Polymer Graphite Pencil Lead As a Cheap Alternative for Classic Conductive SPM Probes

nanomaterials

Article Polymer Graphite Pencil Lead as a Cheap Alternative for Classic Conductive SPM Probes

Alexandr Knápek 1,*, Dinara Sobola 2,3, Daniel Burda 1,2, Aleš Da ˇnhel 4, Marwan Mousa 5 and Vladimír Kolaˇrík 1

1 Institute of Scientiﬁc Instruments of the Czech Academy of Sciences, Královopolská 147, 612 64 Brno, Czech Republic; [email protected] (D.B.); [email protected] (V.K.) 2 Department of Physics, Faculty of Electrical Engineering and Communication, Brno University of Technology, Technická 8, 616 00 Brno, Czech Republic; [email protected] 3 Central European Institute of Technology, Brno University of Technology, Technická 10, 616 00 Brno, Czech Republic 4 Institute of Biophysics of the Czech Academy of Sciences, Královopolská 135, 612 65 Brno, Czech Republic; [email protected] 5 Department of Physics, Mu’tah University, Al-Karak 61710, Jordan; [email protected] * Correspondence: [email protected]; Tel.: +420-5415-142-58

Received: 13 November 2019; Accepted: 6 December 2019; Published: 10 December 2019

Abstract: This paper presents polymer graphite (PG) as a novel material for the scanning tunneling microscopy (STM) probe. Conductive PG is a relatively modern nanocomposite material used for micro-pencil reﬁlls containing a polymer-based binding agent and graphite ﬂakes. Its high conductivity and immunity against surface contamination, with a low price, make it seem like a highly suitable material for electrode manufacturing in general. For the tip production, three methods were developed and are further described in the paper. For the production, three commercially available polymer graphite rods were used. Each has been discussed in terms of performance within the tunneling microscope and within other potential applications.

Keywords: scanning tunneling microscopy; sharp tip formation; graphite

1. Introduction Graphite is a natural mineral and one of carbon’s allotropes that possesses a layered structure. It’s soft, malleable, and greasy to the touch [1]. Several unique properties make graphite a widely used material: production of electrodes, lubricants, fillers of plastics, neutron moderators in nuclear reactors, and many other applications [2,3]. Graphite is also used in the production of aluminum, synthetic diamonds, and in thermal protection of warheads of ballistic missiles and space modules. Classical pencil leads are made from a mixture of white clay (kaolinite) and graphite [4]. The classical pencil lead in its modern form was invented in 1794 by Nicolas Jacques Conte, a talented French scientist and inventor. Conte developed a recipe for mixing graphite with white clay and produced high-quality rods from these materials. Strength was achieved by heat treatment, at which the clay hardens [4]. Varying the proportion of the mixture made it possible to make rods of different hardness. The composition of modern leads includes polymers, which make it possible to achieve the desired combination of hardness, strength, and elasticity. Carbon itself as well as polymer graphite (PG) also show field-emission behavior as it was described in previous papers [5–7]. Main characteristics of polymer graphite which could be convenient for usage in scanning probe microscopy (SPM) are: heat resistance, considerable durability, resistance to mechanical and hydraulic stress, and, last but not least, corrosion resistance [8]. Also, it has been previously reported that polymer graphite

Nanomaterials 2019, 9, 1756; doi:10.3390/nano9121756 www.mdpi.com/journal/nanomaterials Nanomaterials 2019, 9, 1756 2 of 12 contains up to 80% of sp3 hybridized carbon, moving its properties more towards graphene-based structure [9,10]. For our study, we have chosen a scanning tunneling microscopy (STM) to demonstrate the use of pencil lead in conductive SPM techniques [8]. Whether it’s a study of topography or electrical characteristics, the scanning probe microscopy is an integral part of modern research [11,12]. Possibilities of SPM are defined by both set-up configuration and sample-tip interface [13]. In order to achieve maximum resolution by STM, it is necessary to obtain an extremely sharp conductive tip serving as a probe scanning over a sample surface. The STM probes, which are currently used, are made traditionally of platinum-iridium alloy in proportions of 90:10 or 70:30 (Pt:Ir). This composition benefits from the chemical stability of platinum and an increased hardness. The main disadvantage of this material is its high price. Another commonly used material is tungsten (W) which is used mainly for its high melting point, good conductivity, and a relatively easy way of creating the sharp tip. This is done mostly by incorporating an electrochemical etching method, where the etched tungsten wires dissolute in an alkaline electrolyte yielding a sharp tip by the increased tension of the electrolyte near its surface [14]. Tungsten is still quite an expensive material compared to the graphite pencil lead, even when working with the polycrystalline wire. Tungsten also tends to create surface oxide which blunts the tip and hence decreases the scanning resolution. From the above mentioned factors, the development of new probes is important for surface science because STM is a high-accuracy method for characterization of micromorphology and nanomorphology of surface, defects and fractures of surface, measurements of nano-scaled structures, and, last but not least, understanding mechanisms on the surface [15].

2. Materials and Methods

2.1. Materials The market of pencil leads consists of several popular brands of mechanical pencils (e.g., Pilot, Erich Krause, Proﬀ, Parker, Koh-i-Noor Hardtmuth, Stabilo, Pentel, Staedtler, Faber-Castell, Rotring, Bic, Conte, Index, Lamy, Constructor, and many others). The production of carbon pencils is spread over the world and each product is based on a special fabrication technology. For our experiments, we have used three diﬀerent brands of polymer graphite rod, in particular: Koh-i-Noor, Staedtler, and Pentel Hi-polymer E.

2.2. Equipment and Methods of Analysis For the analysis, several analytical methods were used, providing information based on different physical principles. Such a complementary approach allows for achieving reliable conclusions on the pencil lead’s chemical composition. Raman spectroscopy was carried out in the inVia Spectrometer (Renishaw, Wotton-under-Edge, UK) utilizing green laser (DPSS 532 nm, P = 30 mW) and the exposure of 10 10 s with a 50 objective. × × The Raman spectra were evaluated and fitted with Lorentzian peak shapes using Fityk 1.3.1 software (Marcin Wojdyr, Warsaw, Poland). X-ray photoelectron spectroscopy (XPS) measurements were carried out with Kratos Supra with monochromatic Al-Kα source and photon energy of 1486.7 eV. The survey scans were measured within the binding energy range of 0–1200 eV with the step of 1.0 eV. The spectra that were referenced were evaluated in CasaXPS 2.3.22PR1 software (Casa Software Ltd., London, UK) which allows calculation of atomic percent and many different parameters like Full width at half maximum (FWHM), deconvolution for the peak. The Scanning electron microscopy (SEM) and Energy Dispersive X-Ray Analysis (EDX) measurements were carried out in Tescan Lyra3 scanning electron microscope with an in-built Aztec SSD detector (Oxford Instruments, Abingdon-on-Thames, UK). The EDX spectra were obtained with accelerating voltage of 15 kV at a working distance of 9 mm, view the field of 20 µm, and spot size of 5.5 nm. The ICP-MS measurements were carried out in Agilent 7900 (Santa Clara, CA, USA) that is a Nanomaterials 2019, 9, 1756 3 of 12 quadrupole system offering high matrix tolerance, wide dynamic range, and an effective interference removalNanomaterials for trace2019, 9 elements, x FOR PEER across REVIEW most typical applications. 3 of 12

2.2.1. Raman Spectroscopy 2.2.1. Raman Spectroscopy The Raman spectra of pencil leads show peaks common in polycrystalline graphite. The G peak The Raman spectra of pencil leads show peaks common in polycrystalline graphite. The G peak was observed at 1580 cm 1, which arises from the bond stretching of all pairs of sp2 hybridized atoms; was observed at 1580 cm− –1, which arises from the bond stretching of all pairs of sp2 hybridized atoms; the D peak is around 1360 cm 1, which becomes visible in relation with the defects in sp2 graphite the D peak is around 1360 cm− –1, which becomes visible in relation with the defects in sp2 graphite sheets, in polycrystalline graphite, and graphite-like materials with crystalline defect; the D peak sheets, in polycrystalline graphite, and graphite-like materials with crystalline defect; the D peak overtone called 2D peak is observed at around 2690 cm 1, which on polycrystalline graphite samples overtone called 2D peak is observed at around 2690 cm− –1, which on polycrystalline graphite samples splits into two components: 2D1 and 2D2 [16]. The above-mentioned measurements are illustrated in splits into two components: 2D1 and 2D2 [16]. The above-mentioned measurements are illustrated in Figure1. Figure 1.

Figure 1. Stacked Raman spectra of each sample: (A) Pentel Hi-Polymer E (blue); (B) Koh-i-Noor (red); Figure 1. Stacked Raman spectra of each sample: (A) Pentel Hi-Polymer E (blue); (B) Koh-i-Noor (red); (C) STAEDTLER (black). (C) STAEDTLER (black).

D/G and 2D1/2D2 intensity ratios give information about the estimated size and turbostraticity of layersD/G of polycrystalline and 2D1/2D2 intensity graphite ﬂakes,ratios give which information may play aabout signiﬁcant the estimated role in the size fabrication and turbostraticity of sharp of layers of polycrystalline graphite flakes, which may play a significant role in the fabrication of STM tip. Properties of analysed pencil leads include Raman ID/IG intensity ratio, calculated crystallite size,sharp and STM relative tip. atomicProperties concentration of analysed (derived pencil fromleads XPSincl measurements)ude Raman ID/I areG intensity illustrated ratio, in Tablecalculated1. crystallite size, and relative atomic concentration (derived from XPS measurements) are illustrated in

TableTable 1. 1. Properties of analysed pencil leads, Raman ID/IG intensity ratio, calculated crystallite size, and relative atomic concentration derived from XPS measurements. Table 1. Properties of analysed pencil leads, Raman ID/IG intensity ratio, calculated crystallite size, and relative atomicRaman concentration derived EDX from Contaminants XPS measurements. XPS [Atomic %] Crystallite Side Spot at the Sample I /I C O Si Fe F D GRamanSize [nm] SurfaceEDX ContaminantsRod Axis XPS [atomic %] Crystallite Size O, Na, Si, O, Na,Spot Si, at P, the Pentel 0.60 63 65.3 23.4 8.8 0.6 1.3 Sample ID/IG SideP, Fesurface Zn C O Si Fe F [nm] rod axis O, Na, O, Na,O, Na, Mg, Si, P, Mg, Al, Koh-i-NoorPentel 0.60 0.3663 101 O, Na, Si, P, FeAl, Si, P, S, Cl, 67.265.3 23.9 23.4 8.0 8.80.7 0.6 - 1.3 Si, P, S, Cl, Zn K, Ca, Fe K, Ca, Fe O, Na, Mg, Al, O, Na, Mg, Koh-i- O, Na, 0.36 101 Si, P, S, Cl, K, O, Na,Al, Si, Si, S, P, S, 67.2 23.9 8.0 0.7 - Staedtler 0.35 102 Mg, Al, Si, 48.2 24.1 15.6 0.8 1.3 Noor Cl, Ca S,Ca, Cl, CaFe Cl, K, Ca, Fe O, Na, Mg, Al, O, Na, Si, S, Staedtler 0.35 102 48.2 24.1 15.6 0.8 1.3 Si, S, Cl, Ca Cl, Ca

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Nanomaterials 2019, 9, x FOR PEER REVIEW 4 of 12 2.2.2. EDX and XPS Analysis 2.2.2. EDX and XPS Analysis The pencil leads were also thoroughly investigated using EDX and XPS with the aim to obtain complexThe information pencil leads about were bothalso thoroughly the surface investigated and the bulk using properties. EDX and Compositional XPS with the aim analysis to obtain of dicomplexﬀerent carbon information leads was about presented both the originally surface by and Kariuki the bulk [17] and properties. Navratil Compositional et al. [10]. analysis of differentHere, wecarbon compare leads XPS was spectra presented of each originally tip by measuring by Kariuki in [1 the7] and area Navratil where the et lead al. [1 has0]. been etched in a hydroxideHere, we solution compare versus XPS spectra in the unprocessedof each tip by area measuring of the lead, in the yielding area where several the double lead spectrahas been foretched each samplein a hydroxide presented solution in Figure ver2s.us The in the background unprocessed for thearea all of etched the lead areas, yielding tends several to be higher double thanspectra for the for unprocessed each sample areas. presented It was in observed Figure 2. that The Koh-i-Noor background pencil for the leads all areetched more areas stable tends against to be wethigher etching than since for the the unprocessed spectra before areas. and It after was etchingobserved show that moreKoh-i- similaritiesNoor pencil to leads each are other more (small stable backgroundagainst wet for etching etched since area, the samespectra shape before of peaks)and after compared etching to show Staedtler more and similarities Pentel Hi-Polymer to each other E leads.(small Furthermore, background XPS for analysisetched area, revealed the same iron contaminationshape of peaks) present compar ated the to side Staedtler surface and of the Pentel pencil Hi- leads,Polymer most E likely leads. due Furthermore, to the speciﬁc XPS method analysis of revealed fabrication iron of contaminat those pencilion leads. present at the side surface of the pencil leads, most likely due to the specific method of fabrication of those pencil leads.

FigureFigure 2. 2.XPS XPS spectra spectra of of each each sample: sample: ( A(A) Pentel) Pentel Hi-Polymer Hi-Polymer E E (blue); (blue); (B ()B Koh-i-Noor) Koh-i-Noor (red); (red) (;C ()C) StaedtlerStaedtler (black). (black).

TheThe EDX EDX spectra spectra were were obtained obtained from from two two diff erentdifferent spots spots for each for each type oftype pencil of pencil lead, fromlead, thefrom side the surfaceside surface of the rodof the and rod from and the from rod the axis rod after axis breaking after breaking off a small off a piece small of piece it. These of it. These measurements measurements give valuablegive valuable information information about the about surface the and surface bulk; and the measurementsbulk; the measurements taken at the taken rod axis at the are rod illustrated axis are inillustrated Figure3. Quantitatively, in Figure 3. Pentel Quantitatively, Hi-Polymer Pentel E pencil Hi- leadPolymer elemental E pencil composition lead elemental significantly composition di ffers fromsignificantly Koh-i-Noor differs and Staedtlerfrom Koh leads,-i-Noor which and bothStaedtler use clay-like leads, which binder both containing use clay sulfur.-like binder These containing findings aresulfur. in good These agreement findings with are in the good results agreement presented with in [the10]. results presented in [10].

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Figure 3. EDX measurements from the spot at the rod axis of Pentel Hi-Polymer E (A); Koh-i-Noor (B); Figure 3. EDX measurements from the spot at the rod axis of Pentel Hi-Polymer E (A); Koh-i-Noor STAEDTLER (C) pencil leads. (B); STAEDTLER (C) pencil leads. 2.2.3. ICP-MS Analysis 2.2.3. ICP-MS Analysis The preparation of a sample for ICP-MS analysis was done firstly by grinding of micro-pencil rods obtainingThe fine-grained preparation powder. of a sample Following for ICP this,-MS 10 analysis milligrams was of done the powderfirstly by was grinding placed of in amicro 2.5 mL-pencil of polypropylenerods obtaining in fine an- Eppendorfgrained powder. micro tubeFollowing along this with:, 10 250 milligrams microliters of the of concentrated powder was sulfuricplaced in acid a 2.5 (96%)ml of containing polypropylene V2O5 inserving an Eppendorf as a catalytic micro converter, tube along and with: concentrated 250 microliters perchloric of concentrated acid (71%), sulfuric both practicalacid (96%) grade containing chemicals. V2O These5 serving suspensions as a catalytic were converter sonicated, and for 10concentrated min in a laboratory perchloric ultrasound acid (71%), sonicator,both practical then heated grade to chemicals. 90 ◦C and shaken These suspensions at 700 rpm for were 30 min sonicated using an for Eppendorf 10 min inThermoMixer a laboratory (Eppendorfultrasound AG, sonicator, Hamburg, then Germany).heated to 90 After °C and this shaken procedure, at 700 the rpm sample for 30 wasminutes diluted using with an 10Eppendorf mL of distilledThermoMixer water and (Eppendorf poured into AG a, 20 Hamburg mL syringe, Germany equipped). After with athis nylon procedure, membrane the filter sample (0.45 wasµm) diluted from whichwithit 10 was mL injected of distilled into a water 50 mL and volumetric poured flask.into a After 20 m that,L syringe 3.85 mL equipped of concentrated with a nitricnylon acid membrane (65%) wasfilter added (0.45 to μm) the volumetric from which flask it was in order injected to obtain into a a 50 5% m solutionL volumetric after adding flask. After a particular that, 3.85 volume mL of ofconcentrated distilled water. nitric Three acid 3 (65%) Ml samples was added were to taken the volumetric away from flask the in solution order to and obtain used a for5% Inductivelysolution after coupledadding plasma a particular mass volume spectrometry of distilled (ICP-MS) water. analysis Three 3 yieldingMl samples 3 sets were of taken results away that from were the averaged. solution Fromand theseused measurements,for Inductively the coupled arithmetic plasma mean mass value spectrometry and the standard (ICP- deviationMS) analysis were yielding calculated. 3 sets The of resultsresults are that illustrated were averaged. in Table 2From. these measurements, the arithmetic mean value and the standard deviationThe obtained were calculat resultsed. illustrated The results in are the illustrated Table2 are in in T agreementable 2. with the results provided by the EDX and XPS analyses. Elements like C, Cl, S, and O cannot be detected using ICP-MS. Also, phosphorusTable 2. (P) Results and silicon of quantitative (Si) also determination cannot be detected of 26 elements in our contained setup due in themicro absence-pencil rods of particular by the measurementmeans of standards. chemical dilution Between using particular acids and an brands, additional there analysis is a significantof ICP-MS. difference in chemical composition of the contents, in particular the elements: Na, Li, Mg, Fe, and Zn, which is, in our opinion, ppm = ng·mg–1 Staedtler Koh-i-Noor Pentel Hi-Polymer E caused by a specific composition of a bonding agent for each brand. From the electrochemicalAverage point of view, StaedtlerAverage seems to be most suitableAverage since the majority of Element SD SD SD the components are more(ppm) electrochemically inert; in(ppm) particular, we are referring(ppm) to the elements: Na, Mg, Ca, and FeinLi opposition<0.000 to the Koh-i-NoorN/A brand<0 that.000 containsN/A a high amount404.331 of heavy metals5.519 (Al, Ti, Cr, Mn, Co, Ni, Cu, As, and Bi) which are often more electrochemically active (i.e., whose cations Be <0.000 N/A <0.000 N/A <0.000 N/A are reducible at low potentials, starting at 0–1.2 V). B <0.000 N/A <0.000 N/A <0.000 N/A Na 2604.058 425.603 <0.000 N/A <0.000 N/A Mg 126.655 32.525 256.163 9.741 <0.000 N/A Al <0.000 N/A 302.484 20.995 33.291 7.363 K <0.000 N/A <0.000 N/A <0.000 N/A Ca 296.026 53.874 79.924 34.929 <0.000 N/A

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Table 2. Results of quantitative determination of 26 elements contained in micro-pencil rods by the means of chemical dilution using acids and an additional analysis of ICP-MS.

ppm = Staedtler Koh-i-Noor Pentel Hi-Polymer E ng mg 1 · − Average Average Average Element SD SD SD (ppm) (ppm) (ppm) Li <0.000 N/A <0.000 N/A 404.331 5.519 Be <0.000 N/A <0.000 N/A <0.000 N/A B <0.000 N/A <0.000 N/A <0.000 N/A Na 2604.058 425.603 <0.000 N/A <0.000 N/A Mg 126.655 32.525 256.163 9.741 <0.000 N/A Al <0.000 N/A 302.484 20.995 33.291 7.363 K <0.000 N/A <0.000 N/A <0.000 N/A Ca 296.026 53.874 79.924 34.929 <0.000 N/A Ti 2.810 0.929 49.382 5.577 3.685 1.313 V <0.000 N/A 262.205 41.036 <0.000 N/A Cr 7.240 0.751 5.273 0.209 5.571 0.857 Mn 10.734 1.252 43.852 0.869 2.166 1.112 Fe 168.076 48.795 4186.431 30.789 346.161 36.538 Co <0.000 N/A 5.192 0.065 <0.000 N/A Ni 3.978 1.266 11.485 0.380 3.979 1.023 Cu <0.000 N/A 46.328 3.203 <0.000 N/A Zn <0.000 N/A <0.000 N/A 2945.454 38.845 As <0.000 N/A 1.079 0.144 <0.000 N/A Se <0.000 N/A <0.000 N/A <0.000 N/A Sr 2.475 0.967 1.326 0.669 1.938 1.542 Mo 0.563 0.097 34.617 0.430 2.689 0.096 Cd <0.000 N/A <0.000 N/A <0.000 N/A Ba 3.594 1.925 6.339 1.544 73.393 2.737 Tl <0.000 N/A <0.000 N/A <0.000 N/A Pb <0.000 N/A <0.000 N/A <0.000 N/A Bi 0.451 0.033 2.405 0.026 45.076 0.585

2.3. Tip Aroduction

2.3.1. Electrochemical Etching The basic method that has been tested for preparation of the graphite sharp tip is based on electrochemical etching, as it was published earlier [14]. This method is well known and also used for field-emission microscope production. The electrochemical method of tip preparation and sharpening provides good reproducibility of the tip shape and sharpness. Type and concentration of hydroxide (NaOH, KOH) allow to control the tip quality. An electrochemical etching station for probe production of NT-MDT Company (Moscow, Russia) was used. The device was partially modified by adding a precise clamp-holder to fix the graphite rod during etching. The potential between two electrodes (one is a graphite rod and the other is a metal ring) is 12 V with alternative current. This allows a precisely perpendicular attachment of the tip towards the electrolyte surface, as illustrated in Figure4, and hence increases the symmetry of the produced tip. During the etching, one of the graphite rod ends passes through a conducting diaphragm that keeps a drop of alkali solution, providing necessary surface tension cutting the rod as follows. As the bottom part falls down due to its own weight, the electric circuit providing the etching current switches off. The main requirement for adjustment of the existing setup for etching and sharping of graphite probes was the positioning of the pencil lead perpendicular to the ring electrode. Nanomaterials 2019, 9, x FOR PEER REVIEW 7 of 12 Nanomaterials 2019, 9, x FOR PEER REVIEW 7 of 12 requirement for adjustment of the existing setup for etching and sharping of graphite probes was the requirementNanomaterials 2019 for, 9 adjustment, 1756 of the existing setup for etching and sharping of graphite probes was7 ofthe 12 positioning of the pencil lead perpendicular to the ring electrode. positioning of the pencil lead perpendicular to the ring electrode.

Figure 4. Precise clamp holder extension intended for 0.2–0.4 mm diameter wires providing precisely Figure 4. Precise clamp holder extension intended for 0.2–0.4 mm diameter wires providing precisely perpendicularFigure 4. Precise fixation clamp towards holder extension the electrolyte intended surface. for 0.2–0.4 mm diameter wires providing precisely perpendicular fixation towards the electrolyte surface. perpendicular fixation towards the electrolyte surface. From the following figures, it can be seen that for each material the produced tip is of a different From the following figures, it can be seen that for each material the produced tip is of a different shapeFrom based the on following different figures, materials it can of the be usedseen rod.that for The each tips material were made the ofproduced three di fftiperent is of micropencil a different shape based on different materials of the used rod. The tips were made of three different micropencil shapebrands: based Pentel on different Hi-Polymer materials 0.3 HB of (Figure the used5), rod. Koh-i-Noor The tips 0.3were HB made (Figure of three6), and different Staedtler micropencil 0.3 HB brands: Pentel Hi-Polymer 0.3 HB (Figure 5), Koh-i-Noor 0.3 HB (Figure 6), and Staedtler 0.3 HB (Figurebrands:7 Pentel). In the Hi left-Polymer part of 0.3 each HB figure, (Figure the 5), overall Koh-i view-Noor of 0.3 the HB tip (Figure is shown. 6), and The Staedtler surface detail 0.3 HB is (Figure 7). In the left part of each figure, the overall view of the tip is shown. The surface detail is (illustratedFigure 7). inIn athe right left part part of of image. each figure, the overall view of the tip is shown. The surface detail is illustrated in a right part of image. illustrated in a right part of image.

(a) (b) (a) (b) Figure 5. SEM image of the tip produced by electrochemical etching (a) and the tip surface detail (b) Figure 5. SEMSEM image image of of the the tip tip produced produced by by electrochemical electrochemical etching etching (a) (anda) and the the tip tipsurface surface detail detail (b) showing graphite flakes. Material used for this tip is Pentel Hi-Polymer 0.3 HB. (showingb) showing graphite graphite flakes. ﬂakes. Material Material used used for this for this tip is tip Pentel is Pentel Hi-Polymer Hi-Polymer 0.3 0.3HB. HB.

(a) (b) (a) (b) Figure 6. SEM image of the produced tip by electrochemical etching (a) and the tip surface detail (b) Figure 6. SEMSEM image image of of the the produced produced tip tip by by electrochemical electrochemical etching etching (a) (anda) and the the tip tipsurface surface detail detail (b) showing graphite flakes. Material used for this tip is Koh-i-Noor 0.3 HB. (showingb) showing graphite graphite flakes. ﬂakes. Material Material used used for this for this tip is tip Koh is Koh-i-Noor-i-Noor 0.3 0.3HB. HB.

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(a) (b) (a) (b) Figure 7. SEM image of the tip produced by electrochemical etching (a) and the tip surface detail Figure 7. SEM image of the tip produced by electrochemical etching (a) and the tip surface detail Figure(b) showing 7. SEM graphite image offlakes. the tip Material produced used by for electrochemical this tip is Koh etching-i-Noor( 0.3a) and HB. the tip surface detail (b) showingshowing graphitegraphite ﬂakes. flakes. Material Material used used for for this this tip tip is Koh-i-Nooris Koh-i-Noor 0.3 HB.0.3 HB. 2.3.2. Mechanical Sharping 2.3.2. Mechanical Mechanical S Sharpingharping Mechanical sharping achieved by sanding is the simplest method. The only disadvantage is Mechanical sharpingsharping achievedachieved by by sanding sanding is is the the simplest simple method.st method. The The only only disadvantage disadvantage is the is the low reproduciblity of the probe’s shape and the absence of a tip-sharping control (Figure 8); thelow low reproduciblity reproduciblity of the of probe’s the probe’s shape shape and the and absence the absence of a tip-sharping of a tip-sharping control (Figurecontrol8 );(Figure however, 8); however, the tip sharpness may be sufficient for measurements performed in atmospheric pressure however,the tip sharpness the tip sharpness may be su ﬃmaycient be forsufficient measurements for measurements performed performed in atmospheric in atmospheric pressure and pressure just for and just for preliminary control of large area samples with a large range of heights and valleys. The preliminaryand just for preliminary control of large control area samplesof large witharea samples a large range with of a large heights range and of valleys. heights The and sharping valleys. could The sharping could be carried out by grinding, for example, on sandpaper or by forming with a sharpingbe carried could out by be grinding, carried for out example, by grinding, on sandpaper for example, or by forming on sandpaper with a sharpener. or by forming with a sharpener. sharpener.

(a) (b) (c) (a) (b) (c) Figure 8. SEM image of the tip produced by mechanical sharping from the Hi-Polymer rod (a); Figure 8.8. SEMSEM imageimage of of the the tip tip produced produced by by mechanical mechanical sharping sharping from from the Hi-Polymer the Hi-Polymer rod (a );rod by ( thea); by the mechanical sharping from Koh-i-Noor rod (b); and the mechanical sharping from Staedtler bymechanical the mechanical sharping sharping from Koh-i-Noor from Koh- rodi-Noor (b); androd ( theb); mechanicaland the mechanical sharping sharping from Staedtler from rodStaedtler (c). rod (c). 2.3.3.rod Focused (c). Ion Beam Milling 2.3.3. Focused Ion Beam Milling 2.3.3.The Focused probes Ion were Beam prepared Milling by Focused Ion Beam (FIB) at microscope Helios (FEI production, Hillsboro,The probes OR, USA). were Theprepared advantage by Focused of this Ion microscope Beam (FIB is) the at microscope possibility ofHelios the automation (FEI production of tip, The probes were prepared by Focused Ion Beam (FIB) at microscope Helios (FEI production, sharpeningHillsboro, OR, and USA the preparation). The advantage of tips of with this desired microscope shape. is FIBthe processingpossibility providesof the automation control of shapeof tip Hillsboro, OR, USA). The advantage of this microscope is the possibility of the automation of tip andsharpening sharpness and of thethe probespreparation (Figures of9 – tip11).s with The disadvantage desired shape. of this FIB process processing is a high provides price of control the ﬁnal of sharpening and the preparation of tips with desired shape. FIB processing provides control of product.shape and We sharpness used preliminarily of the probes mechanically (Figures 9 sharpened–11). The disadvantage probes to decrease of this the process time of is milling. a high price shape and sharpness of the probes (Figures 9–11). The disadvantage of this process is a high price of the final product. We used preliminarily mechanically sharpened probes to decrease the time of of the final product. We used preliminarily mechanically sharpened probes to decrease the time of milling. milling.

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(a) (b) (a) (b) Figure 9. SEM image of the tip produced by FIB milling (a) and the tip surface detail (b) showing FigureFigure 9. SEM 9. SEM image image of of the the tip tip produced produced byby FIBFIB milling (a (a) )and and the the tip tip surface surface detail detail (b) showing (b) showing graphite flakes from top. Material used for this tip is Pentel Hi-Polymer 0.3 HB. graphitegraphite ﬂakes flakes from from top. top. Material Material used used for for thisthis tiptip is Pentel Pentel Hi Hi-Polymer-Polymer 0.3 0.3 HB. HB.

(a) (b) (a) (b) Figure 10. SEM image of the tip produced by FIB milling (a) and the tip surface detail (b) showing FigureFigure 10. SEM10. SEM image image of of the the tip tip produced produced byby FIBFIB milling (a (a) )and and the the tip tip surface surface detail detail (b) showing (b) showing graphite flakes from top. Material used for this tip is Koh-i-Noor 0.3 HB. graphitegraphite ﬂakes flakes from from top. top. Material Material used used for for thisthis tiptip is Koh Koh-i-Noor-i-Noor 0.3 0.3 HB. HB.

(a) (b) (a) (b) Figure 11. SEM image of the produced by FIB milling (a) and tip surface detail (b) showing graphite Figure 11. SEM image of the produced by FIB milling (a) and tip surface detail (b) showing graphite Figureflakes 11. fromSEM top image before of milling. the produced Material by used FIB for milling this tip (a is) andSTAEDTLER tip surface 0.3 detailHB. (b) showing graphite flakes from top before milling. Material used for this tip is STAEDTLER 0.3 HB. flakes from top before milling. Material used for this tip is STAEDTLER 0.3 HB. 3. Results and Discussion 3. Results3. Results and and Discussion Discussion The results published in this paper are partially based on preliminary results that were obtained The results published in this paper are partially based on preliminary results that were obtained Thein our results previous published paper which in this discu papersses are field partially emission based behavior on preliminary of an ultra results-sharp polymer that were-graphite obtained in in our previous paper which discusses field emission behavior of an ultra-sharp polymer-graphite tip [7]. Electrical measurements proved that the graphite flakes behave like metallic conductors while our previoustip [7]. Electrical paperwhich measurements discusses proved field that emission the graphite behavior flakes of behave an ultra-sharp like metal polymer-graphitelic conductors while tip [7]. the bonding agent used for bonding the graphite flakes within the polymer-graphite compound Electricalthe bonding measurements agent used proved for bonding that the the graphite graphite flakes flakes behave within the like polymer metallic-gr conductorsaphite compou whilend the behaves more like an electron trap affecting current stability of the total emission current. However, bondingbehaves agent more used like for an bonding electron trap the graphiteaffecting current flakes withinstability the of the polymer-graphite total emission current. compound However, behaves in the region of tunneling currents, the emission showed quasi-stable behavior. morein like the anregion electron of tunneling trap a ffcurrents,ecting currentthe emission stability showed of thequasi total-stable emission behavior. current. However, in the To test functionality, three types of probes for SPM measurements were used: the tips prepared by To test functionality, three types of probes for SPM measurements were used: the tips prepared by regionelectrochemical of tunneling etching currents, (Figures the emission 5–7), the tips showed prepar quasi-stableed by mechanical behavior. sharping (Figure 8), and the tips electrochemical etching (Figures 5–7), the tips prepared by mechanical sharping (Figure 8), and the tips Toprepared test functionality, by FIB milling three (Figure typess of9–1 probes1). Each for of SPM the tip measurementss were succes weresfully used: used for the SPM tips prepared images by prepared by FIB milling (Figures 9–11). Each of the tips were successfully used for SPM images electrochemicalacquisition. The etching sharpness (Figures of all5– our7), thetips tipsproved prepared to be sufficient; by mechanical however, sharping the sharpness (Figure depended8), and the acquisition. The sharpness of all our tips proved to be sufficient; however, the sharpness depended tips prepared by FIB milling (Figures9–11). Each of the tips were successfully used for SPM images acquisition. The sharpness of all our tips proved to be sufficient; however, the sharpness depended strongly on the brand of the chosen pencil lead. The lowest sharpness was obtained with Nanomaterials 2019, 9, 1756 10 of 12

Pentel HiPolymer rod. Koh-i-Noor and Stadler allow preparation of tips with small cultivate radius. NanomaterialsNanomaterials 2019 2019, 9,, x9 ,FOR x FOR PEER PEER REVIEW REVIEW 10 of10 12 of 12 FIB processing and chemical etching of the tips ensure reliable obtaining of the tips with demand geometry.stronglystrongly Nevertheless, on on the the brand brand even of of the the the chosen mechanical chosen pencil pencil sharping lead. lead. The proved lowest lowest sharpness tosharpness be suffi was cient was obtained obtained during with STMwith Pentel inPentel air when sub-nanometerHiPolymerHiPolymer rod. resolution rod. Koh Koh-i--Noori- isNoor not and and demanded. Stad Stadlerler allowallow The preparationpreparation probes are ofof also tiptipss stablewith with small small against cultivate cultivate oxidation radius. radius. andFIB FIB can be repeatedlyprocessingprocessing sharpened and and chemical chemical for continuous etching etching of of measurement the the tips tips ensure in reliable a simple obtaining obtaining sharping of of the procedures. the tips tips with with demand The demand shape of geometry. Nevertheless, even the mechanical sharping proved to be sufficient during STM in air the tipgeometry. apex is usually Nevertheless, created even by thea single mechanical flake of sharping graphite proved and determines to be sufficient the output during qualitySTM in ofair SPM, when sub-nanometer resolution is not demanded. The probes are also stable against oxidation and when sub-nanometer resolution is not demanded. The probes are also stable against oxidation and while thecan precise be repeatedly shape of sharpe thene tipd does for continuous not play measurementa significant in role. a simple sharping procedures. The can be repeatedly sharpened for continuous measurement in a simple sharping procedures. The In ordershape of to the demonstrate tip apex is usually the simplicity created by a of single working flake of with graphite PG probes,and determines all the the results output presented quality were shape of the tip apex is usually created by a single flake of graphite and determines the output quality obtainedof usingSPM, while mechanically the precise sharpened shape of the tips tip does that not can play be prepareda significant almost role. effortlessly just by sharpening of SPM,In while order the to demonstrateprecise shape the of simpl the tipicity does of working not play with a significant PG probes,2 role. all the results presented were a PG rod using sandpaper of high granularity (>1600 grains cm− ). The probes were tested within obtainedIn order using to demonstrate mechanically the simplsharpenedicity oftips working that can with be PG prepared· probes, all almost the results effortlessly presented just bywere an SPM NT-MDT Nanoeducator II (Moscow, Russia). As a reference sample for estimation of our obtainedsharpening using a PG mechanically rod using sandpaper sharpened of high tips granularity that can (> be1600 prepared grains·cm almost–2). The probes effortlessly were tested just by probes,sharpeningwithin a surface an a SPM PG of a rodNT compact- usingMDT Nanoeducatorsandpaper disc (CD) of and highII (Moscow highly granularity,oriented Russia (>)1600. As pyrolytic a grains· referencecm graphite –sample2). The probesfor (HOPG) estimation were were tested of chosen (Figurewithin our12a). pranobes, TheSPM a HOPGNT surface-MDT surfaceof Nanoeducator a compact (Figure disc II 12(CD) (Moscowb) wasand highly used, Russia asoriented). aAs reference a referencepyrolytic sample graphitesample for for(HOPG)di estimationfferent were types of of electricalourchosen pr characterizationobes, (Figure a surface 12a). of The in a probe compactHOPG microscopy surface disc (CD) (Figure and [8]. 1 2highlyb) was oriented used as apyrolytic reference graphite sample for(HOPG) different were chosentypes (Figure of electrical 12a). characterization The HOPG surface in probe (Figure microscopy 12b) wa [8]s. used as a reference sample for different types of electrical characterization in probe microscopy [8].

(a) (b) Figure 12. Scanning probe microscopy (SPM) image of the reference sample obtained by polymer Figure 12. Scanning probe microscopy (SPM) image of the reference sample obtained by polymer graphite (PG) tip: ( (aa)) the surface of a compact disc; (b) the surface of a highly(b) oriented pyrolytic graphite (PG) tip: (a) the surface of a compact disc; (b) the surface of a highly oriented pyrolytic Figuregraphite 12. S(HOPGcanning). probe microscopy (SPM) image of the reference sample obtained by polymer graphitegraphite (HOPG). (PG) tip: (a) the surface of a compact disc; (b) the surface of a highly oriented pyrolytic graphiteIn order (HOPG to ) demonstrate. the tip’s performance on a regular-structured surface, a special In ordercalibration to demonstrate sample that is the prepared tip’s performance by electron-beam on lithography a regular-structured and intended surface, for testing a special a scanning calibration sampleelectron thatIn orderis preparedmicroscope’s to demonstrate by resolution electron-beam the was tip’s used. performance lithography The calibration onand sample, a intended regular which-structured for is illustrated testing surface, a in scanning Figure a special 13 electron, microscope’scalibrationcontains resolution samplegrids of that various was is prepared used. sizes. TheFor by our electron calibration measurements,-beam sample, lithography we whichhave andused is inte illustrateda 10,nded 5, and for intesting2.5 Figure μm agrid scanning 13s ,to contains demonstrate tip’s resolution. gridselectron of various microscope’s sizes. For resolution our measurements, was used. The we calibration have used sample, a 10, 5,which and 2.5is illustratedµm grids in to Figure demonstrate 13, tip’s resolution.contains grids of various sizes. For our measurements, we have used a 10, 5, and 2.5 μm grids to demonstrate tip’s resolution.

(a) (b) (c) Figure 13. SEM image of the special calibration sample showing various grid sizes: (a) 10 μm grid; (b) 5 μm grid; (c) 2.5 μm grid. (a) (b) (c)

FigureFigure 13. SEM 13. SEM image image of of the the special special calibration sample sample showing showing various various grid sizes: grid ( sizes:a) 10 μm (a) grid; 10 µ (mb) grid; 5 μm grid; (c) 2.5 μm grid. (b) 5 µm grid; (c) 2.5 µm grid.

Nanomaterials 2019, 9, 1756 11 of 12 Nanomaterials 2019, 9, x FOR PEER REVIEW 11 of 12

The results of the measurements are illustrated in the Figure 14 showing the three diﬀerent grid The results of the measurements are illustrated in the Figure 14 showing the three different grid sizes presented above. It can be seen that in all the grids used, the probe was able to follow the surface sizes presented above. It can be seen that in all the grids used, the probe was able to follow the surface details precisely and also to cover a broader area without a change of the spatial sensitivity. details precisely and also to cover a broader area without a change of the spatial sensitivity.

(a) (b) (c)

Figure 14. SPM image of the special calibration grid showing tip performance and spatial resolution: grid. ( a) 10 μmµm grid; (b) 5 µμmm grid;grid; (c) 2.52.5 µμmm 4. Conclusions 4. Conclusions The aim of this study was to introduce different pencil lead probes for both surface visualization The aim of this study was to introduce different pencil lead probes for both surface visualization and modification by SPM conductive techniques. SPM is continuously developing and the methods of and modification by SPM conductive techniques. SPM is continuously developing and the methods probe preparation and shaping are at the center of interest. Pencil lead is a cheap solution as an SPM of probe preparation and shaping are at the center of interest. Pencil lead is a cheap solution as an probe for inspection of surface and its modification. SPM probe for inspection of surface and its modification. Here, we demonstrate that mechanically polished pencil leads are suitable for fast and Here, we demonstrate that mechanically polished pencil leads are suitable for fast and low- low-resolution SPM measurements. These probes proved to be reliable for routine characterization of resolution SPM measurements. These probes proved to be reliable for routine characterization of the the samples by SPM in air, with low resolution of surface features. The cost-effective, easy preparations samples by SPM in air, with low resolution of surface features. The cost-effective, easy preparations make them useful within the education process in laboratory SPM classes. Generally, a tip made of PG make them useful within the education process in laboratory SPM classes. Generally, a tip made of can find application in the education process in the area of scanning probe microscopy techniques or in PG can find application in the education process in the area of scanning probe microscopy techniques the area of topography estimation of conductive samples with high roughness at low resolution of or in the area of topography estimation of conductive samples with high roughness at low resolution texture details. Electrochemical etching and FIB milling allow the preparation of probes for imaging of of texture details. Electrochemical etching and FIB milling allow the preparation of probes for nanoscale surface features. Besides the introduced STM lithography by pencil lead, the probes have imaging of nanoscale surface features. Besides the introduced STM lithography by pencil lead, the a potential for application at nano-grafting at atomic force microscopy techniques. The mechanical probes have a potential for application at nano-grafting at atomic force microscopy techniques. The properties of flexible pencil lead could be also used for preparation in SPM techniques. mechanical properties of flexible pencil lead could be also used for preparation in SPM techniques.

Author Contributions: Contributions:Conceptualization, conceptualization, A.K. A. andK. and D.S.; formal D.S.; f analysis,ormal analysis, A.K., D.S., A D.B..K., and D.S. A.D.;, D.B investigation,. and A.D.; D.S.investigation, and A.D.; D methodology,.S. and A.D.; A.K.;methodology, supervision, A.K V.K.;.; supervision, validation, V D.S.;.K.; v writing-originalalidation, D.S.; w draft,riting A.K.;-original writing-review draft, A.K.; and editing, M.M. writing-review and editing, M.M. Funding: The research was supported by the Ministry of Industry and Trade of the Czech Republic, MPO-TRIO projectFunding: FV10618. The research The research was supported was also by financially the Ministry supported of Industry by theand MinistryTrade of ofthe Education, Czech Republic, Youth MPO and Sports-TRIO ofproject the Czech FV10618. Republic The research under the was project also financially CEITEC 2020 supported (LQ1601), by Grantthe Ministry Agency of of Educ Czechation, Republic Youth underand Sports project of No.the Czech 19-17457S. Republic A part under of the the work project was carriedCEITEC out 2020 with (LQ1601), the support Grant of CEITECAgency Nanoof Czech Research Republic Infrastructure under project (ID LM2015041,No. 19-17457S. MEYS A part CR, of 2016–2019), the work was CEITEC carried Brno out Universitywith the support of Technology. of CEITEC Nano Research Infrastructure (ID Acknowledgments:LM2015041, MEYS CR,We 2016 also–2019), gratefully CEITEC acknowledge Brno University the contribution of Technology. of the late Prof. Pavel Tománek to this paper and the contribution of Mr. Jiˇrí Sýkora for the design of the micro-clamp holder. Acknowledgments: We also gratefully acknowledge the contribution of the late Prof. Pavel Tománek to this Conflicts of Interest: paper and the contributionThe authors of Mr. Jiří declare Sýkora no for conflict the design of interest. of the micro The funders-clamp hadholder. no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publishConflict thes of results. Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to Referencespublish the results.

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