Template-stripped Research Articles

Mixed carboranethiol self-assembled monolayers on gold surfaces

Carboranethiol self-assembled monolayers on metal surfaces have been shown to be very convenient systems for surface engineering. Here we have studied pure and mixed self-assembled monolayers (SAMs) of three different carboranethiol (CT) isomers on gold surfaces. The isomers were chosen with dipole moments pointing parallel to (m-1-carboranethiol, M1), out of (m-9-carboranethiol, M9) and into (o-1-carboranethiol, O1) the surface plane, in order to investigate the effect of dipole moment orientation on the film properties. In addition, influence of the substrate surface morphology on the film properties was also studied by using flame annealed (FA) and template stripped (TS) gold surfaces. Contact angle measurements indicate that in M1/M9 and M1/O1 mixed SAMs, M1 is the dominant species on the surface even for low M1 ratio in the growth solution. Whereas for O1/M9 mixed SAMs no clear evidence could be observed indicating dominance of one of the species over the other one. Though contact angle values were lower and hysteresis values were higher for SAMs grown on TS gold surfaces, the trends in the behavior of the contact angles with changing mixing ratio were identical for SAMs grown on both substrates. Atomic force microscopy images of the SAMs on TS gold surfaces indicate that the films have similar morphological properties regardless of mixing ratio.

Arrays of high quality SAM-based junctions and their application in molecular diode based logic.

This paper describes a method to fabricate a microfluidic top-electrode that can be utilized to generate arrays of self-assembled monolayer (SAM)-based junctions. The top-electrodes consist of a liquid-metal of GaOx/EGaIn mechanically stabilized in microchannels and through-holes in polydimethylsiloxane (PDMS); these top-electrodes form molecular junctions by directly placing them onto the SAM supported by template-stripped (TS) Ag or Au bottom-electrodes. Unlike conventional techniques to form multiple junctions, our method does not require lithography to pattern the bottom-electrode and is compatible with TS bottom-electrodes, which are ultra-flat with large grains, free from potential contamination of photoresist residues, and do not have electrode-edges where the molecules are unable to pack well. We formed tunneling junctions with n-alkanethiolate SAMs in yields of ∼80%, with good reproducibility and electrical stability. Temperature dependent J(V) measurements indicated that the mechanism of charge transport across the junction is coherent tunneling. To demonstrate the usefulness of these junctions, we formed molecular diodes based on SAMs with Fc head groups. These junctions rectify currents with a rectification ratio R of 45. These molecular diodes were incorporated in simple electronic circuitry to demonstrate molecular diode-based Boolean logic.

Reversible Soft Top‐Contacts to Yield Molecular Junctions with Precise and Reproducible Electrical Characteristics

The reproducibility of the electrical characteristics of molecular junctions has been notoriously low. This paper describes a method to construct tunnel junctions based on self‐assembled monolayers (SAMs) by forming reversible electrical contacts to SAMs using top‐electrodes of a non‐Newtonian liquid‐metal (GaOx/EGaIn) stabilized in a microfluidic‐based device. A single top‐electrode can be used to form up to 15–25 junctions. This method generates SAM‐based junctions with highly reproducible electrical characteristics in terms of precision (widths of distributions) and replicability (closeness to a reference value). The reason is that this method, unlike other approaches that rely on cross‐bar or nano/micropore configurations, does not require patterning of the bottom‐electrodes and is compatible with ultra‐flat template‐stripped (TS) surfaces. This compatibly with non‐patterned electrodes is important for three reasons. i) No edges of the electrodes are present at which SAMs cannot pack well. ii) Patterning requires photoresist that may contaminate the electrode and complicate SAM formation. iii) TS‐surfaces contain large grains, have low rms values, and can be obtained and used (in ordinary laboratory conditions) within a few seconds to minimize contamination. The junctions have very good electrical stability (2500 current‐voltage cycles and retained currents for 27 h), and can be fabricated with good yields (≈78%).

Pinhole‐free large‐grained atomically smooth Au(111) substrates prepared by flame‐annealed template stripping

AbstractHigh‐quality atomically flat substrates are critical for the analysis and imaging of surface‐mounted ultrathin films and nanostructures. Here we report significant improvement in the preparation of large areas of atomically smooth Au(111) substrates. A thin layer of gold on silicon is flame‐annealed in air and then stripped from the template. The substrates were analyzed with X‐ray diffraction and high‐resolution atomic force microscopy (AFM). In contrast to the previously reported template stripped gold (TSG) substrates, flame‐annealed template stripped substrates reveal no detectable pinholes. The substrate surface is atomically smooth with most grains being larger than 1 µm2. The entire procedure requires less than 2 h and uses readily available materials and common laboratory equipment. The resulting substrates can be stored for longer periods of time and then used immediately without need for common cleaning procedures. Evidence is provided that self‐assembled monolayers on these substrates are higher quality than those prepared with previously reported gold substrates. Copyright © 2008 John Wiley & Sons, Ltd.

Scanning Tunneling Microscopy of Template-Stripped Au Surfaces and Highly Ordered Self-Assembled Monolayers

Template stripping of Au films in ultrahigh vacuum (UHV) produces atomically flat and pristine surfaces that serve as substrates for highly ordered self-assembled monolayer (SAM) formation. Atomic resolution scanning tunneling microscopy of template-stripped (TS) Au stripped in UHV confirms that the stripping process produces a flat, predominantly 111 textured, atomically clean surface. Octanethiol SAMs vapor deposited in situ onto UHV TS Au show a c(4 x 2) superlattice with (square root 3 x square root 3) R30 degrees basic molecular structure having an ordered domain size up to 100 nm wide. These UHV results validate the TS Au surface as a simple, clean and high-quality surface preparation method for SAMs deposited from both vapor phase and solution phase.

Large Area, Molecularly Smooth (0.2 nm rms) Gold Films for Surface Forces and Other Studies

Using large-area (cm2) single-crystal mica sheets as the templating substrate, we have created correspondingly large template-stripped (TS) gold films (thickness 82 +/- 2 nm) that appear smooth to within 0.2 nm rms roughness over their entire area. These gold films, created without the use of any releasing solvent, are characterized using AFM, X-ray diffraction, multiple beam interferometric fringes of equal chromatic order (FECO), and contact angle measurements. Being molecularly smooth over large areas and (adjustably) semitransparent, these films are especially suitable for use in the surface force balance (SFB), as shown by measurements of the normal force (F) versus distance (D) profiles between such a flat gold surface and a bare mica surface in water. The F(D) profiles are in good agreement with DLVO theory down to molecular contact and indicate that the gold surface is negatively charged under water.

Influence of Defects on the Electrical Characteristics of Mercury-Drop Junctions: Self-Assembled Monolayers of n-Alkanethiolates on Rough and Smooth Silver

This paper compares the structural and electrical characteristics of self-assembled monolayers (SAMs) of n-alkanethiolates, SCn (n = 10, 12, 14), on two types of silver substrates: one used as-deposited (AS-DEP) by an electron-beam evaporator, and one prepared using the method of template-stripping. Atomic force microscopy showed that the template-stripped (TS) silver surfaces were smoother and had larger grains than the AS-DEP surfaces, and reflectance-absorbance infrared spectroscopy showed that SAMs formed on TS substrates were more crystalline than SAMs formed on AS-DEP substrates. The range of current densities, J (A/cm2), measured through mercury-drop junctions incorporating a given SAM on AS-DEP silver was, on average, several orders of magnitude larger than the range of J measured through the same SAM on TS silver, and the AS-DEP junctions failed, on average, 3.5 times more often within five current density-voltage (J-V) scans than did TS junctions (depending on the length of the alkyl chains of the molecules in the SAM). The apparent log-normal distribution of J through the TS junctions suggests that, in these cases, it is the variability in the effective thickness of the insulating layer (the distance the electron travels between electrodes) that results in the uncertainty in J. The parameter describing the decay of current density with the thickness of the insulating layer, beta, was either 0.57 A-1 at V = +0.5 V (calculated using the log-mean of the distribution of values of J) or 0.64 A-1 (calculated using the peak of the distribution of values of J) for the TS junctions; the latter is probably the more accurate. The mechanisms of failure of the junctions, and the degree and sources of uncertainty in current density, are discussed with respect to a variety of defects that occur within Hg-drop junctions incorporating SAMs on silver.

Optimization of in-vacuo template-stripped Pt surfaces via UHV STM

A recently demonstrated [1] in-vacuo template-stripping process is applied to the study of platinum films stripped from ultra-flat silicon-oxide surfaces. Template-stripped (TS) Pt surfaces, prepared with a range of post-deposition annealing times prior to being stripped from the templating surface in an ultra-high vacuum (UHV) environment, are examined by UHV scanning tunneling microscopy (STM). These studies reveal that without post-deposition annealing, TS Pt surfaces are largely made up of poorly-ordered, granular nanostructures undesirable for many applications. The post-deposition annealing treatments explored in the study result in the emergence and continuous growth of large smooth crystallites. Issues with crystallite orientation relative to the TS surface and artefacts arising as a result of the epoxy used in the template-stripping process are presented and discussed in relation to optimizing the template-stripping procedure for specific applications such as self-assembled monolayer (SAM) formation for molecular electronics.

Template stripping using cold welding

A method for fabricating ultra-flat template-stripped (TS) metal surfaces on standard silicon/silicon-oxide wafers by means of gold cold-welding is presented, and cold-welded template-stripped (CWTS) platinum surfaces are demonstrated as an example. Due to the lack of any adhesives, the final TS metal “sandwiches” are fully compatible with all organic solvents, as well as ultra-high vacuum (UHV). The produced CWTS platinum surfaces are demonstrated to have the same surface properties—most importantly single-angstrom rms roughness—as previously studied TS platinum. The effectiveness of the cold-welding technique is shown to decrease as a function of time exposed to ambient laboratory conditions after removal from vacuum and prior to pressurized cold-welding. Contact angle measurements demonstrate this decrease to be due to a gradual increase in surface contamination of the evaporated gold layers prior to their being joined by cold-welding. The potential versatility and scalability of the CWTS fabrication process is briefly discussed.

Ultra-flat platinum surfaces from template-stripping of sputter deposited films

We report the fabrication of platinum (Pt) surfaces with single-angstrom flatness by template-stripping sputtered films from silicon wafers. For comparison, template-stripped (TS) gold (Au) is also produced from sputtered films using the same procedure. The surface wetting properties, refractive indices and extinction coefficients of the TS platinum are presented and compared against those of non-TS platinum films. The observed optical constants, as well as X-ray crystallography data, suggest that macroscopic properties of the sputtered TS platinum films are similar to previously studied evaporated platinum films [Surf. Interf. Anal. 29 (2000) 179]. Finally, scanning probe microscopy studies of the topography of TS Pt and TS Au sputtered metal surfaces are presented. Immediately after stripping, the TS platinum exhibits ∼1 Å root-mean-square roughness over one square micron and demonstrates higher fidelity to the templating silicon wafer than TS gold. The TS platinum surface also shows high physical stability in ambient laboratory conditions over the period of a week, whereas considerable topographical aging is observed in the case of the TS gold surface. The results are discussed in relation to the potential of TS platinum for use in molecular electronic systems.