Tip-Based Nanomachining on Thin Films: A Mini Review

As one of the most widely used nanofabrication methods, the atomic force microscopy (AFM) tip-based nanomachining technique offers important advantages, including nanoscale manipulation accuracy, low maintenance cost, and flexible experimental operation. This technique has been applied to one-, two-, and even three-dimensional nanomachining patterns on thin films made of polymers, metals, and two-dimensional materials. These structures are widely used in the fields of nanooptics, nanoelectronics, data storage, super lubrication, and so forth. Moreover, they are believed to have a wide application in other fields, and their possible industrialization may be realized in the future. In this work, the current state of the research into the use of the AFM tip-based nanomachining method in thin-film machining is presented. First, the state of the structures machined on thin films is reviewed according to the type of thin-film materials (i.e., polymers, metals, and two-dimensional materials). Second, the related applications of tip-based nanomachining to film machining are presented. Finally, the current situation of this area and its potential development direction are discussed. This review is expected to enrich the understanding of the research status of the use of the tip-based nanomachining method in thin-film machining and ultimately broaden its application.

Severe tip damage occurs during direct machining on hard substrates, such as silicon (Si) or silicon dioxide (SiO 2 ). Therefore, machining on soft polymer films that are spin-coated on the surface of hard substrates first and subsequently transferred to the target hard substrates via etching may be a good approach to reducing tip wear. Nanopatterns of varying dimensions, including nanopits/dots [9,13], nanogrooves/lines [14,15], and bundles [16], have been achieved on polymer films via TBN and have been applied to data storage [17], etch masks [18], sacrificial layers for lift-off [19], and so on. As for machining on metal films, numerous TBN methods have been employed, and they include constant force mode [20], dynamic plowing lithography (DPL) [21], phase mode nanofabrication [22], vibration-assisted nanomachining [23], AFM electric lithography [24], and coupling AFM lithography [25]. Reports have shown that 2DMs demonstrate immense potential in the fabrication of nanobiosensors [26] and nanoelectronic sensors [2]. However, only a few studies have utilized TBN for 1 3 direct machining on 2DMs in contact mode. Most scholars have used AFM tips for sliding on 2DMs to investigate their frictional behavior [27][28][29][30] and their possible application to the reduction of friction and wear [31,32]. Much effort is needed to realize the industrialization of 2DMs.
In the present work, the current status of the application of the TBN method to machine on thin-film materials is reviewed. The thin-film materials include polymers, metal and metal-based thin films, and 2DMs. Nanostructures fabricated on films by using heated AFM tip-based thermomechanical lithography are also presented. At the end of the article, a summary of the current study and possible future directions are discussed.

Machining on Polymer Thin Films
Polymer thin films are widely used as an anti-etching resist because of their stable machining characteristics, low costs, and good light transmittance. They are usually obtained by spin coating. Moreover, the structures machined on polymer thin films can be easily transferred to other substrates. Traditional polymers include polymethyl methacrylate (PMMA), polystyrene, and polycarbonate (PC). To date, nanodots/pits, nanogrooves/lines, and even three-dimensional (3D) complex structures have been achieved on these films by using the TBN method. Various types of nanomachining methods based on tips involving static scratch [33][34][35], DPL [9,13,14,36,37], and vibration-assisted approach [22,[38][39][40][41][42][43][44][45][46], especially the ultrasonic vibration-assisted (UV-assisted) method, have been used to machine nanopatterns on polymer films (Table 1). Heated tips have been found to offer important advantages in machining polymer film materials. They also lead to different results when the material properties are changed. Specifically, the heat from the tip can soften the sample surface to induce material deformation or thermal decomposition.
Tip wear may be caused when the tip is used to machine structures on hard materials, such as Si and SiO 2 . Therefore, machining patterns first on a soft resist (e.g., polymers) and subsequently transferring them to the target substrate may be a good solution. As an excellent etch mask, nanopatterns on polymer films can be transferred to other materials via etching and lift-off and can be applied to the field of nanooptics. Recently, Kim et al. fabricated plasmonic nanostructures, such as nanodisks and nanotriangle arrays, on a glass substrate by combining nanotip indentation lithography (NTIL) and wet etching [1].
As shown in Fig. 1a, a two-layer polymer film was spincoated on the substrate; the top one is polymer B that acted as the etch mask, while the bottom layer is polymer A that served as the sacrificial layer. They indented polymer B by using the AFM tip with a conical shape or pyramidal shape to form the corresponding shape of a nanohole. Then, the sample was soaked into AZ 300 MIF Developer to etch the sacrificial layer. Thereafter, a layer of Au measuring around dozens of nanometers was deposited on the sample surface. The bilayer of the polymer film was removed via lift-off, and the plasmonic nanostructures were transferred to the glass substrate successfully, as shown in Fig. 1b-e. These Au nanostructures exhibited excellent plasmonic properties ( Fig. 1f-h) and thus presented immense potential in the field of nanooptics. Although the operation of this method is relatively complex, it can guarantee nanopatterns with highly precise dimensions and shape controllability and reproducibility; it can also adequately reduce tip wear. Thus, this  [13] technique is expected to be used widely in the fabrication of structures on hard substrates, such as Si and glass. On the surface of polymer films, nanodot arrays can be fabricated via tip nanoindentation directly or through the overlap of two machined nanogrooves by a single scratch performed twice in different directions [13,36]; a third scratch may be employed to improve density [13]. Nanopits have been obtained using a tip that is pressed into a sample, similar to nanoindentation [1] or DPL based on tapping [9,37]. Some scholars have found that heated tips can modify polymer surfaces by changing their properties by heating. Chang et al. recently proposed the use of thermomechanical lithography to regulate the formation of nanopatterns, such as nanowires, nanogrooves, and nanopits, on PMMA films via single scratch [6]. Nanopits were machined on the basis of a stick-slip mechanism, as shown in Fig. 2a. They found that the formed nanopits resulting from the material used are softened by tip heating; meanwhile, a previous study reported that no nanopits are machined when samples are not heated [43]. As shown in Fig. 2b-d, nanopits with various dimensions can be achieved by adjusting the coupling of scratching velocity and heated temperature to change the heat transfer between the tip and the film. In addition, nanopit arrays can be machined in one single scratch with a velocity of several to dozens of microns per second. Therefore, this method is expected to be widely used to fabricate nanopits with high efficiency. Fabrication of plasmonic nanostructures on glass substrate through the combination of nanotip indentation lithography (NTIL) and wet etching: a the schematic of using NTIL to fabricate plasmonic nanostructures; b AFM and SEM images of nanodisk plasmonic nanostructures; c AFM and SEM images of nanotriangle plasmonic nanostructures; d bright field optical microscopy images of eight nanodisk arrays and enlarged view of the selected areas; e diameters of the nanodisks (top) and their coefficient of variation (CV); f and g dark-field images of various nanodisk arrays prepared with composite materials of Ag (f) and Au (g); h the scattering spectrum obtained experimentally. Republished with permission from Ref. [1], Copyright 2021 RSC Pub Although various nanopatterns have been achieved on traditional polymer films, limitations such as poor dimension consistency, bad machining quality, elastic recovery, and pile-up have been observed. Therefore, scholars have been focused on identifying polymers with good machining quality. In 2010, Pires et al. realized patterns at a halfpitch down to 15 nm without proximity corrections on a type of molecular glass resist by using a heated AFM tip [52], the machining accuracy of which is comparable to that of electron beam lithography. The molecular glass resist was specifically tailored in the lab, and its hydrogen bonds were broken down by heating at several microseconds and evaporated to the environment for complete removal. The research results were published in Science and have since attracted increasing attention from scholars. However, this material is difficult to synthesize; thus far, it has been replaced by polyphthalaldehyde (PPA), which can be purchased easily.
PPA is a type of thermal-induced depolymerization polymer, and the material of which can be removed during machining using a heated tip. As an etch mask, PPA performs well in acting as a resist for thermal scanning probe lithography (t-SPL). In this process, polymer chains are broken down into small molecules at high temperatures and evaporate into the environment to realize the removal of the sample material. This approach is thus a good solution for eliminating pile-up in machining on traditional polymers. Given these strengths of t-SPL, scholars have attempted to broaden its application by integrating it with other etching techniques. Nanopatterns are machined first on a PPA surface by using a heated AFM tip and are then transferred to the target substrate through etching. Aminzadeh et al. fabricated a 3D Fresnel zone plate on a PPA film that was spincoated on a Si 3 N 4 substrate first and then transferred to the underlying Si 3 N 4 surface via reactive ion etching (RIE), as shown in Fig. 3a [18]. By using a heated tip, Rawlings et al. formed 30-nm-deep Gaussian-shaped twin microcavities on PPA, etched them into SiO 2 , and then embedded them in a distributed Bragg reflector stack (Fig. 3b) [53]. Photonic molecules were created by adjusting the distance between the twin microcavities. In addition, the combination of t-SPL and etching was found to have potential applications in data storage and recording. As shown in Fig. 3c, the archival data were first patterned on PPA and then transferred to a poly-Si-SiO 2 substrate through RIE to realize the permanent storage of data [47]. This study provided a simple method for storing data with high efficiency, and the method is expected to be widely used in the data recording field. Attempts have been made to fabricate electron microscopy phase masks by using focused (Ga + -) ion beam milling of films. However, ion implantation may influence the phase masks. To rise from the limitations, Hettler et al. proposed using t-SPL integrated with RIE to fabricate phase masks, as shown in Fig. 3d [54]. They achieved a highly accurate control of patterns and depths and thus predicted the wide use of the method in the future. Cheong et al. fabricated lines on a PPA film with a half-pitch of 27.5 nm and then transferred the pattern into a Si substrate with excellent accuracy via RIE (Fig. 3e) [50]. In the following studies, Cho et al. reduced the half-pitch of lines to sub-10 nm (Fig. 3f) and thus greatly improved the resolution of patterns fabricated by t-SPL [51].
Thus far, t-SPL has been utilized to fabricate extremely tiny structures with special properties because of its sub-10-nm resolution. Moreover, t-SPL integrated with etching has been utilized to prepare micro/nanoelectronic and optical to create an indentation on a PPA film to generate an ultrasmall sharp hole, which served as the mold for preparing Ag Fig. 3 Applications of PPA as an etch mask. a 3D Fresnel zone plate fabricated on PPA film and transferred to Si 3 N 4 via reactive ion etching (RIE). Reprinted with permission from Ref. [18]. Copyright 2019, American Vacuum Society. b Gaussian-shaped twin microcavities machined on PPA and transferred on SiO 2 to create photonic molecules. Reprinted under CC BY 4.0 License from Ref. [53], Copyright 2017 Springer Nature. c Archival data patterned on PPA and transferred to poly-Si-SiO 2 substrate through RIE to realize the permanent storage of data. Reprinted from Ref. [47], with the permis-sion of AIP Publishing. d Using thermal scanning probe lithography integrated with RIE to fabricate phase masks. Reprinted with permission from Ref. [54], Copyright 2019 Elsevier. e Dense lines written on PPA with a half-pitch of 27.5 nm and then transferred into Si with excellent accuracy via RIE. Reprinted with permission from Ref. [50]. Copyright 2013 American Chemical Society. f Fabrication of Si nanowire with a half-pitch of sub-10 nm. Reprinted with permission from Ref. [51]. Copyright 2017 American Chemical Society electrodes for use in advanced memristor devices (Fig. 4) [48]. The sharp hole pattern was transferred to the underlying SiO 2 by RIE. After metal deposition, the Ag electrode embedded in SiO 2 was formed. The distance between the Ag electrode and the vertical Pt electrode was only 1 nm, which meets the requirement for reliable switching OFF/ON control even at voltages as low as 100 mV. This application of t-SPL machining may inspire the preparation of other similar electronic devices. Another typical example is the combined use of t-SPL and laser writing to write patterns on PPA that are then transferred to the SiO 2 surface via RIE to fabricate silicon point contact quantum-dot transistors [56]. This method takes full advantage of the 10-nm high resolution of t-SPL and the high efficiency of laser machining for large-area structures. It is expected to be applied broadly in the fabrication of large-scale nanodevices.
Nanostructures with a high aspect ratio were fabricated in a study by combining t-SPL and dry etching [49]. As shown in Fig. 5a, a nanogroove with a depth of 38 nm was first machined on 73-nm-thick PPA by t-SPL. The residual PPA layer was then etched through O 2 plasma to expose the underlying SiO 2 substrate. The pattern was transferred to Si via dry etching at a depth that was 4 μm and 100 times that on the PPA. The depth was thus greatly improved. Through dry etching, Lisunova et al. transferred a 3D nanostructure machined by t-SPL on PPA to Si at a depth of 45 nm (Fig. 5b) [55]. The nanopattern on Si reached 420-480 nm in depth. These examples demonstrate the combination of t-SPL and etching as a promising technique to fabricate structures with high aspect ratios. Moreover, they show that the use of a heated tip to machine high-resolution patterns on PPA resist has become a popular technique that is expected to be applied widely in the future.

Machining on Metal and Metal-Based Thin Films
Various AFM tip-based nanomachining methods have been used to fabricate nanostructures on metal films. These methods include scratching based on a constant force mode, DPL, phase mode nanofabrication, vibration-assisted nanomachining, AFM electric lithography, and coupling AFM lithography (Table 2). Thus, patterns are machined on metal films to mainly serve as a mask for etching. Meanwhile, machining on metal-based thin films mainly promotes these materials' wide applications according to their special physical characteristics.
When scratching based on the constant force mode, the machined depth is dependent on the force, therefore, the machined depth can be controlled with high precision by adjusting the normal load. In addition, the constant force machining system does not require leveling, particularly, the machining accuracy on the curved surface can easily reach less than 100 nm. However, the machining system based on the constant force mode is complex and requires force feedback system to realize force control. Moreover, another disadvantage is that it is hard to machine structures with high aspect ratios on metal films using the constant force mode due to its lower stiffness. The tip wear may also be a point of concern due to the relatively large hardness of metal films.
However, DPL may be a good solution to reduce the tip wear to a certain degree when machining on the metal film. This is because DPL is based on the tapping mode, and cantilever vibrates at resonance frequency during machining to realize intermittent contact between tip and sample. Simultaneously, using the DPL method, the cantilever of the tip is driven to oscillate at several thousands of Hertz, which results in the tip interacting with the sample surface many times in a short period. Therefore, the DPL method can machine nanostructures on the metal film with high throughput. Moreover, structures with characteristic sizes ranging from a few nanometers to tens of nanometers on the metal film can be easily achieved via DPL method. When using the DPL method to machine on the metal film, it is hard to achieve patterns with larger depth due to the relatively large hardness of metal films and the relatively light force between the tip and sample. Therefore, this method is usually to be used to machine on the polymer film, while there are few reports on metal films.
In the phase mode nanomachining method, the phase feedback signal from cantilever was extracted by the lockin amplifier, and then the feedback signal was sent to the PID controller to compare with the preset value. The phase response signal will be affected by the machined depth; therefore, the machined depth can be regulated by real-time monitoring of the phase feedback signal. Therefore, the machined depth can be monitored accurately to avoid the tip from penetrating the film completely and contacting the underlying hard substrate, which can protect the tip from wearing as enough as possible. Furthermore, based on the phase mode, the machining results are not affected by the debris and pile-up, which overcomes the limitation of constant force mode. However, this machining system requires phase response as feedback signal to achieve closed-loop control, the configuration of which may be a little complex.
The vibration is introduced to achieve larger depth even under a relatively small force, and increase the material removal efficiency. Another strength of vibration-assisted nanomachining method is that the width of structure can be controlled by adjusting the vibration amplitude in the x-y plane. Similar to DPL, the z-axis high-frequency vibration machining method makes the probe in intermittent contact with the sample surface, which can reduce the tip wear and increase the material removal efficiency as well.
Compared to traditional AFM mechanical machining, AFM electric lithography shows potential in improving material removal efficiency and the quality of machining structure on the metal film owing to the existence of both bias voltage effect and the force effect. However, it is difficult to precisely control the machined depth due to the complex coupling effect of bias voltage and force.
The coupling AFM lithography is developed to machine the structures with deeper area through applying both force and bias voltage simultaneously. It has been demonstrated that under the same machining condition, the patterns machined by the coupling AFM lithography are much deeper and with lower roughness than that of the AFM mechanical lithography and the AFM electric lithography. However, the weakness of this method is that the consistency and stability of machined structure still need to be further improved.
The metal film is a good mask for etching because of its excellent etching resistance, and the nanopatterns can be easily transferred to other substrates for fabricating structures with high aspect ratios [38]. Metal thin films are usually prepared by electron beam deposition and magnetron sputtering. The adhesion between films and various substrates tends to differ, and the differences greatly affect the machining results when TBN is used. In addition, considerable silicon tip wear can be observed when scratching on metal films because of the materials' high hardness. Therefore, a diamond tip is usually employed by scholars for direct machining. Peng et al. scratched distributed submicron grooves with good machining quality on an Al film by using a diamond tip; these patterns can be transferred to Si to fabricate structures with high aspect ratios [57]. Scholars have recently found that local stress exerts an obvious effect on the polarization of ferroelectric materials [59] and that it may promote the application of such materials to data storage. Zhang et al. fabricated a nanocapacitor array with individually addressable ferroelectric elements on a BiFeO 3 -Cr-Au film [60] and reported its possible use in the field of storage data in the future. Another work described the machining of The University College Dublin's harp logo, a 100-nm nanoisland array comprising circles with a diameter of 2.5 μm and parallel lines, on ferroelectric films [60].
However, the size of patterns machined using a diamond tip may be too large to meet the demand of small structures in some specific applications. A possible solution for this is to use DPL to machine ultra-small patterns on metal films. DPL is based on the tapping mode. In the process, the cantilever vibrates at a resonance frequency during machining to realize intermittent contact between the tip and the sample and thereby reduce tip wear to a certain degree. Thus, this method can realize high-resolution nanomachining. Schumacher et al. employed DPL to machine 5.4-nm-deep holes and 50-nm-wide grooves on a Pt/Co/Pt trilayer [21]. These patterns served as mesoscopic defects to modulate the propagation of domain walls in a magnetic memory prototype, as well as adjust and optimize domain shapes during the reversal of magnetization. This research laid the foundation for the use of TBN to control the reversal of magnetization at the microscopic scale.
As a type of novel and advantageous nanomachining method, nanomilling reaches great depths even under a relatively small force, and the material removal efficiency can be increased. Moreover, the width of a structure can be controlled by adjusting the vibration amplitude in the x-y plane. Similar to DPL, the z-axis high-frequency vibration machining method makes the probe come into contact with the sample surface intermittently, so as to reduce the tip wear and increase the material removal efficiency. Zhang et al. introduced ultrasonic vibration to assist nanomachining on Al films [39]. They fixed an Al film on the surface of a nanovibrator, which vibrated in the z-axis at 3 MHz to regulate the machined depth; in the x-y plane, it vibrated at 4 kHz, which was set to adjust the width of the machined patterns. They found that the normal and lateral forces during UV-assisted nanomachining are significantly lower than those of the traditional setpoint-force control. Hence, the method shows great potential in reducing tip wear. In addition, a greater reduction in tip wear has been observed when using the UV-assisted nanomachining method to fabricate patterns on PMMA films than when using the traditional setpoint force control. The mechanism of tip wear reduction can be explained as follows: only a certain amount of the material determined by the feed is removed at one vibration period in the x-y plane; this condition reduces the contact force between the tip and the sample dramatically. Furthermore, this method can yield structures in the nanometer scale that are much smaller than those machined using a diamond tip. It is also able to meet the requirement of smallsized patterns. Therefore, the UV-assisted nanomachining method is expected to be widely used in the machining of nanostructures on metal films. Following the study of Zhang et al. [39], Park et al. employed a vibration-assisted nanomachining method to fabricate nanogrooves on an Au film by using a diamond tip [23]. As shown in Fig. 6a, they fixed the vibration stages on top of the AFM device to realize the excitation of the sample in the x, y, and z directions. The groove depth was controlled by regulating the amplitude in the direction of the z-axis. Figure 6b illustrates the motion of the tip when machining. Comparing Figs. 6c and d indicate that the vibration-assisted nanomachining method, relative to the approach without vibration, can increase the machining depth while reducing the interaction force. Moreover, the vibration-assisted nanomachining approach is a good solution for fabricating patterns on metal films, as it can balance the size of the structure and tip wear.
In addition, multiple furrows [20] have been machined on Al films, and vector scan method-based scratching has been applied to Ni-Fe films to fabricate nanoconstriction [58]. However, in using the aforementioned force control method, one cannot easily determine whether the film is cut through or not. Moreover, the direct contact between the tip and the substrate leads to severe tip wear and undesired machining results. To address this limitation, a previous study proposed the phase method and obtained nanogrooves with desirable depths on Au thin films [22]. In the study, a control loop based on phase mode was built for machining on the sample (Fig. 7a). The phase feedback signal from the cantilever was extracted by a lock-in amplifier, and the feedback signal was sent to the PID controller to be compared with the preset value. The phase response signal was found to be affected by the machined depth; therefore, the machined depth was regulated by performing a real-time monitoring of the phase feedback signal. Figure 7b shows that the machined depth on the Au film matched the theoretical model well and that the machining depth was accurately controlled. The results also indicated the feasibility of using this method to predict machined depths without imaging. The phase mode nanomachining approach exhibits great potential in predicting machined depths and may thus be widely used as a type of nanomachining method for metal films with minimal tip wear.
In addition to using an AFM tip to machine directly on metal films, other types of energy, such as electric field, have been introduced to facilitate nanomachining. In an existing study, square and "V" patterns were machined on a Cu film under the effect of force and bias voltage via AFM electric lithography [24]. Moreover, 2.5D square patterns, which showed a larger depth and lower surface roughness than those machined by AFM mechanical lithography, on Pt and Cu films were obtained using the

Machining on 2DMs
Since the first successful separation of graphene in 2004 [61], 2DMs such as graphene have become a research hotspot owing to their unique properties in electricity, magnetism, heat, force, and light [62,63]. AFM is widely used to investigate the friction and wear characteristics of 2DMs or cut them into desired patterns on the basis of mechanical lithography because of its nanoscale machining accuracy. Heat [64][65][66] and pressure [67] have been integrated into the AFM tip to actuate a chemical reaction on 2DMs and thereby realize patterning. The relevant research status is summarized in Table 3.

Machining Based on Traditional AFM Tip
This section mainly introduces a probe that is applied directly to the film surface. On the one hand, 2DMs were cut into preset shapes via mechanical machining. On the Elsevier. b Depth of 3D structure amplified through etching to around ten times that of the structure originally machined on PPA by thermal scanning probe lithography. Reprinted with permission from Ref. [55], Copyright 2018 Elsevier The phase response of the cantilever is used to modulate the machining force to achieve the desired machined depth Nanogrooves [22] Vibration-assisted nanomachining External vibration actuates the work piece in elliptical motions Nanogrooves [23,39] AFM electric lithography Applying bias voltage and force to realize patterning "V" word patterns [24] Coupling AFM lithography Material is removed by the coupling effects of mechanical force and electric field 2.5D square patterns [25] other hand, an AFM tip was used for sliding on the 2DMs to reveal their friction and wear characteristics.

Mechanical Cutting of 2DMs
Graphene has shown huge potential in nanosensors [68][69][70], field-effect transistors [71], solar cells [72], and others. However, the lack of a bandgap in graphene limits its application in the preparation of traditional semiconductor devices. Therefore, breaking the bandgap of graphene is important in extending its application. The electrical characteristics of graphene are related to its configurations. If the graphene can be cut into a desired shape, then it exhibits semiconducting properties. Zhang and 90° to investigate the relationship between the cutting force and the lattice orientations of graphene. They cut the monolayer graphene into nanopatterns of various shapes, including circles, squares, and ribbons [11]. Li et al. compared the fabrication results of nanogrooves on molybdenum disulfide (MoS 2 ) by using the phase and force modes (Fig. 8). They verified the effectiveness of the phase mode-based fabrication technique in layer control and debris avoidance [12].
As for its application, graphene has been employed to prepare nanoelectronic devices because of its excellent electrical conductivity. Graphene interdigitated electrodes have been achieved by using AFM to cut graphene according to the designed machining paths; the method provides a flexible approach to preparing single-walled carbon nanotubegraphene field-effect transistors [73][74][75]. Although the AFM tip for machining on 2DMs has been used in certain applications, its industrialization will take time.  Interplanar sliding friction between 2DMs and nonlayered materials [32] Influence of fold deformation on real contact area and friction [29,30,76,[81][82][83] Tribological properties of graphene in liquid [27,28] Effect of surface functional group modification on the friction properties of graphene [84][85][86][87] Effect of substrate on the friction of graphene [88][89][90] Study on wear properties of graphene [91][92][93][94] Heated AFM tip-based nanomachining Thermo-induced material denaturation The heat field was introduced to drive the occurrence of chemical reactions on the surface of 2DMs Conversion of GO to rGO [65,66] Conversion of GF to rGF [64]

Friction and Wear Characteristics
In 2010, Lee et al. first characterized the friction properties of 2DMs, such as graphene, MoS 2 , and hexagonal boron nitride (h-BN) [76]. Their experimental results showed that although these 2DMs are only a few atomic layers thick, they have the same friction reduction effect as bulk lubricated materials. The excellent mechanical and tribological properties of 2DMs require further study. Meanwhile, a series of unique frictional behaviors and mechanisms of 2DMs have been successively discovered and reported [77][78][79][80]. For example, an AFM tip was used to study the friction and wear properties of 2DMs, such as graphene, in nanoscale. Herein, six aspects of the existing research about the friction and wear properties of 2DMs related to the use of AFM tips are introduced: the interplanar sliding friction between 2DMs and nonlayered materials, the influence of fold deformation on the real contact area and friction, tribological properties of graphene in liquid, the effect of surface functional group modification on the friction properties of graphene, the effect of substrates on the friction of graphene, and wear properties of graphene. The interplanar sliding friction phenomenon between 2DMs and nonlayered materials has been studied. The experimental study of Kawai et al. showed that the superlubrication properties between layers of non-commensurate graphene could also be extended to other crystalline materials. They prepared a regular graphene nanoribbon on the surface of Au (111) and anchored the ends of the graphene nanoribbon using the AFM tip in a low-temperature and ultra-high-vacuum environment. Then, the graphene nanoribbon was dragged to reciprocate the motion on the surface of Au (111) by the probe (Fig. 9a). Through the real-time measurement of the tangential force of the probe, they found that the static friction and dynamic friction between the nanoribbon and Au (111) were less than 100 pN (Fig. 9b). The stick-slip phenomenon in the solid-solid connection of the quasi-transmission system was also observed during the sliding process (Fig. 9c) [32].
As 2DMs are only a few nanometers thick, their out-ofplane bending stiffness is very low. When the AFM tip slides on the 2DM surface, it can easily be sucked up to generate puckering under the action of van der Waals force. Lee et al. used AFM to characterize the frictional properties of several mechanically stripped 2DMs, such as graphene, h-BN, and MoS 2 (Fig. 10) [76]. Their experimental results revealed for the first time that the surface friction of 2DMs on weakly adhered substrates is strongly dependent on the number of molecular layers. That is, the greater the number of layers is, the thicker the sample and the smaller the surface friction resistance will be. When the number of molecular layers is greater than four, the surface friction properties tend to be similar to those of bulk materials (Fig. 10d). This trend is independent of the applied pressure load, scanning speed, and probe material [76]. Following the study of Lee et al., Deng et al. employed AFM to measure the friction properties of graphene on a SiO 2 substrate, and their experimental results also reproduced the trend of friction reduction with the increase of graphene layers [81]. Paolicelli et al. characterized the friction of graphene in the atmosphere, dry nitrogen, and vacuum, and their results showed that the variation trends of the graphene surface friction with the number of Fig. 8 Fabrication of nanogrooves on MoS 2 using phase mode and force mode: a and b phase-mode fabrication; c and d force-mode fabrication. Reprinted with permission from Ref. [12], Copyright 2018 Elsevier layers were similar in different atmospheres [82]. In addition to causing the friction of graphene surface to vary with the number of sample layers, the out-of-plane folds in graphene contact sliding also provide its surface friction a unique anisotropy. The correlation between the friction and wrinkles on the surface of graphene has been revealed using AFM tips [30]. Choi et al. developed a method for quickly determining the orientation of the wrinkles on the graphene surface by using the longitudinal force scanning model of AFM, thereby providing a new idea to study the deformation behavior of graphene wrinkles [29].
Although humidity exerts a significant effect on frictional behavior at the macroscale, its effect on the frictional behavior of 2DMs at the nanoscale remains unknown. To address this problem, Ye et al. used an AFM tip in their study of the friction of submerged graphene and found that nanoscale friction exhibits hysteresis in the process of normal load loading and unloading (Fig. 11a) [27]. They also reported that as long as water molecules exist, the hysteresis of friction occurs; the degree of hysteresis depends on whether the graphene surface is hydrophilic or hydrophobic (Fig. 11b), i.e., the contact angle of the surface (Fig. 11c). The authors Fig. 9 Using AFM tip to study the interplanar sliding friction between two-dimensional materials and nonlayered materials: a graphene nanoribbon sliding on Au (111) surface; b relationship between static friction and length of graphene nanoribbon; c fre-quency variation of graphene nanoribbon when reciprocating sliding at different pull-off heights. From Ref. [32]. Reprinted with permission from AAAS also systematically analyzed the influence mechanism of humidity in combination with molecular dynamics. Vilhena et al. used an AFM tip to slide on graphene that was fully immersed in water to study the effect of humidity on its friction properties [28]. The subaqueous atomic-scale stick-slip curve was achieved in water, and the result showed a high agreement with that measured under ultra-high vacuum conditions (Fig. 11d). Hence, friction force microscopy in water may serve as an alternative to ultra-high vacuum measurement. The results demonstrated that water plays a purely stochastic role in the friction production process. In sum, humidity does not seem to have a significant effect on the friction of 2DMs at the nanometer scale.
Surface chemical modification and functionalization are important means to regulate the physical properties of 2DMs [95][96][97]. The friction of graphene treated by perfluorination [84][85][86], hydrogenation [86,87], and oxidation [86] has been investigated. 2DMs are generally prepared through mechanical exfoliation from flakes or via chemical vapor deposition, and they can be transferred to various types of substrates. The interaction between 2DMs and substrates can affect the continuous deformation of 2DMs and, subsequently, their frictional behavior. Many scholars have applied AFM to explore the influence of substrates on the friction characteristics of the graphene surface [88][89][90]. As a potential atomically thin solid lubricant, 2DMs have attracted much attention for their ability to resist wear during friction sliding. Therefore, the wear resistance test has been conducted via AFM tips on 2DM surfaces [91][92][93][94].  [76]. Reprinted with permission from AAAS 2DMs with a layered structure generally form a planar layer through strong 2D (or quasi-2D) chemical bonds, and their in-plane strength and surface chemical stability are generally high. Thus, the surface lubrication effect is obvious and exhibits huge potential in reducing friction. Zhang et al. utilized an AFM tip for sliding on mica and monolayer MoS 2 to investigate the friction behavior and found that the surface friction of monolayer MoS 2 can be reduced by about 30% relative to that of a mica substrate [86].

Surface Modification Based on Heated AFM Tip
The heat field has been introduced to drive the occurrence of chemical reactions on the surface of 2DMs and then change their electrical conductivity. A study used a heated tip to locally reduce the highly insulating graphene oxide (GO) to a conductive graphene-like material (reduced GO, rGO) [65,66].
Wei et al. first used a heated AFM tip for scanning on a GO film. As shown in Fig. 12a, patterns such as crosses, squares, and zigzag-shaped nanoribbons were written at different temperatures [66]. During the scanning process, the lateral force of the tip was observed to decrease with an increase in temperature. They believed that this result might be due to GO being replaced by rGO, which has relatively low friction, as the temperature increased. The electrical properties on the rGO surface were characterized by conductive AFM and Kelvin probe force microscopy (KPFM), and the results proved the reduction of the insulating GO to conductive rGO. Their results also demonstrated that rGO could serve as a substitute for graphene to address the electronic gaps in the preparation of flexible electronic devices. Following the study of Wei et al., Carroll et al. used a heated tip array for scanning on GO deposited on SiO 2 in a zigzag path [65]. The position on the surface of GO written by the heated tip was reduced to conductive rGO. The current sensing AFM image in Fig. 12b was used to measure the current between the tip and GO. The contact potential difference for the zigzag nanostructures in rGO was also characterized via KPFM (Fig. 12b). The change in the current and contact potential demonstrated the reduction of the insulating GO to the conductive rGO. Similarly, the highly insulating graphene fluoride (GF) was locally converted to the conductive reduced graphene fluoride (rGF) by Lee et al. [64]. As shown in Fig. 12c, an rGF nanoribbon with a width of 120 nm and depth of 0.65 nm was achieved. The friction of rGF was also studied through writing rectangles at temperatures in the range of 200-550 °C. The friction of the patterns on the rGF decreased with an increase in temperature; the result was also compared with that on rGO in the research of Wei et al. [66]. The resistance of graphene nanoribbons was also measured using a nanotip system. The study generated graphene nanoribbons via writing on the GF using a heated tip. The method is expected to be applicable to modulating devices. Society. c Simulation of contact angle under loading and unloading. Reprinted with permission from Ref. [27]. Copyright 2016 American Chemical Society. d Friction behavior of submerged graphene. Reprinted with permission from Ref. [28]. Copyright 2016 American Chemical Society

Conclusions
To date, the TBN method has been demonstrated as a powerful approach to fabricate nanopatterns on films, which include polymers, metals, and 2DMs. In this work, the current development status of this method for machining on such films and its applications were reviewed from the following aspects: reported the direct mechanical cutting of 2DMs. In addition, the heat field has been introduced to drive the occurrence of chemical reactions on the surface of 2DMs and then change their electrical conductivity. The application of TBN to machining on 2DMs is mostly focused on its surface lubrication effect, which has been considered in reducing friction and wear. In addition, graphene has been employed to prepare nanoelectronic devices because of its excellent electrical conductivity. Much effort is needed to broaden the application of TBN to machining on 2DMs.
Although using the TBN method to machine on the film material has been developed for decades, there is still a long way to go to realize industrialization. Further study in this area is yet to be carried out and the possible development directions in the future can be given as follows: 1. Development of theoretical models. The existing depth prediction model based on the constant force mode is not suitable for machining on the polymer film due to its large elastic recovery. The existence of high viscoelasticity makes the machining on polymer films more complex. Therefore, more factors such as the adhesion force, the elastic recovery, the ploughing process, and so forth needed to be considered to establish the theoretical model for more machining conditions, such as the phase mode, DPL, vibration-assisted, t-SPL, and so forth. 2. Reduction of tip wear. In order to achieve higher machining accuracy and meet the requirement of structure with dozens of nanometers, the silicon tip is usually used to act as the machine tool. However, for machining on thin films, large tip wear will be caused when the tip completely penetrates the film and contacts directly with the underlying hard substrate. Poor machining results will be caused by the worn tip. Thus, some measures should be taken to protect the tip from wearing as enough as possible. One of them is to establish a more accurate depth prediction model to protect the tip from penetrating the film. Moreover, machining the film under liquid may be a possible development direction to reduce tip wear. This is because the existence of water may act as a lubricant for tip cutting and reducing the friction between tip and sample effectively. Although some methods involving employing the diamond tip, introducing vibration to assist machining, and so forth have been successfully studied, the tip wear mechanism and new method to reduce tip wear still need to be further studied. 3. Improvement of machining accuracy. It is difficult to achieve the structure with a transverse dimension of less than 100 nm via the existing AFM tip-based mechanical nanomachining methods, which limits its further appli-cation in some area where there is a need for small-size patterns, such as nanooptics. To rise above the limitations, the multi-tip probe may be an important growth factor for future TBN methods. This is because the original tip with apex radius of several tens of nanometers can be divided into several tips with apex radius of more than a dozen nanometers through focused ion beam lithography. The multi-tip probe can achieve the nanostructure with a transverse characteristic size of dozens of nanometers, improving the precision of transverse machining greatly. 4. Promotion of wider applications. It can be found that the application of machining on the film materials has not been industrialized. Though machining on the polymer film has been applied in the field of nanooptics [1,18], nanoelectronics [2,48], data storage [47], and so forth, one point that needs to be noted is that in these works the polymer film is acting as the etching mask and only when other nanofabrication methods such as etching technique, lift-off process, ultra-violet lithograph, and so forth are integrated with the TBN method can these applications be achieved. However, machining on metal films and 2DMs has not obtained such a wide application like polymer film. In the future, machining on the metal film may be used as an etching mask to achieve patterns with high aspect ratio on the arbitrary substrate based on its stable machining characteristics and excellent etching resistance. In addition, preparing flexible wearable nanosensors may be an important development direction for machining on 2DMs. Availability of data and material The authors ensured that all data and materials support our published claims and comply with field standards.

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