Polymorphic self-assembly of pyrazine-based tectons at the solution–solid interface

• Achintya Jana,
• Puneet Mishra and
• Neeladri Das

Beilstein J. Nanotechnol. 2019, 10, 494–499, doi:10.3762/bjnano.10.50

• polymorphic self-assembly of these molecules on a HOPG substrate. Two different molecular packing structures with equal distribution are observed. Detailed analysis of the STM images emphasizes the crucial role of weak intermolecular hydrogen bonding, and molecule–substrate interactions in the formation of

• the observed polymorphs. Such weak hydrogen bonding interactions are highly desirable for the formation of modular supramolecular architectures since they can provide sufficiently robust molecular structures and also facilitate error correction. Keywords: highly oriented pyrolytic graphite (HOPG

• –HOPG interface using scanning tunneling microscopy (STM) technique under ambient conditions. The molecules belong to a new class of pyrazine/triazine-based molecules, containing two or more pyridine pendant units, and can act as a precursor to several two- and three-dimensional supramolecular

Investigation of CVD graphene as-grown on Cu foil using simultaneous scanning tunneling/atomic force microscopy

• Majid Fazeli Jadidi,
• Umut Kamber,
• Oğuzhan Gürlü and
• H. Özgür Özer

Beilstein J. Nanotechnol. 2018, 9, 2953–2959, doi:10.3762/bjnano.9.274

• an inequivalent electronic structure in HOPG or multilayer graphene due to the presence of a carbon atom or a hollow site underneath. In this work, we report small-amplitude, simultaneous STM/AFM imaging using a metallic (tungsten) tip, of the graphene surface as-grown by chemical vapor deposition

• topography corresponds to the middle of the two C atoms. The observation of STM topography maxima between a-type and b-type carbon atoms in HOPG is predicted to be possible in the work by Teobaldi and co-workers [38]. Upon obtaining remarkable contrast variations with the bias voltage in low-temperature STM

• experiments, the authors calculated and discussed the role of bias voltage and tip termination on the atomic contrast in constant tunnel current images of HOPG. They chose different terminations of W tips and different relative orientations of the tip and graphite surface. Over a wide range of bias voltages

Disorder in H⁺-irradiated HOPG: effect of impinging energy and dose on Raman D-band splitting and surface topography

• Lisandro Venosta,
• Noelia Bajales,
• Sergio Suárez and
• Paula G. Bercoff

Beilstein J. Nanotechnol. 2018, 9, 2708–2717, doi:10.3762/bjnano.9.253

• . 8400 San Carlos de Bariloche, Argentina 10.3762/bjnano.9.253 Abstract Disorder was induced in pristine highly oriented pyrolytic graphite (HOPG) by irradiation with H+ ions with energies of 0.4 MeV and 1 MeV, and doses of 1014 ions/cm2 and 1016 ions/cm2. Raman spectroscopy was used as the main

• component than to the D1 component. SQUID measurements of the irradiated samples showed an enhancement in the normalized remanence, as well as an increment in coercivity compared to pristine HOPG, consistent with H+-induced point-like defects as well as C–H bonds. AFM scanning after Raman and SQUID

• engineering in carbon-based materials. Keywords: disorder; highly oriented pyrolytic graphite (HOPG); ion–solid interactions; Raman spectroscopy; topography; Introduction The development of novel methods to control the properties of carbon-based materials by introducing disorder is currently a subject of

Characterization of the microscopic tribological properties of sandfish (Scincus scincus) scales by atomic force microscopy

• Weibin Wu,
• Christian Lutz,
• Simon Mersch,
• Richard Thelen,
• Christian Greiner,
• Guillaume Gomard and
• Hendrik Hölscher

Beilstein J. Nanotechnol. 2018, 9, 2618–2627, doi:10.3762/bjnano.9.243

• conducted additional adhesion experiments with a sharp silicon tip on scales of S. diadema and on surfaces of some tribological relevance (PMMA, Teflon, highly oriented pyrolytic graphite (HOPG) and silicon). Figure 4d provides the averaged adhesion forces (n = 10) on 15 arbitrarily chosen positions

• on HOPG (225.2 nN) is much higher, nearly 3.4-times of that of the sandfish scale. Nonetheless, the adhesion forces on sandfish scales are not found to be exceptionally low. We extended our analysis by measuring adhesion with a sand probe also on scales of four snakes (Figure 5). The scale samples

• . Figure 6a displays the friction-vs-load curves of the investigated samples comprising Teflon, PMMA, silicon, sandfish, S. diadema and HOPG. All of these measurements were conducted with the same sharp silicon tip. For each sample, we measured a friction loop [17] for each load value and calculated the

Metal-free catalysis based on nitrogen-doped carbon nanomaterials: a photoelectron spectroscopy point of view

• Mattia Scardamaglia and
• Carla Bittencourt

Beilstein J. Nanotechnol. 2018, 9, 2015–2031, doi:10.3762/bjnano.9.191

• potential for the ORR and might convert the ORR reaction mechanism from a two-electron process to a four-electron process. More recently, Guo and co-workers reported a detailed study on highly oriented pyrolytic graphite (HOPG) doped with well-defined N configurations [115]. They achieved the synthesis of

• model HOPG catalysts with almost only one kind of nitrogen dopant: pyridinic N or graphitic N, with different concentrations up to 11 atom %. By measuring the ORR activity by CV in acidic electrolyte (0.1 M H2SO4), they demonstrated that the HOPG with pyridinic N is the best catalyst for the ORR, with

Recent highlights in nanoscale and mesoscale friction

• Andrea Vanossi,
• Dirk Dietzel,
• Andre Schirmeisen,
• Ernst Meyer,
• Rémy Pawlak,
• Thilo Glatzel,
• Marcin Kisiel,
• Shigeki Kawai and
• Nicola Manini

Beilstein J. Nanotechnol. 2018, 9, 1995–2014, doi:10.3762/bjnano.9.190

• experiments of gold and antimony nanoparticles on highly oriented pyrolithic graphite (HOPG) [46], where the precise value of γ was found to depend sensitively on the crystallinity of the particles. As predicted theoretically [66][67], γ = 0.5 was found for the case of amorphous Sb nanoparticles, whereas

• for sliding depends sensitively on the shape of the particle [68][69]. Perfectly geometrical structures such as Au triangles on HOPG show sharp and defined maxima as a function of the relative rotation angle, whereas rounded edges smoothen out the angular corrugation and additionally increase the

• effectively mediate the contact between incommensurate surfaces [66] and lead to the breakdown of superlubricity. This effect was held responsible, e.g., for the frictional behavior of Sb-nanoparticles on HOPG, where early UHV experiments only yielded a small fraction of particles sliding superlubrically [44

Defect formation in multiwalled carbon nanotubes under low-energy He and Ne ion irradiation

• Santhana Eswara,
• Jean-Nicolas Audinot,
• Brahime El Adib,
• Maël Guennou,
• Tom Wirtz and
• Patrick Philipp

Beilstein J. Nanotechnol. 2018, 9, 1951–1963, doi:10.3762/bjnano.9.186

• graphite (HOPG) and SWCNT and the modification of the Raman spectra when introducing defects in the different materials. Furthermore, HOPG and SWCNT spectra differ from those of graphite and nanocarbon [1]. The Raman spectrum of MWCNTs is in between those of graphite and nanocarbon. All these materials

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

• were acquired under the same illumination conditions and with the same cantilever on a highly oriented pyrolytic graphite (HOPG) substrate (Figure S3, Supporting Information File 1). The surface potential displays no shift under illumination (which also confirms the absence of any carrier

• originate from a thermal expansion effect. Note here that the HOPG substrate displays a thermal expansion coefficient [22] in the out-of-plane direction close to that of the MAPbBr3 crystal [23] and that both samples are relatively similar in terms of size (0.5 mm thick for the HOPG vs ≈1 mm for the MAPbBr3

• crystal). These comparative measurements on HOPG show that at high fluence, the thermal detuning of the cantilever can induce a slow evolution of the z level under illumination and a subsequent slow return to equilibrium in dark conditions. Nevertheless, this extrinsic z-change has no impact on the SP

Heterostuctures of 4-(chloromethyl)phenyltrichlorosilane and 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine prepared on Si(111) using particle lithography: Nanoscale characterization of the main steps of nanopatterning

• Phillip C. Chambers and
• Jayne C. Garno

Beilstein J. Nanotechnol. 2018, 9, 1211–1219, doi:10.3762/bjnano.9.112

• functionalized phthalocyaninato-polysiloxane was studied with STM on surfaces of highly oriented pyrolytic graphite (HOPG) by Samori et al. [22]. Photoelectronic devices of porphyrin polymers containing oligothienyl bridges were prepared as microscopic junction chips and as layered diodes by Shimadzu et al. [23

Scanning speed phenomenon in contact-resonance atomic force microscopy

• Christopher C. Glover,
• Jason P. Killgore and
• Ryan C. Tung

Beilstein J. Nanotechnol. 2018, 9, 945–952, doi:10.3762/bjnano.9.87

• [23] was used to drive the cantilever-holder actuator and to excite and track the first and second CR frequencies while the scan velocity was randomly varied between 0.05 and 100 μm/s. The experiments were repeated on a sample of highly oriented pyrolytic graphite (HOPG). In this set of experiments

• conditions, we may be able to enhance our simple slider model by using higher-order models that extend beyond the continuum assumptions for the confined fluid. The scan speed phenomenon was not observed on mica at 5% RH or on HOPG at 4.3% RH. We believe that the absence of the observed phenomenon is due to

• values for both mica and HOPG. On the hydrophilic mica sample, a distinct increase of measured adhesion forces is apparent with increasing relative humidity. This suggests the growth of the thin water film on mica with increasing relative humidity. The observed behavior of the adhesion force for

Kinetics of solvent supported tubule formation of Lotus (Nelumbo nucifera) wax on highly oriented pyrolytic graphite (HOPG) investigated by atomic force microscopy

• Sujit Kumar Dora,
• Kerstin Koch,
• Wilhelm Barthlott and
• Klaus Wandelt

Beilstein J. Nanotechnol. 2018, 9, 468–481, doi:10.3762/bjnano.9.45

• chloroform-based solutions on an HOPG surface at room temperature was studied by atomic force microscopy (magnetic AC mode) taking series of consecutive images of the formation process. The growth of the tubules oriented in an upright fashion follows a sequential rodlet→ring→tubule behavior. The influence of

• taken in account to understand how different factors affecting self-assembly of these tubules on HOPG. Koch et al. [21] demonstrated that self-assembly of nonacosan-10-ol tubules resulted in an upright orientation of tubules on HOPG. By employing tapping mode atomic force microscopy (AFM), they observed

• the continuous growth of tubules after applying a 10 µL droplet (conc. 1.5 mg/mL) onto an HOPG surface. The growth followed a behavior where wax molecules first aggregated to form rodlets which then changed to a circular structure before growing longitudinally in an upright fashion to the surface by

Blister formation during graphite surface oxidation by Hummers’ method

• Olga V. Sinitsyna,
• Georgy B. Meshkov,
• Anastasija V. Grigorieva,
• Alexander A. Antonov,
• Inna G. Grigorieva and
• Igor V. Yaminsky

Beilstein J. Nanotechnol. 2018, 9, 407–414, doi:10.3762/bjnano.9.40

• important role of the basal-plane surface in the transport of reagents inside the crystals. Eklund et al. [7] noted that sulfuric acid intercalates highly oriented pyrolythic graphite (HOPG) through the basal-plane surface, where the penetration sites are probably grain boundaries, microcracks and atomic

• Offeman, in which graphite is treated with a mixture of concentrated sulfuric acid, sodium nitrate, and potassium permanganate and then washed with water [8]. Traditionally, HOPG is used as a model material to study the physical and chemical processes occurring on graphite surfaces [9]. HOPG consists of

• oriented crystallites with almost parallel c-axes. The inclination angle between the crystallites is characterized by a mosaic spread, which is between 0.1° and 3° for HOPG [10]. In this paper, we used a new material, highly annealed pyrolythic graphite (HAPG). The mosaic spread of a flat HAPG film can be

Robust nanobubble and nanodroplet segmentation in atomic force microscope images using the spherical Hough transform

• Yuliang Wang,
• Tongda Lu,
• Xiaolai Li,
• Shuai Ren and
• Shusheng Bi

Beilstein J. Nanotechnol. 2017, 8, 2572–2582, doi:10.3762/bjnano.8.257

• nanostructures on polymer [15] and highly oriented pyrolytic graphite (HOPG) surfaces [16]. In general, NBs and NDs are 100–800 nm in width and 10–100 nm in height. They are generally studied by atomic force microscopes (AFM) due to their high spatial measurement resolution. The morphological characterization of

• scanners, noise [24][25] or the actual topography of the sample surfaces (e.g., HOPG). In general, it is difficult to establish one source of the uneven background from the others in AFM images. Practically, researchers use a plane fitting method to aggressively flatten AFM images to improve the contrast

• (Mallinckrodt Chemical) to obtain the solution for spin coating. Upon the immersion of the PS film in deionized (DI) water, NBs are spontaneously generated on the PS surface. NDs were obtained on a freshly cleaved HOPG surface. To form the NDs, 10 µL of poly(dimethyl siloxane) (PDMS) solution was dissolved in

Comparing postdeposition reactions of electrons and radicals with Pt nanostructures created by focused electron beam induced deposition

• Julie A. Spencer,
• Michael Barclay,
• Miranda J. Gallagher,
• Robert Winkler,
• Ilyas Unlu,
• Yung-Chien Wu,
• Harald Plank,
• Lisa McElwee-White and
• D. Howard Fairbrother

Beilstein J. Nanotechnol. 2017, 8, 2410–2424, doi:10.3762/bjnano.8.240

• , Si/Mo multilayer mirror substrates [47] were used for most depositions, although highly ordered pyrolytical graphite (HOPG) and SiO2 substrates were used for a few depositions. The Ru-capped, Si/Mo multilayer mirror substrate was preferred due to the smoothness and ease with which deposits could be

Ester formation at the liquid–solid interface

• Nguyen T. N. Ha,
• Thiruvancheril G. Gopakumar,
• Nguyen D. C. Yen,
• Carola Mende,
• Lars Smykalla,
• Maik Schlesinger,
• Roy Buschbeck,
• Tobias Rüffer,
• Heinrich Lang,
• Michael Mehring and
• Michael Hietschold

Beilstein J. Nanotechnol. 2017, 8, 2139–2150, doi:10.3762/bjnano.8.213

• (undecan-1-ol or decan-1-ol), coadsorbed out of a solution of the acid within the alcohol at the interface of highly oriented pyrolytic graphite (HOPG) (0001) substrate. The monoester was observed promptly after reaching a threshold either related to the increased packing density of the adsorbate layer

• . Here we present a chemical reaction (esterification) between trimesic acid (benzene-1,3,5-tricarboxylic acid; TMA) dissolved in an alcoholic solvent (undecan-1-ol or decan-1-ol) on a highly oriented pyrolytic graphite (HOPG) (0001) substrate. The reaction proceeds without catalyst and is controlled by

• Figure 2a. This is consistent with the reported linear pattern (LP) of alcohol and TMA coadsorbed on the HOPG (0001) surface [26][27]. TMA interacts with undecanol via noncovalent hydrogen bonding and forms the observed LP. We call this structure LP0, where 0 indicates no previous sonication of the

A systematic study of the controlled generation of crystalline iron oxide nanoparticles on graphene using a chemical etching process

• Peter Krauß,
• Jörg Engstler and
• Jörg J. Schneider

Beilstein J. Nanotechnol. 2017, 8, 2017–2025, doi:10.3762/bjnano.8.202

• ] and later demonstrated by Geim and Novoselov in 2004 using sophisticated and skillfull mechanical exfoliation of highly oriented pyrolytic graphite (HOPG) [5]. This seminal discovery enabled the research field of two-dimensional materials on a broader scope, leading to the dissemination of several top

Identifying the nature of surface chemical modification for directed self-assembly of block copolymers

• Laura Evangelio,
• Federico Gramazio,
• Matteo Lorenzoni,
• Michaela Gorgoi,
• Francisco Miguel Espinosa,
• Ricardo García,
• Francesc Pérez-Murano and
• Jordi Fraxedas

Beilstein J. Nanotechnol. 2017, 8, 1972–1981, doi:10.3762/bjnano.8.198

• (continuous red line), after EBL (continuous blue line) and with a freshly cleaved highly-oriented pyrolytic graphite (HOPG) surface (discontinuous black line). The surface modified by EBL shows a relatively large broadening and a strong shift towards lower binding energies, as compared to the sample modified

• by EBL and oxygen plasma. Binding energies have been referenced to the Si 2p3/2 peak (99.3 eV) from the buried silicon substrate. The mentioned shift towards lower binding energies denotes the increasing presence of sp2 bonding based on the comparison with the results from a freshly cleaved HOPG

• by EBL (continuous blue line), and HOPG (discontinuous black line) using a PHOIBOS150 analyzer and monochromatic 1486.6 eV excitation. (b) SEM image of a 22 nm pitch PS-b-PMMA aligned in a pattern created by direct electron beam exposure. (a) AFM topography image and profile of the PS–OH brush

Evolution of the graphite surface in phosphoric acid: an AFM and Raman study

• Rossella Yivlialin,
• Luigi Brambilla,
• Gianlorenzo Bussetti,
• Matteo Tommasini,
• Andrea Li Bassi,
• Carlo Spartaco Casari,
• Matteo Passoni,
• Franco Ciccacci,
• Lamberto Duò and
• Chiara Castiglioni

Beilstein J. Nanotechnol. 2016, 7, 1878–1884, doi:10.3762/bjnano.7.180

• situ several micro-Raman spectra with different excitation wavelengths to get information about the different regions of the sample. We analyzed the HOPG sample after fifteen CV cycles in the positive potentials range (see Figure 1a), focusing the 457.9 nm laser at the A, B and C regions. The four

• spectra compared in Figure 5 are representative of the pristine HOPG and of the A-, B-, C-regions displayed in Figure 2. Similar observations can be made, independently on the excitation energy adopted. Indeed, spectra recorded with 632.8 nm and 784.5 nm excitations show a qualitative behavior similar to

• HOPG, a rather strong and broad new component appears (hereafter referred as G* band) at higher Raman shifts. Similar features can be observed in the B-region. However, in this case both the D line and the G* components are less pronounced than in the A-region. Moreover, an additional new line is

Phenalenyl-based mononuclear dysprosium complexes

• Yanhua Lan,
• Andrea Magri,
• Olaf Fuhr and
• Mario Ruben

Beilstein J. Nanotechnol. 2016, 7, 995–1009, doi:10.3762/bjnano.7.92

• such as HOPG, graphene and CNT substrates are expected [20]. The significance of the phenalenyl unit in the diverse research areas ranging from chemistry and materials chemistry to device physics is closely linked to its essential role as an electronic reservoir that has driven the development of the

Optical absorption signature of a self-assembled dye monolayer on graphene

• Tessnim Sghaier,
• Sylvain Le Liepvre,
• Céline Fiorini,
• Ludovic Douillard and
• Fabrice Charra

Beilstein J. Nanotechnol. 2016, 7, 862–868, doi:10.3762/bjnano.7.78

• level of control of the self-assembly geometry is possible by exploiting the interactions between alkyl side chains and the surface of highly oriented pyrolytic graphite (HOPG) [30]. Based on these principles, it has been possible to design molecular building blocks that arrange spontaneously according

• graphitic substrates, highly oriented pyrolytic graphite (HOPG) and monolayers of CVD graphene transferred either onto fused silica (“optical quartz”) or PET. The resolution of carbon atoms is easily obtained on both systems. On CVD graphene samples, an additional moderate roughness is observed, which is

• graphene. The monolayer structures have been studied by STM at the solution–substrate interface. Intramolecular resolution is possible both with HOPG and graphene as substrates (Figure 2). As expected from the atomically flat surface of HOPG, this substrate produces the largest domains. It permits an

Length-extension resonator as a force sensor for high-resolution frequency-modulation atomic force microscopy in air

• Hannes Beyer,
• Tino Wagner and
• Andreas Stemmer

Beilstein J. Nanotechnol. 2016, 7, 432–438, doi:10.3762/bjnano.7.38

• in air or liquid have been reported so far, for example on mica [13][15], Si(111) [16], on a grating [17], HOPG, and DNA origami [18]. Froning et al. [18] also discussed the influence of the environmental conditions on the sensor properties. Temperature and humidity changes lead to variations in

• into a tip–sample force: where f0 is the resonance frequency, k the stiffness, A the amplitude, and z the tip–sample distance. A Δf–z curve on HOPG with calculated Fts at an amplitude of 1.1 nm is shown in Figure 4. Only a small attractive force regime is present, which can be explained by the high

• -resolution capability in air, a clean HOPG surface was investigated. The topography feedback gains were set low, resulting in a quasi-constant height mode measurement. Starting from a low positive frequency shift setpoint, the tip–sample distance was gradually decreased until atomic contrast was observed

Synthesis and applications of carbon nanomaterials for energy generation and storage

• Marco Notarianni,
• Jinzhang Liu,
• Kristy Vernon and
• Nunzio Motta

Beilstein J. Nanotechnol. 2016, 7, 149–196, doi:10.3762/bjnano.7.17

• , highly ordered pyrolytic graphite (HOPG) is chosen in order to guarantee a product of high quality graphene crystallites. The main advantages of this method are the ability to complete this process at room temperature and with inexpensive equipment. However, in terms of scalability, it performs the worst

• process [96][97]. In the electrochemical exfoliation method, the graphite or HOPG is usually connected to an electrode (anode). The counter electrode (cathode) is usually a platinum (Pt) wire and the setup is usually placed in an acidic solution (Figure 16). The complete exfoliation takes place in 15–30

Stick–slip behaviour on Au(111) with adsorption of copper and sulfate

• Nikolay Podgaynyy,
• Sabine Wezisla,
• Christoph Molls,
• Shahid Iqbal and
• Helmut Baltruschat

Beilstein J. Nanotechnol. 2015, 6, 820–830, doi:10.3762/bjnano.6.85

• . There are only a few older publications reporting on AFM friction studies on HOPG (highly oriented pyrolytic graphite) and polycristalline Ag under electrochemical conditions, examining the dependence of friction on potential and the adsorption of anions [7][8][9]. Our group recently started to study

• was observed by Meyer and coworkers [33]: Upon an increase in normal load on a NaCl(001) surface a transition to multiple slip was found. According to [34], who predicted such transitions from theory for low damping conditions and also observed it on HOPG, this process is based on energy minimisation

A versatile strategy towards non-covalent functionalization of graphene by surface-confined supramolecular self-assembly of Janus tectons

• Ping Du,
• David Bléger,
• Fabrice Charra,
• Vincent Bouchiat,
• David Kreher,
• Fabrice Mathevet and
• André-Jean Attias

Beilstein J. Nanotechnol. 2015, 6, 632–639, doi:10.3762/bjnano.6.64

• molecular in-plane-confined self-assembly on substrates (such as HOPG) can be directly transferred to graphene substrates, as was recently demonstrated for a few molecules. There are several examples regarding the formation of well-ordered 2D molecular adlayers self-assembled via hydrogen bonding [21] or

• ) for the precise nanometer-scale 2D decoration of flat sp2-hybridized carbon supports (such as HOPG and graphene) with periodic arrays of functional 3D building blocks, known as Janus tectons [25]. Here, we summarize this general, versatile, and convenient approach for simultaneously (i) generating

• demand” series of 0D, 1D or 2D topologies, based on a single rigid molecular core on HOPG. These achievements are based on the rational design of a novel functional molecular group, which turns into a non-covalent clip-like bond activated by graphite (Figure 1). Among the interactions available for

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

• . Measurements were performed in air and milli-Q water with the same Pt/Ir coated probe. Figure 1a,b shows the bias dependence of Aω for highly ordered pyrolytic graphite (HOPG) and Au surfaces, respectively. For both HOPG and Au, a linear dependence of Aω to the applied Vdc was observed in air, and the minimum

• of Aω corresponds to the measured CPD of the probe–sample system, as described by Equation 2. Thus, the CPD was estimated to be ≈420 mV for HOPG and ≈−60 mV for Au. To investigate the operation of KPFM in liquid, the measurements were repeated in milli-Q water, which has a minimum ion concentration

• of ≈4 × 10−7 M [51]. In a first set of experiments, small bias sweeps (±300 mV) were performed to reduce the likelihood of inducing irreversible electrochemical processes. For HOPG, a minimum was observed at ≈370 mV, close to the measured CPD in air, but no minimum was observed for Au. The observed