NANO LETTERS
Imaging Molecular Orbitals by Scanning Tunneling Microscopy on a Passivated Semiconductor
2009 Vol. 9, No. 1 144-147
Amandine Bellec,*,† Francisco Ample,‡ Damien Riedel,† Ge´rald Dujardin,† and Christian Joachim‡ Laboratoire de Photophysique Mole´culaire, CNRS, Bat. 210, UniVersite´ Paris Sud, 91405 Orsay, France, and CEMES-CNRS, 29 rue J. MarVig, P.O. Box 4347, F-31055 Toulouse Cedex, France Received September 4, 2008; Revised Manuscript Received November 24, 2008
ABSTRACT Decoupling the electronic properties of a molecule from a substrate is of crucial importance for the development of single-molecule electronics. This is achieved here by adsorbing pentacene molecules at low temperature on a hydrogenated Si(100) surface (12 K). The low temperature (5 K) scanning tunneling microscope (STM) topography of the single pentacene molecule at the energy of the highest occupied molecular orbital (HOMO) tunnel resonance clearly resembles the native HOMO of the free molecule. The negligible electronic coupling between the molecule and the substrate is confirmed by theoretical STM topography and diffusion barrier energy calculations.
Understanding the electronic properties of individual molecules on a surface is of crucial importance for the development of molecular electronics devices.1 The scanning tunneling microscope (STM) is one of the sole powerful techniques to investigate these properties and help getting such a comprehension. As the STM requires a tunneling current to stabilize the tip apex over a surface, a large majority of STM images of individual molecules were recorded on metal2,3 or semiconductor surfaces.4-6 However, as demonstrated in recent works,7-10 a molecule can be electronically decoupled from a metallic surface by inserting an ultrathin insulating layer. At bias voltages corresponding to negative and positive ion resonances, STM topographies of at least the highest occupied and the lowest unoccupied molecular orbitals (HOMO and LUMO, respectively) of the adsorbed molecule on such ultrathin layer have been recorded.7,8 They have been shown to resemble the monoelectronic orbitals of the corresponding free molecule with a little deformation due to the tip-electronic interaction with these orbitals. In this article, we show that molecular orbitals can also be decoupled from the surface of a semiconductor. This is obtained by the hydrogenation of a clean silicon surface, which results in the passivation of its surface states. In the present work, the sole presence of a single layer of H atoms which passivate the silicon surface states allows the same * Corresponding author,
[email protected]. † Laboratoire de Photophysique Mole´culaire, CNRS, Bat. 210, Universite´ Paris Sud. ‡ CEMES-CNRS. 10.1021/nl802688g CCC: $40.75 Published on Web 12/17/2008
2009 American Chemical Society
insulating efficiency as observed with composite epitaxial growth on metal surfaces. It confirms our previous theoretical work11 by means of the ESQC12 (electron scattering quantum chemistry) technique in which it is shown that the HOMO of the pentacene molecule could be imaged by STM on a passivated H-terminated Si(100) surface. However, contrary to the case of NaCl on copper surfaces, the LUMO of the pentacene cannot be imaged on the Si(100):H surface because its energy is located inside the semiconductor surface energy band gap. First, we describe the experimental and computational details used in this work. Then the experimental results are discussed together with theoretical calculations. Experiments are performed using a LT-STM (Createc) at 5 K under ultrahigh vacuum (UHV) conditions.13 Si(100) samples are As-doped (n-type), with a conductivity of 5 mΩ·cm. After preparing a clean Si(100)-2×1 reconstructed surface under UHV (base pressure 1 × 10-10 Torr),13 hydrogenation of the Si(100) surface is performed as previously reported with the sample kept at 650 K.14,15 This preparation enables us to have a 2×1 reconstruction of the hydrogenated Si(100) surface.16 After hydrogenation, the pentacene molecule is evaporated at 430 K using an effusion cell (MBE Komponeneten evaporator with a pyrolytic BN cell). The sublimation temperature of the pentacene was adjusted by using a quartz microbalance, placed 10 cm from the crucible, so that a rate of 0.6 Å·min-1 is reached under a pressure of 5 × 10-10 Torr. Three different coverages of approximately 0.02, 0.15, and 0.50 monolayers (ML) have been used. For two of them, the substrate temperature is kept
at 12 K during the evaporation, while the third one is performed at 300 K in order to help the diffusion of the pentacene molecule on the surface. The conformation of the pentacene molecule on the Si(100):H surface was obtained by the semiempirical atom superposition and electronic delocalization molecular orbital (ASED-MO) approach completed with a description of van der Waals forces, ASED+.17 In the ASED-MO approach, the Slater exponents of the valence orbitals and the corresponding ionization potentials (IP) are introduced as input data. For the van der Waals interaction between the surface hydrogen atoms and the molecule, a pairwise potential function was added to the standard ASED potential. For large surfaces, this calculation technique offers a very important computing time reduction associated with the possibility to adjust the interaction parameters using DFT techniques. A detailed description of the method and the parameters used in this work is given in ref 11. The passivated H-terminated silicon surface was described by five layers of silicon atoms with 48 atoms per layer. When introducing the step, one Si and one H additional layers were added respecting the experimental crystallographic orientation. The silicon atoms at the edge of the slab are all hydrogen terminated. The relaxed Si-Si dimer bond length for the passivated surface is 2.52 Å, and the Si-H bond distance on a dimer is 1.50 Å. To optimize the structures, the forces on the atoms are calculated to reach a convergence for a threshold of 0.01 eV/Å. To compute the STM images, we used the ESQC12 technique, based (i) on the transfer matrix approach for the scattering problem, (ii) on the effective Hamiltonian technique to describe the surface molecule-tip interaction in this scattering problem, and (iii) on the extended Huckel (EHMO) semiempirical approach to compute the molecular orbitals of this junction and of the periodic parts of the surface and tip bulks. Experimentally, for the different pentacene exposures studied, unusual rapid instabilities are observed in the tunnel current during imaging. We believe that they are due to the pentacene molecules diffusing on the hydrogenated Si surface. In front of this problem, we used a feedback current of the order of 10 pA, which is the lowest current affordable with our present LT-STM machine configuration. Another possibility to decrease the interaction between the tip and the molecule is to record the topography in a constant height mode. But, even in these conditions, the observed molecules are very mobile on the surface and thus very difficult to be imaged. However, as shown in Figure 1, it is possible to observe some isolated pentacene molecules when they are near a step edge. Even so, the molecule can easily move along the step during the imaging (Figure 1a, translation, or Figure 1b, rotation). This indicates that the molecules are physisorbed and that the surface diffusion barrier is very low.11 The pentacene molecule is never observed to be adsorbed on the silicon dangling bonds that remain on the Si(100):H surface after its preparation because such a chemisorption will require at least two dangling bonds and a large deformation of the pentacene molecule.11 Nano Lett., Vol. 9, No. 1, 2009
Figure 1. STM topographies of the pentacene molecule on the Si(100):H surface. (a, b) Occupied states (VS ) -3 V; It ) 27 pA; 6.5 × 5.5 nm2) showing the unstability of a pentacene molecule: (a) translation along the step edge and (b) rotation. (c) Occupied states VS ) -3 V; It ) 27 pA; 4.9 × 2.7 nm2. (d) Unoccupied states VS ) 2.5 V; It ) 27 pA; 4.9 × 2.7 nm2.
Figure 2. (a) Line profile over the pentacene molecule topography shown in Figure 1c perpendicular to the step edge. (b) (dI/dV)/(I/ V) spectroscopy at the center of a pentacene molecule compared to the Si:H surface. In (a), a STM ESQC constant current scan over the pentacene molecule is superimposed optimizing the pentacene tilt angle according to the STM experimental scan (see also Figure 3a).
Figure 1 presents the STM topographies of a single pentacene molecule obtained for the occupied (Figure 1c) and unoccupied states (Figure 1d). On the STM topography of the occupied states (Figure 1c), the pentacene molecule appears as two rows of five lobes. This feature is consistent with the ESQC calculations of the HOMO of the pentacene molecule on Si(100):H as predicted in ref 11. But on the Si(100):H surface, the molecule is usually observed to sit astride a step edge as shown in Figure 1. A topographic profile performed perpendicularly to the step edge (Figure 2a) shows that the lobes of the molecule on the bottom terrace are slightly lower than the ones on the upper terrace. We have imaged the pentacene molecule at an energy higher than the bottom of the conduction band. But in the STM 145
Figure 3. (a) Calculated adsorption structure of pentacene molecule over a step edge of a passivated Si(100):H surface. (b-d) ESQC calculated constant current STM image of pentacene at the -1.7 eV HOMO tunnel resonance on (b) terrace, (c) at step, and (d) 1 eV up from the bottom of the Si conduction band on a step relative to the Fermi level. The structure in black lines corresponds to a pentacene skeleton and the hydrogenated Si dimers. Notice that the calculated scan presented in Figure 2a was calculated with a 10° less pentacene tilt angle as compared to that obtained in Figure 3a.
topography (Figure 1d), it was not possible to resolve any node of the unoccupied orbitals of the pentacene molecule on this Si(100):H surface. dI/dV spectroscopy curves acquired on the Si:H surface and on the pentacene molecule are shown in Figure 2b. The (dI/dV)/(I/V) curve on the Si:H surface presents a surface gap on the order of 1.5 eV as expected from the passivation of the Si(100) surface by the hydrogen atoms. In order to obtain sufficient signal, the tip-surface distance is much smaller when recording the dI/dV spectrum on the Si(100):H surface than on the pentacene molecule. The (dI/dV)/(I/V) curve on the pentacene molecule shows a unique peak at -2.5V, which can be assigned to the HOMO of the pentacene molecule. At positive surface voltage, the (dI/dV)/ (I/V) curve does not show any specific peak, which confirms that it is not possible to observe the LUMO of pentacene molecule. Theoretical calculations show that polyacene molecules are physisorbed on a fully H-terminated Si(100) surface.11,18 On a terrace, the calculated height of a pentacene molecule is 3.32 Å with respect to the H surface layer and 4.72 Å with respect to the first Si layer. At this height, there is no hybridization between the pentacene molecular orbitals and the Si surface electronic states preserving the electronic structure of the pentacene. Here we want to emphasize that the pentacene molecule is not anchored to the step edge as it is exempt of any chemical bond with the surface. Figure 3b shows the calculated STM image of the pentacene HOMO tunnel resonance when adsorbed on a terrace by the ESQC technique. This calculated image ressembles the native HOMO of the free pentacene molecule.7 Unfortunately, this configuration could not be observed experimentally. Indeed, the adsorption energy of a pentacene molecule on a Si(100):H surface computed with ASED+ is 0.11 eV. This 146
value is in agreement with the work of Tsetseris et al.18 using a DFT method completed by a pairwise dispersion term to describe the van der Waals interactions.19 The computed barrier for the diffusion of pentacene on a terrace of the Si(100):H surface with ASED+ is found to be 0.01 eV. This extremely small barrier produces a high mobility of the molecule as observed experimentally (Figure 1, panels a and b) inducing that the molecule can only be imaged on the step edge. The STM topography of the pentacene molecule observed on a step edge (Figure 1c) is very similar to the calculated image shown in Figure 3b. However, the experimental profile shown in Figure 2a reveals that one-half of the molecule overlaps the upper terrace. Therefore, we have performed the calculation of the minimum energy configuration of a pentacene molecule over the step edge (Figure 3a) with the corresponding ESQC image (Figure 3c). The good agreement between the experimental STM topography (Figure 1) and the HOMO orbital of the pentacene molecule calculated on a terrace (Figure 3b) and over a step edge (Figure 3c) demonstrates that the inherent electronic properties of the free molecule are preserved over a passivated Si(100):H surface. The LUMO image of the pentacene cannot be calculated on this surface by the ESQC technique since its energy is located in the semiconductor surface energy band gap11 where no propagative Bloch state is available to detect a tunneling current. However, the experimental STM topographies can be calculated for tunnel electrons taken at higher energy than the bottom of the conduction band. The calculated image (Figure 3d) obtained is similar to the experimental one (Figure 1d) but does not carry any information about a molecular orbital structure. In conclusion, imaging molecular orbitals by STM can be done on a passivated Si(100):H surface at low temperature. The STM image of a pentacene molecule at the energy of the HOMO tunnel resonance clearly shows the native HOMO of the free molecule. Polyacene molecules as pentacene are physisorbed on a fully H-terminated Si(100) surface. On this surface, the pentacene molecule is 4.72 and 3.32 Å from the Si and H layer, respectively. At this height, there is no effective mixing between molecular orbitals and the surface states. Therefore, the native electronic properties of the free molecule are preserved. The LUMO of the pentacene cannot be imaged because its energy is located in the semiconductor surface energy band gap. Due to the high mobility of the molecule on the surface even at 5 K, imaging orbitals of this molecule is a very difficult work. However, this may be improved in the future by using larger conjugated molecules having stronger van der Waals interactions or by decreasing the tunnel current in the junction down to range a few hundred femtoamperes. These experimental and theoretical results open up an original way to control the electronic decoupling of molecular architectures compared to composite insulting layer adsorbed on metals. This passivated surface allows coupling, on demand, the physisorbed molecule with the silicon surface states or atomic wires fabricated by locally desorbing hydrogen atoms with the STM tip. Nano Lett., Vol. 9, No. 1, 2009
Acknowledgment. This work is supported by the European Integrated project PicoInside (Contract No. FGP015847) and the ANR N3M (Contract No. ANR-05NANO020-01).
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