A Scanning Tunneling Microscope as a Tunable Nanoantenna for

pubs.acs.org/NanoLett

A Scanning Tunneling Microscope as a Tunable Nanoantenna for Atomic Scale Control of Optical-Field Enhancement Damien Riedel,*,† Roger Delattre,† Andrey G. Borisov,† and Tatiana V. Teperik‡ †

Institut des Sciences Mole´culaires d’Orsay, ISMO, UMR 8214, CNRS-Université Paris-Sud, Bâtiment 210, 91405 Orsay Cedex, France, and ‡ Institut d’Electronique Fondamentale, IEF, UMR 8622, CNRS-Université Paris-Sud, 91405 Orsay Cedex ABSTRACT The high stability of a low temperature (9 K) scanning tunneling microscope junction is used to precisely adjust the enhancement of an external pulsed vacuum ultraviolet (VUV) laser. The ensuing VUV optical-field strength is mapped on an hydrogenated Si(100) surface by imprinting locally one-photon atomic scale hydrogen desorption. Subsequent to irradiation, topography of the Si(100):H surface at the reacted area revealed a desorption spot with unprecedented atomic precision. Our results show that the shapes, positions, and sizes of the desorption spots are correlated to the calculated optical-field structure, offering real control of the optical-field distribution at molecular scale. KEYWORDS Nanoantennas, scanning tunneling microscope, near-field enhancement tunability, 3D optical-field calculation, near-field mapping

T

he controllable enhancement of a confined opticalfield at the nanoscale opens up a wide range of applications including the detection of single molecular elements, near-field nanoimaging, or the atomic-scale photochemistry.1-3 While the issue of the sole confinement is successfully solved via the concept of nanoantennas, simultaneous tuning and monitoring of the optical-field enhancement is still a challenging objective.4 Thoroughly studied in photonics, metallic nanoantennas make excellent devices for the confinement and enhancement of optical fields into the spot with a characteristic size much smaller than the exciting wavelength. This property can be mostly attributed to the excitation of specific plasmon modes, although the lighting rod effect can also provide sought-after optical-field enhancement.2 For most of these prefabricated structures, a predefined geometry usually prevents the possibility to tune in situ their optical response. This has made the use of a scanning probe microscope (SPM) tip as a nanoantenna particularly attractive because of the possibility to control various photoinduced processes in the SPM junction.5-9 In this context, the interaction of an external coherent pulsed optical field with the junction of a SPM offers the combination of an ultimate spatial resolution with the various flexible parameters of the laser beam (i.e., wavelength, spectral line width or pulse duration, polarization, pulse shaping).10,11 Hence, controlling the confinement of an optical field at the nanoscale with a SPM is particularly crucial for providing a unique tool to study electronic,

photonic, or charge transfer properties of single nano-objects and, most importantly, to allow driving coherently their dynamics.7,11,12 This work shows that the junction of a LT-STM can be used to precisely tune the enhancement of an external optical field and, simultaneously, photochemically reveal its strength at the surface. This atomically resolved desorption process requires the choice of a specific wavelength (157 nm, 8 eV) to efficiently desorb H atoms in a dissociative onephoton σ f σ* antibonding transition at the Si-H bond.13 A constant dose of VUV pulses is accumulated on the STM junction while the STM tip is located at a fixed position over the Si(100):H surface (Figure 1a). To avoid any possible combined effects of photoinduced hot tunnel electrons6 with the confined VUV optical field, these experiments are carried out with a surface voltage Vs ) 0 (i.e., with no tunnel current) while the STM feedback loop is open. Such experimental conditions imply that the STM tip surface distance is precisely controlled when Vs ) 0 and remains steady with or without laser irradiation. The issue of the photothermal STM tip expansion is thus crucial for these experiments. We have shown that in our experimental conditions (i.e., low temperature, short wavelength, low focusing), STM tip expansion can be extremely weak and thus negligible, contrary to other experiments,14,15 if the laser repetition rate does not exceed 20 Hz16,17 (see Supporting Information). The laser irradiation is combined with a surface voltage pulse, i.e., the controlled variation of the silicon surface voltage. During the whole period of this voltage pulse varying from the scanning bias to Vs ) 0, the STM junction is illuminated with a pulsed VUV irradiation train triggered at a chosen location. Due to our specific experimental conditions, our

* To whom correspondence should be addressed. Received for review: 04/9/2010 Published on Web: 09/08/2010 © 2010 American Chemical Society

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FIGURE 1. (a and b) 16 × 16 nm2 STM topographies (Vs ) -1.7 V, I ) 70 pA) of the same Si(100):H surface area, before and after a VUV laser irradiation dose, respectively, at Vs ) 0. The irradiation dose corresponds to a burst of 3400 VUV pulses at 20 Hz with a relative STM tip surface distance ∆z ) -3 Å. (c) 3D sketch of the incoming VUV laser beam orientations compared to the photoinduced dehydrogenated ellipsoidal spot and the STM tip.

measurements are related to a constant irradiation dose of 3400 laser shots at 20 Hz. A detailed description of this procedure is given in the Supporting Information. Figure 1 presents our main finding and sketches the geometry used in our experiment (Figure 1c). For the STM tip located at a defined position above the Si(100):H surface (marked by the red dot in Figure 1a) the STM junction is irradiated with p-polarized VUV photons. Subsequent imaging of the same silicon surface area reveals a homogeneous light gray ellipsoidal spot of ∼38 × 56 Å2 in the corresponding STM topography (Figure 1b). Hence, the laser irradiation induces hydrogen desorption beneath the STM tip where the ensuing localized dehydrogenated zone is a clean Si(100)-2 × 1 surface area that appears brighter in the STM topography due to the presence of the silicon surface states.18 As supported by our calculations presented below, during p-polarized laser irradiation of the STM junction (Figure 1c), the confined optical field at the STM tip apex is strongly polarized perpendicularly to the silicon surface, in the direction of the Si-H bond axis.19,20 This results in a very favorable electric field orientation for photodesorbing hydrogen atoms.13 Here, the role of photoelectrons emitted from the STM tip apex to induce hydrogen desorption is considered as negligible. Indeed, the photoemission yield is extremely weak at the STM tip apex in the present experimental conditions. Besides, the ensuing photoelectrons © 2010 American Chemical Society

kinetic energy (∼3.2 eV) is too low to induce efficient hydrogen desorption.18,21 Finally, it is shown that electronically induced H desorption is spatially much less defined than the observed optical process (see Supporting Information). Note that an experiment with a s-polarized incident VUV beam has been realized with a similar irradiation dose without inducing dehydrogenation on the Si(100):H surface. This is consistent with the fact that, due to symmetry, the electric field component perpendicular to the silicon surface equals zero in the incidence (x, z) plane, below the STM tip apex. (See Supporting Information for the calculated distribution of the optical-field in the case of s-polarized incident VUV radiation.) To control the optical-field enhancement, the relative STM tip surface distance ∆z (i.e., compared to its initial position fixed by the scanning conditions) is precisely tuned. The series of STM topographies presented in panels a-e of Figure 2 show the size variation of the optically induced dehydrogenated silicon area as the relative STM tip surface distance ∆z decreases. The ∆z spans the range from +3 Å (i.e., the STM tip is retracted away from its initial position) to -7 Å (i.e., the STM tip is approached toward the surface). Larger relative STM tip surface distances (∆z > +3 Å) could not induce hydrogen desorption for our irradiation dose. For decreasing tip-surface distance, the STM topography series in Figure 2a-e, shows dehydrogenated areas of increasing 3858

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FIGURE 2. (a-e) 9.2 × 9.2 nm2 and 12.5 × 12.5 nm2 STM topographies (Vs ) -1.7 V, I ) 70 pA) of the Si(100):H surface following 3400 VUV laser shots at 20 Hz and Vs ) 0, for various relative STM tip-surface distances of +3, 0, -3, -5, and -7 Å, respectively, compared to the initial scanning conditions. The blue dots indicate the STM tip apex position. (f) Sketch of the STM junction geometry indicating the various parameters taken into account in our theoretical calculations: the STM tip radius R, the tip-surface distance d, and the STM tip length h. Note that the z axis is oriented toward the vacuum and starts at the silicon surface (z ) 0). (g) 628 × 628 nm2 calculated (x, y) section of the VUV |Ez|2 component distribution at the Si(100):H surface (z ) 0) for a STM tip radius R ) 20 nm and a STM tip-surface distance d ) 2.5 nm. (h-l) 30 × 30 nm2 calculated (x, y) sections (loge scale) of the VUV |Ez|2 optical-field distributions for R ) 20 nm and d ) 10, 5, 2.5, 1.25, and 0.75 nm, respectively. The dotted white lines cross at the STM tip apex location. (m-q) 30 × 50 nm2 calculated (x, z) section of the VUV |Ez|2 distributions for the various STM tip-surface distances (R ) 20 nm).

sizes. It varies from a few dispersed dangling bonds to a ∼62 × 123 Å2 dehydrogenated spot. Note that the same voltage pulses performed at similar relative STM tip-surface distances without laser radiation do not induce any hydrogen desorption. Here we wish to stress that changing the tip-surface distance allows control of the optical-field enhancement in the STM junction and thus modifies the size of the dehydrogenated surface area. Calculations of the three-dimensional distribution of the electromagnetic field resulting from the VUV irradiation of the STM junction fully support the above conclusions and © 2010 American Chemical Society

allow our findings to be clarified. Our numerical approach is based on the Lippmann-Schwinger formalism in scattering theory formulated in the time domain so that the pseudospectral solver of the Maxwell equations can be applied.22 In parallel with time-domain study, the fields in the junction have been computed with rigorous solution of Maxwell’s equations in the frequency domain using the boundary element method as implemented in BEMAX code.23 For the dielectric constant of tungsten and silicon at 157 nm, we use εw ) -3.38 + i 3.83 and εSi ) -3.70 + i 2.2, respectively.24 The STM junction is represented by a 3859

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silicon surface separated by a gap of variable size d from the parabolic tungsten tip with ending radius of curvature R and height h of typically 1.9λ ) 298 nm (Figure 2f). Calculations performed with different h showed that, for h > λ, the fields in the junction do not depend on the particular choice of the STM tip height.19 It is important that the chemical etching method used for the STM tip preparation is precise and results in STM tip apex radii ranging from 10 to 80 nm (see Supporting Information for further details). Thus, various STM tips have been tested. Yet, all of them could not induce efficient hydrogen desorption under VUV irradiation. The experimental results presented here concern three tested STM tips that could efficientlyinducephotodesorption.Asfollowsfromournumerical study of the optical-field confinement, only STM tips with relative small R values lead to the noticeable amplification of the optical fields in the junction. Thus, selecting STM tips inducing efficient photodesorption corresponds, in fact, to the selection of STM tips with relatively small R values. In order to illustrate this point and to analyze the tip-surface distance dependence of our experimental data, we have performed calculations for STM tip radii R ) 10, 20, and 40 nm. The calculated profiles of the optical field at the Si(100):H surface look very similar for the different tips with the absolute value of the optical-field increasing with decreasing tip radius. In Figure 2 we show the detailed results for R ) 20 nm tip that matches best with experimental data. In Figure 2g, we have plotted the large scale (x, y) plane image (628 × 628 nm2) of the calculated component of the electric field |Ez|2, perpendicular to the surface, for a unitary incident electric field (R ) 20 nm, d ) 2.5 nm). The electric field distribution of |Ez|2 at the silicon surface shows a double scale structure: On a large scale (Figure 2g), we can see a stationary distribution of ripples with |Ez|2 < 2.5 with large spatial periods (∼80 nm) while in the lower half-figure a shadow is produced by the STM tip.25 On a much smaller scale, just beneath the STM tip apex, a spot is formed with higher optical field (|Ez|2 ) 7.5). Panels h-l of Figure 2 show the detailed optical-field distribution (x, y) plane, at small scale (30 × 30 nm2), for various tip-surface distances d and for R ) 20 nm. Panels m-q of Figure 2 complete the information R ) 20 nm on the structure of the fields and present |Ez|2 in the (x, z) incident plane. For the largest calculated STM tip-surface separation (d ) 10 nm), the optical-field remains weak at the Si(100):H surface (Figure 2h) while it is slightly increased in the immediate vicinity of the STM tip because of the lighting rod effect (Figure 2m). As the STM tip-surface separation decreases, the optical field turns out to be strongly enhanced at the silicon surface forming an ellipsoidal spot (Figure 2l). For d ) 0.7 nm, the calculated enhancement F, defined as the maximum value |EzMax|2 at the silicon surface, is 45 for R ) 40 nm, is 120 for R ) 20 nm, and can reach 150 for R ) 10 nm (not shown in the figure). Note that the blossom shape of the optical-field © 2010 American Chemical Society

distribution for the small d values (Figures 2l) arises from the log scale representation. Contrary to earlier reported data at visible and infrared frequencies, we emphasize here that the observed pronounced optical-field enhancement at the surface is not due to plasmon resonance.19,26,27 Rather, this is a consequence of the STM tip lighting rod effect and the strong near field coupling between the STM tip and the silicon surface.28,32 At the present wavelength, the dielectric properties of the silicon surface are very close to a metal. For low dielectric surface, i.e., in the absence of the near-field coupling, one expects much weaker field enhancement as the STM tip approaches the surface.32,36 The strong near-field coupling between the STM tip and the surface is confirmed by plotting the variation of the calculated optical-field enhancement F at the Si(100):H surface (z ) 0) for various STM tip radii (R ) 10, 20, and 40 nm, see Figure 3a) as a function of the tip-surface distance d. Indeed, it follows from the results presented in Figure 3a that the optical-field enhancement is sharply increasing for decreasing tip-surface distances. Thus, it can be precisely tuned for a very small range of d values. The much lower field values attained with R ) 40 nm STM tip are consistent with experimental observation in which some of the tested STM tips could hardly induce the photodesorption. We then attribute the low efficiency of these STM tips to their relatively large radius of curvature. In our experiments the dehydrogenated spot areas are increasing when the tip-surface distance decreases. This is arising from the dehydrogenation process that is linked to a threshold behavior where desorption occurs with probability close to one only at locations where a given optical-field threshold (i.e., a given number of photons per Si-H bond) is reached owing to the field enhancement in the junction. For higher enhancement, i.e., for shorter tip-surface distances, the optical-field threshold is reached at the silicon surface for larger areas (see Figure 2). In (Figure 3b) we show that the shape and the area of the experimentally measured dehydrogenated spot can be correctly reproduced theoretically for R ) 20 nm and d ) 0.7 nm when |Ez|2 ) |Eth|2 ) 32, threshold value for which hydrogen desorption is assumed. Considering the selectively tested STM tips, it is thus reliable to compare our experimental results with the calculation performed at R ) 20 nm. The robustness of our results is thus demonstrated by repeating the same experiments with several different tungsten tips and silicon samples. The subsequent variations of the dehydrogenated spot sizes as well as the corresponding shift δs between the STM tip position (blue dots in Figure 3b) and the center of the dehydrogenated spot, i.e., the center of the ellipse, are reported in Figure 3c for various relative tip-surface distances ∆z. Results are averaged over three tested STM tips inducing efficient photodesorption. Slight variations in the observed sizes of the dehydrogenated areas or apex shifts can be observed and attributed to the 3860

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FIGURE 3. (a) Calculated variation of G as a function of the STM tip-surface distance d for three different STM tips radii R ) 10, 20, and 40 nm (circles and triangles). (b, left) 13 × 13 nm2 STM topography (Vs ) -1.7 V, I ) 70 pA) on the Si(100):H surface following 3400 VUV laser shots at 20 Hz, Vs ) 0, and ∆z ) -7 Å showing the elliptic shape of a dehydrogenated spot and the shift δs between the STM tip apex (blue dot) and the center of the ellipse. (b, right) 19 × 19 nm2 (x, y) section of the optical-field distribution calculated at the Si(100):H surface (R ) 20 nm, d ) 0.7 nm) and the ensuing shift δs. The white ellipse represents the area where the optical field is higher or equal to the chosen threshold |Eth|2 ) 32. (c) Variation of the measured dehydrogenated spot areas (results are averaged over three tested tips) as a function of the variation of the relative STM tip-surface distance ∆z (circles). The resulting averaged distance δs between the STM tip apex position and the center of the dehydrogenated zone is plotted as well (squares). The blue curve is to guide the eyes. (d) Variation of the calculated spot areas as a function of the tip-surface distance d for R ) 20 nm and two arbitrary optical-field threshold values |Eth|2 ) 30 and 40. The corresponding δs values are also reported for d ) 0.7, 0.9, and 1.25 nm and for |Eth|2 ) 40. They are similar for the two considered thresholds.

estimated to be 12 ( 1 nm2/Å. In Figure 3d, we depict the variation of the calculated spot area for R ) 20 nm and for two optical-field threshold values. In both cases, the calculated spot areas are increasing with decreasing d with an average slope of 5 ( 0.1 nm2/Å, i.e., of the same order of magnitude as the measured one shown in Figure 3c. Taking into account uncertainties in the absolute value of the experimental tip-surface distance, the role of the exact shape of the STM tip, as well as the approximate description of the junction with macroscopic dielectric constants, we consider that the correspondence between calculated and measured data is quite satisfactory. In conclusion, this work represents an original experimental and theoretical study of the control of a photoinduced surface chemical reaction at the nanoscale. This is realized by mapping the strength distribution of a confined optical-field when a VUV laser irradiates the junction of a low temperature STM. We intend to show that the optical-field enhancement can be tuned precisely by controlling, at low temperature, the tip-surface distance. The calculated size of the enhanced optical-field spot, its elliptic shape, as well as its variation with tip-surface distance closely matches

STM tip apex radius or shape variations, conferring acceptable reproducibility to our measurements. Comparison between measured and calculated data requires the absolute value of the initial tip surface distance in experiment which is delicate to determine exactly. However, I-Z measurements allow one to estimate the initial tip-surface distance as ∼10-15 Å37 (see Supporting Information). On the basis of this assessment, it appears that our mapping method can tune 75% of the whole range of the calculated enhancement variations (gray rectangle in Figure 3a). We now turn to the detailed comparison between measured (Figure 3c) and calculated data (Figure 3d). Both sets of results show the displacement of the spot δs in the direction of the laser beam propagation. For the STM tip approaching the surface, δs is decreasing nearly linearly with tip-surface distance, with similar measured and calculated slope in the range -4 Å/Å to -6 Å/Å. As to the measured dehydrogenated spot area, it increases with the STM tip approaching the surface. For ∆z varying in the range -4 to -8 Å the measured dehydrogenated spot area increases nearly linearly with decreasing tip-surface distance. The ensuing slope (see dotted green curve in Figure 3c) can be © 2010 American Chemical Society

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experimental observations. The present approach can be extended to other reactive surfaces.38 Moreover, it will open up new perspectives in the control of surface patterning at very small scales39 or nonlinear optical processes40 but will also inspire innovative methods and strategies in application of scanning probe microscopes for the understanding and optical control of the dynamics of individual molecular architectures.41 Acknowledgment. This work was supported by the European Integrated project PicoInside (Contract No. FGP015847). D.R. would like to thank Drs. M. C. Castex and S. Chenais from Laboratoire de Physique des Lasers, Universite´ Paris Nord, Institut Galile´e, 93430 Villetaneuse, France, for the kind loan of a very pure MgF2 Pellin-Brocca prism used in these experiments. D.R. is grateful to Dr. Costel Subran, Director of the OLI Company for the loan of an Excimer laser during preliminary experiments. Supporting Information Available. Description of experimental and theoretical methods and figures showing the control of the STM tip height method, the |Ez|2 component of the VUV optical-field for s-polarized incident beam, and variation of the dehydrogenated spot size as a function of the enhancement F and its numerical adjustment method as a function of the tip-surface are available. This material is available free of charge via the Internet at http://pubs. acs.org. REFERENCES AND NOTES (1)

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