Article pubs.acs.org/JPCC
Electronically Driven Single-Molecule Switch on Silicon Dangling Bonds Anja Nickel,† Thomas Lehmann,† Jörg Meyer,† Frank Eisenhut,† Robin Ohmann,†,# Dmitry A. Ryndyk,† Christian Joachim,‡,§ Francesca Moresco,*,† and Gianaurelio Cuniberti†,∥ †
Institute for Materials Science, Max Bergmann Center of Biomaterials, and Center for Advancing Electronics Dresden, TU Dresden, 01062 Dresden, Germany ‡ GNS & MANA Satellite, CEMES, CNRS, 29 rue J. Marvig, 31055 Toulouse Cedex, France § International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan ∥ Dresden Center for Computational Materials Science (DCMS), TU Dresden, 01062 Dresden, Germany ABSTRACT: We demonstrate that a single 4-acetylbiphenyl molecule adsorbed along the dimer row of a Si(100)-(2 × 1) surface can be reversibly switched between two stable conformations using the tunneling current of a scanning tunneling microscope. The experiment supported by density functional theory calculations demonstrates that the molecule by switching selectively passivates and depassivates a dangling-bond pair on the silicon surface, opening new routes for the logical input in dangling-bond-based atomic-scale circuits.
I. INTRODUCTION For more than 40 years, the possibility to replace solid-state transistors by molecular electronic devices in ultraminiaturized circuits has been actively explored.1,2 Serious limitations of this approach are the low conductance and low power gain of a molecular device,3,4 together with the problem of tunneling leakage currents between the devices and between the interconnections.5 An alternative approach is to develop an atomic-scale technology, where surface electronic circuits are constructed by manipulating atoms of a passivated semiconductor surface. In this case, the Boolean logical status of each device of the atomic-scale circuit can be ideally controlled by the position of a few atoms along a given conductive atomic wire.5,6 A possible route in such direction is to use dangling bond wires built on a Si(100)-(2 × 1)-H surface.7−9 Kawai et al. theoretically designed switches and logic gates using the dangling-bond (DB) dimer electronic states on Si(100)-(2 × 1)-H.5 Experimentally, Kolmer et al. recently realized a prototype of a planar Quantum Hamiltonian Computing (QHC) Boolean logic gate constructed on the atomic scale on the same surface.10 In such DB quantum circuits, molecular switches can provide the logical input to the surface logic gate by passivating and depassivating the DB dimer involved. An ideal candidate for this input function, which can be called a molecular latch, should consist of a molecular chemical group strongly bonded to the surface (the anchoring part of the molecular latch) and a second group weakly bonded to the DB dimer, allowing its electronic passivation or depassivation (the moving part of the molecular latch). This weak bonding needs © XXXX American Chemical Society
to be broken and should be able to reform nonthermally to perform the “0” and “1” logical input. The corresponding switching process has to be reversible and externally controllable. On metal surfaces, conformational changes and reversible switching of organic molecules are well investigated and can be induced by the tip of a scanning tunneling microscope (STM), either by mechanical manipulation11 or by applying voltage pulses at a specific location on the molecule.12,13 On the Si(111)-7 × 7 surface the dissociation and the switching of molecules have been demonstrated.14−17 On the Si(100) surface, some examples of manipulation and switching were reported, including the lateral rotation of a biphenyl molecule around one phenyl group,18 the molecular motion of 4methoxystyrene,19 and the cis−trans isomerization of an azobenzene-derivative by light irradiation.20 Also, the attachment of halogen atoms to silicon surface atoms by using halogenated molecules has been shown.21,22 Furthermore, a negative differential resistance has been observed for styrene on the surface due to configuration changes driven by inelastically scattered electrons.23 Recently, a single pentacene molecule was mechanically pushed on Si(100) to a stable conformation using the tip apex of an STM.24 The adsorption and STM tip-induced manipulation of acetophenone molecules were studied on the clean and passivated Si(100)-(2 × 1) surface,25−27 showing that a single acetophenone, strongly coupled to the Si substrate, can Received: June 6, 2016 Revised: November 7, 2016
A
DOI: 10.1021/acs.jpcc.6b05680 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Kohn−Sham energy. The Si(100) surface was modeled by a periodic slab of six layers. On the uppermost layer the dimer reconstruction lines are formed, while the three lowest layers are fixed during relaxation. In the adsorption process, one hydrogen is transferred from the acetyl group of the molecule to the surface.27 This oxidization is confirmed by our calculations to be energetically favorable. The remaining anchoring group CH2−CO binds to the silicon via one Si−O bond and one Si−C bond to the methylene, thus preventing diffusion of the molecule, as confirmed by molecular dynamics simulations. While several local minima exist, molecular dynamics and the nudged elastic band method (NEB)37 were applied to further explore the ground-state potential energy surface of the adsorbed ABP molecule. Herein the effect of the electric field on the geometry was found to be weak becauue the molecule is adsorbed perpendicular to the field with a very small extension in height relative to the surface dimers.
be manipulated through its phenyl ring assuming an up-right surface conformation.26 However, such switching process is not reversible, and the molecule remains in a stable adsorption conformation after manipulation. Considering also the possible surface leakage current between neighbor molecules, the phenyl switching part of the acetophenone molecule can be too small to be used as a reliable molecular latch for a logical input on a QHC Si(100)-(2 × 1)-H atomic-scale quantum circuit. A better candidate for a molecular latch on the Si(100) surface is the 4-acetylbiphenyl (ABP) molecule (Figure 1). One
III. RESULTS AND DISCUSSION In the STM images (Figure 2a) a single ABP molecule shows upon adsorption two lobes and is nearly aligned along the (2 ×
Figure 1. Molecular structure of the 4-acetylbiphenyl (ABP) molecule.
of its phenyl rings can provide a reversible bonding to a DB dimer, and the acetyl group is still expected to work as an anchoring group for the latch, similarly to acetophenone and other ketone molecules.25,26,28 ABP molecules were recently studied on the Au(111) surface and form supramolecular nanostructures that can be controllably moved by inelastic electrons and are able to transport single atoms.29,30 In this work, we present the electrically reversible switching of single ABP molecules on the Si(100)-(2 × 1) surface by STM. Density functional theory (DFT) calculations were performed to identify the ABP molecular conformations on the surface and the switching reaction path on its ground-state potential energy surface. We show that isolated ABP molecules anchor on the Si(100)-(2 × 1) surface by a Si−O covalent bond and that ABP can be switched between two stable conformations on the surface by inelastic tunneling current. In this way, the ABP molecule can selectively passivate and depassivate a DB dimer only two dimers away from its anchoring group on the same dimer line, acting as a flip-flop like molecular latch for performing a logical input on an atomicscale circuit.
Figure 2. STM image of an ABP molecule adsorbed on the Si(100) surface in the two possible conformations (a) II and (b) I. Line scans of the two conformations taken along the black and the violet lines are presented in (c). VSample = −2.6 V, I = 33 pA, image sizes = 5 nm × 3 nm. Switching at V = −4.0 V.
1) dimer rows. Other adsorbates and defects appear as single protrusions and depressions with different apparent sizes in the STM images, as commonly observed for Si(100).38 A second stable conformation showing a single lobe (Figure 2b and line scans in Figure 2c) can be observed only after switching the molecule by voltage pulses, as described in the following. For simplicity, the conformation with a single lobe is called “I” and the two-lobe initial adsorption conformation “II”. It is known from the work performed on acetophenone26 that the acetyl group of the molecule is strongly electronically coupled to the Si(100)-(2 × 1) surface by means of a dative adsorption followed by a proton shift. As a result, this molecule loses a proton (H+) during the adsorption and is then covalently bonded to the surface, while the detached H+ is transferred to a negatively polarized, up-buckled silicon atom.27 To induce the switching of the ABP molecule, voltage pulses with constant voltages between −2.6 and −4.0 V at a constant current of ∼50 pA were applied to the molecule using the STM tip. We observed a threshold voltage for switching the molecule from conformation II to I and back at −2.7 V. As a consequence, we could image the molecule in both conformations at a bias voltage of −2.6 V without switching it. Figure 3 shows a cycle of reproducible and reversible switching events between two stable conformations (II ↔ I) of
II. METHODS All experiments were performed in a custom-built low temperature STM (5 K) under ultrahigh vacuum conditions (
