carrier lifetime; ion migration; Kelvin probe force microscopy (KPFM); noncontact atomic force microscopy (nc-AFM); organic–inorganic hybrid perovskites; photostriction; single crystals; surface photovoltage (SPV); time-resolved surface photovoltage; Introduction Organic–inorganic hybrid

Artifacts in time-resolved Kelvin probe force microscopy

• Sascha Sadewasser,
• Nicoleta Nicoara and
• Santiago D. Solares

Beilstein J. Nanotechnol. 2018, 9, 1272–1281, doi:10.3762/bjnano.9.119

• 10.3762/bjnano.9.119 Abstract Kelvin probe force microscopy (KPFM) has been used for the characterization of metals, insulators, and semiconducting materials on the nanometer scale. Especially in semiconductors, the charge dynamics are of high interest. Recently, several techniques for time-resolved

• measurements with time resolution down to picoseconds have been developed, many times using a modulated excitation signal, e.g., light modulation or bias modulation that induces changes in the charge carrier distribution. For fast modulation frequencies, the KPFM controller measures an average surface

• potential, which contains information about the involved charge carrier dynamics. Here, we show that such measurements are prone to artifacts due to frequency mixing, by performing numerical dynamics simulations of the cantilever oscillation in KPFM subjected to a bias-modulated signal. For square bias

Anchoring of a dye precursor on NiO(001) studied by non-contact atomic force microscopy

• Sara Freund,
• Antoine Hinaut,
• Nathalie Marinakis,
• Edwin C. Constable,
• Ernst Meyer,
• Catherine E. Housecroft and
• Thilo Glatzel

Beilstein J. Nanotechnol. 2018, 9, 242–249, doi:10.3762/bjnano.9.26

• recorded by Kelvin probe force microscopy (KPFM) [68]. The CPD arises from the work function difference between the tip and the substrate and is altered by surface charges or dipoles. The voltage needed to compensate for the electrostatic forces due to this potential difference is measured in KPFM. The CPD

• qualitative models, that the adsorption symmetries of DCPDMbpy with respect to the crystallographic direction on the (001) terraces of NiO are the same, regardless whether they are single, forming windmill clusters or packing in islands. By combined nc-AFM and KPFM measurements an average charge transfer from

Analysis and modification of defective surface aggregates on PCDTBT:PCBM solar cell blends using combined Kelvin probe, conductive and bimodal atomic force microscopy

• Hanaul Noh,
• Alfredo J. Diaz and
• Santiago D. Solares

Beilstein J. Nanotechnol. 2017, 8, 579–589, doi:10.3762/bjnano.8.62

• of PSCs [15], thus providing insights into the operating mechanism of PSCs. Among all the available modes of AFM, conductive AFM (C-AFM) and Kelvin probe force microscopy (KPFM) are the most widely used for PSC research, since the measured current and electrical potential can reveal local charge

• , the solubility differences of donor and acceptor materials and the boiling point of the solvents affect the morphology and thus the performance of PSCs [20]. The PCDTBT:PCBM blend fabricated with the DCB solvent has shown better performance than the one cast from CB [21]. We performed KPFM

• topography changes on the aged sample. The features in this sample exhibit an interesting behavior. They gradually disappear over consecutive KPFM scans (see Figure S5, Supporting Information File 1). This phenomenon only happens for the aged samples, whereas fresh samples do not change over time

Scanning probe microscopy studies on the adsorption of selected molecular dyes on titania

• Jakub S. Prauzner-Bechcicki,
• Lukasz Zajac,
• Piotr Olszowski,
• Res Jöhr,
• Antoine Hinaut,
• Thilo Glatzel,
• Bartosz Such,
• Ernst Meyer and
• Marek Szymonski

Beilstein J. Nanotechnol. 2016, 7, 1642–1653, doi:10.3762/bjnano.7.156

• results, the next step should be aimed on monitoring the influence of the oxygen vacancies density on the averaged level alignment between a semiconducting substrate and a full organic layer. Another method used to understand the molecule-substrate interactions is Kelvin probe force microscopy (KPFM

• , i.e., there is no deprotonation [60]. From KPFM and the calculations, Jöhr et al. concluded that a charge is transferred to the substrate upon adsorption and a dipole moment that points away from the surface is formed, regardless of the adsorption geometry. The presence of this type of dipole moment

Advanced atomic force microscopy techniques III

• Thilo Glatzel and
• Thomas Schimmel

Beilstein J. Nanotechnol. 2016, 7, 1052–1054, doi:10.3762/bjnano.7.98

• presented by Andrzej J. Kulik and co-workers [30]. High-resolution measurements of the adhesion effect of a water film on CaF2 [31], electric and transport phenomena determined by liquid KPFM in ionically-active and -inactive liquids [32], the spray deposition of single molecules to insulating and ionic

High-resolution noncontact AFM and Kelvin probe force microscopy investigations of self-assembled photovoltaic donor–acceptor dyads

• Benjamin Grévin,
• Pierre-Olivier Schwartz,
• Laure Biniek,
• Martin Brinkmann,
• Nicolas Leclerc,
• Elena Zaborova and
• Stéphane Méry

Beilstein J. Nanotechnol. 2016, 7, 799–808, doi:10.3762/bjnano.7.71

• dyads are used as model nanostructured heterojunctions for local investigations by noncontact atomic force microscopy (nc-AFM) and Kelvin probe force microscopy (KPFM). With the aim to probe the photo-induced charge carrier generation, thin films deposited on transparent indium tin oxide substrates are

• elementary building block level. Keywords: donor–acceptor co-oligomers; donor–acceptor lamellae; donor–acceptor-ordered bulk heterojunction; Kelvin probe force microscopy (KPFM); noncontact atomic force microscopy (nc-AFM); organic photovoltaics; surface photo-voltage (SPV); Introduction Nowadays, with

• ) materials, arranged in interpenetrated networks at the 10 nm scale to efficiently separate the excitons into free charges at the D–A interface. In the past decade, several studies demonstrated that Kelvin probe force microscopy (KPFM) can be powerfully combined with atomic force microscopy (AFM) to

Large area scanning probe microscope in ultra-high vacuum demonstrated for electrostatic force measurements on high-voltage devices

• Urs Gysin,
• Thilo Glatzel,
• Thomas Schmölzer,
• Adolf Schöner,
• Sergey Reshanov,
• Holger Bartolf and
• Ernst Meyer

Beilstein J. Nanotechnol. 2015, 6, 2485–2497, doi:10.3762/bjnano.6.258

• range of 100 μm in lateral and 25 μm in vertical direction as well as an additional fibre optics. This enables the illumination of the tip–sample interface for optically excited measurements such as local surface photo voltage detection. Results: We present Kelvin probe force microscopy (KPFM

• ) measurements before and after sputtering of a copper alloy with chromium grains used as electrical contact surface in ultra-high power switches. In addition, we discuss KPFM measurements on cross sections of cleaved silicon carbide structures: a calibration layer sample and a power rectifier. To demonstrate

• materials), separated (e.g., electrostatic forces from magnetic forces), or be dynamically compensated (e.g., by tuning the bias voltage in Kelvin probe force microscopy (KPFM)) and measured together with the topological information. For all these properties various experimental approaches have been

Graphene on SiC(0001) inspected by dynamic atomic force microscopy at room temperature

• Mykola Telychko,
• Jan Berger,
• Zsolt Majzik,
• Pavel Jelínek and
• Martin Švec

Beilstein J. Nanotechnol. 2015, 6, 901–906, doi:10.3762/bjnano.6.93

• specifically chosen for the conditions when the graphene contrast provided by the Δf is not giving atomic resolution in the attractive regime. Kelvin probe force measurements (KPFM) were also done in the constant-height mode, with a slow feedback between the measurement points, to compensate the tilt of the

• sample. KPFM parabola was measured by sweeping the bias voltage and measuring the Δf value [24]. In order to estimate the SLG corrugation we carried out large scale total energy density functional theory (DFT) calculations. We used local-orbital FIREBALL code [25][26]. FIREBALL uses an optimized [27

Kelvin probe force microscopy in liquid using electrochemical force microscopy

• Liam Collins,
• Stephen Jesse,
• Jason I. Kilpatrick,
• Alexander Tselev,
• M. Baris Okatan,
• Sergei V. Kalinin and
• Brian J. Rodriguez

Beilstein J. Nanotechnol. 2015, 6, 201–214, doi:10.3762/bjnano.6.19

• (KPFM) has emerged as a powerful technique for probing electric and transport phenomena at the solid–gas interface. The extension of KPFM capabilities to probe electrostatic and electrochemical phenomena at the solid–liquid interface is of interest for a broad range of applications from energy storage

• to biological systems. However, the operation of KPFM implicitly relies on the presence of a linear lossless dielectric in the probe–sample gap, a condition which is violated for ionically-active liquids (e.g., when diffuse charge dynamics are present). Here, electrostatic and electrochemical

• and non-polar liquids), KPFM and EcFM are both feasible, yielding comparable contact potential difference (CPD) values. In ionically-active liquids, KPFM is not possible and EcFM can be used to measure the dynamic CPD and a rich spectrum of information pertaining to charge screening, ion diffusion

Highly NO₂ sensitive caesium doped graphene oxide conductometric sensors

• Carlo Piloto,
• Marco Notarianni,
• Mahnaz Shafiei,
• Elena Taran,
• Dilini Galpaya,
• Cheng Yan and
• Nunzio Motta

Beilstein J. Nanotechnol. 2014, 5, 1073–1081, doi:10.3762/bjnano.5.120

• microscopy (FESEM), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), Raman spectroscopy and Kelvin probe force microscopy (KPFM). XPS data were acquired using a Kratos Axis ULTRA X-ray photoelectron spectrometer incorporating a 165 mm hemispherical electron energy analyser. The incident

• performed by using an ‘‘inVia Renishaw Raman microscope’’ with λ = 532 nm operated at 35 mW, with a 1 μm spot size, to investigate bond changes and defects in the material. The KPFM was performed with a commercial AFM (Cypher-Asylum Research) equipped with an air temperature controller (ATC). The ATC flows

• temperature regulated, HEPA (High-Efficiency Particulate Absorption) filtered air through the Cypher enclosure. Closed-loop temperature control isolates the AFM from room temperature variations, minimizing thermal drift for imaging. During measurements the temperature was kept constant at 26 °C. For all KPFM

Uncertainties in forces extracted from non-contact atomic force microscopy measurements by fitting of long-range background forces

• Adam Sweetman and
• Andrew Stannard

Beilstein J. Nanotechnol. 2014, 5, 386–393, doi:10.3762/bjnano.5.45

• exception to this is the discussion that has surrounded Kelvin probe force microscopy (KPFM) where accurate modelling of this long-range regime is critical to interpreting results [8][9][10]. Nonetheless, long-range forces are readily subtracted in the literature using this method, often using simplistic

Effect of contaminations and surface preparation on the work function of single layer MoS₂

• Oliver Ochedowski,
• Kolyo Marinov,
• Nils Scheuschner,
• Artur Poloczek,
• Benedict Kleine Bussmann,
• Janina Maultzsch and
• Marika Schleberger

Beilstein J. Nanotechnol. 2014, 5, 291–297, doi:10.3762/bjnano.5.32

• materials employed in electronic devices in order to optimize their performance. In this work we used NC-AFM in combination with quantitative KPFM to study the influence of the substrate material and the processing on single layer MoS2 during device fabrication. We find a strong influence of contaminations

• impact on the surface potential. Second, that by choosing appropriate materials the work function can be modified to reduce contact resistance. Keywords: KPFM; MoS2; NC-AFM; surface potential; work function; Introduction Due to their unique properties which can differ a lot compared to bulk materials

• , e.g., lower the contact resistance and improve their performance. First experiments adressing this issue for MoS2 by using Kelvin probe force microscopy (KPFM) have already been reported [28][29]. However, these measurements were not done on SLM but bilayer MoS2 (BLM) and higher layer numbers and the

Noncontact atomic force microscopy II

• Mehmet Z. Baykara and
• Udo D. Schwarz

Beilstein J. Nanotechnol. 2014, 5, 289–290, doi:10.3762/bjnano.5.31

• how new experimental methods that are based on the working principle of NC-AFM are continuously being developed, which is documented by a number of papers dealing with multi-frequency AFM as well as Kelvin probe force microscopy (KPFM). We thank all the scientists who have submitted their outstanding

Routes to rupture and folding of graphene on rough 6H-SiC(0001) and their identification

• M. Temmen,
• O. Ochedowski,
• B. Kleine Bussmann,
• M. Schleberger,
• M. Reichling and
• T. R. J. Bollmann

Beilstein J. Nanotechnol. 2013, 4, 625–631, doi:10.3762/bjnano.4.69

• Kelvin probe force microscopy (KPFM). SHI irradiation results in rupture of the SLG sheets, thereby creating foldings and bilayer graphene (BLG). Applying the other modification methods creates enlarged (twisted) graphene foldings that show rupture along preferential edges of zigzag and armchair type

• ) measured by KPFM. Keywords: graphene; Kelvin probe force microscopy (KPFM), local contact potential difference (LCPD); non-contact atomic force microscopy (NC-AFM); SiC; Introduction Since its discovery in 2004 [1], graphene, the 2D crystal with a honeycomb lattice of sp2-bonded carbon atoms, has been

• different BLG stackings, we investigate the topography by non-contact atomic force microscopy (NC-AFM) combined with measuring the local contact potential differences (LCPD) using Kelvin probe force microscopy (KPFM). Experimental Graphene is exfoliated from a HOPG crystal (Momentive Performance Materials

Kelvin probe force microscopy of nanocrystalline TiO₂ photoelectrodes

• Alex Henning,
• Gino Günzburger,
• Res Jöhr,
• Yossi Rosenwaks,
• Biljana Bozic-Weber,
• Catherine E. Housecroft,
• Edwin C. Constable,
• Ernst Meyer and
• Thilo Glatzel

Beilstein J. Nanotechnol. 2013, 4, 418–428, doi:10.3762/bjnano.4.49

• built-in potential on the DSC performance at the TiO2/SnO2:F interface, investigated on a nanometer scale by KPFM measurements under visible light illumination, has not been resolved so far. Keywords: atomic force microscopy (AFM); dye-sensitized solar cells (DSC); Kelvin probe force microscopy (KPFM

• select surface treatments that are beneficial for the DSC performance. In order to achieve a nanometer scale resolution, SPV spectroscopy can be combined with Kelvin probe force microscopy (KPFM) [30][31][32], an atomic force microscopy (AFM) technique that was introduced in 1991 [33]. KPFM is a surface

• -potential detection method that determines the contact potential difference (CPD) during scanning by compensating the electrostatic forces between a microscopic tip and the sample [34]. Figure 2a illustrates a schematic band diagram for a KPFM tip in close proximity to a semiconductor sample surface with

Graphite, graphene on SiC, and graphene nanoribbons: Calculated images with a numerical FM-AFM

• Fabien Castanié,
• Laurent Nony,
• Sébastien Gauthier and
• Xavier Bouju

Beilstein J. Nanotechnol. 2012, 3, 301–311, doi:10.3762/bjnano.3.34

• sample upon interaction into account. One of the main difficulties here is to handle the different time scales that characterize the different dynamic behaviors of the oscillator and the AFM junction atoms. Finally, a Kelvin probe force microscopy module (KPFM) [86][87][88] will be included in a near

• implemented with the n-AFM, advanced first-principles methods [92] are well adapted to deal with local changes of electronic structure when the tip interacts with the sample surface, especially for KPFM [93][94]. For weak chemical interactions and van der Waals forces, theoretical studies have demonstrated

Junction formation of Cu₃BiS₃ investigated by Kelvin probe force microscopy and surface photovoltage measurements

• Fredy Mesa,
• William Chamorro,
• William Vallejo,
• Robert Baier,
• Thomas Dittrich,
• Alexander Grimm,
• Martha C. Lux-Steiner and
• Sascha Sadewasser

Beilstein J. Nanotechnol. 2012, 3, 277–284, doi:10.3762/bjnano.3.31

• distribution for In2S3 compared to that of CdS and ZnS. For In2S3 and CdS buffer layers the KPFM experiments indicate negatively charged Cu3BiS3 grain boundaries resulting from the deposition of the buffer layer. Macroscopic measurements of the surface photovoltage at variable excitation wavelength indicate

• films and different buffer layers, investigated by KPFM, locally resolved SPV measurements, and macroscopic spectral SPV measurements. Results and Discussion Chemical surface analysis by X-ray photoelectron spectroscopy (XPS) For the validity and interpretation of surface-sensitive KPFM measurements, it

• is important to know the state of the surface of the examined sample. Surface oxidation can modify the work function of the sample and complicate data analysis. To clean the surface of Cu3BiS3 samples, we used an NH3 treatment prior to introduction into the ultrahigh-vacuum (UHV) system of the KPFM

An NC-AFM and KPFM study of the adsorption of a triphenylene derivative on KBr(001)

• Antoine Hinaut,
• Adeline Pujol,
• Florian Chaumeton,
• David Martrou,
• André Gourdon and
• Sébastien Gauthier

Beilstein J. Nanotechnol. 2012, 3, 221–229, doi:10.3762/bjnano.3.25

• designed molecule, consisting of a flat aromatic triphenylene core equipped with six flexible propyl chains ending with polar cyano groups, is investigated by using atomic force microscopy in the noncontact mode (NC-AFM) coupled to Kelvin probe force microscopy (KPFM) in ultrahigh vacuum at room

• room for progress as shown by the impressive submolecular resolution that has been demonstrated in recent works on the adsorption of pentacene [14] or decastarphene [15] molecules on Cu(111) and on a NaCl(001) bilayer on Cu(111). During the same period, Kelvin probe force microscopy (KPFM) has been

• first results of a coupled NC-AFM and KPFM study of the adsorption on KBr(001) of 2,3,6,7,10,11-hexa(cyanopropyloxy)triphenylene (HCPTP), presented in Figure 1. This molecule was designed to adsorb strongly on an alkali halide surface in the hope of blocking its diffusion at room temperature. It is

Quantitative multichannel NC-AFM data analysis of graphene growth on SiC(0001)

• Christian Held,
• Thomas Seyller and
• Roland Bennewitz

Beilstein J. Nanotechnol. 2012, 3, 179–185, doi:10.3762/bjnano.3.19

• decomposition in ultrahigh vacuum and in an argon atmosphere are compared and the respective growth mechanisms discussed. Keywords: FM-AFM; graphene; 6H-SiC(0001); KPFM; SPM; Introduction Graphene grows epitaxially on the Si face of 6H-SiC(0001) by thermal decomposition in vacuum or an inert atmosphere

• identification of the graphene layer thickness from the local contact potential as determined by means of Kelvin probe force microscopy (KPFM) [11][12]. As a further advantage, KPFM determines step heights more accurately than STM or AFM with constant bias [13] and is therefore employed in this study to

• almost the same carbon density as one layer of graphene [16]. Experimental Noncontact atomic force microscopy (NC-AFM) measurements were performed in ultrahigh vacuum (UHV, p < 2·10−10 mbar) by means of a home-built microscope similar to the one described in [17]. Kelvin probe force microscopy (KPFM