We have studied the DNA bases adenine, thymine, guanine, and cytosine adsorbed onto Au( 1 1 1) using in situ scanning tunneling microscopy (STM), atomic force microscopy (AFM), and cyclic voltammetry. Adenine, guanine, and cytosine adsorbed spontaneously onto the electrode with the counter electrode disconnected and yielded stable images over a range of electrode potentials. Thymine was not adsorbed until an electron-transfer reaction occurred at +0.4 V (SCE). Adenine and guanine formed polymeric aggregates in which the bases stacked with repeat distances of 3.4 i 0.2 A (adenine) and 3.3 f 0.3 A (guanine). Adenine aggregates aligned along the Au[lTO] directions and also formed a complex that decorated the path of the p X .\/3 (p Y 23) reconstruction. Guanine aggregates were disordered with respect to the underlying gold. Cytosine formed an oblique lattice with a = 10.5 f 0.2 A, b = 9.5 f 0.2 A, and y = 103 f 3O, oriented at 30° to the Au [liO] directions. It is fitted well by a model in which every other molecule inverted to bond 0 2 to N4 and N1 to N3, followed by N4 to 0 2 and N 3 to N1. Thymine also formed an oblique lattice with a = 6.5 f 0.5 A, b = 7.1 i 0.5 A, and y = 105 i 5 O . The STM corrugation measured over adenine, cytosine, and guanine adsorbates was -1 A but only -0.1 A over thymine. Thymine contrast also changed with the electrochemical potential of the electrode. We discuss the possibility of a connection between electrochemical electron transfer and STM contrast.

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Substrates of aluminum (Al) deposited by physical vapor deposition onto Si substrates and then chemically reacted with perfluorodecylphosphonic acid (PFDPAlSi), decylphosphonic acid (DPAlSi), and octadecylphosphonic acid (ODPAlSi) were studied by x-ray photoelectron spectroscopy (XPS), contact angle measurements, atomic force microscopy (AFM), and friction force microscopy, a derivative of AFM, to characterize their surface chemical composition, roughness, and micro-/nanotribological properties. XPS analysis confirmed the presence of perfluorinated and nonperfluorinated alkylphosphonate molecules on the PFDPAlSi, DPAlSi, and ODPAlSi. The sessile drop static contact angle of pure water on PFDPAlSi was typically more than 130 degrees and on DPAlSi and ODPAlSi typically more than 125 degrees indicating that all phosphonic acid reacted AlSi samples were very hydrophobic. The surface roughness for PFDPAlSi, DPAlSi, ODPAlSi, and bare AlSi was approximately 35 nm as determined by AFM. The surface energy for PFDPAlSi was determined to be approximately 11 mNm by the Zisman plot method compared to 21 and 20 mNm for DPAlSi and ODPAlSi, respectively. Tribology involves the measure of lateral forces due to friction and adhesion between two surfaces. Friction, adhesion, and wear play important roles in the performance of micro-/nanoelectromechanical systems. PFDPAlSi gave the lowest adhesion and coefficient of friction values while bare AlSi gave the highest. The adhesion and coefficient of friction values for DPAlSi and ODPAlSi were comparable.

The performance of micro- and nanoelectromechanical systems depends on the surface properties of the substrate material, such as chemical composition, roughness, friction, adhesion, and wear. Substrates of aluminum deposited onto Si (Al/Si) have been chemically reacted with perfluorodecyldimethylchlorosilane (PFMS), octadecylphosphonic acid (ODP), decylphosphonic acid (DP), octylphosphonic acid (OP), and perfluorodecylphosphonic acid (PFDP) and then characterized by X-ray photoelectron spectroscopy (XPS), contact angle measurements, and atomic force microscopy (AFM). PFMS/Al self-assembled monolayers (SAMs) were studied by friction force microscopy, a derivative of AFM, to better understand their micro- and nanotribological properties. The adhesion forces for PFMS/Al SAMs were found to be lower when compared to those of bare Al/Si; however, the coefficient of friction for both was comparable. XPS analysis revealed the presence of the corresponding alkyl chain molecules on PFMS/Al, ODP/Al, DP/Al, OP/Al, and PFDP/Al SAMs. The sessile drop static contact angle of pure water demonstrates that all the SAMs are extremely hydrophobic, giving contact angles typically >130° on PFDP/Al, ODP/Al, and PFMS/Al SAMs and >125° on DP/Al and OP/Al SAMs. The surface energy of PFMS/Al SAMs determined by the Zisman plot method is 16.5 ± 2 mJ/m2 (mN/m). The rms surface roughness of ODP/Al, DP/Al, OP/Al, PFMS/Al, and PFDP/Al SAMs, before exposure to warm nitric acid (pH 1.8, 30 min, 60−95 °C), as well as bare Al, is less than 40 nm as determined by AFM. The XPS data and stability against harsh chemical conditions indicate that a type of bond forms between a phosphonic acid or silane molecule and the oxidized Al/Si surface. Stability tests using warm nitric acid (pH 1.8, 30 min, 60−95 °C) show ODP/Al SAMs to be the most stable followed by PFDP/Al, DP/Al, PFMS/Al, and OP/Al SAMs.

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We describe a procedure for reversible adsorption of DNA onto a gold electrode maintained under potential control. The adsorbate can be imaged by scanning probe microscopy in situ. Quantitative control of a molecular adsorbate for microscopy is now possible. We found a potential window (between 0 and 180 mV versus a silver wire quasi reference) over which a gold (111) surface under phosphate buffer is positively charged, but is not covered with a dense adsorbate. When DNA is present in these conditions, molecules adsorb onto the electrode and remain stable under repeated scanning with a scanning tunneling microscope (STM). They become removed when the surface is brought to a negative charge. When operated at tunnel currents below approximately 0.4 nA, the STM yields a resolution of approximately 1 nm, which is better than can be obtained with atomic force microscopy (AFM) at present. We illustrate this procedure by imaging a series of DNA molecules made by ligating a 21 base-pair oligonucleotide. We observed the expected series of fragment lengths but small fragments are adsorbed preferentially.

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Formation of stable and hydrophobic self-assembled monolayers (SAMs) on an oxidized copper (Cu) surface has been accomplished via reaction with 1H,1H,2H,2H-perfluorodecyldimethylchlorosilane (PFMS). The perfluoroalkyl SAMs showed sessile drop static contact angles of more than 125°for pure water and stability against exposure to boiling water, boiling nitric acid solution, and warm sodium hydroxide solution for up to 30 min. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared reflection/absorption spectroscopy (FT-IRRAS) data reveal a coordination of the PFMS silicon (Si) atom with a cuprate (CuO) molecule present on the oxidized copper substrate. The data give good evidence that the stability of the SAM film on the PFMS-modified oxidized Cu surface is largely due to the formation of a siloxy-copper (-Si-O-Cu-) bond via a condensation reaction between silanol (-Si-OH) and copper hydroxide (-CuOH). The extremely hydrophobic and stable SAMs on oxidized Cu could have useful applications in corrosion inhibition for micro/nanoelectronics and/or heat exchange surfaces exploiting dropwise condensation.