“…Previous studies have prepared I-terminated Si samples by immersing the H-terminated surfaces into solutions of iodine dissolved in ethanol or methanol. ,,, However, it was also reported that the resulting surfaces have strongly adsorbed alkoxy groups, with only ∼0.25 monolayer of iodine. , We therefore examined iodine termination in less reactive solvents. Previous studies under ultrahigh vacuum conditions have shown that benzene will bond to silicon surfaces; however, desorption occurs at a relatively low temperature of 350 K with no fragmentation. , This reversible behavior, together with the high solubility of iodine in benzene, suggested the possibility of using benzene as an alternative solvent for iodine termination of Si. 1 Iodine(3d) and carbon(1s) XP spectra of Si(111) sample reacted with saturated solutions of iodine/benzene and iodine/methanol, for 20 min: (a) I(3d), immersed in iodine/benzene; (b) I(3d), immersed in iodine/methanol; (c) C(1s), immersed in iodine/benzene; and (d) C(1s), immersed in iodine/methanol.…”
The use of iodine as a photolabile passivating agent for photochemical modification of silicon surfaces is demonstrated. X-ray photoelectron spectroscopy measurements show that iodine termination using iodine dissolved in benzene leads to Si surfaces exhibiting higher iodine surface coverages and lower levels of carbon contamination than previous methods. When exposed to 514 nm light in the presence of a suitable reactive molecule, such as an organic alkene, the surface iodine is removed and the reactive molecule links to the silicon surface. The results of experiments in which the polarization and angle of the incident light were varied show that the reaction mechanism is mediated by absorption of light in the bulk Si. A much greater photoattachment efficiency is obtained on heavily doped n-type silicon than on p-type silicon. It is proposed that on n-type silicon samples the photogenerated minority carriers (holes) accumulate near the surface, making the surface more susceptible to nucleophilic attack by the alkene molecules. The use of this method for photopatterning a Si surface with specific reactive groups is demonstrated.
“…Previous studies have prepared I-terminated Si samples by immersing the H-terminated surfaces into solutions of iodine dissolved in ethanol or methanol. ,,, However, it was also reported that the resulting surfaces have strongly adsorbed alkoxy groups, with only ∼0.25 monolayer of iodine. , We therefore examined iodine termination in less reactive solvents. Previous studies under ultrahigh vacuum conditions have shown that benzene will bond to silicon surfaces; however, desorption occurs at a relatively low temperature of 350 K with no fragmentation. , This reversible behavior, together with the high solubility of iodine in benzene, suggested the possibility of using benzene as an alternative solvent for iodine termination of Si. 1 Iodine(3d) and carbon(1s) XP spectra of Si(111) sample reacted with saturated solutions of iodine/benzene and iodine/methanol, for 20 min: (a) I(3d), immersed in iodine/benzene; (b) I(3d), immersed in iodine/methanol; (c) C(1s), immersed in iodine/benzene; and (d) C(1s), immersed in iodine/methanol.…”
The use of iodine as a photolabile passivating agent for photochemical modification of silicon surfaces is demonstrated. X-ray photoelectron spectroscopy measurements show that iodine termination using iodine dissolved in benzene leads to Si surfaces exhibiting higher iodine surface coverages and lower levels of carbon contamination than previous methods. When exposed to 514 nm light in the presence of a suitable reactive molecule, such as an organic alkene, the surface iodine is removed and the reactive molecule links to the silicon surface. The results of experiments in which the polarization and angle of the incident light were varied show that the reaction mechanism is mediated by absorption of light in the bulk Si. A much greater photoattachment efficiency is obtained on heavily doped n-type silicon than on p-type silicon. It is proposed that on n-type silicon samples the photogenerated minority carriers (holes) accumulate near the surface, making the surface more susceptible to nucleophilic attack by the alkene molecules. The use of this method for photopatterning a Si surface with specific reactive groups is demonstrated.
“…To measure the barrier between chemisorbed and physisorbed chlorobenzene, we examined the rate of thermally excited molecular diffusion as a function of temperature, which in the case for benzene on the Si(111)-7 × 7 surface is known to proceed via the physisorbed state . Since benzene and chlorobenzene have similar (di-σ) chemisorption configurations, physisorbed precursor states and nearly identical binding energies for both chemisorbed (benzene = 0.98 ± 0.06 eV, , chlorobenzene = 0.98 ± 0.08 eV) and physisorbed (benzene = 0.46 ± 0.01 eV, , chlorobenzene = 0.52 ± 0.06 eV) species, it seems reasonable that chlorobenzene molecules also diffuse across the surface via thermal excitation to the physisorbed state. The diffusion rate was obtained by comparing a sequence of STM images of precisely the same surface area (as used above to determine the desorption barrier).…”
We report a new mechanism of (bond-selective) atomic manipulation in the scanning tunneling microscope (STM). We demonstrate a channel for one-electron-induced C-Cl bond dissociation in chlorobenzene molecules chemisorbed on the Si(111)-7 × 7 surface, at room temperature and above, which is thermally activated. We find an Arrhenius thermal energy barrier to one-electron dissociation of 0.8 ± 0.2 eV, which we correlate explicitly with the barrier between chemisorbed and physisorbed precursor states of the molecule. Thermal excitation promotes the target molecule from a state where one-electron dissociation is suppressed to a transient state where efficient one-electron dissociation, analogous to the gas-phase negative-ion resonance process, occurs. We expect the mechanism will be obtained in many surface systems, and not just in STM manipulation, but in photon and electron beam stimulated (selective) chemistry.
“…We note that these slower desorbing sites correspond well with the shoulder observed in TDS. 12 One somewhat surprising feature of the desorption data is that the curves describing the center faulted and center unfaulted sites cross. The center faulted site displays the most rapid desorption and is therefore the weakest binding, yet, at high total coverage this site shows the highest coverage of the various site types.…”
Benzene adsorption on Si͑111͒-7ϫ7 is studied with scanning tunneling microscopy. Benzene diffusion is found to be inhibited. Ordinarily surface diffusion is controlled by a substantially lower energy of activation than is desorption. In this case diffusion is frustrated by a barrier to diffusion that is comparable to that for desorption. Both desorption and diffusion are monitored. On average, for every two adsorbate disappearances, one readsorption is observed and one molecule desorbs. Site-specific activation barriers of 0.94Ϯ0.01 eV and 0.95Ϯ0.01 eV for center faulted and corner faulted adatom sites, respectively, are extracted. Residence times increase as coverage decreases, implying adsorbate crowding causes the strength of the surface-adsorbate bond to weaken. Diffusion is generally found to involve jumps to sites beyond nearest neighbors. It emerges that the adsorbate largely breaks its existing bond to a surface site before forming a substantial bonding interaction with a new site. We surmise that this ''break before make'' scheme leaves the adsorbate in an intermediate, essentially physisorbed state, where it is sufficiently mobile to make longer than nearest neighbor jumps, or from which it desorbs.
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