Imaging Three-Dimensional Surface Objects with Submolecular

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Letter pubs.acs.org/NanoLett

Imaging Three-Dimensional Surface Objects with Submolecular Resolution by Atomic Force Microscopy César Moreno,*,†,‡ Oleksandr Stetsovych,†,§ Tomoko K. Shimizu,†,§ and Oscar Custance† †

National Institute for Materials Science (NIMS), 1-2-1 Sengen, 305-0047 Tsukuba, Ibaraki Japan International Center for Young Scientists, NIMS, 1-2-1 Sengen, 305-0047 Tsukuba, Ibaraki Japan § Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnicka 10, Prague, Czech Rebublic § JST, PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan ‡

S Supporting Information *

ABSTRACT: Submolecular imaging by atomic force microscopy (AFM) has recently been established as a stunning technique to reveal the chemical structure of unknown molecules, to characterize intramolecular charge distributions and bond ordering, as well as to study chemical transformations and intermolecular interactions. So far, most of these feats were achieved on planar molecular systems because high-resolution imaging of three-dimensional (3D) surface structures with AFM remains challenging. Here we present a method for high-resolution imaging of nonplanar molecules and 3D surface systems using AFM with silicon cantilevers as force sensors. We demonstrate this method by resolving the stepedges of the (101) anatase surface at the atomic scale by simultaneously visualizing the structure of a pentacene molecule together with the atomic positions of the substrate and by resolving the contour and probe-surface force field on a C60 molecule with intramolecular resolution. The method reported here holds substantial promise for the study of 3D surface systems such as nanotubes, clusters, nanoparticles, polymers, and biomolecules using AFM with high resolution. KEYWORDS: Noncontact atomic force microscopy (NC-AFM), submolecular resolution, three-dimensional dynamic force spectroscopy, high-resolution imaging ince the first demonstration of atomic resolution imaging using dynamic force microscopy,1 several landmark works have contributed to push the resolution limits2,3 and versatility4−10 of this technique. A long-lasting endeavor was, however, the achievement of intramolecular resolution, which was accomplished by scanning tunnelling microscopy (STM) long ago.11,12 Recently, Gross and co-workers amazed the scientific community by reporting the first atomic force microscopy (AFM) images clearly resolving the chemical structure of a pentacene molecule,13 revealing the chemical structure of unknown molecules,14 and characterizing the intramolecular charge distributions15 and bond ordering16 of organic molecules. The key strategy to accomplish these results was a functionalization of the forefront part of the AFM probe by picking up a relatively inert atom or molecule, such as xenon and carbon monoxide (CO), which was coadsorbed on the same surface as the organic molecules. The bending of this CO molecule17 enables exploring the repulsive interaction forces responsible for submolecular contrast13 while preventing unintentional modifications of the atomic and molecular arrangements at both probe and surface. This probe functionalization strategy has been adopted by many groups, and it has helped produce quite relevant results in the field of nanoscience.18−23

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© XXXX American Chemical Society

The functionalization of the AFM probe with a CO molecule requires operation of dynamic AFM at cryogenic temperatures (typically below 15 K) using a stiff piezoelectric sensors2 driven at a few tens of picometers of oscillation amplitude; implementation that also enables reliable STM data acquisition. Submolecular resolution at liquid nitrogen temperature without deliberate tip functionalization has been also accomplished with such stiff piezoelectric sensor.24,25 Intramolecular resolution with AFM is normally achieved by imaging in constant height mode, that is, scanning the probe in a plane parallel to the surface with a topographic open feedback loop. This method is necessary to record nonmonotonic variations of the AFM main observable over the molecules that otherwise would yield an unstable topographic feedback. Imaging in constant-height mode, however, limits the applicability of the method to study planar molecules or small planar regions of three-dimensional (3D) surface structures.13−16,19−22 To overcome this limitation, there have been proposed several multipass routines that by tracing the same line scan multiple times with different setting parameters enable Received: October 31, 2014 Revised: February 27, 2015

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Figure 1. Multipass method for intramolecular resolution using AFM with silicon cantilevers as force sensors. (a), Schematic representation of the approach used in this work. For each position along the slow scan direction of an image, the fast line scan is performed twice: first with a closed feedback loop, and then in an open feedback loop following the topography recorded during the first line scan but closer to the surface by applying a constant offset distance. (b), Topographic image (Z[Δf ]) associated with the first line scans over a pentacene molecule adsorbed on the (101) anatase surface. (c) and (d), Images of the variation of the resonance frequency of the first (Δf) and second (Δf 2) cantilever longitudinal flexural modes, respectively, simultaneously acquired during the second line scans. The offset distance the probe was approached toward the surface during the second line scan was 0.3 nm. The inset in (d) is a ball-and-stick model of the pentacene molecule. Image dimensions are (3 × 3) nm2. The topographic set point for the first line scans is Δf = −4.5 Hz. Acquisition parameters: free-oscillation first ( f) and second (f 2) longitudinal resonance frequencies of the cantilever are f = 159202 Hz and f 2 = 983324.1 Hz; cantilever oscillation amplitudes for the first (A) and second (A2) longitudinal resonance modes are A = 169.1 Å and A2 = 27 pm; the cantilever stiffness (K) is K = 26.9 N·m−1; the bias applied to compensate the probe-surface contact potential difference is VCPD = +100 mV.

offset distance has to be adjusted depending on the surface system explored, and it is experimentally determined with support from force spectroscopy measurements4 as the separation that provides better contrast difference in the Δf signal among the objects imaged. This procedure generates several simultaneous images that map exactly the same surface location. The first line scan produces topographic pictures that are almost featureless and inform mainly about the surface corrugation. Figure 1b is an example of such topographic maps and represents a pentacene molecule adsorbed on the (101) surface of a titanium dioxide anatase natural single crystal. The pentacene molecule is homogeneously imaged as a ∼0.24 nm protrusion over an almost featureless surface. The Δf and Δf 2 images (Figure 1, panels c and d, respectively) obtained from the second line scan show intramolecular details over the pentacene molecule that resemble its chemical structure, as well as simultaneous atomic resolution of the anatase substrate. Variations of the van der Waals forces a few nanometers away from the surface in ultrahigh vacuum conditions are normally dominated by the topography rather than by compositional changes; the first typically obeying an inverse-square law with the probe-surface separation, while the latter can be considered as a minimal change in the coefficient given by the Hamaker constant.4 Therefore, the topography measured over the first line scan accounts for a fair cancellation of corrugation effects during the second line scan. At the boundary between two surface objects showing a considerable height difference between them, the probe-surface separation during the second line scan will slightly differ from the value over planar areas. Still, this method is good enough to image 3D structures with high-resolution. Figure 2a−c displays a set of images acquired using the multipass approach described above over a C60 molecule adsorbed on a (101) anatase surface. The topographic image obtained during the first line scans (Figure 2a) does not show any intramolecular characteristics. The Δf and the Δf 2 images (Figure 2b,c, respectively) simultaneously obtained during the second line scans reveal features that can be ascribed to the carbon−carbon bonds forming the contours of the hexagonal

exploring a surface with the probe following the topography in an open feedback loop.14,26−28 However, the imaging methods that have been implemented so far fail in providing a quasiconstant probe-surface separation (within the precision required for intramolecular resolution) along the trajectory followed by the probe. The main drawback of most of these imaging routines is that the STM signal, which has an exponential dependence with the tip−sample distance, was used to compensate for the topography. Force spectroscopy protocols using STM to regulate the initial tip−surface separation have been also presented, which can resolve submolecular features and atomic structures of the substrate simultaneously.29,30 These spectroscopic approaches require long acquisition times and, when aiming for high-resolution, their applicability is limited to explore small surface areas. In this work, we demonstrate that when taking advantage of the gentle variation of the long-range van der Waals force with the probe-surface separation to compensate for the surface corrugation, it is possible to image 3D surface structures with high resolution over large surface regions. Results and Discussion. Our approach to image a surface following the topography in an open feedback loop is sketched out in Figure 1a, and it is based on multipass methods commonly used in conventional surface force microscopy to minimize topographic contributions in magnetic or electrostatic measurements.31 We operate the AFM in bimodal frequency modulation detection mode,32,33 recording the variation of the resonance frequency of the first (Δf) and second (Δf 2) longitudinal flexural modes of the cantilever with respect to the free oscillation values (see Methods for details). For each point along the slow scan direction of an image, we perform two consecutive line scans over the fast scan direction (Figure 1a). The first line scan is carried out with a closed feedback loop using a Δf value for the topographic set point corresponding to a probe-surface separation regime where the interaction responsible for atomic contrast4 is barely sensed and the van der Waals force dominates. The second line scan is performed in open feedback loop following the topography registered during the first line scan but applying a constant offset distance that further approaches the probe toward the surface. This B

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Figure 2. Intramolecular resolution and probe-surface force volume characterization on a C60 molecule. (a) Topographic image (Z[Δf ]) of a C60 molecule deposited on the (101) anatase surface obtained by applying the multipass method described in the text. The inset is a ball-and-stick model of the C60 molecule. (b,c) Δf and Δf 2 signals simultaneously recorded during the second line scans. The distance offset for the second line scans was 0.21 nm toward the surface with respect to the topography measured during the first line scans. Image dimensions are (3 × 3) nm2. (d) Probesurface short-range forces obtained from a force volume measured over a C60 molecule deposited on the (101) anatase surface. (e) Force map obtained at a x = 1.54 nm position of the force volume, highlighted with a dashed line rectangle in (d). The force scale has been cut to a −0.25 nN maximum attractive force for visualization purposes. The same map in full scale can be found in Supporting Information Figure S3. Dashed pink and green lines mark the probe-surface separation contours of the first and second line scans over the C60 molecule, respectively. (f) Details of representative force curves obtained over the C60 molecule (black and red solid lines) and the (101) anatase substrate (green solid line). The color code for these force curves is identical for panels (d−f). The black curve was obtained over a carbon−carbon bond site [bright rings in (b)] and the red curve was obtained at a position halfway between the carbon−carbon bonds and the bright feature at the center of the ring, [dark contrast rings in (b)]. The inset in (f) displays a fit to the long-range contribution (blue line) subtracted to the total force curve (gray line) to obtain the shortrange force curve labeled in the main graph as C60-dark ring. Pink and green vertical lines mark the probe-surface separation for the first and second line scans, respectively. (g) Measured frequency shift signal corresponding to the force map shown in (e) over the full tip−surface separation distance explored in the force volume. The pink solid line marks the probe-surface separation trajectory of the first line scans over the C60 molecule that correspond to a topographic set point value of Δf = −3.2 Hz. The topographic set point for the acquisition of the force volume is Δf = −3.5 Hz. Acquisition parameters are f = 162299 Hz; f 2 = 1005462 Hz; A = 127.2 Å; A2 = 36 pm; K = 28.5 N·m−1; VCPD = −350 mV.

the Δf 2 images barely showing any intramolecular features. On the contrary, further approaching the probe toward the surface during the second line scans results in a considerable deformation of the molecule and the blurring of the intramolecular features observed in Figure 2b (see Supporting Information Figure S1). This behavior and the submolecular contrast of the Δf and Δf 2 images can be understood by analyzing force spectroscopy data obtained from a force volume9,10 measured on top of a C60 molecule. Figure 2d displays the probe-surface short-range forces obtained over a (3 nm × 3 nm × 2.6 nm) volume on top of a C60 molecule adsorbed on a (101) anatase surface. The top part of the cube corresponds to strong attractive forces between the probe and the anatase surface (red-brown) with the blue depression representing forces close to the repulsive part of the

and pentagonal rings that give shape to the structure of the C60 molecule. In these images, a decrease in the absolute value of the Δf and the Δf 2 signals is also visible at the center of these rings, which is in agreement with previous works.16 The possibility of partially resolving the structure of the molecule, not only at the topmost part but also to the sides of the molecule, enables to unambiguously identify the adsorption configuration26 of the molecule on the surface (see Figure S1 at the Supporting Information for a direct comparison of Figure 2b with a C60 structural model). The correct determination of the distance the probe should approach toward the surface during the second line scans (0.21 nm for this experiment) is critical to obtain intramolecular resolution in the case of C60. Performing the second line scans at slightly farther separation from the surface produces Δf and C

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unsaturated bonds at the step edges often produces the modification of the probe apex. Figure 3 shows an example of

interaction obtained over the C60 molecule (see Figure 2f and Supporting Information Figure S3 for more details). A vertical slice of this cube at a x = 1.54 nm position (rectangle in black dotted line in Figure 2d) produces a two-dimensional force map across the center of the C60 molecule (Figure 2e). The corresponding Δf signal measured during the realization of the force volume is displayed in Figure 2g. The top part of the map is flat due to the absence of perceptible probe-surface interaction forces. As the separation between the probe and the surface decreases, forces start developing over the molecule until reaching the maximum attractive interaction force region, which extends over the whitish stripe following the contour of the molecule in Figure 2e. By further approaching the probe toward the surface, forces start to increase toward values close to the onset of repulsive interaction. This behavior is summarized by three vertical line profiles extracted from this two-dimensional force map shown in Figure 2f. These profiles correspond to a force curve measured over the anatase substrate (green), and two curves crossing over a maximum (red), and an adjacent saddle point (black) found within the maximum attractive force region on top of the molecule. The probe-surface separation corresponding to the first line scans over the C60 molecule in the imaging experiment (Figure 2a−c) is highlighted as a pink dotted line in Figure 2e−g. This trajectory was obtained by correlating the data point matching the topographic image set point value (−3.2 Hz) in each of the curves composing the Δf map (Figure 2g) with the corresponding force point in the map displayed in Figure 2e. The first line scans were performed at a distance where the van der Waals force dominates over the short-range force (the latter barely sensed), as it is quantified in the inset graph of Figure 2f. The path followed by the probe during the second line scans (highlighted by a green dotted line) can be obtained by adding an offset of 0.21 nm toward the surface to each point of the trajectory for the first line scan, mimicking the conditions of the imaging experiment. According to these trajectories and the force curves displayed in Figure 2f, the intramolecular contrast obtained over the C60 molecule arises from the variation of the Δf and Δf 2 signals generated by the force difference between the maxima and saddle points observed along the maximum attractive interaction force region outlining the C60 molecule in Figure 2e. The similarity of the force values for probe-surface separations slightly larger and smaller than the distance at which this maximum attractive interaction force region develops explains the blurring of the intramolecular features when further approaching the probe toward the surface and the vanishing of the patterns when retracting the probe from the sample. The modulation of the contour lines at both sides of the maximum attractive interaction force region denotes, however, subtle differences in the slopes of the attractive and repulsive part of the tip−surface interaction force over different parts of the molecule.26 The decrease in the absolute value of the Δf and the Δf 2 signals at the center of the hexagonal and pentagonal rings of the molecule is probably due to a competition between attractive short-range van der Waals forces9 and slightly repulsive interatomic forces between the probe forefront atom with the equidistant carbon−carbon bonds forming the rings, although atomic relaxations at the probe apex may also have some influence.16,17,26,34 The multipass approach reported here is particularly useful to study the structure and properties of surface steps. Topographic AFM imaging of surface steps is usually challenging, especially on reactive surfaces, because the interaction with atoms having

Figure 3. Resolving terraces and steps with atomic resolution using multipass scan. (a) Topographic image (Z[Δf ]) across five terraces of a (101) anatase surface with pentacene molecules adsorbed on it. (b) Δf signal obtained during the second line scans showing atomic resolution on the surface terraces as well as at the step edges. Acquisition parameters are f = 162282 Hz; A = 122.0 Å; and, K = 28.5 N·m−1. The topographic set point value for the first line scans was Δf = −2.7 Hz, and the second line scans were carried out by approaching the probe 0.32 nm toward the surface with respect to the topography recorded during the first line scans. Image dimensions are (40 × 29) nm2; VCPD = +600 mV.

multipass imaging across five terraces of the (101) anatase surface on which a low coverage of pentacene molecules have been deposited. The pentacene molecules appear as bright elongated protrusions on the terraces and at the steps (Figure 3a). The Δf signal acquired during the second line scans appears as a continuous surface terrace (Figure 3b) because the topography variation between the different terraces is compensated. The image clearly reveals the atomic structure of the surface at the terraces, details about the molecules adsorbed on the terraces and at the step edges, as well as the atomic structure of the steps in regions that are not decorated with molecules. We have demonstrated the capability of obtaining intramolecular resolution using commercial microfabricated silicon cantilevers (widely used in standard AFM setups) at 78 K and without an intentional functionalization of the AFM probe (see Supporting Information Figure S2). These experimental conditions are less demanding than those of previous works on submolecular resolution AFM imaging.13−16,19−22 The method we propose for imaging three-dimensional surface structures is based on performing multiple line scans for each point along the slow scan direction of the image. We make use of the gentle variation of the van der Waals force a few nanometers away from the surface to characterize the topography of the surface during a first line scan. In a second line scan, the probe follows the topography recorded during the D

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Nano Letters first line scan in an open feedback loop but approaching the probe toward the surface by a given offset distance that provides high-resolution imaging. The smaller the surface corrugation and the probe-surface force difference between the features to be imaged, the more precision in the determination of the offset separation for the second line scans is required. This offset can be determined from force spectroscopy measurements, or alternatively, by gradually decreasing the probe surface separation for the second line scan until highresolution contrasts in Δf is obtained. The method reported here holds promise to study nonplanar nanostructures such as nanotubes, nanoparticles, polymers, and biomolecules using AFM with unprecedented resolution. Methods. AFM Measurements. A UNISOKU Ltd. ultrahigh vacuum (UHV) cryogenic dynamic AFM with a homebuilt optical interferometer for the detection of the cantilever dynamics and a commercial SPM controller (Nanonis SPM Control System, SPECS, Germany) was used for the experiments and the in situ sample preparation. Measurements were carried out at a 78 K probe-sample temperature using platinum−iridium coated silicon cantilevers (PPP-NCLPt-20, Nanosensors, Switzerland) and bimodal frequency modulation detection.32,33 The first and second longitudinal flexural modes of the cantilever were instantaneously and simultaneously excited keeping the oscillation amplitude of each mode independently constant. The shift of the resonance frequency for the first (Δf) and second (Δf 2) longitudinal flexural modes of the cantilever with respect to the corresponding freeoscillation values were measured as the main AFM observables. The Δf 2 signal was primarily used to monitor the onset and evolution of the short-range forces during the experiments. Force spectroscopy4 was carried out by recording the Δf signal as a function of the probe-sample relative vertical displacement (Z). Force volume measurements9,10 were accomplished by the acquisition of Δf [Z] curves over each of the positions of a grid of (36 × 36) points. The topographic feedback loop was closed for the movement of the probe between the points of the grid. The determination of the corresponding K and A values are described elsewhere.6 No significant dissipation signal during the force spectroscopy experiments was detected (see Supporting Information Figure S3). The absence of any probe or surface modification during the spectroscopic acquisition was carefully checked. During AFM imaging and force spectroscopic measurements, the longrange electrostatic interaction was minimized by compensating the probe-surface contact potential difference (see Supporting Information Figure S4 for more details). The total probesurface interaction force was obtained from Δf [Z] curves by applying an inversion procedure.35 Topographic differences in Z during the spectroscopic acquisition were compensated,6 assuring a common origin with respect to the surface plane for all the force curves (see Supporting Information Figure S3). The probe-surface short-range forces were obtained by subtracting an approximation of the van der Waals force for a sphere-plane geometry fit4 over the long-range interaction region to the total force. For a proper characterization and subtraction of the long-range forces each Δf [Z] curve was measured by retracting the probe from the closest point to the surface until reaching the free oscillation regime. The validity and accuracy of the fits were checked by following the approach reported by Kuhn and Rahe.36 Prior to the measurements, the apex of the cantilever probe was conditioned by performing current-bias spectroscopy in

static STM mode while approaching the probe toward the surface several angstroms until good atomic resolution was obtained. This procedure often results in the probe picking up surface material; holes of a few nanometers diameter were normally detected at the surface after the probe conditioning. Intramolecular resolution imaging with a C60 molecule at the probe apex (see Supporting Information Figure S3) and over a pentacene molecule adsorbed on a Cu(111) substrate (see Supporting Information Figure S5) have been also achieved. Sample Preparation. Natural anatase single crystals cut exhibiting a polished (101) surface were purchased from Surface Net GmbH (www.surfacenet.de). The surface preparation was made by successive cycles of 20 min. Ar+ ion sputtering (1 keV energy and 5 μA ion current measured at the sample) and annealing at 970 K during 30 min in UHV (∼1.2 × 10−8 Pa). Pentacene and C60 molecules were evaporated on top of an atomically clean anatase substrate by means of thermal evaporation in UHV environment.

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ASSOCIATED CONTENT

S Supporting Information *

Additional intramolecular resolution images of pentacene and C60 molecules together with force volume spectroscopy data and the variation of the local contact potential difference over the pentacene molecule shown in Figure 1 are available. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address

(C.M.) Catalan Institute of Nanoscience and Nanotechnology (ICN2), 08193 Bellaterra, Barcelona, Spain. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Leo Gross and Ruben Perez for fruitful discussions, and Emilio Palomares and James William Ryan for supplying the anatase substrates. C.M. was supported by the Japanese Ministry for Education, Science, and Technology through International Center for Young Scientist (ICYS) program. O.S. acknowledges financial support from Charles University (GAUK 339311) and the Charles University-NIMS International Cooperative Graduate School Program. T.K.S. acknowledges the financial support by the Kao Foundation for Arts and Sciences and the MEXT KAKENHI Grant 26104540. Work supported by the NIMS (AA002 and AF006 projects).

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REFERENCES

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