Conference-ISSS-4-Control of probe function in noncontact atomic force microscopy using photo-responsive molecular tip

A tripod molecule with an azobenzene arm was designed as a single-molecular tip for noncontact atomic force microscopy (NC-AFM), where photoswitching of the tip apex can change the probe function by the external stimulus. NC-AFM imaging on the synthesized molecules revealed that they can be strongly fixed on Au surfaces in isolated form and reversibly switch between transand cis-form by irradiation at selected wavelength of light. [DOI: 10.1380/ejssnt.2006.249]


I. INTRODUCTION
Scanning probe microscopies (SPM) have demonstrated their great potential to image the sample surface with atomic scale resolution.It has been recognized that the property of probe apex, which is a gateway of interaction with the sample, is very important.For example, in the case of scanning tunneling microscopy and spectroscopy (STM/STS), the electric state of a tip deeply affects the obtained image and the profile of the spectra.In the case of atomic force microscopy (AFM), the overall shape of the tip, presence of chemical species, electric charge, and unsaturated bonds affect the force to be used for imaging.Recently, noncontact atomic force microscopy (NC-AFM), which can regulate the tip position with very week attractive force working between the tip and the sample, has successfully demonstrated the great imaging capability with atomic scale resolution for various sample surfaces, including insulators [1,2].Onishi and coworkers have recently indicated a possibility to use a chemically modified tip for NC-AFM measurements, where covered organic monolayer shift the contact potential value from that of the original Si tip [3].Sugawara and coworkers have reported that NC-AFM images drastically changed depending on the atom species picked up from the surface on the tip apex [4].These results suggest that controlling of tip apex species in NC-AFM may be applied as a means of chemical identification of surface species and of mesoscopic scale molecules.
In spite of experimental efforts to prepare a sharp and reproducible tip apex [5], strict regulation of tip apex in a sense of structure and chemical property has not been suc-cessful.Recently, Arai and Tomitori demonstrated that a Si nanopillar grown on a commercial Si tip under UHV condition can provide a tip with a high aspect ratio, which has advantages on NC-AFM measurements, and can elongage the lifetime of the tips [6].Another way to control the tip apex of SPM is the use of a single molecule as a tip.Carbon nanotube tip, which can give the high aspect ratio with variety of applications, can be included in this category [7].Recently, the design and synthesis of nanoscale molecules with a potential use as a single molecule tip of AFM have appeared [8,9].These probe technologies, which are developed from the viewpoint of organic synthesis, may provide new functional capacities to the SPM tip.
In this study, we report a novel single molecular tip which can control its functionality by photoirradiation.Photoisomerization is a reversible process accompanied by structural change, which has been widely investigated and received interests as a component of external trigger in molecular machines or manipulators [10,11].We have designed and synthesized a molecule that can be strongly fixed on an Au surface in isolated form to serve as a single molecular tip.It includes azobenzene arm and its photoisomerization enables us to controll the tip apex structure that is responsible for tip-sample interaction.Photoswitching behavior of the molecule adsorbed on Au(111) was examined and reversible switching between transand cis-form was successfully confirmed.

II. EXPERIMENTAL
NC-AFM measurements were performed in an ultrahigh vacuum (UHV) AFM (JEOL JSPM-4610A) at room temperature (RT) unless otherwise noted.The base pressure was 2 × 10 −8 Pa.Conductive Si cantilevers with the resonant frequency of f 0 ∼ 315 kHz and the force constant of k ∼ 48 N/m (MicroMasch) were used as force sensors for observations.Resonant frequency of the cantilever was detected by a frequency modulation (FM) method.The peak-to-peak amplitude of vibration of the cantilever was set approximately at 12 nm.
A 100 W Xe lamp with a cold mirror system and bandpass filters (Asahi Spectra, LAX-101) was used as a light source.Selected wavelength of light at 360 ± 10 nm (ultraviolet region) or 450 ± 10 nm (visible region) was introduced though quartz windows to be focused on the sample during measuring NC-AFM under UHV.The density of photons which reached the sample was roughly estimated to be ca.3.0 × 10 2 photons nm −2 s −1 for both light irradiation.
Au(111) substrates were prepared by vacuum evaporation of gold onto freshly cleaved mica and annealed in a butane flame.Just after the cleaning of an Au(111) surface, synthesized molecules were fixed on the Au surface by adopting the in situ base-promoted deprotection method [12,13].

III. RESULTS AND DISCUSSION
We have designed a rigid and sharp molecule (AZO-SH) shown in Fig. 1(a) suitable for a molecular tip of NC-AFM.The AZO-SH molecule consists of a rigid adamantane core, three linear legs with thiols at the end, and a sharp tip moiety.The AZO-SH molecule can be adsorbed on Au surfaces in isolated form by formation of three Au-S bonds.Similar tripodal molecule that can be fixed on a substrate has been reported by Keana and coworkers [14].The sharp structure of the molecule has advantages in reducing the van der Waals force, which becomes a background force as a result of summation of van der Waals force for the whole atoms involved [1,2].It gains the contribution of a short range force such as a chemical force as a dominant force for imaging of NC-AFM.Furthermore, our molecule includes azobenzene arm for photoswitching of the tip apex structure.Azobenzene derivatives are widely used for photoresponding systems because of its reversible isomerization to cis-form by UV light (ca.350 nm) irradiation and to trans-form by visible light (ca.450 nm) irradiation [10,15].In the case of the AZO-SH molecule, we can expect the tip apex structure can be reversibly controlled by selected wavelength of light as shown in Fig. 1(a).The details of the synthesis and characterization of AZO-SAc molecule will be described elsewhere [16].At present, the apex of AZO-SAc is terminated by trimethylsilyl (TMS) ligand as a protected ligand, which do not make special interaction to the Au surface.It is easy to exchange TMS with other ligands for further applications.
The synthesized AZO-SAc molecules were transformed to AZO-SH and tightly fixed on an Au surface by formation of three Au-S bonds as shown in Fig. 1(b).An in situ base-promoted deprotection method was adopted [13] and dispersed adsorption of AZO-SH succeeded.ductive Si cantilever (I t = 50 pA), the bright protrusions have not been imaged but swept away from the scanned area and following NC-AFM measurements of larger area revealed agglomerates that probably consist of the bright protrusions.These results suggest that the bright protrusions are AZO-SH molecules that are bound strongly enough on Au(111) and stable against the force used for imaging of Fig. 2(a).Line profile analyses of the bright protrusions in Fig. 2(a) revealed that there are two kinds of species with different apparent heights.The apparent height of major one (white arrows) was 3.7 ± 0.1 nm [17] and that of minor one (black arrows) was less than 2.5 nm.From the molecular structure in Fig. 1(a), one can expect that physical topography of a trans-AZO-SH standing on the Au surface by three S-Au bonds is ca.3.9 nm, which is in good agreement with the observed height of the major species.As will be discussed later, the minor one is not a structural isomer (i.e.cis-AZO-SH) but a species with different adsorption conformation.It should be noted that the ap-parent diameters of all the bright protrusions were much larger than those expected from the size of the molecule.This is as a result of convolution of the Si tip apex whose effective radius is estimated to be ca.10 nm.When a protrusion of 3.7 nm high is observed with such size of tip, the lateral size of the protrusion should be at least 16 nm in rough estimation [18].Thus we assigned the major bright protrusion in Fig. 2(a) to be the isolated trans-AZO-SH molecule, which is thermodynamically more stable than the cis-form at room temperature.
Next we examined photoisomerization of the adsorbed AZO-SH molecules by irradiation at selected wavelength of light during NC-AFM measurements.UV-visible spectra of AZO-SAc in CH 2 Cl 2 solution showed that conversion of trans-form to cis-form occurs by ultraviolet light (UV; 360 ± 10 nm) irradiation and the reverse process occurs by visible light (vis; 450 ± 10 nm) irradiation.If the major species in Fig. 2 radiation, the physical topography of the molecule is expected to be decreased by ca.1.0 nm as shown in Fig. 3(c).It was actually observed as shown in Fig. 3(a) and 3(b).After obtaining an NC-AFM image of a molecule whose apparent height was 3.6 nm prior to photoirradiation, scanning was stopped, the surface was irradiated by ultraviolet light (UV; 360 ± 10 nm) for 10 min, and the same molecule was imaged again under illumination with UV light.The apparent height of the molecule was reduced by 0.9 nm, which indicates a photoisomerization of the azobenzene arm from transto cis-form.Besides, the apparent height of the molecule was recovered to the initial value by following the above procedure while changing the wavelength of irradiating light to visible light (vis; 450 ± 10 nm).It can be understood as the reverse photoisomerization from cis-form to trans-form.As shown in the series of NC-AFM images in Fig. 3(a) and 3(b), it was confirmed that the process was reversible against irradiation of selected wavelength of light.The same cyclic behavior was observed as a common process on other molecules whose apparent height were 3.7 ± 0.1 nm prior to light irradiation as shown in Fig. 4.Although there are some deviation of the apparent height of the species, the difference of the apparent height of the molecule under UV irradiation and under visible light irradiation falls in the range of 0.8 ± 0.2 nm.This value is well above the experimental error of the apparent height calculated from the line profile analyses.Crude simulations of the frequency shift due to van der Waals force between the tip and the Au substrate + AZO-SH molecule reproduced the height difference of 1.0 nm between transform and cis-form.Thus we can conclude the assignment of the major species whose apparent height is 3.7 ± 0.1 nm to be the isolated trans-AZO-SH molecule is valid.The simulation also indicated that the slope of frequencyshift vs. distance curve for trans-form molecule and for cis-form molecule are almost the same at the frequency shift used for imaging of the molecule.It means that we cannot expect the reduction of long-range van der Waals force because of presence of a bulky trimethylsilyl (TMS) protecting ligand.This was actually the case observed in Fig. 3(b).If the TMS ligand is removed from the AZO-SH and terminated with H, very sharp tip suitable for high resolution NC-AFM imaging will be realized.
We considered that the minor species in Fig. 2(a) (indicated by black arrows), which showed an apparent height less than 2.5 nm, are not cis-AZO-SH molecules.The apparent height was less than the physical topography of cis-AZO-SH (2.8 ± 0.1 nm) as well as the calculated apparent height of cis-AZO-SH (ca.2.8 nm) in crude simulation of the frequency shift vs. distance curves.As well, they did not change in height either by UV light irradiation or visible light irradiation in contrast to the species that we assigned to be cis-AZO-SH (2.8 ± 0.1 nm high) in Figs.3(a) and 3(b).Potential surface calculation for azobenzene has shown that trans-azobenzene is more stable than cis-azobenzene by ca.0.5 eV in its ground state and the activation energy to convert cis-azobenzene to stable trans-azobenzene is ca.1.0 eV [19].This suggests that trans-azobenzene is majority at RT in dark condition as is usually assumed.We can tentatively assign these low height species to the AZO-SH lying on the Au surface bound through one or two S-Au bonds.Variation of the apparent height may be attributed to different geometry and bonding of the lying AZO-SH molecule.
It was sometimes observed that the cis-AZO-SH spontaneously isomerized to trans-AZO-SH just after stopping of UV irradiation during NC-AFM measurements.Previous spectroscopic studies of azobenzene derivatives in solution and theoretical calculation of potential energy surface have reported that an activation energy barrier for isomerization from cis-form to trans-form was ca.1.0eV [19,20].Thus the life time of cis-form is expected to be the order of several hours (ca.4.5 hours).UV-visible absorption spectroscopy results of AZO-SAc in CH 2 Cl 2 also indicated that cis-form formed by UV light irradiation (360 ± 10 nm) did not covert to trans-form at RT in the order of several minutes.Sigekawa and coworkers have reported STM observations of reversible photoisomerization of an azobenzene derivative molecule embedded at boundary of thiol SAMs, and found a stochastic height change of the molecule induced by electric field and tunneling current flow [21].In our NC-AFM measurements, bias voltage in the range of ±0.5 V to compensate the average contact potential between the tip and the sample depending on the tip state, which is effective to obtain clearer NC-AFM images [1].But observed height change of cis-AZO-SH to that corresponds to trans-AZO-SH is not stochastic event but conversion to stable trans-AZO-SH.Therefore, we consider that it is a thermally activated process, though we are not sure of the actual potential barrier from cis-form to trans-form of AZO-SH.During the UV irradiation, the process may be virtually prohibited because of effective conversion of trans-AZO-SH to cis-AZO-SH by UV irradiation.Preliminary NC-AFM measurements at 78 K showed that the frequency of the spontaneous conversion from cis-form to trans-form was decreased compared with the corresponding experiment at RT, which supports above assumption.Further studies are necessary to discuss this issue in more detail, which includes lifetime estimation, temperature dependent measurements, estimation of quantum efficiency of photoisomerization.http://www.sssj.org/ejssnt(J-Stage: http://ejssnt.jstage.jst.go.jp) e-Journal of Surface Science and Nanotechnology Volume 4 (2006)

IV. CONCLUSIONS
In conclusion, it has been demonstrated that newly designed AZO-SH molecule can be strongly fixed on the Au(111) surface in isolated form standing on the surface through three S-Au bonds.Their photoswitching between transand cis-form was possible by UV (360 nm) and visible (450 nm) light irradiation, which was confirmed as a change in apparent height of the molecule by ca.0.8 nm.We believe this molecule can be fixed at the apex of Au-coated cantilever by the same methods as on Au(111) surface and work as a photoresponding tip.Our designed molecule satisfies the fundamental requirements for using as the single molecular tip and has a possibility to be applied as a functional probe.

FIG. 1 :
FIG. 1: (a) Molecular structures of trans-and cis-azobenzene derivatives (AZO-SH).(b) Schematic of in situ base-promoted deprotection and adsorption of the synthesized AZO-SAc molecule to the Au surface.

FIG. 2 :
FIG. 2: (a) 3D view of NC-AFM image (500×500 nm 2 ) of isolated AZO-SH molecules adsorbed on the Au(111) substrate.∆f = −10 Hz.Two kinds of bright protrusions were observed with an apparent height of 3.7 ± 0.1 nm (white arrows) and less than 2.5 nm (black arrows), respectively.(b) Models of possible adsorption geometry of AZO-SH on the Au substrate observed in (a).

FIG. 3 :
FIG. 3: (a) A series of NC-AFM images (50×50 nm 2 ) of an AZO-SH adsorbed on Au(111) representing the trans form (after visible light (450 ± 10 irradiation) and the cis form (after UV light (360 ± 10 nm) irradiation) and corresponding line profiles are indicated.∆f = −60 Hz.(b) 3D view of NC-AFM images in (a).(c) Models of corresponding molecular structures of AZO-SH fixed on the Au substrate.