Benzene adsorbed on Si(001): The role of electron correlation and finite temperature

Abstract

van der Waals energy-corrected density functional theory (DFT + vdW) as well as the exact exchange with electron correlation in the random-phase approximation are used to study the adsorption of benzene on the Si(001) surface with respect to two controversial adsorption structures (termed "butterfly" and "tight bridge"). Our finding that the tight-bridge structure is energetically favored over the butterfly structure agrees with standard DFT but conflicts with previous vdW-inclusive calculations. However, the inclusion of zero-point energy and thermal vibrations reverses the stability of the two structures with increasing temperature. Our results provide an explanation for the recent experimental observation that both structures coexist at room temperature. The interaction of aromatic hydrocarbon molecules with the Si(001) surface is of considerable interest because of the functionalization of the silicon surface toward molecular control of electronic devices. [1][2][3][4][5] Especially, the adsorption of benzene on the Si(001) surface has become a prototype system to study and understand the interaction of π -conjugated aromatic rings with the dangling bonds of the Si surface dimers. [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] Despite its apparent simplicity, the structure and dynamics of benzene on Si(001) have been controversial between experiment 6-14 and theory. [15][16][17][18][19] Many experimental studies on the adsorption of benzene on Si(001) have been performed at room temperature, using high-resolution electron-energy-loss spectroscopy, 6 thermaldesorption spectroscopy, 6,7 Auger electron spectroscopy, 6 near-edge x-ray-absorption fine structure, 8,9 and optical spectroscopy.10 All of them supported the so-called butterfly (BF) structure, where benzene adsorbs on top of a single Si dimer [see Fig. 1(a)]. However, scanning tunneling microscopy (STM) experiments at room temperature observed both the BF structure and the tight-bridge (TB) structure, where benzene adsorbs across two adjacent Si dimers [see Fig. 1(b)].11-13 It was also observed by STM that the BF structure was initially formed but was gradually converted to the TB structure, 12,13 implying that the BF structure is an intermediate state.Recently, a room-temperature photoelectron diffraction study concluded that the saturated monolayer (ML) of benzene on Si(001) contains both structures with 58 ± 29% of molecules having the BF structure. 14 To date, theoretical works remain divided on the issue of the adsorption structure of benzene on Si(001). [15][16][17][18][19] Standard density functional theory (DFT) calculations using the slab geometry predicted that the TB structure is more stable than the BF structure by 0.07-0.27 eV depending on the coverage, [15][16][17][18][19] and that the initially formed BF structure undergoes a conversion into the more stable TB structure with an activation barrier of ∼1 eV, 17 which seemed to be consistent with STM experiments.12,13 On the other hand, a MP2 cluster calculation (Møller-Plesset ...

Self-consistent van der Waals density functional study of benzene adsorption on Si(100)

“…2, 4,6,[17][18][19][20][21][22][23][24][25][26][27][28][29][30]32 While semiempirical cluster calculations favor unstable adsorption structures such as symmetricand tilted-bridge configurations, 2,4,17-20 density functional theory (DFT) calculations within the local density approximation (LDA) and the generalized gradient approximation (GGA) predict that TB is the most stable adsorption structure, 20,[22][23][24][26][27][28] in good agreement with the STM results.…”

“…For example, the vdW-DF1 and vdW-DF2 results differ from the PBE ones by less than ±0.1 eV, while those obtained with the other vdW-DFs are always larger than the PBE results by 0.4-0.7 eV, consistent with the PBE+vdW and EX+cRPA results. 30 We now investigate the relative stability of BF and TB for each vdW-DF using the energy difference ∆E ads ≡ E ads (BF) − E ads (TB). For vdW-DF1, BF is marginally more stable than TB by ∆E ads = 7 (2) meV in the SC-relaxed (non-SC) result, while less stable by 7 meV in the SC-fixed result.…”

“…Note, however, that the influence of the vdW interaction in the present problem is yet to be fully understood, since the cluster model used in the MP2 calculation 25 corresponds to the low coverage limit, while the vdW-DF results 29 show that BF becomes slightly more stable than TB at almost saturation coverage. Indeed, other DFT calculations with semiempirical dispersion correction 32 and a more sophisticated method based on exact exchange and correlation within the random phase approximation (EX + cRPA) 30 support the TB structure, suggesting that the stability of the two adsorption structures cannot be inverted only by the vdW interaction. Moreover, the above vdW-DF study 29 leaves some ambiguities in its accuracy in retrospect.…”

“…Our results show that the adsorption energies of BF and TB are quite sensitive to the choice of vdW-DF, and in particular, some of the vdW-DFs predict that TB is more stable than BF, in good agreement with accurate EX-cRPA calculations. 30 A more detailed analysis reveals that the importance of the SC treatment of vdW-DFs becomes prominent in interaction between benzene and the Si surface as well as deformation of a benzene molecule.…”

Phase behaviour of self-assembled monolayers controlled by tuning physisorbed and chemisorbed states: A lattice-model view

“…For chemisorption on silicon, it is known that benzene rings can form tight-bridge or butterfly structures [32,33], where the ring undergoes significant distortion from sp2 to sp3 bonding. However, on platinum the ring does not distort and remains sp2-bonded.…”