Abstract:Single and multicomponent mixed layers of silsesquioxane clusters on freshly evaporated gold surfaces have been investigated by X-ray photoelectron spectroscopy and reflection-absorption infrared spectroscopy. Approximately 5-10% of the cluster layers (e.g., H8Si8O12 and H10Si10O15) on gold desorb upon evacuation of the adsorbate from the reaction chamber. These open adsorption sites are an avenue for cluster displacement reactions that yield mixed monolayers (e.g., H8Si8O12/D8Si8O12 and H8Si8O12/C6H13-H7Si8O1… Show more
“…We consider the compounds to be anchored to the gold electrodes either via silyl (SiH 3 ) groups ( 2a – 2d , 3a – 3d , 5 , and 6 ), which allow for direct contact between the σ-bonded silicon framework and the Au electrodes, or via traditional thiophenols ( 4a – 4d and 7 ), which have a mismatch between the conjugation topologies of the terminal and the central molecular segments (π- versus σ-conjugation). With regard to the silyl groups they have not previously been used as anchor groups in single-molecule transport investigations; however, self-assembled monolayers of alkylsilanes (RSiH 3 ) on Au(111) surfaces are well-established even though the attachment, which goes via bond cleavage of the three Si–H bonds, occurs at ultrahigh-vacuum conditions. − At low coverage the RSi moieties prefers to bind strongly in a 3-fold hollow site on the Au(111) surface with an energy of about 3.5 eV (81 kcal/mol), which is higher compared to the gold–thiol bond of about 2 eV (45 kcal/mol) . This makes the RSi moieties less prone to migrate over the gold surface, resulting in a well-defined conductance.…”
Silicon
is still the dominating material in microelectronics, yet
primarily π-conjugated hydrocarbons are investigated in the
field of single-molecule electronics even though linear oligosilanes
are σ-conjugated. A drawback with the latter is their high conformational
flexibility which strongly affects conductance. Here we report on
a first principles density functional theory investigation of a series
of rigid [2.2.2]bicyclic carbosilanes with 3, 2, 1, or 0 disilanylene
bridges, providing all-silicon paths for charge transport. It is explored
if these paths can be seen as independent and equivalent current paths
acting as parallel resistors. For high conductance through the carbosilanes
they need to be anchored to the gold electrodes via groups that are
matched with the σ-conjugated paths of the oligosilane cage
segment, and we find that silyl (SiH3) groups are better
matched than thiophenol groups. Even for the carbosilane with three
disilanylene bridges we find that the most transmitting conductance
channel is not equally distributed on the three parallel bridges.
In addition, there is significant communication between the various
pathways, which results in destructive interference lowering the conductance.
Taken together, the different disilanylene bridges in the cage compounds
do not act as parallel resistors.
“…We consider the compounds to be anchored to the gold electrodes either via silyl (SiH 3 ) groups ( 2a – 2d , 3a – 3d , 5 , and 6 ), which allow for direct contact between the σ-bonded silicon framework and the Au electrodes, or via traditional thiophenols ( 4a – 4d and 7 ), which have a mismatch between the conjugation topologies of the terminal and the central molecular segments (π- versus σ-conjugation). With regard to the silyl groups they have not previously been used as anchor groups in single-molecule transport investigations; however, self-assembled monolayers of alkylsilanes (RSiH 3 ) on Au(111) surfaces are well-established even though the attachment, which goes via bond cleavage of the three Si–H bonds, occurs at ultrahigh-vacuum conditions. − At low coverage the RSi moieties prefers to bind strongly in a 3-fold hollow site on the Au(111) surface with an energy of about 3.5 eV (81 kcal/mol), which is higher compared to the gold–thiol bond of about 2 eV (45 kcal/mol) . This makes the RSi moieties less prone to migrate over the gold surface, resulting in a well-defined conductance.…”
Silicon
is still the dominating material in microelectronics, yet
primarily π-conjugated hydrocarbons are investigated in the
field of single-molecule electronics even though linear oligosilanes
are σ-conjugated. A drawback with the latter is their high conformational
flexibility which strongly affects conductance. Here we report on
a first principles density functional theory investigation of a series
of rigid [2.2.2]bicyclic carbosilanes with 3, 2, 1, or 0 disilanylene
bridges, providing all-silicon paths for charge transport. It is explored
if these paths can be seen as independent and equivalent current paths
acting as parallel resistors. For high conductance through the carbosilanes
they need to be anchored to the gold electrodes via groups that are
matched with the σ-conjugated paths of the oligosilane cage
segment, and we find that silyl (SiH3) groups are better
matched than thiophenol groups. Even for the carbosilane with three
disilanylene bridges we find that the most transmitting conductance
channel is not equally distributed on the three parallel bridges.
In addition, there is significant communication between the various
pathways, which results in destructive interference lowering the conductance.
Taken together, the different disilanylene bridges in the cage compounds
do not act as parallel resistors.
“…Additionally, the site-specific cluster adsorption/desorption behavior yields a discontinuous Au/H 7 Si 8 O 12 adsorbate layer surface containing a complex pattern of holes and channels. The holes present in the discontinuous adsorbate layer may provide an avenue for, or facilitate, known H 8 Si 8 O 12 /H 8 Si 8 O 12 , H 8 Si 8 O 12 /D 8 Si 8 O 12 , and H 8 Si 8 O 12 /C 6 H 13 -H 7 Si 8 O 12 cluster exchange reactions. , These novel surface structures and their relationship to cluster adsorption and desorption are the focus of this paper. 2 H 8 Si 8 O 12 : (a) ball and stick model and (b) space filling model.…”
Scanning tunneling microscopy (STM) data are presented in conjunction with reflection-absorption infrared spectroscopy and X-ray photoemission spectroscopy data investigating the adsorbate layer formation of H8Si8O12 clusters on a clean Au(111) 23× 3 surface. All three experimental techniques independently support desorption of approximately 10-15% of the H8Si8O12 clusters from the Au/H7Si8O12 adsorbate layer surface following evacuation of excess H8Si8O12 cluster pressure from the ultrahigh vacuum reaction chamber. Surprisingly, the STM data indicate that unlike all other reported molecular adsorbates having a strong chemical interaction with the Au(111) surface, the 23× 3 surface reconstruction is preserved following chemisorption of H 8Si8O12 clusters. Additionally, at saturation coverage the clusters are preferentially bound to, and predominantly desorb from, specific sites on the Au(111) 23× 3 reconstructed surface. The preferential cluster adsorption/desorption behavior creates a unique pattern of holes and channels in the Au/H7Si8O12 adsorbate layer surface.
“…To anchor the tetramers to the gold electrodes we used two silyl (SiH 3 ) groups on each side of the tetramer. The silyl group has previously been shown to bind in a 3-fold manner to the Au(111) surface under ultrahigh vacuum conditions (Figure ). − We choose silyl anchors so as to match the σ-conjugation of the tetramers as this has been shown earlier to be important for good conductance . Experimentally, trimethylsilylethynyl groups have previously been used in conductance measurements, and this silyl group gives comparable values to the more commonly used thiol (−SH) or amine (−NH 2 ) anchoring groups .…”
Oligomers of 1,4-disila/germa/stannacyclohexa-2,5-dienes as well as allcarbon 1,4-cyclohexadienes connected via EE single bonds (E = C, Si, Ge, or Sn) were studied through quantum chemical calculations in an effort to identify conformationally flexible molecular wires that act as molecular "electrical cords" having conformerindependent conjugative and conductive properties. Our oligomers display neutral hyperconjugative interactions (σ/π-conjugation) between adjacent σ(EE) and π(CC) bond orbitals, and these interactions do not change with conformation. The energies and spatial distributions of the highest occupied molecular orbitals of methyl-, silyl-, and trimethylsilyl (TMS)-substituted 1,4-disilacyclohexa-2,5-diene dimers, and stable conformers of trimers and tetramers, remain rather constant upon Si−Si bond rotation. Yet, steric congestion may be a concern in some of the oligomer types. The calculated conductances for the Si-containing tetramers are similar to that of a σ-conjugated linear allanti oligosilane (a hexadecasilane) with equally many bonds in the conjugated paths. Moreover, the Me-substituted 1,4-disilacyclohexadiene tetramer has modest conductance fluctuations with Si−Si bond rotations when the electrode−electrode distance is locked (variation by factor ∼30), while the fluctuations under similar conditions are larger for the analogous TMS-substituted tetramer. When the electrode−electrode distance is changed several oligomers display small conductance variations within certain distance intervals, e.g., the mean conductance of TMS-substituted 1,4-disilacyclohexa-2,5-diene tetramer is almost unchanged over 9 Å of electrode−electrode distances.
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