Graphene Coated Nanoprobes: a Review

crystals

Review Graphene Coated Nanoprobes: A Review

Fei Hui, Shaochuan Chen, Xianhu Liang, Bin Yuan, Xu Jing, Yuanyuan Shi and Mario Lanza *

Institute of Functional Nano& Soft Materials, Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215123, China; [email protected] (F.H.); [email protected] (S.C.); [email protected] (X.L.); [email protected] (B.Y.); [email protected] (X.J.); [email protected] (Y.S.) * Correspondence: [email protected]

Academic Editor: Filippo Giannazzo Received: 25 July 2017; Accepted: 28 August 2017; Published: 8 September 2017

Abstract: Nanoprobes are one of the most important components in several fields of nanoscience to study materials, molecules and particles. In scanning probe microscopes, the nanoprobes consist on silicon tips coated with thin metallic films to provide additional properties, such as conductivity. However, if the experiments involve high currents or lateral frictions, the initial properties of the tips can wear out very fast. One possible solution is the use of hard coatings, such as diamond, or making the entire tip out of a precious material (platinum or diamond). However, this strategy is more expensive and the diamond coatings can damage the samples. In this context, the use of graphene as a protective coating for nanoprobes has attracted considerable interest. Here we review the main literature in this field, and discuss the fabrication, performance and scalability of nanoprobes.

Keywords: graphene; nanoprobes; coatings; atomic force microscopy; wear

1. Introduction Sharp probe tips with an apex radius <100 nm are widely used in many different ﬁelds of science including electronics [1], physics [2], chemistry [3], biology [4] and medicine [5], as they allow local characterization and manipulation of materials with a nanometric spatial resolution. Depending on the equipment and application in which they are used, nanoprobes can have a wide variety of shapes, material composition and prizes. One of the main problems of nanoprobes is that they can lose their initial properties (e.g., sharpness, conductivity) after several experiments, leading to false data collection and increases in the cost of the research (i.e., new probes need to be used). Therefore, understanding the degradation process and the speed of nanoprobes during each experiment is essential to ensure high quality, reliable and cheap data collection. Scanning Probe Microscopes (SPM) are among the most advanced equipments for nanoscale characterization and patterning, as they can achieve high spatial resolution both laterally (<0.1 nm) and vertically (<0.01 nm) [6,7]. The nanoprobes used in SPMs consist of a cantilever with a sharp tip at its end, which is the only part that contacts the sample being tested. The lateral resolution of the data collected with an SPM in most cases depends on the sharpness of the nanoprobe at its apex: the smaller the tip radius, the smaller the tip–sample contact area, and therefore the higher the lateral resolution [6]. When using an SPM to analyze the morphological properties of a material (e.g., topography, adhesion force) normally Si or Si3N4 nanoprobes are used, as they can be easily fabricated by standard silicon bulk micromachining technology [8]. The tip radius at the tip apex can be as small as ~2 nm. For electrical modes of SPMs, such as conductive atomic force microscopy (C-AFM), electrical force microscopy (EFM) and Kelvin probe force microscopy (KPFM), the probes need to be conductive, a capability that can be provided by coating the Si or Si3N4 nanoprobes with a thin (~20 nm) conductive varnish [9]. The conductive varnish (normally a metal or doped diamond) should

Crystals 2017, 7, 269; doi:10.3390/cryst7090269 www.mdpi.com/journal/crystals Crystals 2017, 7, 269 2 of 21 be thick enough to provide stable conductivity to the tips under high current densities and frictions, and at the same time thin enough for not increasing the radius at the apex too much (this would result in a loss of lateral resolution). Maintaining the initially high conductivity and sharpness of conductive nanoprobes during several experiments is one of the main problems for the users of electrical modes of SPMs [10]. It is worth noting that the contact area between the tip of an SPM and the sample is typically <100 nm2, and that the minimum current that this equipment can measure is ~1pA. Therefore, the minimum current that an SPM can detect is 1 A/cm2. This is already a very high current density, but it can go even higher, up to 109 A/cm2 if the currents detected with the SPM increase into the milliampere regime [11]. According to the International Technology Roadmap of Semiconductors [12], sub-10 nm resolution and a dynamic range of 1016–1020 A/cm2 is required to acquire electrical information in future nanoscale devices [13,14]. In the market place, one can also ﬁnd conductive nanoprobes for SPMs made of solid metals or doped diamond. However, these nanoprobes show many disadvantages: (i) much higher cost (up to 10-times) due to the use of precious materials and hone techniques; (ii) very few suppliers and a very limited range of spring constants, resonance frequencies and tip radiuses; and (iii) damage to the samples due to high stiffness (for the diamond tips), which not only produces degradation of the sample under test, but also abundant adhesion of particles to the tip apex and subsequent reduction of its sharpness and conductivity. Table1 summarizes the most used conductive nanoprobes for SPMs, and classiﬁes them into four categories depending on their material structure: metal varnished Si probes, doped diamond varnished Si probes, solid metallic probes and solid doped diamond probes. Despite the wide range of conductive nanoprobes available in the market, currently none of them possess high spatial resolution, high conductivity, long durability and low cost at the same time. Therefore the exploration of novel materials with high conductivity and wear resistance is necessary to promote nanoscale characterization techniques.

Table 1. Speciﬁcations of the most used commercial conductive nanoprobes from different manufacturers. The prices represent the cost given by the local distributors in Shanghai.

Tip Tip Coating Bulk Spring k Unit Type Model Radius Freq (kHz) Manufacturer (nm) Materials (N/m) Price ($) (nm) SCM-PIC PtIr n-doped Si 20 + 5 0.2 (0.1–0.4) 13 (10–16) Bruker 41.9 OSCM-PT Pt (20) Si 15 + 10 2 (0.6–3.5) 70 (50–90) Bruker 51.2 SCM-PTSI Pt/Si n-doped Si 15 + 10 2.8 (1–5) 75 (50–100) Bruker 156.7 SMIM-150 TiW Si3N4 50 ± 10 8 (7–9) 75 (70–80) Bruker 139.8 MESP Co/Cr Si 35 + 15 2.8 (1–5) 75 (50–100) Bruker 116.7 Arrow CONTPT Cr/PtIr (5/25) Si 33 ± 10 0.2 (0.06–0.38) 14 (10–19) NanoWorld 38 Metal CONTPT Cr/PtIr (5/25) Si 30 ± 10 0.2 (0.07–0.4) 13 (9–17) NanoWorld 42.98 varnished ATEC-CONTPT Cr/PtIr (5/25) Si 33 ± 10 0.2 (0.02–0.75) 15 (7–25) Nanosensors 41.39 Si tip PPP-CONTPT Cr/PtIr (5/25) Si 30 ± 10 0.2 (0.02–0.77) 13 (6–21) Nanosensors 46.11 PtSi-NCH Pt Si 30 ± 10 42 (10–130) 330 (204–497) Nanosensors 152.08 ACCESSS-NC-GG Au Si 30 113 330 App Nano 53.99 TiN-ACT TiN Si 70 37 300 App Nano 39.5 AC240TM Ti/Pt (5/20) Si 28 ± 10 2 (0.3–4.8) 70 (45–95) Olympus 35.94 NSC14/Pt Pt or Au Si <30 5 (1.8–13) 160 (110–220) µ-Masch 40.3 Electri Tap 190-G Cr/Pt Si <25 48 (20–100) 190 ± 60 Budgetsensors 37.26 Doped CDT-FMR Si 83 ± 17 2.8 (1.2–5.5) 75 (60–90) NanoWorld 143.66 diamond Doped CDT-CONTR Si 83 ± 17 0.2 (0.02–0.77) 13 (6–21) Nanosensors 152.08 diamond Doped Doped CDT-NCLR Si 83 ± 17 48 (21–98) 190 (146–236) Nanosensors 152.08 diamond diamond Doped varnished DD-ACCESS-NC Si 100–300 93 320 App Nano 154.48 diamond Si tip Doped DDESP-FM Si 150 + 50 6.2 (3–11.4) 105 (80–103) Bruker 132.8 diamond Single crystal AD-0.5-AS Si 10 ± 5 0.5 (0.1–1) 30 (10–50) Bruker 186.5 diamond Single crystal AD-0.5-SS Si <5 0.5 (0.1–1) 30 (10–50) Bruker 279.6 diamond Solid metal RMN-12PT400B None Pt 15 ± 5 0.3 (0.18–0.42) 4.5 (3.15-5.85) Bruker 74.5 AFM tip Crystals 2017, 7, 269 3 of 21

Table 1. Cont.

Tip Tip Coating Bulk Spring k Unit Type Model Radius Freq (kHz) Manufacturer (nm) Materials (N/m) Price ($) (nm) Bruker SSRM-DIA None Diamond 5–20 3/11/27 - 372.2 (IMEC) Advanced Solid Creative P-CT1T2S None Diamond - 0.71 50 1050 doped Solution diamond Technology AFM tip Advanced Creative P-CTCR1S None Diamond <10 nm 0.35 35 950 Solution Technology

Researchers have been working on designing prototypes of nanoprobes coated with specific materials. For example, Dai et al. [15] modified conventional SPM probe tips by attaching multiple walled carbon nanotubes (diameter of 5–20 nm and length of 1 µm) to their apex using epoxy and manual manipulation under optical microscope. Afterwards, the attachment method was improved with the assistance of DC current flow [16]. Tay et al. [17] attached single metallic (tungsten or cobalt) nanowires to commercial AFM tips, and successfully used them to profile a steep sidewall structure with high resolution due to their tiny radius of curvature (1–2 nm) and high aspect ratio (length of ~100 nm, height of ~1.5 µm). Bakhti et al. [18] grew a gold nano-filament with radius of <3 nm and length of 10–100 nm on the apex of conductive SPM nanoprobes, and the resulting nanoprobe was shown to be chemically inert with improved lateral resolution (observable from the topographic maps). The emergence of two dimensional (2D) materials with superior properties (mechanical strength, flexibility, transparency, thermal conductivity, chemical stability, among others) [19–22], has also been attractive in the field of conductive nanoprobes engineering. Several works [23–29] have demonstrated that graphene would be an ideal coating material to enhance the lifetime of a conductive nanoprobe, as it can provide high conductivity and mechanical robustness without increasing the tip radius (it is just one atom thick) and/or modifying the spring constant of the cantilever (its mass is negligible, while hard coatings like diamond have a considerable mass that bend the cantilevers and alter their mechanical and dynamic properties). Furthermore, graphene could be used to functionalize the surface of the probes, providing additional properties like hydrophobicity and piezoelectricity. In this review, we present a detailed summary of all the graphene coated nanoprobes developed, and describe several characteristics including the fabrication technologies and performances.

2. Graphene-Coated AFM Probes Production

2.1. Direct Chemical Vapor Deposition of Graphene on AFM Nanoprobes Chemical vapor deposition (CVD) is a widespread methodology used to produce high quality, large area and continuous graphene films on the surface of different metal catalysts (e.g. Cu [30], Fe [31], Ni [32]). To do so, the metallic substrate (typically a foil) is introduced in a tube CVD furnace and heated at high temperatures (>850 °C) while the graphene precursor (typically CH4 or C2H5OH) is introduced in the tube with the assistance of a carrier gas (typically H2/Ar). Using this approach, the precursor seeds (carbon-containing molecules) can precipitate at random locations on the surface of the metallic sample, and they grow until merging into each other, forming a homogeneous (but polycrystalline) graphene film—intuitively this process is similar to placing several ice cubes on a flat table and waiting until they melt and form a homogeneous water film. By controlling the amount of precursor in the chamber, the temperature and growth time, the properties of the graphene sheets can be tuned (e.g. domain size, graphene thickness). As the growthCrystals 2017 takes, 7, 269place at all locations of the sample simultaneously, the growth process is scalable.4 of 21 Wen et al. [23] attempted to grow graphene on the surface of Au-varnished Si nanoprobes via CVD. To do so, they inserted the nanoprobes (from Mikromash model CSC38/Cr-Au) in a tube and ran the CVD process (Figure1a). C 2H5OH was used to supply the carbon source and delivered to furnace and ran the CVD process (Figure 1a). C2H5OH was used to supply the carbon source and the Au-coated nanoprobe by the mixed gases of Ar/H2 for 5 min, under a ﬂow rate of 100 sccm at delivered◦ to the Au-coated nanoprobe by the mixed gases of Ar/H2 for 5 min, under a flow rate of 100750–850 sccm atC. 750–850 The authors °C. The claim authors that, asclaim the that, tips areas th varnishede tips are withvarnished Au (which with Au can (which serve ascan a serve metallic as acatalyst), metallic the catalyst), graphene the would graphene form would on the surfaceform on of the the surface Au and of eventually the Au and cover eventually the entire cover surface the of entirethe nanoprobe. surface of In the order nanoprobe. to demonstrate In order the to correct demonstrate growth the of graphene correct growth the authors of graphene compare the the authors Raman comparespectra of the the Raman graphene spectra ﬁlms of grown the graphene on Au electrodes films grown and Au-varnished on Au electrodes tips, and bothAu-varnished showed peaks tips, −1 andat 1597 both cm showed(Figure peaks1b). at However, 1597 cm−1 the (Figure scanning 1b). However, electron microscopy the scanning (SEM) electron images microscopy of the tip (SEM) apex imagesshown byof the authorstip apexreveal shown the by material the authors deposited reveal onthe the material tip apex deposited appears toon be the very tip un-homogenousapex appears to beand very thick un-homogenous (Figure1c), meaning and thick that this(Figure carbon-rich 1c), meaning material that maythis notcarbon-rich hold the material genuine may properties not hold of the2D graphenegenuine properties sheets. of 2D graphene sheets.

FigureFigure 1. Direct 1. Direct chemical chemical vapor vapor deposition deposition of of graphenegraphene ononmetal-varnished metal-varnished AFM AFM tips. tips. (a) Schematic(a) Schematic representationrepresentation of an of Au-varnished an Au-varnished AFM AFM tip coated tip coated with with graphene graphene that that is isbeing being used used as as top top electrode electrode in a molecularin a molecularjunction; (b junction;) Raman ( bspectrum) Raman of spectrum the graphene of the formed graphene on the formed Au electrode on the Au (red) electrode and Au-coated (red) and AFM tip (blackAu-coated line); ( AFMc) SEM tip image (black of line); the tip (c) apex SEM after image the of CVD the tip proc apexess. afterThe surface the CVD appears process. to Thebe covered surface by a discontinuousappears toand be thick covered layer, by rather a discontinuous than atomically-t and thickhin continuous layer, rather graphene. than atomically-thin Reproduced with continuous permission from [23],graphene. copyright Reproduced Wiley-VCH with (2012). permission from [23], copyright Wiley-VCH (2012).

It shouldshould bebe highlighted highlighted that that the the growth growth of 2D of materials 2D materials on metallic on metallic substrates substrates has been has normally been normallyperformed performed using metallic using foils, metallic not metal-varnished foils, not metal-varnished surfaces. The reasonsurfaces. is that The large reason amounts is that of metallarge amountsneed to be of available metal need on theto be surface available of the on samplethe surf toace avoid of the massive sample diffusion to avoid andmassive de-wetting diffusion at highand de-wettingtemperatures at high >850 temperatures◦C. Very few >850 authors °C. successfullyVery few authors achieved successfully the CVD achieved growth ofthe graphene CVD growth or any of graphene or any other 2D material on metal-varnished samples (300 nm SiO2/Si) [33–35], and in all other 2D material on metal-varnished samples (300 nm SiO2/Si) [33–35], and in all cases the thickness casesof the the metal thickness was >500 of the nm. metal In Ref. was [23 >500], the nm. authors In Ref. used [23], standard the authors Au-varnished used standard Si nanoprobes Au-varnished for SPMs, on which the Au-varnish is <70 nm thick. They recognized discontinuous graphene growth due to uncontrollable Au melting. Despite the authors showing that their graphene coated nanoprobes achieved enhanced performance (90% yield) as a molecular junction, to the best of our knowledge this approach has never been reproduced by these or other authors, and the graphene grown by this method contains a large number of defects, which is indicated by the strong D peak. Therefore, this is not an ideal methodology for graphene coating on the AFM tips.

Si nanoprobes for SPMs, on which the Au-varnish is <70 nm thick. They recognized discontinuous graphene growth due to uncontrollable Au melting. Despite the authors showing that their graphene coated nanoprobes achieved enhanced performance (90% yield) as a molecular junction, to the best of our knowledge this approach has never been reproduced by these or other authors, and the graphene grown by this method contains a large number of defects, which is indicated by the strong D peak. Therefore, this is not an ideal methodology for graphene coating on the AFM tips.

2.2. Transfer of CVD-Grown Graphene onto AFM Probes As the temperatures required for the CVD growth of graphene are very high, avoiding the use of this method directly on the tips is necessary. One of the main advantages of CVD-grown 2D materials is that they can be prepared on a substrate that ensures very high quality (e.g., Cu foils) and is then transferred onto the target device. Lanza et al. [24] coated commercially available AFM probes with a sheet of graphene previously grown on a Cu foil. During the fabrication process two samples were prepared independently and merged (see Figure 2a). Single layer graphene was synthesized via CVD approach on a 25 μm thick Cu foil, using 20 sccm methane gas mixed with 10 sccm hydrogen in a tube furnace working at a growth temperature of 1000 °C for 15 minutes. After Crystalsthat, the2017 furnace, 7, 269 was cooled down to room temperature under the flow rate of 10 sccm hydrogen.5 of 21 During this process, carbon-containing sources precipitated and eventually graphene films formed on both sides of the Cu substrate. After the growth, the graphene/Cu/graphene stack was introduced substrate.to an oxygen After plasma the growth, furnace the to graphene/Cu/grapheneremove one side of the graphene stack was layers. introduced The resulting to an oxygen graphene/Cu plasma furnacestack was to fixed remove in a one spinner side of and the a graphenedrop of PMMA layers. was The deposited resulting graphene/Cuon top of the graphene stack was sheet fixed and in a spinnerspun at 1000 and arpm drop for of 1 PMMA min. Then, was the deposited sample onwas top backed of the on graphene a hot plate sheet at and170 °C spun for at5 min 1000 until rpm the for ◦ 1PMMA min. Then, became the solid, sample resulting was backed in an on average a hot plate thickness at 170 ofC ~200 for 5nm. min The until PMMA/graphene/Cu the PMMA became solid,stack resultingwas deposited in an averageon the thicknesssurface of of a ~200 FeCl nm.3 solution The PMMA/graphene/Cu for etching the Cu stacksubstrate. was deposited After that, on the surfacePMMA/graphene of a FeCl3 solutionstack was for fished etching and the washed, Cu substrate. first in After a 2%that, HCl thesolution PMMA/graphene and later in deionized stack was fishedwater. andMeanwhile, washed, a first commercial in a 2% HCl PtIr-varnished solution and si laterlicon in AFM deionized tip was water. fully Meanwhile,wrapped and acommercial glued on a PtIr-varnishedcleaned piece of silicon Si wafer. AFM To tip do was so, fully the surface wrapped of andthe gluedSi was on first a cleaned covered piece with of a Si~200 wafer. nm Tolayer do so,of thePMMA surface (spun of at the 1000 Si was rpm first for 1 covered min and with baked a ~200 at 170 nm °C layer for 5 ofmin). PMMA Then, (spun the tip at was 1000 fixed rpm manually for 1 min ◦ andsimply baked by pushing at 170 C and for 5partially min). Then, immersing the tip the was chip fixed containing manually simplythe AFM by tip pushing in the andsoft partially PMMA immersingsubstrate. Then the chip the containingresulting sample the AFM was tip covere in the softd again PMMA with substrate. PMMA. ThenDuring the this resulting process, sample the wascantilever covered of againthe AFM with nanoprobe PMMA. During was bent this due process, to the theweight cantilever of PMMA. of the The AFM probe nanoprobe tip used was was bent the dueCONTPt to the from weight Nanosensors, of PMMA. which The probe has the tip usedfollowing was themain CONTPt properties: from tip Nanosensors, radius = 10 nm, which cantilever has the followinglength = 450 main μm, properties: spring constant tip radius = 0.2 = N/m, 10 nm, and cantilever resonanc lengthe frequency = 450 µ m,= 13 spring kHz; constantthe thickness = 0.2 of N/m, the andPtIr resonancevarnish was frequency 20 nm. = 13 kHz; the thickness of the PtIr varnish was 20 nm.

Figure 2. Transfer of CVD-grown graphene onto AFM probe tips. (a) Step-by-step schematic of the process followed to transfer graphene (grown on Cu foils) onto AFM tips. Reproduced with permission from [24], copyright Wiley-VCH (2013): (b) Experimental protocol used to fabricate graphene coated tip arrays. Reproduced with permission from [25], copyright AAAS.

Once the tip is immobilized on the surface of the Si substrate by the bottom and top PMMA layers, it is used as target substrate to pick up the PMMA/graphene sample prepared previously (see Figure 2a), and the entire structure was dried at room temperature. Finally, all the PMMA was removed via acetone vapor. This is the most critical step of this technique, as highlighted by the authors [24]. Basically, a very ingenious method was proposed to remove the PMMA in this work, that is, instead of liquid acetone, a large amount of PMMA was sufficiently removed by boiling acetone vapor (~68 °C) for 30 min. A designed set up with two glass containers was used to keep the AFM tip continuously exposed to the vaporized acetone, which effectively reduced the residual of contamination on the AFM tip. As a result, the geometric shape of the tip was fully wrapped by flexible graphene film and finally achieved the conformal graphene coated AFM tip. This kind of transfer method was also used in the work reported by Shim et al. [25], in which a Crystals 2017, 7, 269 6 of 21 10–20 layer thick graphene stack grown on a 4-inch Ni/Si wafer (Graphene Laboratories Inc.) was used as graphene coating for scanning probe arrays. As usual, a ~70 nm thick PMMA layer was layerspin-coated was spin-coated on as-grown on graphene as-grown/Ni/Si graphene/Ni/Si at 500 rpm for at 50010 s rpm with for a speed 10 s with of 100 a speedrpm, followed of 100 rpm, by followed5000 rpm byfor 500060 s with rpm a for speed 60 s of with 1000 a rpm. speed The of 1000PMMA/graphene/Ni/Si rpm. The PMMA/graphene/Ni/Si stack was preserved stack at room was preservedtemperature at roomfor 24 temperature h and dried for naturally. 24 h and driedAfterwards, naturally. small Afterwards, pieces (1 small × 1 piecescm2) of (1 wafer× 1 cm were2) of waferimmersed were into immersed aqueous into FeCl aqueous3 solution FeCl3 (1solution M) for (1 etching M) foretching away the away Ni the film, Ni film,which which produced produced the theseparation separation of the of PMMA/graphene the PMMA/graphene film from film the from substrate. the substrate. Similarly, Similarly, the PMMA/graphene the PMMA/graphene film was filmrinsed was with rinsed deionized with deionized water and water thenand fished then with fished the withtarget the substrate. target substrate. In this case In this the caseauthors the authorsdesigned designed a target asubstrate target substrate consisting consisting on a transparent on a transparent glass with glass 100 with nm 100 SiO nm2 with SiO2 morewith morethan than4489 4489pyramidal pyramidal tips tips(without (without cantilever, cantilever, see seeFigure Figure 2b)2b) arranged arranged in in a a matrix distribution. ThisThis targettarget substratesubstrate waswas usedused toto fishfish thethe PMMA/graphenePMMA/graphene stack, a process that was done by keeping a constant angleangle ofof 4040°◦ with with respect respect to to the the liquid liquid surface. surface. The The sample sample was was exposed exposed 48 h 48 to airh to atmosphere air atmosphere at room at temperatureroom temperature for natural for natural drying, drying, and the and PMMA/graphene the PMMA/graphene coated coated HSL HSL tip arraytip array was was immersed immersed in acetonein acetone for for 2 h 2 to h removeto remove the the PMMA. PMMA. Finally, Finall they, the sample sample was was cleaned cleaned again again in ethanol. in ethanol.

2.3.2.3. Mold-AssistedMold-assisted Transfer Transfer of CV CVD-GrownD-Grown Graphene onto AFM Probes Martin-OlmosMartin-Olmos et al. [[26]26] developed a different transfertransfer method based on the use of a Cu mold, onon whichwhich thethe graphenegraphene waswas growngrown beforebefore beingbeing ﬁlledfilled withwith SU-8SU-8 photoresist.photoresist. TheThe entireentire fabricationfabrication processprocess isis displayeddisplayed inin FigureFigure3 3a.a.

Figure 3. Schematic of graphene coated SU-8 AFM probes obtained by CVD graphene transfer

using a mold. (a) Fabrication procedure of graphene coated SU-8 AFM probes; (b) SEM images of released graphene coated SU-8 AFM probes (top) and zoom in probe apex (bottom). Reproduced with permission from [26], copyright ACS Nano (2013).

Figure 3. Schematic of graphene coated SU-8 AFM probes obtained by CVD graphene transfer using a mold. (a) Fabrication procedure of graphene coated SU-8 AFM probes; (b) SEM images of released graphene coated SU-8 AFM probes (top) and zoom in probe apex (bottom). Reproduced with permission from [26], copyright ACS Nano (2013).

First, a 100 nm SiO2/Si wafer was patterned with circles of different sizes (a few micrometers) via lithography. The sample was then immersed in potassium hydroxide (KOH) etchant to generate the inverted pyramids with different sizes, which were used as the molds for AFM tips. Thermal thin silicon dioxide (which served as a sacrificial layer) was grown on the patterned wafers, followed by the deposition of a 500 nm thick high-purity copper film, which acted as a catalytic metal for the CVD growth of graphene. In this case, the authors used a suitable metallic thickness that might be able to withstand the thermal heating during the 2D material growth. The molded substrates were heated in a tube furnace up to 800 ℃ under hydrogen gas flow of 5 sccm. Then, 35 sccm of methane (CH4) was introduced into the chamber, which produced the decomposition of methane and Crystals 2017, 7, 269 7 of 21 formation of monolayer graphene. The resulting graphene coated Cu mold was filled with SU-8 photoresist (10 μm) by using a spinner. Finally, the SU-8 photoresist was patterned by lithography wasand the etched: substrate ﬁrst thewas etching etched: offirst SiO the2 using etching KOH of SiO and2 using then KOH the Cu and using then FeCl the 3Cu(more using experimental FeCl3 (more detailsexperimental can be founddetails incan Ref. be [26 found]). The in SEM Ref. images[26]). The of the SEM released images SU-8 of graphenethe released coated SU-8 nanoprobes graphene (Figurecoated 3nanoprobesb) show interesting (Figure and 3b) correct show graphene interesting coating and on correct the SU-8 graphene photoresist, coating which on can the be easilySU-8 distinguishedphotoresist, which by the can characteristic be easily distinguished wrinkles. by the characteristic wrinkles.

2.4. Direct Graphite-LikeGraphite-like Thin Thin Film Film Deposition Deposition on on AFM AFM Nanoprobes Nanoprobes Graphene cancan bebe alsoalso successfully successfully deposited deposited on on the the apex apex of standardof standard nanoprobes nanoprobes without without the usethe ofuse any of any CVD CVD steps. steps. Pacios Pacios et al. et [ 27al.] [27] reported reported the fabricationthe fabrication of an of ultrathinan ultrathin graphite-coated graphite-coated AFM AFM tip usingtip using a sputtering a sputtering deposition deposition followed followed by in situ by annealing.in situ annealing. First, a 30 First, nm thick a 30 amorphous nm thick amorphous carbon film wascarbon deposited film was on deposited the as-received on the spherical as-received Si/SiO spherical2 or rounded Si/SiO Si2 bulkor rounded AFM tips Si viabulk radio AFM frequency tips via (RF)radio sputtering. frequency Then, (RF) asputtering. 100 nm thick Then, platinum a 100 (Pt) nm catalyst thick platinum film was directly(Pt) catalyst deposited film onwas the directly carbon film,deposited also via on RFthe sputtering. carbon film, The also process via RF is highlightedsputtering. The in Figure process4a. Theis highlighted tip is then annealedin Figure in4a. a The quartz tip ◦ tubeis then furnace annealed under in ana quartz argon atmospheretube furnace for under 30 min an atargon a temperature atmosphere of 800for 30C. min This at step a temperature produced the of spreading800 °C. This of step carbon produced from the the graphite spreading film of into carbon metal from catalyst the graphite due to the film high into solubility metal catalyst of carbon due into the high Pt. When solubility cooling of carbon down, in the the carbon Pt. When separates cooling and down, graphitic the carbon flakes separates are generated and graphitic on the surface flakes ofare the generated Pt film (seeon the Figure surface4b). Cross-sectionof the Pt film TEM(see Fi imagesgure 4b). exhibited Cross-section the successful TEM images growth exhibited of layered the graphitesuccessful film growth on the of Pt-coated layered graphite tips (Figure film4c,d). on the The Pt-coated thickness tips of the (Figure ultrathin 4c,d). graphite The thickness film obtained of the byultrathin this method graphite is aroundfilm obtained 20 nm (seeby this Figure method4d). Foris around this experiment, 20 nm (see the Figure authors 4d). usedFor this AFM experiment, tips with differentthe authors tip radiusesused AFM (ranging tips with from diff 2 µmerent to 90tip nm), radiuses heights (ranging (typically from 10–15 2 µμm)m to and 90 force nm), constants heights (ranging(typically from 10–15 48 μ N/mm) and to force 0.2 N/m). constants (ranging from 48 N/m to 0.2 N/m).

Figure 4. Ultrathin graphite growth on high aspect ratio features. (a) Schematic of the growth process on a 3D shaped AFM nanoprobe; (b) Growth mechanism of thin graphite by thin ﬁlm deposition technology and thermal annealing; Cross-sectional TEM image of the resulting graphite coated AFM tip in low (c) and high (d) magniﬁcation. Reproduced with permission from [27], copyright Springer Nature Publishing Group (2016).

The advantages of this work are: (i) unlike thermal evaporation, in which carbon and Pt layers are evaporated in two separated chambers, here sputtering can produce both layers in the same run and the quality of the materials can be precisely controlled by the bias and pressure during the deposition process; (ii) the changeable directionality of sputtering helps to achieve full coverage for the irregular shapes of AFM tips; (iii) as a metal catalyst, platinum can provide better surface morphology and homogeneity for the growth of high quality 2D materials due to its higher melting temperature and lower thermal expansion coefﬁcient; and (iv) Pt cannot be easily oxidized, in contrast Crystals 2017, 7, 269 8 of 21 with what happens to other metals (such as Ni or Cu), and its chemically inert property contributes to decreasing surface irregularities and keeping its catalytic ability. Compared to traditional CVD technology, the authors claim that this method is a clean, simple, less hazardous and reproducible technique, which makes it possible for large-scale manufacturing. In Figure4c,d a cross-section of the graphitic ﬁlm grown on the surface of the tip apex can be seen. Layered structure can be successfully obtained, as observed from the high resolution TEM images.

2.5. Liquid Phase Graphene Flakes Coated AFM Probes Although AFM probes coated with graphene can be achieved by using some of the above- mentioned methodologies, complicated procedures are not advisable for the mass production needed in industry. Recently, high quality solution-processed graphene was used to coat different kinds of nanoprobes [28]. The graphene sheets were synthesized from graphite powder. Based on a series of redox reactions, graphite powder (2 g) was firstly oxidized by the Hummers–Offeman method with the ◦ assistance of H2SO4 (12 mL), K2S2O8 (3.0 g) and P2O5 (3.0 g) at 80 C for 5 h. Then, H2SO4 (150 mL), KMnO4 (25 g) and 30 mL H2O2 were added to the resulting product in order to help to re-oxidize it, and thin flakes of graphene oxide were successfully obtained after: (i) washing (in 1:10 HCl and pure water), (ii) drying naturally, (iii) purifying (by dialysis for 1 week), and (iv) an ultrasonic bath. The 50 mL graphene oxide solution (0.1 mg/mL) was reduced by mixing it with hydration hydrazine (5 mL). The mix was then stirred for 24 h at 80 ◦C, filtered and dried, which resulted in a black powder graphene. Finally, the graphene solution was prepared by mixing the black graphene powder (5 mg) and pure water (1 mL), and sonicating at 50 W for 10 min. The resulting solution contained large amounts of graphene sheets with an average thickness of 0.7 nm and size of <1 µm. Currently there are several suppliers that provide these types of graphene solutions at a very low price. However, when selecting a graphene solution supplier one needs to verify that the solution really contains large area atomically thin graphene sheets, not just thick graphite particles. One recent study analyzed the size and thickness of the sheets in liquid phase exfoliated graphene from more than 20 different companies, and it was concluded that only two were able to provide micrometer scale sheets with thicknesses below 10 layers [36]. Also, several companies add polymers to the solution in order to stabilize it. It is important to select graphene solutions that do not use these polymers, as they may fall between the tip apex and the graphene flake, and result in a thick and non-conductive coating. Once the solution is ready, coating the graphene tip is very simple, cheap and fast. In Hui’s work [28], commercial AFM nanoprobes with a conductive coating of 20 nm Pt or PtIr from different manufacturers (Olympus and Bruker) were immersed in the graphene solution for less than one minute (see Figure5). By swinging the probe, the graphene sheets readily attached to the sharp AFM tip by van der Waals forces. Van der Waals forces are much higher at very sharp morphologies [37], which means that all the flakes tend to attach there (i.e., many other locations of the nanoprobes remain uncoated, but very good conformal coating and high reproducibility is achieved at the tip apex). After that, the graphene coated nanoprobe was left to dry naturally. Alternatively, the nanoprobes coated by this method can also be dried using a N2 gun at a very low gas flow. N2 blowing also enhances the adhesion between the graphene and the tip apex. Additionally, the amount of graphene sheets attached to the tip apex can be tuned by adjusting the concentration of the graphene solution. This cost-efficient methodology could be used to achieve high-yields of graphene coated nanoprobes and facilitate their industrial production. Crystals 2017, 7, 269 9 of 21

Additionally, the amount of graphene sheets attached to the tip apex can be tuned by adjusting the

Crystalsconcentration2017, 7, 269 of the graphene solution. This cost-efficient methodology could be used to achieve9 of 21 high-yields of graphene coated nanoprobes and facilitate their industrial production.

Figure 5. Different commercial Pt-varnished AFM probes coated by solution processed graphene Figure 5. Different commercial Pt-varnished AFM probes coated by solution processed graphene flakes. SEM ﬂakes. SEM images of as-received OMCL-AC240 nanoprobe before (a) and after coating (b); (c,d) are images of as-received OMCL-AC240 nanoprobe before (a) and after coating (b); (c) and (d) are OSCM-PT and OSCM-PT and SCM-PIC AFM nanoprobes coated with low density of graphene sheets (respectively). SCM-PIC AFM nanoprobes coated with low density of graphene sheets (respectively). Reproduced with Reproduced with permission from [28], copyright The Royal Society of Chemistry (2016). permission from [28], copyright The Royal Society of Chemistry (2016).

3. Perspectives on the Fabrication of Graphene Coated AFM Probes Based on the above discussions, liquid-phase exfoliatedexfoliated graphene coating seems to be the most promising methodology due to its low cost, fast coatingcoating processprocess andand excellentexcellent compatibilitycompatibility with industry. Here,Here, thethe possiblepossible manufacturingmanufacturing procedureprocedure based on the current technologies being used by AFM tips manufacturers (such as Nanoworld) is proposed as followsfollows [[38].38]. TheThe basicbasic fabricationfabrication process of standard tips is describeddescribed in Figure6 6.. AA piecepiece ofof siliconsilicon waferwafer isis oxidizedoxidized onon bothboth sidessides (Figure6a). Then, a layer of photo resist is spin-coated onto the back side of the SiO /Si/SiO wafer, (Figure 6a). Then, a layer of photo resist is spin-coated onto the back side of the SiO2 2/Si/SiO2 wafer, and subsequentlysubsequently exposed exposed to to UV UV light light using using a mask a mask (Figure (Figure6b–c). Similarly,6b–c). Similarly, the front the side front is patterned side is bypatterned the same by methodthe same but method using but a different using a maskdifferen (seet mask Figure (see6d–e). Figure The 6d–e). silicon The dioxide silicon on dioxide the front on sidethe front is removed side is removed by isotropic by isotropic wet etching wet methodology etching methodology (Figure6 f)(Figure and then 6f) and the photoresistthen the photoresist on both sideson both is dissolvedsides is dissolved (Figure 6(Figureg). The 6g). formation The formatio of probesn of probes can be can observed be observed after theafter anisotropic the anisotropic wet etchingwet etching of silicon of silicon by KOH by KOH in the in following the following steps, step untils, theuntil oxide the oxide shields shields falls off falls (as off shown (as shown in image in Figureimage 6Figureh,i). A 6h,i). silicon A nitridesilicon layernitride is depositedlayer is deposited to protect to theprotect tip side the oftip the side probe of the from probe the from damage the throughdamage furtherthrough wet further etching wet of etching silicon, whichof silicon, deﬁnes which the defines thickness the of thickness cantilever of (Figure cantilever6j,k). Finally,(Figure silicon6j-k). Finally, bulk probes silicon arebulk isolated probes byare removing isolated by the re siliconmoving nitride the silicon layer. nitride In order layer. to In obtain order functional to obtain coatingfunctional probes, coating metallic probes, or metallic magnetic or materials magnetic are mate requiredrials are to required be deposited to be on deposited top of Si bulkon top probes of Si bybulk speciﬁc probes methods, by specific such methods, as sputtering, such as atomic sputtering, layer depositionatomic layer (ALD) deposition and/or (ALD) E-beam and/or evaporation. E-beam Thisevaporation. process isThis normally process conducted is normally on six-inchconducted wafers on six-inch (see Figure wafers7a),and (see after Figure the 7a), process andthe after chips the areprocess patterned the chips within are the patterned silicon waferwithin (Figure the silicon7b). Then, wafer they (Figure need 7b). to be Then, removed they andneed placed to be removed in sticky boxes (using vacuum tweezers to avoiding scratches) for commercialization. The photograph of a

Crystals 2017, 7, 269 10 of 21 Crystals 2017, 7, 269 10 of 21 Crystals 2017, 7, 269 10 of 21 and placed in sticky boxes (using vacuum tweezers to avoiding scratches) for commercialization. andwaferThe photographplaced patterned in sticky withof a boxeswafer AFM (usingpatterned tips, as vacuum well with as AFM thetweezer zoomed-in tips,s to as avoiding well SEM as imagesthe scratches) zoomed-in of the for chips commercialization.SEM containingimages of the Thechipscantilevers photograph containing and sharp ofthe a cantilevers wafer tips are patterned shown and sharp in with Figure tips AFM7 area,b, tips, shown respectively. as wellin Figure as the 7a,b, zoomed-in respectively. SEM images of the chips containing the cantilevers and sharp tips are shown in Figure 7a,b, respectively.

Figure 6. Schematic illustration of standard AFM probes manufacturing by Nanoworld. (a) Silicon Figure 6. Schematic illustration of standard AFM probes manufacturing by Nanoworld. (a) Silicon wafer with wafer with both sides of silicon dioxides; (b,d) exposure of the back and front sides to photoresist Figureboth sides 6. Schematic of silicon illustrationdioxides; ( b,dof standard) exposure AFM of the probes back manufacturing and front sides by to Nanoworld. photoresist (througha) Silicon a wafermask; with(c,e) through a mask; (c,e) development of the exposed photoresist; (f) isotropic wet etching of silicon bothdevelopment sides of siliconof the exposed dioxides; photoresist; (b,d) exposure (f) isotropic of the backwet etching and front of silicon sides tooxide; photoresist (g) dissolution through of a photo mask; resist; (c,e) oxide; (g) dissolution of photo resist; (h,i) anisotropic wet etching of silicon by KOH and silicon oxide; development(h,i) anisotropic of thewet exposed etching photoresist;of silicon by ( fKOH) isotropic and siliconwet etching oxide; of ( j–lsilicon) deposition oxide; (g and) dissolution isotropic ofwet photo etching resist; of (j–l) deposition and isotropic wet etching of silicon and silicon nitride, respectively. Reproduced with (siliconh,i) anisotropic and silicon wet nitride, etching respectively. of silicon by Reproduced KOH and withsilicon permission oxide; (j–l from) deposition [38], copyright and isotropic Nanoworld wet etching 2008. of permission from [38], copyright Nanoworld 2008. silicon and silicon nitride, respectively. Reproduced with permission from [38], copyright Nanoworld 2008. The coating process via liquid phase exfoliation presented in Ref. [28] has been carried out by manipulatingTheThe coating coating probe process process tips viaone-by-one via liquid liquid phase phasefrom exfoliation commercial exfoliation pr presentedesentedsticky boxes in inRef. Ref.using [28] [28 hasstandard] has been been carriedtweezers, carried out and outby manipulatingimmersingby manipulating them probe probeinto tips tipsaone-by-one tube one-by-one containing from from commercial th commerciale graphene sticky sticky solution.boxes boxes using Despite using standard standardthe tweezers,success tweezers, and immersingreproducibilityand immersing them of them theinto experiments, intoa tube a tube containing this containing method th shou thee grapheneld graphene be optimized solution. solution. for theDespite Despite coating the theof severalsuccess success tips and in reproducibilityparallelreproducibility and avoid of the tip-induced experiments, scratch this in method the chip shou should thatld contains be optimized the cantilever for the coating coatingand tips of of (see several several Figure tips tips 7c). in in parallelThe ideal and is avoidthat avoid AFM tip-induced tip-induced tip manufacturers scratch scratch in in the theincorporat chip chip that thate thecontains contains graphene the the cantilever cantilevercoating process and and tips tips at (see (seethe Figureend Figure of 7c).7thec). Theproduction ideal is chainthat AFM of the tip AFM manufacturers tips. To do incorporat incorporateso, the as-fabricatede the the graphene wafers coating coating containing process process the at at tipsthe the end endshould of of the be productionimmersed in chain a container of the AFM filled tips. with Tographene To do so, the solution as as-fabricated-fabricated for a certain wafers period containing of time. the theThe tips concentration should be immersedofimmersed the graphene in in a a container container solution, filled ﬁlled the immersion with with graphene graphene time solution solutionand the for foruse a a of certain certain agitation period period and/or of of time. time. voltage The The mayconcentration concentration be tuned oftoof theimprove graphene the percentage solution,solution, thethe of immersiontipsimmersion coated time andtime andthe and quality the the use use of of of the agitation agitation coating. and/or and/or voltage voltage may may be be tuned tuned to toimprove improve the the percentage percentage of of tips tips coated coated and and the the quality quality of theof the coating. coating.

Figure 7. As-fabricated AFM probes by Nanoworld manufacturer. (a) Wafer scale of AFM probes; SEM images Figure 7. As-fabricated AFM probes by Nanoworld manufacturer. (a) Wafer scale of AFM probes; Figureof (b) as-fabricated7. As-fabricated AFM AFM probes. probes Reproduced by Nanoworld with manufacturer. permission from (a) Wafer Ref. [38], scale copyright of AFM probes; Nanoworld SEM imagesGmbH SEM images of (b) as-fabricated AFM probes. Reproduced with permission from Ref. [38], copyright of2008; (b) (as-fabricatedc) Individual AFMprobe probes.scratched Reproduced by tweezers with when permi pickingssion itfrom up. ReproducedRef. [38], copyright with permission Nanoworld from GmbH [28], Nanoworld GmbH 2008; (c) Individual probe scratched by tweezers when picking it up. Reproduced 2008;copyright (c) Individual The Royal probe Society scratched of Chemistry by tweezers (2016). when picking it up. Reproduced with permission from [28], copyrightwith The permission Royal Society from [ 28of], Chemistry copyright (2016). The Royal Society of Chemistry (2016).

Crystals 2017, 7, 269 11 of 21

4. Functionalities of Graphene Coated AFM Probes Graphene coated AFM probes have been fabricated successfully by some of the methodologies described above. After fabrication, these devices show enhanced performance in several types of experimentsCrystals such 2017, 7 as,, 269 topographic and current maps, current-voltage curves, force-distance11 of 21 curves, as well as statistical analysis of variability and durability. Graphene coated nanoprobes have been 4. Functionalities of Graphene Coated AFM Probes used in different ﬁelds of science, including electronics, mechanics, and physics [23–29]. Graphene coated AFM probes have been fabricated successfully by some of the methodologies 4.1. High Weardescribed Resistance above. After in Lateral fabrication, Scans these devices show enhanced performance in several types of experiments such as, topographic and current maps, current-voltage curves, force-distance curves, Theas graphene well as statistical coated analysis SU-8 of nanoprobes variability and fabricated durability. Graphene by mold-assisted coated nanoprobes graphene have been transfer [26] exhibitedused mechanical in different propertiesfields of science, and including conductivity electronics, different mechanics, to and those physics of [23–29]. uncoated (graphene-free)

SU-8 probes.4.1. High In Wear experiments Resistance in Lateral both Scans tapping mode and contact mode images are collected on a calibration sample, which consists of 800 nm diameter and 40 nm thick Ag pillars on a SiO substrate, The graphene coated SU-8 nanoprobes fabricated by mold-assisted graphene transfer [26]2 to characterizeexhibited the mechanical polymer properties AFM probes. and conductivity As shown different in Figure to those8a, under of uncoated tapping (graphene-free) mode, both probes can resolveSU-8 the probes. topographic In experiments information, both tapping and themode scanned and contact image mode using images a graphene are collected coated on a SU-8 probe shows similarcalibration resolution sample, (10% which loss) consists as theof 800 uncoated nm diameter probe. and This 40 maynm thick a result Ag pillars of the on slightly a SiO2 increased radius ofsubstrate, the tip apexto characterize caused the by polymer the graphene AFM prob layeres. As (this shown indicates in Figure that8a, under the tapping layer ismode, not as thin as both probes can resolve the topographic information, and the scanned image using a graphene believed) and/orcoated SU-8 non-uniform probe shows surfacesimilar reso oflution the graphene (10% loss) film.as theThe uncoated interesting probe. This point may came a result when of scanning the samplethe in slightly contact increased mode. radius The imagesof the tip scanned apex caus usinged by the uncoated graphene (graphene-free)layer (this indicates SU-8 that the probes show much lowerlayer lateral is not as and thin vertical as believed) resolution and/or non-uniform than those surface collected of the using graphene graphene film. The coated interesting ones, and the images appearpoint came blurry when and scanning with the the profiles sample repeated.in contact Themode. characteristic The images scanned doubled using (repeated) uncoated features in (graphene-free) SU-8 probes show much lower lateral and vertical resolution than those collected the topographicusing graphene scans are coated a clear ones, indication and the images that aappear tip with blurry two and apexes with the has profiles been formed.repeated. ThisThe is related to the removalcharacteristic of some doubled material (repeated) at the features tip apex; in the the topographic volume loss scans in are the atip clear can indication produce that irregular a tip shapes, making itwith possible two apexes that morehas been than formed. one locationThis is related of the to tipthe touchesremoval of the some sample, material which at the resultstip apex; in repeated profiles alongthe volume the scanned loss in the area. tip can produce irregular shapes, making it possible that more than one location of the tip touches the sample, which results in repeated profiles along the scanned area.

FigureFigure 8. 8.Mechanical Mechanical exploration exploration of graphene of graphene coated AFM coated compared AFM with compareduncoated ones. with (a) Topographic uncoated ones. images collected by both tapping mode and contact mode on the sample of Ag/SiO2. Reproduced with (a) Topographic images collected by both tapping mode and contact mode on the sample of Ag/SiO2. permission from [26], copyright American Chemical Society (2013); (b) 400 nm × 250 nm current maps of Reproduced with permission from [26], copyright American Chemical Society (2013); (b) 400 nm × HfO2/SiO2 stack using a new (top) and worn-out commercial PtIr AFM conductive tip (bottom), the schematic in 250 nmthe right current side represents maps of the HfO degree2/SiO of wearing2 stack of using tips; (c) a SEM new images (top) indicate and worn-out the degree of commercial damage of the PtIrPtIr AFM conductive tip (bottom), the schematic in the right side represents the degree of wearing of tips; (c) SEM

images indicate the degree of damage of the PtIr tip with and without graphene coating after use. The scale bars are 5 µm (left) and 3 µm (right), respectively. Reproduced with permission from [24], copyright Wiley-VCH (2013). Crystals 2017, 7, 269 12 of 21

The reason why the tip loses resolution so fast is the low hardness of the SU-8 material (~0.43 GPa [39], compared to Si ~11.9 GPa [40]), leading to the fast wearing of (volume removal) the apex after scanning the sample consecutively. This phenomenon has been conﬁrmed by collecting current maps of 4 nm HfO2/1 nm SiO2 stacks (under a constant bias), using a new (top) and worn out (bottom) commercial conductive AFM (see Figure8b). When the tip is new (top map) small conductive spots can be detected, indicating the ﬂow of tunneling current across the ultra-thin bilayer insulator. On the contrary, the worn tip shows abundant ring-like conducting features (instead of spots, see bottom current map) which is related to the removal of the tip apex (see schematics in Figure8b). Fortunately, the superb mechanical properties of graphene can effectively protect the apex from wearing even after several scans (see Figure8c).

4.2. Avoiding Water Perturbations at the Tip–Sample Junction The interaction between the tips and a silicon substrate has been compared before and after graphene coating. Force-distance (F–Z) curves have been collected at several locations of the sample. The F–Z curves represent the vertical force that the tip applies to the surface of the sample, which varies with the distance between them. The force vertically applied is proportional to the deflection of the cantilever, and for this reason deflection-force (D–Z) curves can also give relevant information (the Y-axis in Figure9a is deflection, not force, but this is also acceptable if relative variations want to be studied). The D–Z collected with the graphene-free Pt-varnished Si tip on the surface of a Si substrate shows the typical shape: (i) the force stays at zero when the tip is far away from the sample surface of the sample (Z << 0); (ii) starts to increase when the tip is in contact with the sample. In theory that should happen for Z > 0, but in Figure9a the deflection starts to increase at Z ~ −25 nm; the reason is that the initial position of the tip is not at Z > 0, but a bit (~25 nm) above the surface of the sample in order to avoid damage of the tip); (iii) during tip retraction, a high negative peak can be observed, which is related to the adhesion (anti-repulsive) force between the tip and the sample and (iv) once the tip is beyond the limited distance this type of force suddenly disappears (a jump in the D-Z curve can be distinguished). As shown in Figure9a, the adhesion force detected in the D–Z curve performed with the Pt-coated AFM tip is 1680 nN. Interestingly, when the same experiment is carried out using a graphene coated tip (obtained with the graphite thin film deposition method explained in Section 2.4) the shape of the D–Z curve is similar, but the adhesion force is much smaller. While different materials at the tip apex may produce different tip-sample interactions, a feasible explanation for such a huge difference in the adhesion peak is as follows. The force that plays the major role in tip–sample adhesion in AFM experiments is due to capillary effects [41]. Basically, when an AFM tip is placed in contact with the sample in normal atmospheric conditions, a water layer related to the relative humidity is formed between the tip and a sample’s surface, which produces a water meniscus at the junction. The depth of the water layers and the size of the water meniscus increases with the relative humidity (see Figure9d) [ 42]. Stukalov et al. [43] demonstrated that the contact force between a Si AFM tip and a mica substrate can increase greatly depending on the relative humidity (see Figure9c). Taking into account that the reduction observed in Figure9a,b is one order of magnitude, this indicates that graphene is an excellent material for avoiding tip-sample capillary effects, which are very annoying in several types of SPM experiments [44–46]. This observation can also be related to the hydrophobic nature of the graphene coating [47]. On the contrary, the Pt coating of the graphene-free tip is highly hydrophilic. Crystals 2017, 7, 269 13 of 21 Crystals 2017, 7, 269 13 of 21

FigureFigure 9. Tip-sample 9. Tip-sample interaction. interaction. Force-distance Force-distance curves collected curves collected on Silicon on substrate Silicon with substrate a Pt coated with a AFM Pt tip (a) andcoated ultrathin AFM graphite tip (a) and coated ultrathin tip (b graphite). Reproduced coated with tip (permissionb). Reproduced from with[27], permissioncopyright Springer from [27 Nature], Publishingcopyright Group Springer (2016); Nature (c) Representative Publishing Group force-distance (2016); (c )curves Representative obtained force-distanceby retracting a curves Si cantilever obtained from a freshly-cleavedby retracting mica a Si cantileversurface at from different a freshly-cleaved RH values mica that surface are listed at different next to RH each values curve. that areReproduced listed next with permissionto each from curve. [43], Reproduced copyright with Amer permissionican Institute from [of43 ],Physics copyright (2006); American (d) Sequence Institute of of images Physics collected (2006); at various(d) relative Sequence humidity. of images The collected scale bars at variousare 1 μm. relative Reproduced humidity. with Thepermission scale bars from are [42], 1 µm. copyright Reproduced American Chemicalwith permissionSociety (2005). from [42], copyright American Chemical Society (2005).

The effect of water meniscus minimization/removal at the tip-sample junction due to the The effect of water meniscus minimization/removal at the tip-sample junction due to the graphene graphene coating have very positive impact in several different types of experiments, especially in coating have very positive impact in several different types of experiments, especially in electronic electronic measurements. Hui et al. [28] collected forward and backward current-voltage (I–V) measurements. Hui et al. [28] collected forward and backward current-voltage (I–V) curves on the curves on the surface of an n-type Si wafer using both standard PtIr varnished AFM tips with and surface of an n-type Si wafer using both standard PtIr varnished AFM tips with and without graphene without graphene coating (solution processed liquid-phase exfoliation graphene). The results are coating (solution processed liquid-phase exfoliation graphene). The results are displayed in Figure 10a. displayed in Figure 10a. The standard tip shows a large onset potential (VON, defined as the minimum voltage that shows The standard tip shows a large onset potential (VON, defined as the minimum voltage that currents above the noise level) followed by a sharp current increase in the forward curve. Then, shows currents above the noise level) followed by a sharp current increase in the forward curve. the backward tip shows a clear shift towards lower potentials, indicating that the tip-sample junction Then, the backward tip shows a clear shift towards lower potentials, indicating that the tip-sample has become more conductive; moreover, the shape of this I–V curve is less sharp, and more similar junction has become more conductive; moreover, the shape of this I–V curve is less sharp, and more to the typical conduction of silicon. On the contrary, the I–V curves collected with graphene coated similar to the typical conduction of silicon. On the contrary, the I–V curves collected with graphene AFM tips show currents similar to the backward curve collected with graphene-free tips. It should be coated AFM tips show currents similar to the backward curve collected with graphene-free tips. It highlighted that the expected conduction mechanism between the PtIr coating (in fact it is 95% Pt) and should be highlighted that the expected conduction mechanism between the PtIr coating (in fact it is the n-type Si substrate (which has no native oxide, as it was removed via hydrofluoric acid) is Schottky 95% Pt) and the n-type Si substrate (which has no native oxide, as it was removed via hydrofluoric conduction, and the observations during the forward I–V curve collected with the graphene-free acid) is Schottky conduction, and the observations during the forward I–V curve collected with the PtIr-varnished tip are unexpected. graphene-free PtIr-varnished tip are unexpected. Nevertheless, this behavior can be explained by the presence of a water meniscus and a nanometric Nevertheless, this behavior can be explained by the presence of a water meniscus and a water layer between the graphene-free PtIr-varnished AFM tip and the sample. The water layer nanometric water layer between the graphene-free PtIr-varnished AFM tip and the sample. The between the tip and the sample increases the resistivity at the junction, which is why VON is larger. water layer between the tip and the sample increases the resistivity at the junction, which is why VON When the electrical field is large enough current starts to flow and the tip physically contacts the n-type is larger. When the electrical field is large enough current starts to flow and the tip physically Si sample, which produces a shift in the I–V curve towards lower potentials. After that, the shape contacts the n-type Si sample, which produces a shift in the I–V curve towards lower potentials. observed in the I–V curve is typical for Schottky conduction. To demonstrate this hypothesis, several After that, the shape observed in the I–V curve is typical for Schottky conduction. To demonstrate I–V curves have been collected at different locations of the sample using the standard (graphene-free) this hypothesis, several I–V curves have been collected at different locations of the sample using the tips, and the forward and backward I–V curves have been fitted to the conduction across a Pt/H O/Si standard (graphene-free) tips, and the forward and backward I–V curves have been fitted 2to the heterojunction (similar to the tip-sample junction) tuning the different H2O thicknesses. The fittings conduction across a Pt/H2O/Si heterojunction (similar to the tip-sample junction) tuning the different were done with the software MDLab [48]. As displayed in Figure 10b, the calculations indicate that the H2O thicknesses. The fittings were done with the software MDLab [48]. As displayed in Figure 10b, the calculations indicate that the forward I–V curves (blue) can only be fitted using a H2O thickness of 10 Å, while the backward I–V curves can be fitted with an almost negligible H2O thickness of 1 Å,

Crystals 2017, 7, 269 14 of 21

Crystalsforward 2017 I–V, 7, 269 curves (blue) can only be fitted using a H2O thickness of 10 Å, while the backward14 of I–V 21 curves can be fitted with an almost negligible H2O thickness of 1 Å, respectively. In contrast, graphene respectively.coated AFM In tips contrast, do not graphene show this coated problem AFM (see tips Figure do not 10 showc,d), andthis problem the currents (see registeredFigure 10c,d), are and real the(correct) currents from registered the initial are (forward) real (correct) I–V curve. from the Again, initial the (forward) hydrophobic I–V curve. nature Again, of the graphenethe hydrophobic coating naturehad a positive of the graphene influence coating on the had measurements. a positive influence on the measurements.

FigureFigure 10. (a) 10.Typical(a) Typicalforward forward and backward and backward I–V curves I–V collecte curvesd with collected standard with and standard graphene and nanoprobes graphene on a piece nanoprobesof n-type silicon; on a piece(b) Fitting of n-type of the silicon; forward (b) and Fitting backward of the forward I–V curv andes collected backward with I–V the curves standard collected tip to the chargewith transport the standard model; tip 3D to schematics the charge of transport (c) standard model; and 3D ( schematicsd) graphene of nanoprobe, (c) standard which and ( dshows) graphene the water resistancenanoprobe, of graphene. which showsReproduced the water with resistance permission of graphene.from [28], Reproducedcopyright The with Royal permission Societyfrom of Chemistry [28], (2016).copyright The Royal Society of Chemistry (2016).

LanzaLanza et al. [[29]29] has provedproved thisthis byby studyingstudying thethe electricalelectrical behaviorsbehaviors ofof HfOHfO22/Si/Si stacks using as-receivedas-received AFM AFM tips tips (Figure (Figure 11a–c) 11a–c) and and graphene graphene coated coated AFM AFM tips tips (Figure (Figure 11d–f), 11d–f), respectively. respectively. As shownAs shown in Figure in Figure 11, 11 topographic, topographic and and current current maps maps were were collected collected simultaneously simultaneously under under a a contact contact forceforce of 0.1 nN andand voltagevoltage ofof 22 VV inin aa highhighvacuum vacuumenvironment environment(10 (10−−7 torr). Comparing Comparing these these two two currentcurrent maps maps (white (white represents represents 0 0 pA), pA), it it is obvi obviousous that the dark spots, which represent the local currentscurrents through through HfO HfO2,2 ,are are smaller smaller in in Figure Figure 11e 11 thane than the the current current collected collected by bythe theuncoated uncoated AFM AFM tip (Figuretip (Figure 11b). 11 Also,b). Also, the statistical the statistical analysis analysis shown shown in Figure in Figure 11c,f 11 corroboratec,f corroborate the smaller the smaller size sizeof the of conductivethe conductive spots spots collected collected using using graphene graphene coated coated tips tips and and an an obvious obvious shift shift to to higher higher currents comparedcompared toto uncoateduncoated onesones (insets (insets in in Figure Figure 11 11c,f),c,f), which which further further shows shows a lower a lower tip-sample tip-sample contact contact area areafor the for graphene-coated the graphene-coated tips due tips to keepingdue to goodkeepin conductivity.g good conductivity. In general, In the general, smoother the topographic smoother topographicsurface and individualsurface and conductive individual spots conductive observed spots in Figure observed 11 demonstrate in Figure that11 demonstrate the graphene-coated that the graphene-coatedAFM tips possess AFM not only tips high possess lateral not resolution, only high but la alsoteral superior resolution, electrical but also performance. superior electrical performance.

Crystals 2017, 7, 269 15 of 21 Crystals 2017, 7, 269 15 of 21

Figure 11. 11. TopographicTopographic (a,d) (anda,d )current and current maps (b,e maps) recorded (b,e) for recorded as-received for (top as-received panel) and (topgraphene-coated panel) and graphene-coatedAFM tips (bottom panel) AFM on tips 2.5 (bottomnm HfO2 panel)/Si stack; on (c,f 2.5) Statistical nm HfO analysis2/Si stack; of the ( c,fconductive) Statistical spots analysis are measured of the when scanning the surface of HfO2/SiO2/Si stack. Reproduced with permission from [29], copyright The Royal conductive spots are measured when scanning the surface of HfO2/SiO2/Si stack. Reproduced with Society of Chemistry (2013). permission from [29], copyright The Royal Society of Chemistry (2013). 4.3. Lower Data Variability 4.3. Lower Data Variability Before analyzing the variability of graphene coated nanoprobes, the intrinsic variability of Beforestandard analyzing (graphene-free) the variability probes should of graphene be discussed. coated The nanoprobes,variability of standard the intrinsic nanoprobes variability is of standardmainly (graphene-free) affected by two probesfactors: (i) should the variability be discussed. of the parameters The variability of the tips of (tip standard radius and nanoprobes spring is mainlyconstant). affected It byis important two factors: to note (i) thethat variability the manufa ofcturers the parameters of AFM probes of the allo tipsw deviations (tip radius of andthe tip spring constant).radius Itup is to important 100% (e.g., to from note 100 that nm the to manufacturers 200 nm [49]), and of AFMthe typical probes rang allowes for deviations spring constant of the tip radiusdeviations up to 100% can be (e.g., as high from as 100one order nm to of 200 magnitude nm [49 ]),(e.g., and from the 1.5 typical N/m to ranges 18.3 N/m for [49]); spring and constant (ii) the presence of a water meniscus at the tip-sample junction. Standard AFM probes varnished with deviations can be as high as one order of magnitude (e.g., from 1.5 N/m to 18.3 N/m [49]); and (ii) metal are hydrophilic (which can form a thick water meniscus at the junction, see Figure 9d), while the presence of a water meniscus at the tip-sample junction. Standard AFM probes varnished with graphene-coated probes are hydrophobic (which repulses the water from the tip and produces a metalclean are hydrophilicjunction). Different (which amounts can form of awater thick at water the junction meniscus can have at the a junction,huge effect see in Figureexperiments,9d), while graphene-coatedespecially when probes measuring are hydrophobic adhesion force (which (see Figure repulses 9) or the currents water (see from Figure the tip 10 andand Refs. produces [50,51]). a clean junction).In this Different section, the amounts effect of of the water graphene at the coatin junctiong on these can have two sources a huge of effect variability in experiments, is analyzed. especially when measuringWhen discussing adhesion the force variability (see Figureof the tip9) orradius currents and spring (see Figureconstant 10 induced and Refs. by the [ 50 graphene,51]). In this section,coating, the effect the observations of the graphene are clear: coating the effect on these of the two graphene sources coating of variability on these is two analyzed. parameters is Whenmuch lower discussing than the the intrinsic variability variability of the provided tip radius by andthe manufacturer. spring constant First, induced the typical by thickness the graphene coating,of the the graphene observations sheets used are clear:to coat thethe tips effect in the of theliquid-phase graphene exfoliated coating method on these (which two is parameters the most is promising) varies from one to five layers, which equals 0.46 nm–2.3 nm. Second, the mass of the much lower than the intrinsic variability provided by the manufacturer. First, the typical thickness graphene nanosheets used in the liquid-phase exfoliated method is negligible, which does not of themodify graphene the spring sheets constant used to of coat thethe cantilever tips in [28]. the liquid-phaseThe effect of the exfoliated graphene method coating (whichmay be ismuch the most promising)more relevant varies fromif its onethickness to five and layers, area is which increased equals (for 0.46 example, nm–2.3 using nm. the Second, graphite-like the mass film of the graphenedeposition nanosheets method used described in the in liquid-phase Section 2.4). exfoliated method is negligible, which does not modify the springRegarding constant the of thevariability cantilever induced [28]. by The the effectwater ofmeniscus, the graphene the use coating of graphene may becoating much is more relevantbeneficial if its thicknessbecause it andreduces area (if is not increased completely (for removes) example, the using presence the graphite-like of water at the film junction. deposition methodTherefore, described this inshould Section result 2.4 ).in even lower variability compared to its graphene-free counterparts. RegardingWen et al. the[23] variabilitystatistically inducedcollected several by the groups water meniscus,of I–V curves the in use octanemonothiol-based of graphene coating is beneficialmolecular because junctions it reduces using both (if not graphene-free completely an removes)d graphene the coated presence Au-varnished of water nanoprobes at the junction. (fabricated via direct CVD-growth on the surface of the Au varnish), leading to two types of Therefore, this should result in even lower variability compared to its graphene-free counterparts. molecular junctions, Au/octanemonothiol/Au (Figure 12a) and Au/octanemonothiol/graphene/Au Wen et(Figure al. [23 12b).] statistically In total, more collected than 1000 several curves groups are co ofllected I–V curves using in18 octanemonothiol-basedtips, and each line in the molecularplots junctions using both graphene-free and graphene coated Au-varnished nanoprobes (fabricated via direct CVD-growth on the surface of the Au varnish), leading to two types of molecular junctions, Au/octanemonothiol/Au (Figure 12a) and Au/octanemonothiol/graphene/Au (Figure 12b). In total, Crystals 2017, 7, 269 16 of 21

Crystals 2017, 7, 269 16 of 21 more thanCrystals 1000 2017, curves 7, 269 are collected using 18 tips, and each line in the plots represents16 the of 21 average ofrepresents 100 I–V curves. the average Comparing of 100 I–V them, curves. larger Comparing tip-to-tip variancethem, larger in currenttip-to-tip up variance to 3 orders in current of magnitude up to has3 orders beenrepresents detectedof magnitude the inaverage the has molecular of been 100 I–Vdetected curves. junctions in Comparing the molecular without them, graphene. junctions larger tip-to-tip without Additionally, variance graphene. in current a relativelyAdditionally, up to small 3 orders of magnitude has been detected in the molecular junctions without graphene. Additionally, variationa relatively of less small than variation 1 order ofof magnitudeless than 1 hasorder been of magnitude detected using has been graphene detected coated using nanoprobes graphene as coateda nanoprobesrelatively small as variationmolecular of junction. less than 1 order of magnitude has been detected using graphene molecularcoated junction. nanoprobes as molecular junction.

FigureFigure 12. 12.Variability Variability andand stability test. test. (a () aAverage) Average I-V I-Vdata data for Au/octanem for Au/octanemonothiol/Auonothiol/Au junctions with junctions 18 Figurewithdifferent 12. 18 differentVariability Au tips; Au and(b tips;) Averagestability (b) Average I-V test. data ( I-Va ) forAverage data Au/octanemonothiol for I-V Au/octanemonothiol/graphene/Au data for/graphene/Au Au/octanem onothiol/Aujunctions with junctions junctions18 different with with 18 different18 differentgraphene Au tips; tips. graphene Reproduced(b) Average tips. with Reproduced I-V permissi data onfor with from Au/octanemonothiol permission [23], copyright from Wiley-VCH [/graphene/Au23], copyright (2012). junctions Wiley-VCH with (2012). 18 different graphene tips. Reproduced with permission from [23], copyright Wiley-VCH (2012). Statistical analyses of the experimental data using nanoprobes coated with graphene via Statistical analyses of the experimental data using nanoprobes coated with graphene via Statisticalliquid-phase analyses exfoliation of methodthe experimental have been alsodata carried using outnanoprobes [28,52]. More coated than with100 I–V graphene curves via liquid-phase exfoliation method have been also carried out [28,52]. More than 100 I–V curves collected liquid-phasecollected exfoliationon a native oxidemethod free haven-type been Si substrate also carried have been out collected [28,52]. (see More Figure than 13a, 100b) for I–V each curves on a nativetype of oxide tip. In free order n-type to avoid Si substratebig differences have rela beented to collected the presence (see of Figurewater between 13a,b) the for tip each and typethe of tip. collected on a native oxide free n-type Si substrate have been collected (see Figure 13a,b) for each In ordersample, to avoid only big backward differences curves related have tobeen the considered presence (see of water also explanations between the related tip and to Figure the sample, 10). only type of tip. In order to avoid big differences related to the presence of water between the tip and the backwardDespite curves using have only been backward considered curves the (see results also explanationsstill show that relatedboth the toonset Figure potential 10). and Despite the using sample,variability only backward is smaller whencurves using have graphene been considered coated nanoprobes. (see also Finally, explanations groups ofrelated I–V curves to Figure have 10). only backward curves the results still show that both the onset potential and the variability is smaller Despitebeen using measured only withbackward the same curves types ofthe tips results on a me sttallicill show substrate. that Theboth plots the show onset saturation potential of theand the whenvariability usingcurrents grapheneis atsmaller ± 5 nA, coatedwhen with a using linear nanoprobes. shapegraphene going Finally, coated from − 5nanoprobes. groups nA to +5 of nA. I–V TheFinally, curves saturation groups have voltage of been I–V (voltage measured curves at have with thebeen same measuredwhich types the of I-Vwith tips curves the on same a reach metallic types 5 nA) substrate.of hastips beenon a The evaluatedmetallic plots substrate. showstatistically. saturation The Figure plots of13c show the shows currentssaturation that the at of ± the5 nA, withcurrents a linearvariability at ± shape 5 nA,of the goingwith data a fromlinearis not − shapeonly5 nA smaller going to +5 fromwhen nA. − Theusing5 nA saturation tographene +5 nA. Thecoated voltage saturation AFM (voltage tips, voltage but at also which (voltage the the at I–V curveswhich reachvariability the 5I-V nA) whencurves has comparing been reach evaluated 5 different nA) has statistically.devices been is evaluatedmuch Figure smaller. statistically.13 Huic shows et al. [28] thatFigure advise the variability13c that shows the smaller ofthat the the data onset voltage of the graphene coated probes may be affected by the different work functions is notvariability only smaller of the whendata is using not grapheneonly smaller coated when AFM using tips, graphene but also coated the variability AFM tips, when but comparingalso the between Pt and graphene tip electrodes. Indeed, the work function of the probes varies from ~6.1 eV differentvariability devices when is comparing much smaller. different Hui etdevices al. [28 is] advisemuch smaller. that the Hui smaller et al. onset [28] advise voltage that of the smaller graphene (Pt) to ~4.8 eV (Pt/graphene) [53], as well as the absence of water perturbations, together coatedonset probes contributingvoltage may of to bethe the affected graphenesmaller byonset thecoated voltage different probes and workthe ma variabilityy functions be affected of I–V between curves. by the Pt anddifferent graphene work tip functions electrodes. Indeed,between the Pt work and graphene function tip of theelectrodes. probes Indeed, varies fromthe work ~6.1 function eV (Pt) of to the ~4.8 probes eV (Pt/graphene) varies from ~6.1 [53 eV], as well(Pt) as to the ~4.8 absence eV of(Pt/graphene) water perturbations, [53], as well together as the contributing absence of to thewater smaller perturbations, onset voltage together and the variabilitycontributing of I–V to the curves. smaller onset voltage and the variability of I–V curves.

Figure 13. 100 backward I–V curves collected with both standard (a) and graphene coated nanoprobes; (b) the onset voltage and the deviation of the graphene nanoprobes is much smaller. Reproduced with permission from [28], copyright The Royal Society of Chemistry (2016); (c) Variance statistics analysis for single and multiple tip measurements at the saturation voltage (I = −5 nA) for fresh probes (in red) or graphene coated (in Figureblue). 13. Reproduced100 backward with permission I–V curves from collected [52], copyright with both Elsevier standard (2016). (a) and graphene coated nanoprobes; Figure 13. 100 backward I–V curves collected with both standard (a) and graphene coated nanoprobes; (b) the (b) the onset voltage and the deviation of the graphene nanoprobes is much smaller. Reproduced with onset voltage and the deviation of the graphene nanoprobes is much smaller. Reproduced with permission permission from [28], copyright The Royal Society of Chemistry (2016); (c) Variance statistics analysis from [28], copyright The Royal Society of Chemistry (2016); (c) Variance statistics analysis for single and for single and multiple tip measurements at the saturation voltage (I = −5 nA) for fresh probes (in red) multiple tip measurements at the saturation voltage (I = −5 nA) for fresh probes (in red) or graphene coated (in or graphene coated (in blue). Reproduced with permission from [52], copyright Elsevier (2016). blue). Reproduced with permission from [52], copyright Elsevier (2016).

Crystals 2017, 7, 269 17 of 21

4.4. High Stability vs. High Currents and Mechanical Strains: Enhanced Lifetime This is probably one of the properties that can bring most benefit to the users of graphene coated nanoprobes. Graphene coated nanoprobes fabricated by various methods have been demonstrated to Crystals 2017, 7, 269 17 of 21 have superior stability when performing different types of experiments (see Figure 14). Lanza et al. [24] performed4.4. an High experiment Stability vs. that High consisted Currents and of Mechanical collecting Strains: several Enhanced I-V curves Lifetime at different (randomly selected) locations of aThis 3 nm is probably thick HfOone 2of stackthe properties (in order that tocan monitor bring most the benefit tunneling to the users currents of graphene and dielectric breakdown)coated using nanoprobes. standard Graphene and graphene coated nanoprobes coated AFM fabricated tips (fabricated by variousvia methods CVD graphenehave been transfer). demonstrated to have superior stability when performing different types of experiments (see Figure The main characteristic was that a source meter was connected directly to the tip of the CAFM, so that 14). Lanza et al. [24] performed an experiment that consisted of collecting several I-V curves at high voltagesdifferent (>10 (randomly V) and high selected) currents locations (up of to a 3 mA) nm thick could HfO be2 stack registered (in order [54 to ].monitor While the the tunneling graphene coated AFM tip iscurrents able toand distinguish dielectric breakdown) the dielectric using standard breakdown and graphene at several coated different AFM tips locations (fabricated (Figurevia 14d), the standardCVD tip graphene cannot transfer). withstand The main the characteristic high currents was that (the a source current meter is was 10− connected4 A, therefore, directly to the current the tip of the CAFM, so that high voltages (>10 V) and high currents (up to mA) could be registered density is 108 A/cm2 because the typical effective area at the tip-sample junction for this sample is [54]. While the graphene coated AFM tip is able to distinguish the dielectric breakdown at several 2 100 nm ), anddifferent shows locations obvious (Figure current 14d), the reduction standard tip with cannot total withstand conductivity the high degradation currents (the current after 5 is I–V curves. In a different10−4 A, therefore, experiment the current by density Hui et is al.108 A/cm [28],2 thebecause durability the typical of effective the graphene-free area at the tip-sample and graphene coated tipsjunction (using for a this liquid-phase sample is 100 exfoliation nm2), and shows method) obvious was current analyzed reduction via with current total conductivity maps collected on degradation after 5 I–V curves. highly conductive samples (graphene/Cu foils) under a constant bias of 1 V and a deflection setpoint In a different experiment by Hui et al. [28], the durability of the graphene-free and graphene of 4 V. Ascoated shown tips in (using Figure a liquid-phase14b,e, while exfoliation the standard method) CAFMwas analyzed tip lost via itscurrent conductivity maps collected in juston 13 scans (Figure 14b),highly the conductive graphene-coated samples (graphene/Cu tip kept good foils) under conductivity a constant evenbias of after1 V and 92 a scansdeflection (Figure setpoint 14 e). In the first experimentof 4 V. As (Figure shown in14 Figurea,d) the14b,e, tip while was the held standard static CAFM at one tip lost position its conductivity on the sample;in just 13 scans therefore, the (Figure 14b), the graphene-coated tip kept good conductivity even after 92 scans (Figure 14e). In the lateral friction was minimal and the wearing was related to the high currents. On the contrary first experiment (Figure 14a,d) the tip was held static at one position on the sample; therefore, the (Figure 14lateralb,e), thefriction most was harmful minimal thingand the for wearing the tips was was related the to friction, the high ascurrents. the currents On the contrary did not increase above the(Figure nanometer 14b,e), scale.the most In harmful any case, thing graphene for the tips coatedwas the friction, AFM tips as the showed currents gooddid not stability increase vs. these harmful tests.above In the Figure nanometer 14c, scale. an AFM In any tip case, degraded graphene incoated a static AFM experiment tips showed good driving stability large vs. currents these can be harmful tests. In Figure 14c, an AFM tip degraded in a static experiment driving large currents can observed via SEM; the metallic varnishes that provide high conductivity melted. In Figure 14f an SEM be observed via SEM; the metallic varnishes that provide high conductivity melted. In Figure 14f an image of aSEM graphene image of coated a graphene tip after coated a sequence tip after a of sequence current of maps current can maps be observed.can be observed. No degradation No is detected, otherdegradation than is small detected, particles other than attached small particles to the attached tip during to the the tip scans.during the scans.

FigureFigure 14. Reliability 14. Reliability and and durability test. test. Current Current vs. Voltage vs. Voltage(I–V) curves (I–V) performed curves on performed different positions on different of positionsHfO2/Si of stack HfO by2/Si (a) stackuncoated by AFM (a) tip uncoated and (d) graphene AFM tip coated and tips. (d )Reproduc grapheneed with coated permission tips. from Reproduced [24], with permissioncopyright Wiley-VCH from [24 (2013).], copyright Current spectra Wiley-VCH and the (2013).insets in ( Currentb) are the spectra1st (blue) and and 13th the insets(red) CAFM in (b ) are the 1stcurrent (blue) maps and collected 13th (red)on graphene/Cu CAFM current foil with mapsa standard collected metal-varnished on graphene/Cu nanoprobe. Current foil with spectra a standardand metal-varnished nanoprobe. Current spectra and the insets in (e) are the 1st (blue) and 92th (red) CAFM current maps collected with a graphene coated nanoprobe. SEM images of AFM tips without (c) and with graphene coating (f) after testing. Reproduced with permission from [28], copyright The Royal Society of Chemistry (2015). Crystals 2017, 7, 269 18 of 21

Apart from all the above benefits, more recently graphene coated colloidal probes, i.e., SiO2 microspheres, were developed to achieve an ultra-low and robust friction coefficient of 0.003 under high contact pressure, in which the super-lubricity of graphite and graphene plays a significant role [55]. Moreover, other properties of graphene, such as, flexibility, transparency, and low toxicity, enable the graphene coated probes to be competitive in the development of flexible and transparency microelectromechanical systems, as well as medical diagnosis.

5. Conclusions In conclusion, different approaches using CVD growth, CVD transfer, sputtering and liquid phase exfoliated graphene have been used to coat nanoprobes with graphene. Direct CVD growth is highly questionable because the high temperatures used can damage the tip, and CVD transfer is too expensive due to the involvement of human labor. Direct sputtering shows good performance but the thermal budget is also high. So far, the liquid phase exfoliated approach has proved to be the cheapest and most industry-compatible method. All graphene coated nanoprobes show lifetimes much longer than their uncoated counterparts when used in different experiments. Speciﬁcally, they possess high wear resistance, stability and mechanical strength, as well as lower variability, which enables the application of the graphene-coated nanoprobes in the ﬁelds of electronics, mechanics and biology in the near future. In order to truly realize the manufacturing of graphene coated probes in industry, systematic techniques need to be further undertaken and optimized for mass production.

Acknowledgments: This work has been supported by the Young 1000 Global Talent Recruitment Program of the Ministry of Education of China, the National Natural Science Foundation of China (grants no. 61502326, 41550110223, 11661131002), the Jiangsu Government (grant no. BK20150343), the Ministry of Finance of China (grant no. SX21400213) and the Young 973 National Program of the Chinese Ministry of Science and Technology (grant no. 2015CB932700). The Collaborative Innovation Center of Suzhou Nano Science & Technology, the Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, the Priority Academic Program Development of Jiangsu Higher Education Institutions, the 111 Project from the State Administration of Foreign Experts Affairs, and the Opening Project of Key Laboratory of Microelectronic Devices & Integrated Technology (Institute of Microelectronics, Chinese Academy of Sciences) are also acknowledged. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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