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0347125266
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A low-density monolayer was prepared by (i) immersing Au(111) in a solution of MHAE (1mM, in ethanol; refer to Materials and Methods for details regarding the synthesis of MHAE) and (ii) removing the acid-labile end group by incubation in trifluoroacetic acid (40% in ethanol). The thickness of the SAM of MHAE was 2.05 (±0.1) nm as examined by ellipsometry. Formation of islands of adsorbed thiolates was ruled out by cyclic voltammetry (Gamry PC4-300 potentiostat; polycrystalline Au counter and Ag-AgCl reference electrodes; scan rate, 50 mV s{%N}-1{%N}; 10 cycles; scan range, -0.2 to 0.5 V w.r.t. Ag-Agl), revealing a shielding of the gold surface by the SAM of MHAE that was comparable to dense SAMs (32).
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0345863971
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In air, the low-density monolayer collapsed into a disordered configuration with a thickness of 1.1 (±0.1) nm as confirmed by ellipsometry. This value was significantly lower than the thickness of a dense SAM [2.03 (±0.1) nm] and indicated the loss of two-dimensional order due to increased spacing between the adsorbed molecules. The collapsed structure was also characterized by IR spectroscopy, which revealed general features of a disordered monolayer, such as the red shift of the methylene bands from 2921 and 2852 cm{%N}-1{%N} (dense SAMs of MHA) to 2929 and 2858 cm{%N}-1{%N}. Furthermore, we detected an increased static contact angle with water of 62° as compared to 10° measured for the dense SAMs of MHA. The carboxylic acid group was deprotonated under argon atmosphere by incubation with a degassed aqueous cesium hydroxide solution (0.1 M), and completeness was verified by IR spectroscopy (BioRad FTS 175, grazing angle of 80°), revealing the absence of the carbonyl signals at 1742 and 1716 cm{%N}-1{%N} and the presence of signals at 1550 and 1440 cm{%N}-1{%N}. The low-density SAM showed chemical stability because it remained unaltered after exposure to ambient conditions for 4 weeks (confirmed by XP5 and IR spectroscopy).
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6744248168
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Y. R. Shen, Nature 337, 519 (1989).
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26
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0347125265
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note
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o is the surface potential (33).
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27
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0001361674
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S. Baldelli, N. Markovic, P. Ross, Y. R. Shen, G. Somorjai, J. Phys. Chem. 103, 8920 (1999).
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0347755602
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Scanning force microscopy did not reveal notable differences in surface roughness between systems configured of molecules in bent [root mean square (RMS) roughness = 1.9 nm] and straight (RMS roughness = 1.5 nm) states. The different states were realized by conducting scanning force microscopy of a low-density SAM in toluene and water, respectively. Our experiments indicated that surface roughness is predominately caused by the intrinsic surface roughness of the gold surface (RMS roughness < 2 nm). All RMS values refer to an area of 500 × 500 nm{%N}2{%N}. Refer to fig. S2 for detailed information.
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31
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33845282373
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39
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0345863969
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note
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We thank S. Baldelli, N. Flynn, R. Raman, M. Stevens, and G. Venkataraman for valuable discussions; G. M. Whitesides and J. Israelachvili for insightful comments on the manuscript; P. E. Laibinis for the use of the IR; and the Materials Research Science and Engineering Center shared facilities (MIT) supported by NSF under award DMR-0213282. J.L. thanks the Fonds der Chemischen Industrie, Germany, for a Justus-Liebig fellowship, and T.-N.T. acknowledges support from the Whitaker Foundation. This work was sponsored in part by the U.S. Army Research Office through the Institute for Soldier Nanotechnologies at MIT under contract DAAD-19-02-002 and NIH. The content of the information does not necessarily reflect the position or the policy of the Government, and no official endorsement should be inferred.
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