1. Self-assembled molecular array in methylamine dissociation on Si(100), J.-H. Cho, L. Kleinman, U. of Texas, Phys. Rev. B 67, 201301(2003). A candidate for ordered arrays of organic molecules NH 3 /Si dimers→ NH 2 on down-buckled side (stabilized through hybridization between lone pair and empty dangling bond state) UP-buckled side(nucleophilic) to be its neighbor Zigzag structure(Queeney, PRL86,1046) NH 2 CH 3 /Si100→H and NHCH 3 (JCP114,10170) Adsorbs on the down-dimer site -> up buckling switch alternative buckling of neifhboring dimers -> NHCH 3 species along the dimer row Methods: total energy using density functional Fig.1. Atomic structure of NH 2 CH 3 /Si100 at 0.5ML -> Eads=1.41 eV > 1.27 eV (NH 3 ) Early experiments; N-CH 3 as well as N-H cleavage Fig.2. E ads =1.33 E b =2.16, E ads =2.61eV for N- CH 3 cleavage → kinetically prohibited, equally probable desorption rather than dissociation E ads =1.40 E b =0.83, E ads =2.06eV for N-H cleavage Fig.3. Nucleophilic up-buckled side for NHCH3 electrophilic down-buckled side for NH2 Buckling energy barrier (0.17 eV) – sufficiently low, alternating buckling favored by 0.05 eV Fig.4. 1ML adsorption energy 2.08 > 2.04 (0.5ML) → hydrogen bonding between H and N, despite the repulsion between methyl group (a) Zigzag adsorption 2.07 eV, (b) Molecular adsorption 1.29 eV < 1.41 eV (c) Si(100) NH 2 CH 3 NH 3 H+NH 2 H+NHCH 3 Conclusion Alternative buckling of Si is favored by 7 times more likely at room temperature. Possible alignment of NHCH 3 moiety, in an extremely slow adsorption limit! If CH 3 is replaced by longer chain of hydrocarbon, steric hindrance or electron donation effect Or CH3-CH2-NH 2, C6H5(benzyl)-NH 2

2. New(√3x √3 )R30 phase of Pb/Ge(111) and its consequence for melting transition,M.F.Reedijk et al.(U. of Nijmegen, The Netherlands)PRL 90,56104 (2003). 4/3ML Pb coverage phase : (√3x√3)R30 (β phase) melting to 1x1 at 270 ℃ Pb occupies H3 and OCT(off-center bridge positions between T1 and T4 Pb deposition at 230 ℃ of at least 4/3ML ((√3x√3)R30 (β ’) phase → irreversible transition to 1x1 at 330 ℃ Surface X-ray diffraction analysis of β and β’ phase (DUBBLE beamline aat the ESRF) X-ray of 1.13 Å, incidence angle =1˚, varying exit angle, Large resemblance between the two phase Except Pb Bragg peak at l =3.4 for β’ (Pb 111 islands) The difference in the (11) rod near l =2 is due to difference in vacancy density Fitting results (surface fraction, height of H3 atom, debye-waller factor of Pb, etc.) Main differnce is the coverage (1.33 vs. 1.28 ML) What causes this large difference in melting temperature? Lower coverage (more vacancies) facilitates melting.(PRL70, 826) Excess Pb (1.5% of surface) forms islands. Adding more Pb on βphase (~0.06ML) converts back into β’ phase. Questions Vacancy(=defects?)