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Scanning probe techniques for nanoscale imaging and patterning
CHAPTER 5
Scanning probe techniques for nanoscale imaging and patterning Antoniu Moldovan, Anca Marinescu, Simona Brajnicov, Nicoleta Dumitrescu, Nicu D. Scarisoreanu, Maria Dinescu National Institute for Lasers, Plasma and Radiation Physics, Magurele, Romania
Chapter outline 5.1 Introduction 5.2 Imaging: Scanning probe microscopy 5.2.1 Scanning tunneling microscopy 5.2.2 Atomic force microscopy 5.2.3 Scanning ion-conductance microscopy 5.3 Patterning: Probe lithography 5.3.1 Patterning by STM 5.3.2 Patterning by AFM 5.3.3 Patterning by SICM 5.4 Conclusions and perspectives Acknowledgments References
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5.1 Introduction The systematic microstructural characterization of materials is a relatively recent scientific endeavor, being adopted in the 19th century—more than two centuries after the development of the first optical microscopes (Pradeep, 2007). The rapid progress in science and technology during the 20th century was largely due to the development of micro- and nanoscale characterization techniques. Nanotechnology, the technological revolution started in the second half of the 20th century, was made possible by the development of measurement and manufacturing methods that allow the visualization and control of matter at very small dimensional scales, with an extremely high precision. In his famous speech, “There’s Plenty of Room at the Bottom” (Feynman, 1992), theoretical physicist Richard Feynman predicted, >50 years ago, many of the possibilities arising from nanoscale miniaturization. When referring to the miniaturization of the computer, Feynman said that “the wires should be 10 or 100 atoms in diameter and the circuits should be a few thousand angstroms across.” As we know, mass production of circuits with 10 nm FinFET technology has already started at the end of 2016 (Samsung press release, 2016). In practice, this means that the industry can mass Functional Nanostructured Interfaces for Environmental and Biomedical Applications https://doi.org/10.1016/B978-0-12-814401-5.00005-0
© 2019 Elsevier Inc. All rights reserved.
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produce circuits incorporating billions of transistors with channel length as small as 10 nm—roughly 30 atoms. Since the development of such small devices requires tools that can measure the properties of materials with subnanometer resolution, scanning probe microscopy is an indispensable technique for the progress of nanoelectronics. Together with electron microscopes, scanning probe microscopes allow scientists to explore materials and phenomena at unprecedented resolution, in a variety of fields, from fundamental science, materials science, medicine, biology, constructions, agriculture, cosmetics, etc.
5.2 Imaging: Scanning probe microscopy Scanning probe microscopy (SPM) relies on the detection and mapping of a local property or phenomenon that manifests itself at the surface of the studied material, usually through the interaction with a physical probe. The first technique in the SPM family, scanning tunneling microscopy (STM), was invented in 1981 by Gerd Binnig and Heinrich Rohrer at IBM Zurich (Binnig et al., 1982a, b). Their invention is one of the key points in the history of nanotechnology and is widely regarded as the birth point of the SPM family. For their invention, Binnig and Rohrer were awarded the 1986 Nobel Prize in Physics, together with Ernst Ruska for the electron microscope. In the same year, Gerd Binnig, Calvin Quate, and Christoph Gerber published the first paper regarding the implementation of the atomic force microscope (AFM) at Stanford University (Binnig et al., 1986). Less than four decades separate the first STM measurement results, which were presented in the form of simple topographic profiles (Binnig et al., 1982b) and recent AFM studies of intra- and intermolecular structure ( Jarvis, 2015). In this relatively short period, the development of SPM techniques went hand in hand with the development of nanotechnologies. Over the years, various SPM techniques were introduced, depending on the type of interaction or phenomenon, on which the measurements are based. The techniques are also called operating modes or simply modes. Table 1 shows some of the most common modes currently available on commercial systems. Advanced modes are usually implemented on homebuilt or personalized commercial systems. STM and AFM are the most widely used SPM techniques. In STM, the intensity of the tunneling current between tip and sample is measured with high accuracy. Since the tunneling current depends sharply on the tip-sample separation, it can be used to create atomic resolution images of surfaces. AFM measures tip-sample interaction forces, which are usually van der Waals forces (short range), capillary forces, and electric and magnetic forces (long range). The results of SPM measurements are usually stored and presented in the form of images, which are two-dimensional maps of the determined values of the local parameters. The resolution of these images ultimately depends on the physical dimensions of the probe, the probe–sample separation, and the type of interaction.
Scanning probe techniques for nanoscale imaging and patterning
Table 1 Some of the most common SPM modes SPM mode
Acronym
Scanning tunneling microscopy Atomic force microscopy Force modulation microscopy Magnetic force microscopy Electric force microscopy Scanning Kelvin probe microscopy Kelvin probe force microscopy Piezoresponse force microscopy Scanning capacitance microscopy Current atomic force microscopy Conductive atomic force microscopy Conductive probe atomic force microscopy Scanning spreading resistance microscopy Scanning thermal microscopy Scanning ion-conductance microscopy Near-field scanning optical microscopy Scanning near-field optical microscopy
STM AFM FMM MFM EFM SKPM KPFM PFM SCM I-AFM C-AFM CP-AFM SSRM SThM SICM NSOM SNOM
Since the earliest reported STM measurements (Binnig et al., 1982b), a common tip preparation method has been the mechanical machining or cutting of conductive wires. Even though these methods are relatively coarse, the resulting tips commonly yield atomic resolution STM images even in ambient air. In fact, these tip processing techniques create local microscopic spikes. As the tunneling current intensity depends exponentially on the distance between tip and sample (Binnig et al., 1982a), tunneling will only occur through the outermost atoms of the spike that is closest to the sample surface. The fabrication of commercial AFM tips is much more complex, involving lithography techniques and ultimately a sharpening process by focused ion beam or electron beam. The resulting tips commonly exhibit a curvature radius <5 nm. Atomic resolution is more difficult to achieve with AFM than STM (Ohnesorge et al., 1993). As will be shown in the following paragraphs, in STM, the measurement of the small tunneling currents, in the subnanoampere domain, is relatively simple even with common low-noise current amplifiers; on the other hand, the detection of the extremely low interaction forces necessary for atomic resolution AFM, in the subnanonewton domain, is more complicated, usually requiring AC modulation techniques (Albrecht et al., 1991).
5.2.1 Scanning tunneling microscopy In scanning tunneling microscopy (STM), a conductive tip is positioned close to the sample surface. In the case of conductive and semiconductor materials, an electric bias applied between tip and sample will give rise to a tunneling current through the gap between the
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two. The tunneling current intensity depends on the applied bias, the width of the gap, and the density of states of the materials that make up the tip and the sample (Garcı´a et al., 1983; Tersoff et al., 1983; Selloni et al., 1985; Tersoff et al., 1985). When scanning the tip above the surface at constant bias, a feedback loop can adjust the tip-sample relative position to maintain a constant tunneling current. This will result in the tip following the topography contour of the surface. More accurately, constant-current “topography” STM images basically show surfaces where the tunneling probability is constant. STM can be performed in vacuum, in ambient air or various gases, and even in liquid environment. The main components of a typical STM instrument are shown in Fig. 1. The coarse positioning mechanism is not shown. The fine tip-sample relative positioning is usually accomplished via a piezo tube, with an accuracy at the subnanometer level laterally and subangstrom vertically. The tunneling current, with intensities ranging from hundreds of picoamperes to a few nanoamperes, is first preamplified by an I/V converter positioned as close to the probe as physically possible in order to avoid picking up electric noise from the surroundings. The value of the tunneling current is compared with the set point value. The difference, also called error signal, is used as driving signal for the topography feedback circuit, which controls the vertical positioning bias applied to the piezo scanner. Some of the most notable early results that had an important contribution to the field of surface science concern the direct visualization of surface reconstruction. One example
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Scanning probe techniques for nanoscale imaging and patterning
is the 7 7 reconstruction of the Si(111) surface. Observations of this reconstruction in real space were first reported in 1983 by Binnig and coworkers, from STM measurements (Binnig et al., 1983; Haneman, 1987). Surface reconstruction had previously been studied extensively by electron diffraction methods: low-energy electron diffraction (LEED) and reflection high-energy electron diffraction (RHEED) (Bennett et al., 1981; Ino, 1977). These techniques offer valuable information about the periodicity of the surface, but are not able to resolve the structural changes at the atomic scale (Kitamura et al., 1991). The first STM observations of the Si(111) 7 7 reconstruction contributed to the dimer-adatom-stacking fault (DAS) model proposed by Takayanagi and coworkers in 1985 (Takayanagi et al., 1985). The 12 adatoms of the 7 7 unit cell and the corner holes are clearly resolved even in the first STM images published on this topic (Binnig et al., 1983). Subsequent STM studies were refined to the point that even the ˚ below, can be resolved either separately rest atoms of the 7 7 unit cell, located 1 A (Sutter et al., 2003) or simultaneously with the adatoms (Wang et al., 2008). Another particular field in which STM and then SPM in general played an important role is the study of carbon nanotubes (CNTs). CNTs were first reported in 1991 by Iijima (1991), and it soon became clear that SPM techniques are an extremely valuable tool for the study of their morphology and structure. Properties of CNTs that make these materials highly interesting for research and industry, such as their high mechanical strength and high thermal and electric conductivity, are influenced by their atomic and electronic structure, which can be directly imaged by scanning probe techniques. The first SPM analyses of CNTs were reported in 1993 (Gallagher et al., 1993; Ge et al., 1993). SPM techniques were able to not only resolve the honeycomb lattice, characteristic of the graphite monolayers that build up carbon nanotubes, but also give information about the helicity (chirality) of the nanotubes (Ge et al., 1993). Since the STM is able to image only conductive and semiconductor materials, it became clear that a new technique was necessary to be able to study the surface of insulating materials.
5.2.2 Atomic force microscopy Atomic force microscopy (AFM) measures the interaction forces between the probe and the sample. In the first AFM experiments, reported by Binnig, Quate, and Gerber in 1986 (Binnig et al., 1986), the forces were detected by measuring the elastic deformation of a cantilever spring using an incorporated STM. The instrument was basically an adaptation of the STM to allow the study of insulating samples: the tunneling current would flow through the STM tip and the AFM cantilever—a thin foil made of gold—instead of the sample. The AFM probe was a diamond tip glued to the end of the cantilever. Interestingly, the tip-sample interaction was detected in what is now commonly called “dynamic mode,” that is, by modulating the tip-sample relative position. Also, this instrument used
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a relatively complex feedback mechanism that allowed separate modulation of the relative position of the sample, the AFM tip, and the STM tip. Currently, one of the most common techniques for force measurement in AFM is the optical lever or light lever: the deformation of the cantilever is determined by reflecting a laser beam off its backside, as shown in Fig. 2. The deviation of the reflected beam is calculated using a position-sensitive photodetector (PSPD): the difference between the illumination on the upper half and bottom half gives the vertical deflection, and the difference between the right half and left half illumination gives the lateral deflection. Cantilevers are usually produced using conventional microfabrication techniques adapted from the microelectronics industry, with the most common materials being silicon; silicon nitride; or metals such as platinum, gold, and tungsten (Akamine et al., 1990; Albrecht et al., 1990; Ximen et al., 1992; Boisen et al., 1996). An optical lever AFM can work in contact mode (where the tip-sample interaction is repulsive) or noncontact mode (where the tip-sample interaction is attractive). In contact mode, the tip is pressed against the surface, causing the cantilever to bend upward. During scanning, the bending is maintained constant by the topography feedback loop, which controls the tip-sample relative position. In noncontact mode, since the attractive forces are extremely weak, the interaction is usually detected using modulation techniques. The cantilever is mechanically modulated at or near its natural resonance frequency, usually with the aid of a small piezoelectric element incorporated in its holder. In the vicinity of the sample surface, the resonance frequency of the cantilever will experience a slight shift due to the mechanical coupling between tip and sample. If the modulation frequency is fixed, the resulting amplitude of the tip oscillation will change. This change can be used as input by the topography feedback loop. This mode is the amplitude-modulated AFM (AM-AFM) (Martin et al., 1987). Alternatively, in frequency-modulated AFM (FM-AFM), the frequency shift
Fig. 2 Detection of the forces acting on the tip, using the optical lever technique: first, the laser beam and the PSPD are aligned so that the reflected beam is centered on the PSPD; forces acting on the tip cause bending and twisting of the cantilever, which deviate the reflected beam; the vertical deflection is proportional to the normal component of the force, and the lateral deflection is proportional to the lateral component of the force.
Scanning probe techniques for nanoscale imaging and patterning
of the cantilever is directly detected by an FM demodulator and is used as input for topography feedback (Albrecht et al., 1991). FM-AFM is more suitable for working in vacuum environment. In vacuum, the damping of the cantilever oscillation is extremely low (the quality factor of the cantilever is greatly enhanced), so it takes longer for the oscillation amplitude to respond to tip-sample interaction forces, making AM-AFM impractical (Albrecht et al., 1991). Since the Si(111) 7 7 reconstructed surface provided a suitable benchmark for testing the accuracy of STM, the same structure was readily adopted by the AFM community for the same purpose. The first results showing true atomic resolution AFM on Si(111) 7 7 surfaces were published in 1995 by Giessibl (1995). Imaging of the Si(111) 7 7 surface was carried out in UHV, using piezoresistive cantilevers (Tortonese et al., 1993; Giessibl et al., 1994) and FM detection. In this approach, the deformation of the cantilever is detected from the strain of an incorporated piezoresistive element, which is created by doping one side of the cantilever, and is part of a Wheatstone bridge. The output of the Wheatstone bridge is a direct measure of the cantilever deflection and can be used for topography feedback. Considering that AFM can also be operated in liquid environment, it became an interesting tool for studies related to biological materials—including, but not limited to, living cells. The first AFM studies of cells were published as early as 1990 and 1991 (Butt et al., 1990; Gould et al., 1990a, b; H€aberle et al., 1991). Interestingly, the AFM system used in the work reported by H€aberle et al. (1991), although working in liquid buffer, employed a force measurement mechanism based on a tunneling tip, similar to the one used in the first reported AFM (Binnig et al., 1986). In this work, H€aberle and coworkers envisioned the possibility of probing with the AFM the structure of the cytoskeleton or performing dynamic studies of cellular processes, at the nanoscale. In the context of the present book, some of the results of studies related to biomaterials carried out in the authors’ group are presented. Magnetospirillum gryphiswaldense, a strain of magnetotactic bacteria, was imaged in ambient air, on glass slides, and in dried state. The images were obtained with a commercial AFM (Nomad, Quesant Instrument Corp.) in intermittent-contact AFM, with standard silicon tips (Q-WM190, Budget Sensors). Fig. 3 shows topography AFM images of a 40 μm 40 μm area (A) and a 3-D rendering of a 10 μm 10 μm area (B) of the sample. Individual cells and agglomerated cells can be observed. Some small particles are also present between the cells— these might be magnetosomes or agglomerations of magnetosomes.
5.2.3 Scanning ion-conductance microscopy In scanning ion-conductance microscopy (SICM), the probe is a micro- or nanopipette with an inner electrode. Measurements are made in an electrolyte solution by monitoring the ionic current that passes through the pipette opening. A schematic view of the
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Fig. 3 Intermittent-contact AFM images of M. gryphiswaldense, a strain of magnetotactic bacteria. (A) 40 μm 40 μm top view; (B) 10 μm 10 μm 3D rendering.
Scanning probe techniques for nanoscale imaging and patterning
Fig. 4 Principle of operation of SICM. The sample is submerged in an electrolyte solution—usually NaCl or a growth medium in the case of biological samples. The electrodes are usually Ag/AgCl. The nanopipette is moved using piezoelectric positioners.
working principle of SICM is shown in Fig. 4. The pipette electrode is biased relative to a reference electrode placed inside the solution. For constant bias, the ionic current depends on the separation between the pipette opening and the sample surface: when the pipette is far from the surface, the ionic current flows unobstructed through the opening; as the pipette is brought closer to the surface, the current flow is gradually obstructed. Topography feedback is achieved by adjusting the pipette-surface separation to keep the ionic current constant. The resolution of SICM is limited, in principle, by the dimension of the pipette opening. SICM was first reported in 1989 by Hansma and coworkers (Hansma et al., 1989) although, according to the authors, preliminary results were already obtained in their laboratory as early as 1986. Using a large-scale model, with a pipette having an inner diameter of 0.71 mm and an outer diameter of 1 mm, the authors showed that the system is able to resolve surface features having the same dimensions as the inner diameter. In the mentioned study and in subsequent works (Prater et al., 1991; Proksch et al., 1996), SICM was already seen as a suitable method for the study of biological samples. This was due to its proved ability to measure the topography of delicate structures, such as porous membranes in liquid, and also to map ionic currents through the pores of such structures. However, the first results regarding SICM imaging of live cells were published only in 1997 by Korchev and coworkers (Korchev et al., 1997a, b). For these studies, a patchclamp system was modified by adding specific components for SICM imaging. Murine melanocyte cells, human colon cancer cells, cardiomyocytes, and smooth muscle cells were scanned in liquid environment, maintaining their viability. The topography of the cells, with relatively high corrugations (>30 μm), was successfully imaged with high resolution. Some dynamic changes of the cell surfaces were also observed.
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Besides being able to determine the topography of delicate structures such as living cells or other biological materials, SICM can detect and map the activity of single ionic channels on the surface of intact cellular membranes, as shown in the work published in 2000 by Korchev and coworkers (Korchev et al., 2000). In order to achieve this, SICM was carried out simultaneously with patch-clamp experiments. The SICM pipette, while scanning the topography of the cell, also acted as a highly localized source of K+ ions. The electric response of the cell, quantified by the K+ ionic current, was recorded via the patch-clamp micropipette. The local maxima of the ionic current indicated the position of the active ionic channels. SICM has been used to study structures such as microvilli and their dynamics (Gorelik et al., 2003), amyloid fibrils (Zhang et al., 2012), and primary cilia (Zhou et al., 2018) and to monitor dynamic processes such as phospholipid-induced morphological changes of red blood cells (Zhu et al., 2018). Aside from imaging and current sensing, SICM has proved to be a promising technique for patterning, as will be shown in Section 5.3.3.
5.3 Patterning: Probe lithography In probe lithography, patterns are created on surfaces by various methods. Local mechanical stress can be applied to create dots, scratches, etc. A heated tip can also pattern the surface by changing the chemical state or simply by melting the material. A biased tip can transfer local charge to an insulating material, or it can change the local polarization state of a ferroelectric material. The next paragraphs will show some of the early advances in patterning at the atomic scale with STM and AFM.
5.3.1 Patterning by STM Perhaps, the best known example of atomic-scale manipulation is the famous IBM logo written with 35 atoms, created by Eigler and Schweizer at the IBM laboratories in Almaden (Eigler et al., 1990). Previous to this, the controlled positioning of atoms (Becker et al., 1987) or molecules (Foster et al., 1988) had been reported. However, the chemical nature of the manipulated material was uncertain. In the work described by Becker et al. in 1987 (Becker et al., 1987), the deposited atoms are hypothesized to originate from the surface material itself (germanium), being previously transferred to the scanning tip (tungsten). The atoms are deposited on the surface by raising the tip to surface bias from the value of 1 V used during imaging to 4 V. In the experiments described by Foster et al. in 1988 (Foster et al., 1988), controlled pinning to the substrate and removal of previously pinned molecules or molecule fragments are achieved. The molecules are believed to originate from the liquid in which the measurements are taking place (di(2-ethylhexyl) phthalate). The molecules are pinned during imaging by applying a short voltage pulse to the tip (3.7 V, 100 ns). Subsequently,
Scanning probe techniques for nanoscale imaging and patterning
such pinned structures can be removed (completely or partially) by applying again a similar pulse as the tip scans over them. For the atomic IBM logo, xenon atoms were repositioned on a single-crystal nickel surface, in ultrahigh vacuum at low temperature (4 K). First, a scan was performed to locate several xenon atoms on the surface, which were then individually positioned to create the IBM logo. When the STM tip approached the surface sufficiently close (closer than it was required for imaging), the force exerted by the tip allowed xenon atoms to be dragged across the surface (Fig. 5). The logo consisted of 35 atoms, with each letter having a height of 5 nm. The atoms were positioned in a rectangular grid with periodicity ˚. 14 12.5 A Subsequently, in 1991, Eigler and coworkers created the “atomic switch” (Eigler et al., 1991). Using the same materials as for the IBM logo, they showed that an adsorbed xenon atom can be repeatedly and reproducibly transferred from the substrate to the STM tip and back. The transfer was achieved by applying short bias pulses of opposite sign to the tip, which was kept at tunneling distance above the surface. The presence and absence of the xenon atom on the surface were confirmed by taking successive STM images. The conductance state of the switch was monitored throughout the process by measuring the tunneling current: with the xenon atom bound to the surface, the switch was in the low-conductance state, and it changed to the highconductance state after the transfer of the xenon atom to the tip. A variation of the current by almost one order of magnitude between the two conduction states was reported.
Fig. 5 Repositioning of one Xe atom on the Ni substrate, for Eigler’s “IBM” logo written with 35 atoms. (Modified from Eigler, D.M., et al., 1990. Nature 344, 524–526. https://doi.org/10.1038/344524a0.). First, the Xe atom is located on the surface by standard STM imaging (site A); next, the tip is positioned above the atom and is lowered until the tip-atom interaction is high enough to allow dragging the atom across the surface to the desired location (site B), where the tip is raised. (The atomic lattice of the Ni substrate is not resolved in the STM images taken throughout the experiment.)
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5.3.2 Patterning by AFM As in the case of imaging, atomic-scale patterning by AFM proved relatively more difficult to achieve than with STM (Custance et al., 2009). The first results concerning the manipulation of individual selected atoms were published in 2003 (Oyabu et al., 2003). In this work, carried out at low temperature, single silicon adatoms of the Si(111) 7 7 reconstructed surface were removed leaving behind vacancies or deposited on previously created vacant spots. The authors termed the method “soft nanoindentation”: the tip was gently approached to the surface while being oscillated at the cantilever’s resonance frequency, and the amplitude and frequency shift of the oscillation were monitored. The approach was stopped as soon as the frequency shift experienced a sudden jump—indicating the reorganization of the local tip-sample atomic arrangement. A notable example of lateral manipulation at the atomic scale is the induced interchange of different adatom species, reported in 2005 (Sugimoto et al., 2005). In this study, atomic-scale shapes were created by successively interchanging the positions of substitutional tin adatoms and germanium adatoms on the Ge(111)-c(2 8) reconstructed surface. The interchange between two neighboring tin and germanium atoms was induced by the attractive interaction with the scanning tip. The tip was gradually brought closer to the surface while repeatedly scanning along the same line, which was appropriately chosen to pass through the centers of both atoms. Some of the unpublished results obtained by the authors of the present chapter, related to AFM patterning of biological structures, will be presented hereafter. The results were obtained using a commercial AFM (XE100, Park Systems), with the dedicated nanolithography software. One such example is the creation of patterns on the surface of red blood cells. Human red blood cells were obtained from a fresh blood sample that was drop cast on a glass slide directly from the harvesting syringe. In order to avoid the formation of large clumps of cells, the fresh drop was translated (smeared) across the surface of the slide with the syringe needle. The cells were then left to dry in ambient air for approximately 30 min before being imaged by NC-AFM. Standard silicon tips (ACTA, Applied NanoStructures Inc.) were used for AFM imaging. Fig. 6A shows the image of a 50 μm 50 μm area of the sample. Since the blood sample was not subject to centrifugation for the separation of serum, the cells are surrounded by a matrix resulting from the dried serum. One of the cells in the highlighted area was patterned with the AFM tip, using the AFM’s nanolithography software. Patterns were created by pressing the tip against the surface and dragging it laterally. Subsequently, the highlighted area was scanned again in NC-AFM (Fig. 6B). The resulting patterns were up to 50 nm deep and 150 nm wide.
5.3.3 Patterning by SICM SICM-based patterning can be accomplished in a number of ways. Mechanical deformation exerted by the scanning pipette was employed in the work described in
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B€ ocker et al. (2009). Lipid bilayer membranes, suspended over the pores of highly ordered porous silicon substrates, were first imaged by SICM. Subsequently, in selected locations, the surface of the membranes was ruptured by gradually approaching the nanopipette while monitoring the ionic current, which showed a sharp increase at the moment of rupture. Desired patterns were created, with the pores acting as “pixels.” Alternatively, the SICM can be used as a nanoscale “3D printer,” as reported in a recent work by Momotenko and coworkers (Momotenko et al., 2016). A dual-channel or dual-barrel nanopipette was used: one of the channels served for positioning SICM feedback and the other as a source of precursor ions (Cu2+) for material deposition. The electrochemical deposition was highly localized by the size of the nanopipette opening and also depended on the substrate potential. The deposition was initiated with the pipette approached in the vicinity of the substrate. As the material was gradually deposited under the pipette, the SICM feedback automatically lifted the pipette away from the substrate, leading to the deposition of high-aspect-ratio structures in the form of pillars, from several micrometers to >20 μm long and <1 μm wide. By combining the vertical movement of the pipette with lateral translation, zigzag structures were also created.
5.4 Conclusions and perspectives Scanning probe microscopy offered scientists tools that allow them to analyze and control matter at scales that span more than four orders of magnitude: from visualizing and manipulating atoms at the subnanometer level to measuring the shape, dynamics, and interactions of living cells. With the continuous development of control electronics, SPMs are becoming more and more reliable, versatile, and easy to use. They are already indispensable tools in a number of fields, from probing mechanical and electric properties of materials used in next-generation electronics, to helping cellular biologists probe antigen-antibody interactions at cellular level, to aiding in art preservation efforts. An important trend in the development of SPM is the increase of overall acquisition speed, in order to allow the visualization of a larger variety of real-time processes. Also, control over the functionalization of the probe at the atomic level would allow new insights in the study of chemical reactions and routinely carry out SPM with chemical sensitivity at the molecular and atomic scale.
Acknowledgments This work was supported by the Romanian National Authority for Scientific Research and Innovation, in the frame of the “Nucleus” Programme, and grants of the Romanian Ministry of Scientific Research and Innovation, CCCDI—UEFISCDI, project number PN-III-P1-1.2-PCCDI-2017-0637 (MultiMonD2), PN-III-P1-1.2-PCCDI-2017-0172 (TESTES), and PN-III-P1-1.2-PCCDI-2017-0755 (MALASENT), within PNCDI III.
Scanning probe techniques for nanoscale imaging and patterning
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Scanning probe techniques for nanoscale imaging and patterning Antoniu Moldovan, Anca Marinescu, Simona Brajnicov, Nicoleta Dumitrescu, Nicu D. Scarisoreanu, Maria Dinescu National Institute for Lasers, Plasma and Radiation Physics, Magurele, Romania
Chapter outline 5.1 Introduction 5.2 Imaging: Scanning probe microscopy 5.2.1 Scanning tunneling microscopy 5.2.2 Atomic force microscopy 5.2.3 Scanning ion-conductance microscopy 5.3 Patterning: Probe lithography 5.3.1 Patterning by STM 5.3.2 Patterning by AFM 5.3.3 Patterning by SICM 5.4 Conclusions and perspectives Acknowledgments References
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5.1 Introduction The systematic microstructural characterization of materials is a relatively recent scientific endeavor, being adopted in the 19th century—more than two centuries after the development of the first optical microscopes (Pradeep, 2007). The rapid progress in science and technology during the 20th century was largely due to the development of micro- and nanoscale characterization techniques. Nanotechnology, the technological revolution started in the second half of the 20th century, was made possible by the development of measurement and manufacturing methods that allow the visualization and control of matter at very small dimensional scales, with an extremely high precision. In his famous speech, “There’s Plenty of Room at the Bottom” (Feynman, 1992), theoretical physicist Richard Feynman predicted, >50 years ago, many of the possibilities arising from nanoscale miniaturization. When referring to the miniaturization of the computer, Feynman said that “the wires should be 10 or 100 atoms in diameter and the circuits should be a few thousand angstroms across.” As we know, mass production of circuits with 10 nm FinFET technology has already started at the end of 2016 (Samsung press release, 2016). In practice, this means that the industry can mass Functional Nanostructured Interfaces for Environmental and Biomedical Applications https://doi.org/10.1016/B978-0-12-814401-5.00005-0
© 2019 Elsevier Inc. All rights reserved.
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produce circuits incorporating billions of transistors with channel length as small as 10 nm—roughly 30 atoms. Since the development of such small devices requires tools that can measure the properties of materials with subnanometer resolution, scanning probe microscopy is an indispensable technique for the progress of nanoelectronics. Together with electron microscopes, scanning probe microscopes allow scientists to explore materials and phenomena at unprecedented resolution, in a variety of fields, from fundamental science, materials science, medicine, biology, constructions, agriculture, cosmetics, etc.
5.2 Imaging: Scanning probe microscopy Scanning probe microscopy (SPM) relies on the detection and mapping of a local property or phenomenon that manifests itself at the surface of the studied material, usually through the interaction with a physical probe. The first technique in the SPM family, scanning tunneling microscopy (STM), was invented in 1981 by Gerd Binnig and Heinrich Rohrer at IBM Zurich (Binnig et al., 1982a, b). Their invention is one of the key points in the history of nanotechnology and is widely regarded as the birth point of the SPM family. For their invention, Binnig and Rohrer were awarded the 1986 Nobel Prize in Physics, together with Ernst Ruska for the electron microscope. In the same year, Gerd Binnig, Calvin Quate, and Christoph Gerber published the first paper regarding the implementation of the atomic force microscope (AFM) at Stanford University (Binnig et al., 1986). Less than four decades separate the first STM measurement results, which were presented in the form of simple topographic profiles (Binnig et al., 1982b) and recent AFM studies of intra- and intermolecular structure ( Jarvis, 2015). In this relatively short period, the development of SPM techniques went hand in hand with the development of nanotechnologies. Over the years, various SPM techniques were introduced, depending on the type of interaction or phenomenon, on which the measurements are based. The techniques are also called operating modes or simply modes. Table 1 shows some of the most common modes currently available on commercial systems. Advanced modes are usually implemented on homebuilt or personalized commercial systems. STM and AFM are the most widely used SPM techniques. In STM, the intensity of the tunneling current between tip and sample is measured with high accuracy. Since the tunneling current depends sharply on the tip-sample separation, it can be used to create atomic resolution images of surfaces. AFM measures tip-sample interaction forces, which are usually van der Waals forces (short range), capillary forces, and electric and magnetic forces (long range). The results of SPM measurements are usually stored and presented in the form of images, which are two-dimensional maps of the determined values of the local parameters. The resolution of these images ultimately depends on the physical dimensions of the probe, the probe–sample separation, and the type of interaction.
Scanning probe techniques for nanoscale imaging and patterning
Table 1 Some of the most common SPM modes SPM mode
Acronym
Scanning tunneling microscopy Atomic force microscopy Force modulation microscopy Magnetic force microscopy Electric force microscopy Scanning Kelvin probe microscopy Kelvin probe force microscopy Piezoresponse force microscopy Scanning capacitance microscopy Current atomic force microscopy Conductive atomic force microscopy Conductive probe atomic force microscopy Scanning spreading resistance microscopy Scanning thermal microscopy Scanning ion-conductance microscopy Near-field scanning optical microscopy Scanning near-field optical microscopy
STM AFM FMM MFM EFM SKPM KPFM PFM SCM I-AFM C-AFM CP-AFM SSRM SThM SICM NSOM SNOM
Since the earliest reported STM measurements (Binnig et al., 1982b), a common tip preparation method has been the mechanical machining or cutting of conductive wires. Even though these methods are relatively coarse, the resulting tips commonly yield atomic resolution STM images even in ambient air. In fact, these tip processing techniques create local microscopic spikes. As the tunneling current intensity depends exponentially on the distance between tip and sample (Binnig et al., 1982a), tunneling will only occur through the outermost atoms of the spike that is closest to the sample surface. The fabrication of commercial AFM tips is much more complex, involving lithography techniques and ultimately a sharpening process by focused ion beam or electron beam. The resulting tips commonly exhibit a curvature radius <5 nm. Atomic resolution is more difficult to achieve with AFM than STM (Ohnesorge et al., 1993). As will be shown in the following paragraphs, in STM, the measurement of the small tunneling currents, in the subnanoampere domain, is relatively simple even with common low-noise current amplifiers; on the other hand, the detection of the extremely low interaction forces necessary for atomic resolution AFM, in the subnanonewton domain, is more complicated, usually requiring AC modulation techniques (Albrecht et al., 1991).
5.2.1 Scanning tunneling microscopy In scanning tunneling microscopy (STM), a conductive tip is positioned close to the sample surface. In the case of conductive and semiconductor materials, an electric bias applied between tip and sample will give rise to a tunneling current through the gap between the
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two. The tunneling current intensity depends on the applied bias, the width of the gap, and the density of states of the materials that make up the tip and the sample (Garcı´a et al., 1983; Tersoff et al., 1983; Selloni et al., 1985; Tersoff et al., 1985). When scanning the tip above the surface at constant bias, a feedback loop can adjust the tip-sample relative position to maintain a constant tunneling current. This will result in the tip following the topography contour of the surface. More accurately, constant-current “topography” STM images basically show surfaces where the tunneling probability is constant. STM can be performed in vacuum, in ambient air or various gases, and even in liquid environment. The main components of a typical STM instrument are shown in Fig. 1. The coarse positioning mechanism is not shown. The fine tip-sample relative positioning is usually accomplished via a piezo tube, with an accuracy at the subnanometer level laterally and subangstrom vertically. The tunneling current, with intensities ranging from hundreds of picoamperes to a few nanoamperes, is first preamplified by an I/V converter positioned as close to the probe as physically possible in order to avoid picking up electric noise from the surroundings. The value of the tunneling current is compared with the set point value. The difference, also called error signal, is used as driving signal for the topography feedback circuit, which controls the vertical positioning bias applied to the piezo scanner. Some of the most notable early results that had an important contribution to the field of surface science concern the direct visualization of surface reconstruction. One example
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Fig. 1 The main components of a typical STM.
Scanning probe techniques for nanoscale imaging and patterning
is the 7 7 reconstruction of the Si(111) surface. Observations of this reconstruction in real space were first reported in 1983 by Binnig and coworkers, from STM measurements (Binnig et al., 1983; Haneman, 1987). Surface reconstruction had previously been studied extensively by electron diffraction methods: low-energy electron diffraction (LEED) and reflection high-energy electron diffraction (RHEED) (Bennett et al., 1981; Ino, 1977). These techniques offer valuable information about the periodicity of the surface, but are not able to resolve the structural changes at the atomic scale (Kitamura et al., 1991). The first STM observations of the Si(111) 7 7 reconstruction contributed to the dimer-adatom-stacking fault (DAS) model proposed by Takayanagi and coworkers in 1985 (Takayanagi et al., 1985). The 12 adatoms of the 7 7 unit cell and the corner holes are clearly resolved even in the first STM images published on this topic (Binnig et al., 1983). Subsequent STM studies were refined to the point that even the ˚ below, can be resolved either separately rest atoms of the 7 7 unit cell, located 1 A (Sutter et al., 2003) or simultaneously with the adatoms (Wang et al., 2008). Another particular field in which STM and then SPM in general played an important role is the study of carbon nanotubes (CNTs). CNTs were first reported in 1991 by Iijima (1991), and it soon became clear that SPM techniques are an extremely valuable tool for the study of their morphology and structure. Properties of CNTs that make these materials highly interesting for research and industry, such as their high mechanical strength and high thermal and electric conductivity, are influenced by their atomic and electronic structure, which can be directly imaged by scanning probe techniques. The first SPM analyses of CNTs were reported in 1993 (Gallagher et al., 1993; Ge et al., 1993). SPM techniques were able to not only resolve the honeycomb lattice, characteristic of the graphite monolayers that build up carbon nanotubes, but also give information about the helicity (chirality) of the nanotubes (Ge et al., 1993). Since the STM is able to image only conductive and semiconductor materials, it became clear that a new technique was necessary to be able to study the surface of insulating materials.
5.2.2 Atomic force microscopy Atomic force microscopy (AFM) measures the interaction forces between the probe and the sample. In the first AFM experiments, reported by Binnig, Quate, and Gerber in 1986 (Binnig et al., 1986), the forces were detected by measuring the elastic deformation of a cantilever spring using an incorporated STM. The instrument was basically an adaptation of the STM to allow the study of insulating samples: the tunneling current would flow through the STM tip and the AFM cantilever—a thin foil made of gold—instead of the sample. The AFM probe was a diamond tip glued to the end of the cantilever. Interestingly, the tip-sample interaction was detected in what is now commonly called “dynamic mode,” that is, by modulating the tip-sample relative position. Also, this instrument used
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a relatively complex feedback mechanism that allowed separate modulation of the relative position of the sample, the AFM tip, and the STM tip. Currently, one of the most common techniques for force measurement in AFM is the optical lever or light lever: the deformation of the cantilever is determined by reflecting a laser beam off its backside, as shown in Fig. 2. The deviation of the reflected beam is calculated using a position-sensitive photodetector (PSPD): the difference between the illumination on the upper half and bottom half gives the vertical deflection, and the difference between the right half and left half illumination gives the lateral deflection. Cantilevers are usually produced using conventional microfabrication techniques adapted from the microelectronics industry, with the most common materials being silicon; silicon nitride; or metals such as platinum, gold, and tungsten (Akamine et al., 1990; Albrecht et al., 1990; Ximen et al., 1992; Boisen et al., 1996). An optical lever AFM can work in contact mode (where the tip-sample interaction is repulsive) or noncontact mode (where the tip-sample interaction is attractive). In contact mode, the tip is pressed against the surface, causing the cantilever to bend upward. During scanning, the bending is maintained constant by the topography feedback loop, which controls the tip-sample relative position. In noncontact mode, since the attractive forces are extremely weak, the interaction is usually detected using modulation techniques. The cantilever is mechanically modulated at or near its natural resonance frequency, usually with the aid of a small piezoelectric element incorporated in its holder. In the vicinity of the sample surface, the resonance frequency of the cantilever will experience a slight shift due to the mechanical coupling between tip and sample. If the modulation frequency is fixed, the resulting amplitude of the tip oscillation will change. This change can be used as input by the topography feedback loop. This mode is the amplitude-modulated AFM (AM-AFM) (Martin et al., 1987). Alternatively, in frequency-modulated AFM (FM-AFM), the frequency shift
Fig. 2 Detection of the forces acting on the tip, using the optical lever technique: first, the laser beam and the PSPD are aligned so that the reflected beam is centered on the PSPD; forces acting on the tip cause bending and twisting of the cantilever, which deviate the reflected beam; the vertical deflection is proportional to the normal component of the force, and the lateral deflection is proportional to the lateral component of the force.
Scanning probe techniques for nanoscale imaging and patterning
of the cantilever is directly detected by an FM demodulator and is used as input for topography feedback (Albrecht et al., 1991). FM-AFM is more suitable for working in vacuum environment. In vacuum, the damping of the cantilever oscillation is extremely low (the quality factor of the cantilever is greatly enhanced), so it takes longer for the oscillation amplitude to respond to tip-sample interaction forces, making AM-AFM impractical (Albrecht et al., 1991). Since the Si(111) 7 7 reconstructed surface provided a suitable benchmark for testing the accuracy of STM, the same structure was readily adopted by the AFM community for the same purpose. The first results showing true atomic resolution AFM on Si(111) 7 7 surfaces were published in 1995 by Giessibl (1995). Imaging of the Si(111) 7 7 surface was carried out in UHV, using piezoresistive cantilevers (Tortonese et al., 1993; Giessibl et al., 1994) and FM detection. In this approach, the deformation of the cantilever is detected from the strain of an incorporated piezoresistive element, which is created by doping one side of the cantilever, and is part of a Wheatstone bridge. The output of the Wheatstone bridge is a direct measure of the cantilever deflection and can be used for topography feedback. Considering that AFM can also be operated in liquid environment, it became an interesting tool for studies related to biological materials—including, but not limited to, living cells. The first AFM studies of cells were published as early as 1990 and 1991 (Butt et al., 1990; Gould et al., 1990a, b; H€aberle et al., 1991). Interestingly, the AFM system used in the work reported by H€aberle et al. (1991), although working in liquid buffer, employed a force measurement mechanism based on a tunneling tip, similar to the one used in the first reported AFM (Binnig et al., 1986). In this work, H€aberle and coworkers envisioned the possibility of probing with the AFM the structure of the cytoskeleton or performing dynamic studies of cellular processes, at the nanoscale. In the context of the present book, some of the results of studies related to biomaterials carried out in the authors’ group are presented. Magnetospirillum gryphiswaldense, a strain of magnetotactic bacteria, was imaged in ambient air, on glass slides, and in dried state. The images were obtained with a commercial AFM (Nomad, Quesant Instrument Corp.) in intermittent-contact AFM, with standard silicon tips (Q-WM190, Budget Sensors). Fig. 3 shows topography AFM images of a 40 μm 40 μm area (A) and a 3-D rendering of a 10 μm 10 μm area (B) of the sample. Individual cells and agglomerated cells can be observed. Some small particles are also present between the cells— these might be magnetosomes or agglomerations of magnetosomes.
5.2.3 Scanning ion-conductance microscopy In scanning ion-conductance microscopy (SICM), the probe is a micro- or nanopipette with an inner electrode. Measurements are made in an electrolyte solution by monitoring the ionic current that passes through the pipette opening. A schematic view of the
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Fig. 3 Intermittent-contact AFM images of M. gryphiswaldense, a strain of magnetotactic bacteria. (A) 40 μm 40 μm top view; (B) 10 μm 10 μm 3D rendering.
Scanning probe techniques for nanoscale imaging and patterning
Fig. 4 Principle of operation of SICM. The sample is submerged in an electrolyte solution—usually NaCl or a growth medium in the case of biological samples. The electrodes are usually Ag/AgCl. The nanopipette is moved using piezoelectric positioners.
working principle of SICM is shown in Fig. 4. The pipette electrode is biased relative to a reference electrode placed inside the solution. For constant bias, the ionic current depends on the separation between the pipette opening and the sample surface: when the pipette is far from the surface, the ionic current flows unobstructed through the opening; as the pipette is brought closer to the surface, the current flow is gradually obstructed. Topography feedback is achieved by adjusting the pipette-surface separation to keep the ionic current constant. The resolution of SICM is limited, in principle, by the dimension of the pipette opening. SICM was first reported in 1989 by Hansma and coworkers (Hansma et al., 1989) although, according to the authors, preliminary results were already obtained in their laboratory as early as 1986. Using a large-scale model, with a pipette having an inner diameter of 0.71 mm and an outer diameter of 1 mm, the authors showed that the system is able to resolve surface features having the same dimensions as the inner diameter. In the mentioned study and in subsequent works (Prater et al., 1991; Proksch et al., 1996), SICM was already seen as a suitable method for the study of biological samples. This was due to its proved ability to measure the topography of delicate structures, such as porous membranes in liquid, and also to map ionic currents through the pores of such structures. However, the first results regarding SICM imaging of live cells were published only in 1997 by Korchev and coworkers (Korchev et al., 1997a, b). For these studies, a patchclamp system was modified by adding specific components for SICM imaging. Murine melanocyte cells, human colon cancer cells, cardiomyocytes, and smooth muscle cells were scanned in liquid environment, maintaining their viability. The topography of the cells, with relatively high corrugations (>30 μm), was successfully imaged with high resolution. Some dynamic changes of the cell surfaces were also observed.
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Besides being able to determine the topography of delicate structures such as living cells or other biological materials, SICM can detect and map the activity of single ionic channels on the surface of intact cellular membranes, as shown in the work published in 2000 by Korchev and coworkers (Korchev et al., 2000). In order to achieve this, SICM was carried out simultaneously with patch-clamp experiments. The SICM pipette, while scanning the topography of the cell, also acted as a highly localized source of K+ ions. The electric response of the cell, quantified by the K+ ionic current, was recorded via the patch-clamp micropipette. The local maxima of the ionic current indicated the position of the active ionic channels. SICM has been used to study structures such as microvilli and their dynamics (Gorelik et al., 2003), amyloid fibrils (Zhang et al., 2012), and primary cilia (Zhou et al., 2018) and to monitor dynamic processes such as phospholipid-induced morphological changes of red blood cells (Zhu et al., 2018). Aside from imaging and current sensing, SICM has proved to be a promising technique for patterning, as will be shown in Section 5.3.3.
5.3 Patterning: Probe lithography In probe lithography, patterns are created on surfaces by various methods. Local mechanical stress can be applied to create dots, scratches, etc. A heated tip can also pattern the surface by changing the chemical state or simply by melting the material. A biased tip can transfer local charge to an insulating material, or it can change the local polarization state of a ferroelectric material. The next paragraphs will show some of the early advances in patterning at the atomic scale with STM and AFM.
5.3.1 Patterning by STM Perhaps, the best known example of atomic-scale manipulation is the famous IBM logo written with 35 atoms, created by Eigler and Schweizer at the IBM laboratories in Almaden (Eigler et al., 1990). Previous to this, the controlled positioning of atoms (Becker et al., 1987) or molecules (Foster et al., 1988) had been reported. However, the chemical nature of the manipulated material was uncertain. In the work described by Becker et al. in 1987 (Becker et al., 1987), the deposited atoms are hypothesized to originate from the surface material itself (germanium), being previously transferred to the scanning tip (tungsten). The atoms are deposited on the surface by raising the tip to surface bias from the value of 1 V used during imaging to 4 V. In the experiments described by Foster et al. in 1988 (Foster et al., 1988), controlled pinning to the substrate and removal of previously pinned molecules or molecule fragments are achieved. The molecules are believed to originate from the liquid in which the measurements are taking place (di(2-ethylhexyl) phthalate). The molecules are pinned during imaging by applying a short voltage pulse to the tip (3.7 V, 100 ns). Subsequently,
Scanning probe techniques for nanoscale imaging and patterning
such pinned structures can be removed (completely or partially) by applying again a similar pulse as the tip scans over them. For the atomic IBM logo, xenon atoms were repositioned on a single-crystal nickel surface, in ultrahigh vacuum at low temperature (4 K). First, a scan was performed to locate several xenon atoms on the surface, which were then individually positioned to create the IBM logo. When the STM tip approached the surface sufficiently close (closer than it was required for imaging), the force exerted by the tip allowed xenon atoms to be dragged across the surface (Fig. 5). The logo consisted of 35 atoms, with each letter having a height of 5 nm. The atoms were positioned in a rectangular grid with periodicity ˚. 14 12.5 A Subsequently, in 1991, Eigler and coworkers created the “atomic switch” (Eigler et al., 1991). Using the same materials as for the IBM logo, they showed that an adsorbed xenon atom can be repeatedly and reproducibly transferred from the substrate to the STM tip and back. The transfer was achieved by applying short bias pulses of opposite sign to the tip, which was kept at tunneling distance above the surface. The presence and absence of the xenon atom on the surface were confirmed by taking successive STM images. The conductance state of the switch was monitored throughout the process by measuring the tunneling current: with the xenon atom bound to the surface, the switch was in the low-conductance state, and it changed to the highconductance state after the transfer of the xenon atom to the tip. A variation of the current by almost one order of magnitude between the two conduction states was reported.
Fig. 5 Repositioning of one Xe atom on the Ni substrate, for Eigler’s “IBM” logo written with 35 atoms. (Modified from Eigler, D.M., et al., 1990. Nature 344, 524–526. https://doi.org/10.1038/344524a0.). First, the Xe atom is located on the surface by standard STM imaging (site A); next, the tip is positioned above the atom and is lowered until the tip-atom interaction is high enough to allow dragging the atom across the surface to the desired location (site B), where the tip is raised. (The atomic lattice of the Ni substrate is not resolved in the STM images taken throughout the experiment.)
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5.3.2 Patterning by AFM As in the case of imaging, atomic-scale patterning by AFM proved relatively more difficult to achieve than with STM (Custance et al., 2009). The first results concerning the manipulation of individual selected atoms were published in 2003 (Oyabu et al., 2003). In this work, carried out at low temperature, single silicon adatoms of the Si(111) 7 7 reconstructed surface were removed leaving behind vacancies or deposited on previously created vacant spots. The authors termed the method “soft nanoindentation”: the tip was gently approached to the surface while being oscillated at the cantilever’s resonance frequency, and the amplitude and frequency shift of the oscillation were monitored. The approach was stopped as soon as the frequency shift experienced a sudden jump—indicating the reorganization of the local tip-sample atomic arrangement. A notable example of lateral manipulation at the atomic scale is the induced interchange of different adatom species, reported in 2005 (Sugimoto et al., 2005). In this study, atomic-scale shapes were created by successively interchanging the positions of substitutional tin adatoms and germanium adatoms on the Ge(111)-c(2 8) reconstructed surface. The interchange between two neighboring tin and germanium atoms was induced by the attractive interaction with the scanning tip. The tip was gradually brought closer to the surface while repeatedly scanning along the same line, which was appropriately chosen to pass through the centers of both atoms. Some of the unpublished results obtained by the authors of the present chapter, related to AFM patterning of biological structures, will be presented hereafter. The results were obtained using a commercial AFM (XE100, Park Systems), with the dedicated nanolithography software. One such example is the creation of patterns on the surface of red blood cells. Human red blood cells were obtained from a fresh blood sample that was drop cast on a glass slide directly from the harvesting syringe. In order to avoid the formation of large clumps of cells, the fresh drop was translated (smeared) across the surface of the slide with the syringe needle. The cells were then left to dry in ambient air for approximately 30 min before being imaged by NC-AFM. Standard silicon tips (ACTA, Applied NanoStructures Inc.) were used for AFM imaging. Fig. 6A shows the image of a 50 μm 50 μm area of the sample. Since the blood sample was not subject to centrifugation for the separation of serum, the cells are surrounded by a matrix resulting from the dried serum. One of the cells in the highlighted area was patterned with the AFM tip, using the AFM’s nanolithography software. Patterns were created by pressing the tip against the surface and dragging it laterally. Subsequently, the highlighted area was scanned again in NC-AFM (Fig. 6B). The resulting patterns were up to 50 nm deep and 150 nm wide.
5.3.3 Patterning by SICM SICM-based patterning can be accomplished in a number of ways. Mechanical deformation exerted by the scanning pipette was employed in the work described in
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B€ ocker et al. (2009). Lipid bilayer membranes, suspended over the pores of highly ordered porous silicon substrates, were first imaged by SICM. Subsequently, in selected locations, the surface of the membranes was ruptured by gradually approaching the nanopipette while monitoring the ionic current, which showed a sharp increase at the moment of rupture. Desired patterns were created, with the pores acting as “pixels.” Alternatively, the SICM can be used as a nanoscale “3D printer,” as reported in a recent work by Momotenko and coworkers (Momotenko et al., 2016). A dual-channel or dual-barrel nanopipette was used: one of the channels served for positioning SICM feedback and the other as a source of precursor ions (Cu2+) for material deposition. The electrochemical deposition was highly localized by the size of the nanopipette opening and also depended on the substrate potential. The deposition was initiated with the pipette approached in the vicinity of the substrate. As the material was gradually deposited under the pipette, the SICM feedback automatically lifted the pipette away from the substrate, leading to the deposition of high-aspect-ratio structures in the form of pillars, from several micrometers to >20 μm long and <1 μm wide. By combining the vertical movement of the pipette with lateral translation, zigzag structures were also created.
5.4 Conclusions and perspectives Scanning probe microscopy offered scientists tools that allow them to analyze and control matter at scales that span more than four orders of magnitude: from visualizing and manipulating atoms at the subnanometer level to measuring the shape, dynamics, and interactions of living cells. With the continuous development of control electronics, SPMs are becoming more and more reliable, versatile, and easy to use. They are already indispensable tools in a number of fields, from probing mechanical and electric properties of materials used in next-generation electronics, to helping cellular biologists probe antigen-antibody interactions at cellular level, to aiding in art preservation efforts. An important trend in the development of SPM is the increase of overall acquisition speed, in order to allow the visualization of a larger variety of real-time processes. Also, control over the functionalization of the probe at the atomic level would allow new insights in the study of chemical reactions and routinely carry out SPM with chemical sensitivity at the molecular and atomic scale.
Acknowledgments This work was supported by the Romanian National Authority for Scientific Research and Innovation, in the frame of the “Nucleus” Programme, and grants of the Romanian Ministry of Scientific Research and Innovation, CCCDI—UEFISCDI, project number PN-III-P1-1.2-PCCDI-2017-0637 (MultiMonD2), PN-III-P1-1.2-PCCDI-2017-0172 (TESTES), and PN-III-P1-1.2-PCCDI-2017-0755 (MALASENT), within PNCDI III.
Scanning probe techniques for nanoscale imaging and patterning
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