Phenyl Attachment to Si(001) via STM Manipulation of Acetophenone

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Phenyl Attachment to Si(001) via STM Manipulation of Acetophenone Steven Richard Schofield, Oliver Warschkow, Daniel Raymond Belcher, Kian Adam Rahnejat, Marian W. Radny, and Philip V. Smith J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp311261r • Publication Date (Web): 22 Feb 2013 Downloaded from http://pubs.acs.org on February 25, 2013

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The Journal of Physical Chemistry

Phenyl Attachment to Si(001) via STM Manipulation of Acetophenone Steven R. Schofield,∗,†,§ Oliver Warschkow,‡ Daniel R. Belcher,¶ K. Adam Rahnejat,†,§ Marian W. Radny,¶,k and Philip V. Smith¶ London Centre for Nanotechnology, University College London, London, WC1H 0AH, UK, Centre for Quantum Computation and Communication Technology, School of Physics, The University of Sydney, Sydney, NSW 2006, Australia, and School of Mathematical and Physical Sciences, The University of Newcastle, Callaghan, NSW 2308, Australia E-mail: [email protected]

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whom correspondence should be addressed for Nanotechnology, University College London, London, WC1H 0AH, UK ‡ Centre for Quantum Computation and Communication Technology, School of Physics, The University of Sydney, Sydney, NSW 2006, Australia ¶ School of Mathematical and Physical Sciences, The University of Newcastle, Callaghan, NSW 2308, Australia § Department of Physics & Astronomy, University College London, London, WC1E 6BT, UK k Institute of Physics, Poznan University of Technology, Poznan, Poland † London Centre

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Abstract The attachment of organic molecules to semiconductor surfaces and their measurement using scanning tunneling microscopy and spectroscopy (STM/STS) is considered to be a potential route toward conductance measurements of single molecules with known structural and electronic configuration. Here, we investigate a model system—acetophenone on Si(001)— and demonstrate that this adsorbate can be manipulated using the STM such that it adopts a configuration where it stands upright with a free-standing phenyl ring and strong Si-O linkage to the substrate. For the structural identification we combine STM imaging with density functional theory (DFT) calculations to describe the adsorbate structures that were observed in our experiments and the reaction pathways that link them; of these the upright configuration is the most thermodynamically stable. The chemical structure of the upright configuration suggests that π -conjugation within the adsorbate extends to the silicon surface resulting in strong hybridisation of the molecular states with the substrate. This is supported by the absence of any significant features in STS curves recorded over the adsorbate. The structure of this adsorbate and its robust attachment to silicon makes it attractive for future STM/semiconductor-based molecular conductance measurements.

Keywords: Silicon; Molecular manipulation; Scanning tunneling microscopy; Molecular electronics

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Introduction The relentless miniaturisation of electronic components toward the atomic scale provides compelling motivation to seek out novel device architectures. Proposals to use organic molecules in electronic devices date back to the 1950s 1 and the concept of single molecule electronics was firmly established in 1974 with the seminal design of a single molecule current rectifier by Aviram and Ratner. 2 The power of this idea lies in its utilisation of modern synthetic chemistry to construct complex organic molecules as a means of providing access to a large range of potential device functionalities. Experimental techniques capable of measuring single molecule conductance include mechanical 3 and elecromigration 4 break junctions, and scanning tunneling microscopy/spectroscopy 5 (STM/STS). Notable measurement highlights include the observation of Coulomb blockade and Kondo scattering, 6,7 vibrational excitation, 8 and mode counting, 9 among others. 1 Notwithstanding these successes, an outstanding challenge is to measure the conductance of a single molecule with structurally defined linkage to the electrodes. This is important because carefully designed molecular quantum states count for little unless the atomic and electronic structure of the contacts and their effect on the molecular states are taken into account.

Figure 1: Single molecule junctions for molecular conductance measurements. (a) An organic molecule attached to a semiconductor surface and probed with a metal STM tip. The attachment of the molecule to the surface is via a contact group that determines the structural and electronic properties of the molecule-semiconductor interface. (b) 1,4-benzenedithiol attached to two gold electrodes, similar to that found in Ref. 10 A potential route to this goal is to construct single molecule junctions by controllably adsorbing organic molecules to semiconductor surfaces and probing them with STM (1a). The method is conceptually simple: a STM is used to image an isolated adsorbed molecule and identify its 3



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structure and bonding to the surface in combination with complementary techniques such as density functional theory (DFT). If required, the STM can be used to manipulate an adsorbate into a desired configuration. The STM tip is then positioned at a fixed point in space directly above an isolated adsorbate and the current through the junction is recorded as a function of the applied bias between the tip and substrate. This method can allow the measurement of molecular conductance for junctions where the configuration of the molecule and its contact to one of the electrodes (the substrate) is known precisely. Care can be taken to ensure that the STM tip, i.e., the second electrode, is as uniform and reproducible as possible (e.g., see Methods), and the coupling between the STM tip and the adsorbate can be continuously varied from tunneling to point-contact by variation of the tip-adsorbate separation. 11 The attachment of an organic molecule to a surface is facilitated by a reactive functional group, which we call the contact group (1a); this can be compared to the often employed gold-thiol linkage used in break juction techniques (1b). In order for a particular adsorbate to be useful for molecular conductance measurements this contact group should form strong covalent bonds with the surface such that: (1) the adsorbate adopts a desirable structural conformation, (2) the adsorbate is stable within a requisite range of applied bias and current, and (3) the electronic coupling between the substrate and the adsorbate is appropriate for the type of conductance measurement that is sought; e.g., to measure intrinsic molecular conductance it is desirable to have strong electronic coupling, 9 while weak electronic coupling is required to measure resonant molecular tunneling. 12,13 Here, we provide an example of the manipulation of a molecular adsorbate (acetophenone) on a silicon surface into a thermodynamically stable configuration that satisfies the above criteria of surface attachment for conductance measurements: its phenyl ring is vertically upright and the acetyl group of the molecule forms an effective coupling group producing strong electronic coupling to the substrate. The Si(001) surface was chosen because its properties are well known, 14 it is ubiquitous in the semiconductor industry, and there is a long history of the investigation of organic molecules on this surface; e.g., adsorption via alkene; 15,16 halide; 17 carbonyl; 18 sulfide; 19 amine; 20 nitrile; 21 acetyl; 22,23 and carboxyl 24 functional groups has been studied with STM. We 4


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also note the potential to develop hybrid semiconductor-organic technologies 25 beyond singlemolecule devices, e.g., sensor, energy conversion and light emission technologies.

Methods The STM experiments were performed at room temperature and under ultra-high vacuum (< 5 × 10−11 mbar) using an Omicron GmbH variable temperature scanning tunneling microscope (VTSTM). Silicon samples were cleaved from (001) orientated Antimony doped (0.08 − 0.1 Ωcm −1 ; Virginia Semiconductor) and prepared by degassing at 600◦ C overnight followed by flash annealing to 1200◦ C and controlled cooling to room temperature. 26 Acetophenone molecules were deposited from vapor using an UHV precision leak valve following preparation using several freezepump-thaw cycles and purity checking with residual gas analysis in vacuum. STM biases are quoted as the bias applied to the sample. STM tips were prepared by electrochemically etching 0.25 mm diameter tungsten wire in 3 M potassium hydroxide (KOH). Tips were visually inspected, rinsed in deionised water and loaded into UHV where they were degassed overnight at 150◦ C. Individual tips were then positioned within 1 to 2 mm of a tungsten filament and electron bombardment annealed by applying a 400 V bias to the STM tip and increasing the filament current until a ∼ 50 µ A emission current was reached. This was maintained for 5 minutes. The filament was then grounded and the tip field emitted by applying voltage sweeps of 0 to −1 kV to the tip. Field emission current sweeps were recorded and repeated until no discernable differences occurred between successive sweeps, indicating that a stable tip had been reached. The tips were then loaded into the STM for measurement. Tips prepared in this way exhibited a high probability for atomic resolution imaging and reproducible spectroscopy upon approach to clean Si(001) surfaces. Individual adsorbates were manipulated using the following manipulation pulse procedure. We imaged an isolated adsorbate and then positioned the STM tip near the centre of its brightly imaging allyl orbital. We then switched the STM feedback loop off so that the tip would remain

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at a fixed point in space while we ramped the bias voltage from 0 to −3 V. A sharp discontinuity in the measured current during the voltage sweep indicated a change in the configuration of the adsorbate, which in almost all cases occurred between −2.2 and −2.8 V bias, in agreement with previous observations of STM induced Si-C bond breaking in π -conjugated organic molecules attached to Si(001). 27 Density functional theory (DFT) calculations were carried out using the Gaussian 03 software 28 and a combination of a small 3-dimer Si20 H21 and a larger 5-dimer Si65 H52 cluster representation of the Si(001) surface. Single-point energy calculations were performed using the 3-dimer cluster, the hybrid exact-exchange B3LYP functional, and a large 6-311G(2df,2pd) basis set with additional ‘++’ diffuse functions placed on the atoms of the adsorbate molecule and the three Si-Si dimers. Structure optimisations and vibrational frequency calculations were conducted using a more compact basis set, namely 6-311++G(d,p) for the adsorbate and Si-Si dimer atoms, 6-311G(d,p) for second layer (i.e. subsurface) silicon atoms, and LANL2DZ for all other atoms. The larger 5-dimer cluster and the generalised gradient approximation (PW91 functional) are used to calculate a cluster size correction which accounts for the effects of a longer and thicker cluster (see Ref. 23 for details). All energies reported in the text include a harmonic vibrational zero point correction and are given relative to the sum of energies of a bare surface cluster and a gas-phase acetophenone molecule.

Results & Discussion 2a,b shows empty- and filled-state images of a Si(001) surface that has been lightly dosed with acetophenone at room temperature. These images, and those shown later in Figs. 3 and 4, are representative of a large set of images acquried with many STM tips and samples. The majority of the adsorbates appear as double-lobed protrusions with a characteristic nodal plane in the dimer row direction, labelled ‘AR’, which in all cases are accompanied by a secondary protrusion (in filled-state images), labelled ‘HH’. We attribute these features to the dimer-bridge allyl radical

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Figure 2: Si(001) lightly dosed with acetophenone and imaged at room temperature. (a) Emptyand (b) filled-state STM images (±1.5 V, 200 pA). The majority of adsorbates appear as doublelobed protrusions due to the formation of an allyl radical adsorbate structure (labelled AR; see text). A small minority of adsorbates appear as a single protrusion of lesser intensity; an example is shown in the insets to panels a and b. (c,d) Top and side views of the calculated spin density (isosurface 0.024 e/Å3 ) that corresponds to the two half-occupied molecular orbitals of the allyl radical (AR) and the hemihydride (HH). (e) A textbook style linear combination of atomic orbitals picture for three p-orbitals, which provides physical insight into the allyl radical orbital structure. (f) Filled-state image (−1.5 V, 200 pA), rendered in three dimensions for clarity, of two allyl radical adsorbates that differ in the position of the hemihydride dimer produced by the dissociated hydrogen: the left adsorbate has its dissociated hydrogen on the neighboring row, while the right adsorbate has its dissociated hydrogen on the same row. (g-j) Reaction pathway from the molecular adsorbed, dative configuration to the dimer bridge allyl radical that is observed as a kinetically stable intermediate at room temperature. structure shown in 2c,d,j. In this configuration, the acetophenone acetyl group forms a bidentate linkage with one silicon dimer, while carbon atoms 2 and 6 of the phenyl ring form covalent bonds with an adjacent dimer. This assignment is compelling for several reasons: the allyl radical arises as a simple, yet interesting variation of the known pathways of the chemically analogous molecules 7



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acetone 23 and actaldehyde 29 on Si(001); its structure provides a straightforward explanation of the observed appearance; and its bonding and energetics explains STM manipulation experiments that turn the allyl radical into new configurations as we describe below. The reaction proceeds as illustrated in 2g–j. The initial attachment to the surface is via a dative bond between the carbonyl oxygen atom and a down-buckled dimer atom. This dative configuration (2g) is rapidly stabilised into an enolate (2h) by loss of a hydrogen atom, creating a nearby hemihydride dimer. The enolate in turn stabilises by forming a Si-C bond between its methylene (=CH2 ) carbon atom and a surface silicon atom, producing the intermediate structure in 2i. Up to this point the reaction is entirely analogous to the acetone 23 and acetaldehyde 29 dissociation pathway. However, the next step is unique for acetophenone as it involves the phenyl ring, which the smaller molecules lack, forming two new covalent bonds to an adjacent dimer to produce the allyl radical structure described above and shown in 2c,d,j. As such, this allyl radical structure is a kinetically favored diversion from the main acetone/acetaldehyde path. The observation of these allyl radical structures at room temperature suggests that the energy barriers for onwards conversion are larger than the available thermal energy; this is consistent with our DFT calculations that show a large formation energy for this structure (−2.44 eV). The double-lobe appearance of this structure in the STM images results from the half-occupied molecular orbital of the allyl radical, as is evident in the calculated spin density shown in 2c,d. Physical insight into the structure of this orbital can be gained through a linear combination of atomic orbitals description: 30 the three carbon atoms at the vertex of the phenyl ring are not involved in bonding to the surface and remain sp2 -hybridised. The combination of their half-filled p-orbitals produces molecular orbitals as illustrated in 2e. The three available electrons fill the bonding orbital and half-fill the non-bonding orbital. Tunneling into or out of the latter produces the double lobe appearance in the STM images as this orbital exhibits density maxima on the outer two carbon atoms and a nodal plane going through the central carbon atom. The secondary protrusion associated with each allyl-radical feature (‘HH’ in 2a,b) originates from the dangling bond of the hemihydride. 31 The spin density of this dangling bond orbital is 8


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shown together with the allyl radical spin density in 2c,d; the result is an excellent match to the appearance of the features in the STM images. Furthermore, hemihydride dimers are known to induce static dimer pinning, 14,32 producing a slightly zig-zagged appearance to the dimer row in their immediate vicinity, and this is also evident in our STM data (2a,b). The hemihydride dimer of the allyl radical most often occurs in the same row as the adsorbate; however, occasionally the hydrogen atom is dissociated to the neighboring dimer row, as seen in 2f. The characteristic appearance of this hemihydride combined with its occasional observation on the neighboring dimer row provides a neat visual confirmation of our proposed reaction pathway and confirms the assignment of the features seen in the STM images to the allyl-radical structure described above. We have manipulated individual adsorbates using a voltage pulse procedure that is described in the Methods section. 3 shows an interesting example where we change the orientation of an allylradical adsorbate on the surface. In 3a the adsorbate is orientated such that the carbon ring of the allyl-radical adsorbate lies on the left of the image. By the application of the manipulation pulse we were able to “flip” the adsorbate into a mirror image configuration where the ring is orientated to the right (3c). We applied a manipulation pulse to this adsorbate four times and achieved a flip of the adsorbate on each attempt. We also observed spontaneous flipping of the adsorbate subsequent to a manipulation pulse where it reversed its orientation a few seconds after the initial flipping. An example of this is shown in 3b: here, the manipulation pulse was applied half way through the acquisition of the image (note that the slow scan direction runs diagonally; see caption) and induced the adsorbate to change from left to right orientation. The adsorbate was imaged in the right orientation for several scan lines before the adsorbate spontaneously flipped back to its original orientation. Such spontaneous orientation changes were only observed subsequent to the application of a manipulation pulse and the adsorbates were otherwise stable when imaged at −1.8 V and 100 pA. These observations suggest a lowering of the effective barrier for flipping that persists for a short time after the application of the manipulation pulse, possibly due to local heating by inelastic tunneling. A new structure, shown in 3d, was formed when a fifth manipulation pulse was applied to the 9



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Figure 3: Manipulation of the orientation of an allyl-radical adsorbate using STM. (a-d) A dimerbridge allyl radical is induced to change configuration on the surface through the application of STM tip voltage pulses. Images are filled state (−1.8 V, 100 pA; note these parameters also determine the tip height during the manipulation pulse). The images have been rotated such that the slow scan direction in the images as presented in the figure is diagonal from bottom left to top right. The location of the hemihydride (HH) and a dimer vacancy (DV) defect are highlighted as a reference for the adsorbate motion. The fixed position of the Si-O bond is indicated by a white triangle in the STM images and the neighboring chemical schematics. A slight “double-tip” imaging artifact can be seen above the brightly imaging allyl lobes in panels a and c. The images are presented as raw data without any correction for subtle distortions due to thermal drift. (e-i) DFT calculated structures and formation energies that describe the nature of the changes observed in the STM data: the adsorbate is induced to flip and rotate about its stable Si-O bond (see text). Abbreviations DB and EB are for ‘dimer bridge’ and ‘end bridge’, respectively. allyl radical shown in 3a-c. This new structure has a double lobed protrusion that has its nodal plane at 90◦ to that of the nodal plane of the original feature. We will show below that this is a rotated allyl radical that we call the end-bridge allyl radical, where the acetyl group bonds across two adjacent dimers of the same dimer row. These rotated structures also formed randomly from dimer-bridge allyl-radical structures when the surface was imaged continuously with filled-state sample biases of magnitude 2.2 V or greater; however, the reverse transition from an end-bridge to a dimer-bridge allyl radical was never observed. This indicates that the end-bridge allyl radical has a greater

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thermodynamic stability than the dimer-bridge allyl, but that a kinetic energy barrier between the two structures limits the formation of the end-bridge allyl upon deposition at room temperature; similar behaviour has been reported for other adsorbates on Si(001), see e.g.,Refs. 22,23 We have elucidated the nature of the configuration changes shown in 3 using DFT. The structural models of the left and right orientated allyl radical (3e,g) show that a necessary requirement for the left to right flipping observed in experiment (3a-c) is that the two Si-C bonds between the phenyl ring and the surface are broken; this will transiently produce the dimer-bridge intermediate structure (3f), which can relax to produce either the left- or right-orientated dimer-bridge allyl radical. A similar situation exists if all three Si-C bonds of the dimer-bridge allyl radical are broken to produce the enolate intermediate shown in 3h. However, this structure presents additional pathways for relaxation because it introduces a rotational degree of freedom about its Si-O bond. In particular, it opens the possibility of the adsorbate reforming in a configuration chemically similar to the dimer-bridge allyl radical but rotated 90◦ with respect to the dimer rows to produce the endbridge allyl radical structure shown in 3i. It is this structure that we observe in the data shown in 3d. The end-bridge allyl radical was found to be 0.15 eV more stable than the dimer-bridge allyl radical, confirming its increased thermodynamic stability as observed in our experimental data. We note that the left to right flipping seen in 3a-c requires that the hemihydride is formed on the adjacent row, which occurs in only a minority of cases; in contrast, the 90 ◦ rotated end-bridge allyl radical is able to form regardless of the position of the dissociated H. STM manipulation of dimer-bridge allyl radicals sometimes led to another structure, which we call the tall feature (labelled ‘TF’ in 4) due to its large height profile. An example of the formation of this tall feature through the application of a manipulation pulse is shown in 4a-c. The increased height of the tall feature compared to the allyl radical is illustrated in the perspective view and line profiles shown in 4g and 4i, respectively. We assign this tall feature to an upright standing adsorbate whose phenyl ring is not directly bound to the substate. This assignment is based on the exceptionally large height profile of this feature (at 2.3 Å, as shown in 4i, the tall feature is a full angstrom taller than the Si(001) step height), and by analogy with the reaction of acetone 23 and 11



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Figure 4: An allyl-radical (AR) adsorbate is converted into a tall feature (TF) through STM manipulation. (a–c) Filled-state images before, during and after the conversion; the manipulation pulse was applied halfway through the acquisition of the image shown in (b). The slow scan directions for each image are from bottom to top for (a) and (c), and from top to bottom for (b). Image parameters for all images: −1.8 V and 100 pA. A slight “double-tip” imaging artifact can be seen to the lower left of the brightly imaging allyl lobes and tall feature in panels a–c. (d-f) Calculated chemical structural models and formation energies indicating the transition sequence that we assign to the image data. (g) Three dimensional rendering of a filled-state STM image (−2 V, 100 pA) showing a dimer-bridge allyl radical and tall feature side by side that provides a visual indication of the height difference between the two features. (h) Tunneling spectroscopy measurement taken above a tall feature and the clean Si(001) surface; regulation conditions were −1.6 V and 100 pA . (i) Line profiles taken across the allyl radical and tall feature in panel (g). acetaldehyde 29 on Si(001). The three molecules differ only in the residue R of the acetyl group, with R being phenyl in acetophenone; methyl (−CH3 ) in acetone; and hydrogen in acetaldehyde. We have previously shown that the two smaller molecules become locked into an upright orientation such that their acetyl-residues are not attached to the Si(001) surface when a second hydrogen atom is transferred to the surface 22,23,29 and we attribute the tall feature to the analogous structure 12


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for acetophenone. The reaction sequence is indicated in 4d-f. As with the flipping transitions discussed in 3, the reaction requires that the Si-C bonds between the phenyl ring and the surface are broken (4e). The new feature is formed when a hydrogen atom is transferred from the methylene group (−CH2 −) of this structure to a surface silicon atom, as shown in 4f. Overall, the transition is driven by a significant increase in thermodynamic stability (calculated DFT reaction energy ∆E = −1.19 eV from the dimer-bridge allyl radical to the tall feature); in fact, preliminary experiments where the sample was annealed for short periods at temperatures < 200 ◦ C suggest that mild thermal annealing increases the number of tall features on the surface relative to allyl structures. While this suggests agreement with our calculated formation energies for these structures, further experiments are required before this can be stated definitively. The large aspect ratio of this feature precludes the direct observation of the location of the second dissociated hydrogen atom since the protrusion of the adsorbate is effectively “sharper” than the imaging apex of the STM tip. However, in most cases we observe the disappearance of the zig-zagged dimer pinning that is associated with the hemihydride dimer, which provides indirect evidence that the second hydrogen atom attaches to the surface to form a monohydride dimer 32 as shown in 4f. In summary, the large height profile of these features, together with their calculated thermodynamic stability (4f), and the analogy with the established reaction pathways for acetaldehyde and acetone, provide compelling evidence that the tall feature corresponds to the upright standing configuration shown in 4f. A comparison of 1a and 4f reveals that the tall feature corresponds to our desired device geometry as outlined in the introduction. Furthermore, the strong Si-O bond and the high thermodynamic stability of the tall structure makes this adsorbate configuration quite stable under the application of biases and currents. Indeed, it is the strong Si-O bond that inhibits desorption of the adsorbate and facilitates the manipulations described in 3 and 4a-f. Underlying these transitions is that Si-C bonds are prone to break under STM manipulation, 27 while the much stronger Si-O bond is stable. This suggests that Si-O bonding is very attractive for the attachment of organic molecules to silicon for device applications and that further study of O based attachment of organic molecules to silicon is warranted. 13



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In order to gain insight into the electronic coupling between the adsorbate and the substrate we performed tunneling spectroscopy measurements with the tip positioned over the tall feature. We recorded the tunneling current as a function of the applied bias voltage at a fixed tip position and numerically differentiated the resulting data to obtain the differential conductance (dI/dV ) as a function of the applied bias; such traces are considered to be an approximate measurement of the local density of states at the tip position. 14 The result of the measurement above the tall feature is shown in 4h together with a measurement recorded over an adjacent area of clean Si(001) surface. We observe that the tall feature exhibits an apparent surface band gap that is ∼ 0.35 eV larger than the measured clean surface band gap of 0.6 eV. The tall feature also exhibits an increased differential conductivity at high bias magnitudes compared to the clean surface; however, it does not exhibit any significant structure within the bias range of our measurement. The increased surface band gap and the increased differential conductivity at high bias for the tall feature compared with the clean Si(001) surface suggests the removal of the surface π and π ∗ states due to the formation of Si-O, Si-C and Si-H bonds and their replacement with electronic states due to the adsorbate. The fact that the measured differential conductivity of the tall feature is monotonically increasing without any significant peaks or shoulders is an indication that strong electronic coupling exists between the π -conjugated system of the molecule with the electronic states of the substrate. This may at first seem surprising since the phenyl ring of the adsorbate is not directly bonded to the surface; however, upon closer examination we notice that π -conjugation within the molecule extends to the substrate (i.e., a π -bond exists within the contact group; 4f). Thus, strong hybridisation of the molecular π states with those of the substrate can be expected.

Conclusion In summary, we have described a method for constructing and measuring atomically precise singlemolecule junctions using organic molecules on semiconductor surfaces and scanning tunneling microscopy. The concept builds on established ideas for semiconductor-based single molecule

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electronics (e.g., Refs. 11,12,16,19 ) and extends them to incorporate the potential for controlling the electronic coupling of the adsorbate to the substrate via a contact group and STM-based molecular manipulation. We have provided a detailed analysis of a model system—acetophenone on Si(001)—using both STM/STS and DFT to describe the adsorbate structures and reaction pathways for this molecule/substrate combination. Through this we were able to demonstrate that this adsorbate can be made to stand upright on the surface through direct STM manipulation to form the desired device geometry where a π -conjugated molecular system is sandwiched between the substrate and the STM tip as contact electrodes. Measurement of the differential conductivity of the adsorbate in this upright configuration in the tip tunneling regime indicated that the molecular

π states are strongly hybridised with the substrate. This result is consistent with the elucidated chemical structure model that suggests that the π -conjugation within the adsorbate extends to the substrate surface, even though the phenyl ring itself is not in direct contact with the surface. We are currently performing detailed low temperature tunneling spectroscopy measurements of this and other organic adsorbates on the Si(001) surface.

Acknowledgements We acknowledge financial support from the Engineering and Physical Sciences Research Council (EP/H003991/1), the Australian Research Council (DP0557331). SRS, DRB, KAR, OW and MWR are respectively supported by an EPSRC Career Acceleration Fellowship, an Australian Postgraduate Award, an EPSRC Doctoral Training Grant, the ARC Centre of Excellence for Quantum Computation and Communication Technology (CE110001027), and the Polish MNiSW (DS 62-176/13).

References (1) Cuevas, J. C.; Scheer, E. Molecular electronics: an introduction to theory and experiment; World Scientific Publishing Co. Pte. Ltd., 2010. 15



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Figure 5: Table of contents graphic

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Phenyl Attachment to Si(001) via STM Manipulation of Acetophenone

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