Mechanisms of Molecular Manipulation with the Scanning Tunneling Microscope at Room Temperature: Chlorobenzene=Si111-7  7 P. A. Sloan, M. F.G. Hedouin, and R. E. Palmer * Nanoscale Physics Research Laboratory, School of Physics and Astronomy, The University of Birmingham, Birmingham B15 2TT, United Kingdom M. Persson Department of Applied Physics, Chalmers/Go ¨teborg University, S-412 96 Go ¨teborg, Sweden (Received 9 December 2002; published 9 September 2003) We report a systematic experimental investigation of the mechanism of desorption of chlorobenzene molecules from the Si1117  7 surface induced by the STM at room temperature. We measure the desorption probability as a function of both tunneling current and a wide range of sample bias voltages between 3V and 4V. The results exclude field desorption, thermally induced desorption, and mechanical tip-surface effects. They indicate that desorption is driven by the population of negative (or positive) ion resonances of the chemisorbed molecule by the tunneling electrons (or holes). Density functional calculations suggest that these resonant states are associated with the  orbitals of the benzene ring. DOI: 10.1103/PhysRevLett.91.118301 PACS numbers: 82.37.Gk, 34.80.Ht, 68.37.Ef, 68.43.Rs The manipulation of individual atoms and molecules with the STM is one of the great scientific achievements of the last 20 years. Beginning with the lateral manipu- lation of physisorbed Xe atoms on the Ni(110) surface at cryogenic temperatures [1], Eigler and colleagues have demonstrated how to assemble predesigned, nanometer- scale structures which can be employed to trap electrons in ‘‘textbook’’ quantum boxes [2], or to perform logic operations by sequential electromechanical molecular manipulation [3]. While the extension of such methods to room temperature systems is far from trivial, signifi- cant progress has been reported, especially in relation to vertical manipulation, i.e., the STM-induced desorption of surface or adsorbed atoms [4–7]. Underpinning all these examples is the question of the mechanism of atomic manipulation [8,9], not always evident from the experiments and sometimes the subject of contro- versy [10,11]. In this Letter we bring a combination of systematic experimental investigations and density functional calcu- lations to bear on the problem of the mechanism of desorption of chemisorbed chlorobenzene (C 6 H 5 Cl) mole- cules from the reconstructed Si111-7  7 surface at room temperature. This model system is also relevant to the interface between molecular electronics and conven- tional silicon devices [12]. Measurements of the desorp- tion probability over an unusually wide range of both positive and negative sample bias voltages (from 3V to 4V) establish asymmetric threshold voltages for desorption. Measurements as a function of tunneling current rule out vibrational heating, electric field, and mechanical (i.e., tip-surface force) mechanisms. We de- duce that desorption is driven by the population of nega- tive (or positive) ion resonance states of the chemisorbed chlorobenzene molecule. Comparison with the density of states (DOS) calculated from density functional theory leads us to propose further that these resonance states are associated with the  orbitals of the molecular ben- zene ring. The room temperature STM was housed in a UHV chamber [13] with a base pressure of 1  10 10 Torr. The silicon (111) samples, cut from n-type and p-type wafers 0.38 mm thick, had resistivities of 1–30  cm. To create extremely large (10 000 A ˚ ) and almost defect-free terraces, the samples were initially degassed overnight at 700  C and subsequently flashed for 30 s to temperatures increasing to 1300  C (by resistive heating). The final step was a flash to 1300  C for 30 s followed by quick cooling to 960  C and further cooling (1  Cs 1 ) to room tem- perature. The STM tips, produced by electrochemical etching (2 M NaOH) of polycrystalline tungsten wire, before electron bombardment in the UHV chamber, were characterized by their field emission properties. Thus electron bombardment was stopped after oxide removal but before the tip apex began to melt appreciably. Chlorobenzene was subject to repeated freeze-pump- thaw cycles before dosing into the vacuum chamber by means of a leak valve. The typical dosage was 50 s  2  10 8 Torr (i.e., 1 Langmuir), corresponding to about 0.05 monolayer (ML)—1 ML corresponds to one mole- cule per 1  1 unreconstructed unit cell of the Si(111) surface [14]. Previous experiments [15] have demonstrated that chlorobenzene can be dissociated on the Si111-7  7 surface under an STM tip by the application of (  4V) voltage pulses to the sample, thereby generating chemi- sorbed chlorine atoms. We find that the dominant channel of STM manipulation, i.e., desorption versus dissociation, PHYSICAL REVIEW LETTERS week ending 12 SEPTEMBER 2003 VOLUME 91, NUMBER 11 118301-1 0031-9007= 03=91(11)=118301(4)$20.00  2003 The American Physical Society 118301-1