J. Phys. Chem. C 2008, 112, 1493-1497
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Configuration Specific Desorption by Scanning Tunneling Microscope in Organic-Semiconductor Hybrid Systems Ansoon Kim,*,† Sung-Soo Bae,‡ Dae Sik Choi,‡ and Sehun Kim*,‡ Nano-Bio Electronic DeVices Team, Electronics and Telecommunications Research Institute (ETRI) and Department of Chemistry and School of Molecular Science (BK21), Korea AdVanced Institute of Science and Technology, Daejeon 305-701, Republic of Korea ReceiVed: July 16, 2007; In Final Form: October 29, 2007
Configuration-specific desorption of ethylene on a Ge(100) surface has been controlled at the nanoscale, induced using a scanning tunneling microscope (STM) tip at room temperature. Ethylene was found to adsorb in two distinct bonding geometries: (i) on top of a single Ge-Ge dimer (OT) and (ii) in a paired end-bridge between two neighboring Ge dimers within the same dimer row (PEB). Only OT configuration desorbs effectively at the sample bias voltages between -2.9 V and -3.1 V, tunneling current of 50 pA, and room temperature. The desorption yield for each configuration was measured as a function of sample bias voltages, where the voltage dependences of desorption yields show rapid increases between -2.9 V and -3.4 V for OT desorption, whereas between -3.2 V and -3.7 V, the PEB desorption increases rapidly. We have found that the applied sample bias voltages induce σGe-C hole-resonant inelastic tunneling, resulting in the dissociation of the Ge-C bonds. This selective, STM-induced desorption makes it possible to apply local control of surface reactions and to develop nanoscale lithography for molecular electronic devices.
1. Introduction The chemistry of molecule-semiconductor surface systems determines the fundamental and technological aspects that must be taken into account when developing new semiconductor devices that incorporate particular electronic properties, optical responses, and biological activity. Interest in the use of nanoscale lithographic techniques1-3 and surface reaction control4,5 in the fabrication of nanoscale molecular electronics has recently been growing. Scanning tunneling microscopy (STM) is crucial to nanoscale science because of its atomic scale resolution. The development of STM has made possible not only real-space imaging but also manipulations such as rotation, dissociation, desorption, and association of individual atoms and molecules at crystal surfaces. In particular, manipulation with atomic-scale precision enables the fabrication of atomic-scale structures and the probing of the chemical and physical properties of materials at the atomic level, which opens up entirely new possibilities in nanoscience and nanotechnology. Surface reactions of organic molecules on a semiconductor have been investigated in numerous studies6-10 to add new functionalities of the molecules to the semiconductor. However, most surface-reaction studies showed multiple adsorption configurations on the surface. In order to improve the quality of molecule-based devices, it is important to form surface products with a single adsorption configuration on the devices. Surface reaction products can be controlled by using variations in the reactivity of reagents, and with changes in temperature or catalyst. However, it is difficult to control surface reactions on the nanoscale with these methods, so the use of STM has significant advantages. Various STM-induced manipulations * To whom correspondence should be addressed. Ansoon Kim, Tel: (+82) 42-860-1850, Fax: (+82) 42-860-5404, E-mail:
[email protected]; Sehun Kim, Tel: (+82) 42-869-2841, Fax: (+82) 42-869-2810. † Electronics and Telecommunications Research Institute. ‡ Korea Advanced Institute of Science and Technology.
through control of tip-adsorbate interaction forces, the evaporating electric field, or tunneling electrons have been studied. More recently, inelastic electron tunneling has been recognized as one of the most important topics in nanoscale physics and is thought to play an important role in the manipulation of adsorbed atoms and molecules on surfaces with STM11-15 or lasers.16,17 Among the works that are relevant to the present study, many studies have examined simple systems such as hydrogen13,18-22 and oxygen23,24 on surfaces, whereas few molecule/semiconductor systems11,14 have been studied. In particular, selective STM tipinduced semiconductor-molecule bond breaking of different adsorption structures is in its infancy and has the potential to create a novel form of lithography for use in molecular electronics. Herein, we present configuration-specific desorption in a nanoscale induced by STM in a C2H4/Ge(100) hybrid system. We demonstrated selective STM-induced desorption of ethylene from a Ge(100) surface under moderate conditions, which enables the formation of single C2H4 products on the surface. Although the concept of STM-induced desorption and reaction has been demonstrated for a number of years for the silicon surface,11,14,20,25-27 this may be the first time that selective desorption of one surface configuration over another using the STM tip has been shown both at a negative bias voltage and on the Ge(100) surface. In a previous study,9 ethylene was found to adsorb in two distinct bonding geometries: (i) on top of a single Ge-Ge dimer (OT) and (ii) in a paired end-bridge between two neighboring Ge dimers within the same dimer row (PEB). We determined the desorption yield for each configuration as a function of the applied sample bias voltage at a tunneling current of 50 pA. It was demonstrated that only the OT configuration desorbs effectively at sample bias voltages between -2.9 V and -3.1 V at RT. On the basis of our density functional calculations and previous photoemission studies,10 selective STM-induced desorption is related to σGe-C hole-
10.1021/jp075540y CCC: $40.75 © 2008 American Chemical Society Published on Web 01/11/2008
1494 J. Phys. Chem. C, Vol. 112, No. 5, 2008
Kim et al.
resonant inelastic tunneling, which results in dissociation of Ge-C bonds. 2. Experimental and Computational Details The Ge(100) crystal (0.1∼0.39 Ωcm, p-type, B-doped) was cleaned in ultrahigh vacuum (base pressure
