Molecular Transport from an Atomic Force Microscope Tip: A

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Molecular Transport from an Atomic Force Microscope Tip: A Comparative Study of Dip-Pen Nanolithography Peter V. Schwartz* Department of Physics, California Polytechnic State University, San Luis Obispo, California 93407 Received November 6, 2001. In Final Form: March 6, 2002 Dip-pen nanolithography (DPN), the direct transport of a “molecular ink” onto a substrate from an AFM tip, is of interest both as a potential industrial patterning tool and as a means to study molecular behavior. We investigated DPN for the patterning of both octadecanethiol (ODT) and mercaptohexadecanoic acid (MHA). The molecular transport of ODT and MHA does not require a water meniscus but takes place because the molecules are individually mobile. The effects of relative humidity and temperature on the patterning of each molecule were investigated. While the molecular transport rate for MHA increases slightly with increased relative humidity, the rate of molecular transport for ODT exhibits no dependence on relative humidity (although relative humidity affects the resulting molecular distribution). The transport rate of both molecules increases exponentially with temperature, consistent with an activation barrier of about 0.75 eV. Although the molecular fluxes of MHA and ODT vary with tip preparation and atmospheric conditions, both molecules diffuse from an AFM tip at a rate of about 105 molecules/s, corresponding to a patterning time of 100 years/cm2.

Introduction Dip-pen nanolithography (DPN)1-6 has shown potential as a fast and simple surface patterning technology applicable for a variety of molecular “inks”. In order for DPN to be developed for industrial applications, a better understanding of the process must be established. The degree of patterning reproducibility and effects of environmental parameters, such as temperature, humidity, and concentration of vapors or coadsorbed solvents must be determined. The process may also vary over time with changes in concentration gradients, depletion of molecular ink, and evaporation of coadsorbed solvents. It has been proposed that the water meniscus, present on all surfaces at nonzero relative humidities,7,8 serves as the medium that transports molecules from the atomic force microscope (AFM) tip to the substrate. According to this theory,1-6 the water meniscus is pulled from the AFM tip to the substrate, carrying with it the molecules adsorbed on the AFM tip.9 To better understand the DPN process, we have conducted DPN experiments with two different molecular inks, octadecanethiol (ODT) and mercaptohexadecanoic acid (MHA). The molecules have about the same mass11 and differ only in the presence of a terminal carboxylic acid group in MHA, giving MHA a 65 °C melting * E-mail: [email protected]. (1) Piner, R. D.; Zhu, J.; Hong, S.; Mirkin, C. A. Science 1999, 283, 661. (2) Hong, S.; Zhu, J.; Mirkin, C. A. Science 1999, 286, 523. (3) Mirkin, C. A. MRS Bull. 2000, 25, 43. (4) Mirkin, C. A. Inorg. Chem. 2000, 39, 2258. (5) Hong, S.; Zhu, J.; Mirkin, C. A. Langmuir 1999, 15, 7897. (6) Maynor, B. W.; Li, Y.; Liu, J. Langmuir 2001, 17, 2575. (7) Piner, R. D.; Mirkin, C. A. Langmuir 1997, 13, 6864. (8) (a) Hu, J.; Xiao, X.-d.; Ogletree, D. F.; Salmeron, M. Science 1995, 268, 267. (b) Xu, L.; Lio, A.; Hu, J.; Ogletree, D. F.; Salmeron, M. J. Phys. Chem. 1998, 102, 540. (9) This molecular transport model is supported by reports of the humidity dependence of the molecular transport rate for octadecanethiol (ODT) onto gold. See refs 4 and 10. (10) http://www.sciencemag.org/feature/data/1044170.shl Supplemental Material on the web for ref 2. (11) Mass of octadecanethiol (ODT), 286 AMU; mass of mercaptohexadecanoic acid (MHA), 289 AMU.

Figure 1. Atmospheric conditions were controlled by means of a small enclosure around the AFM. Dry nitrogen either directly entered the chamber or was bubbled through water or ethanol. A heat exchanger controls temperature.

temperature compared to 32 °C for ODT. We studied the patterning process as a function of temperature, atmospheric vapor constituents (water and ethanol), and protocol for coating the AFM tip with the molecular ink. The patterning process proved to be much more complex than we anticipated. In fact, our findings indicate that the water meniscus is not universally responsible for DPN molecular transport, and we present an alternative model consistent with our observations. Experimental Section DPN patterning and AFM imaging were performed using a Thermomicroscopes “CP” AFM and silicon nitride gold-coated Microlever probes (using Microlever “A”). A small chamber (Figure 1) was constructed for the AFM to control temperature and atmospheric vapors during the patterning process. Dry nitrogen could be either fed into the chamber or bubbled through either Nanopure water or ethanol, allowing control of vapor constituents. Temperature was controlled with a heat exchanger immersed in either liquid N2 or hot water. The small size of the chamber (about 3 L) allowed for rapid change of environmental conditions. Humidity and air temperature were monitored with a humidity probe,12 and the temperature of the gold substrate was monitored with a thermocouple. Polycrystalline gold surfaces (root mean square roughness 2-3 nm) were formed via vacuum evaporation of about 60 nm of gold after evaporation of a 10 nm titanium adhesion layer onto atomically smooth Si(100) wafers at room temperature, without breaking vacuum between the two evaporations. (12) Fisher Scientific, Fisherbrand Certified Traceable Digital Hygrometer/Thermometer, Instant model.

10.1021/la011652j CCC: $22.00 © 2002 American Chemical Society Published on Web 04/17/2002

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Solution Deposition. AFM tips were coated without prior cleaning by dipping for a few seconds in a 1 mM solution of ODT or MHA in acetonitrile. Generally, tips were dipped three times before being used for patterning. The tips were gently blown dry with difluoroethane13 immediately after each dipping. Vapor Deposition. After the microlevers were soaked for 30 min in piranha solution14 and rinsed with Nanopure water, they were placed in a small (∼10 mL) metal container with 200 mg of ODT. The temperature was then raised to 45 °C for about 10 min.15 Although MHA was successfully applied via vapor deposition at 80 °C, the molecule proved to be immobile unless deposited on the AFM tip via acetonitrile solution deposition (see section Patterning of MHA: Dependence on Method for Coating AFM Tip). Patterning and Imaging Protocol. Dots and lines were DPN patterned onto a gold surface with an AFM tip coated with either ODT or MHA. The contact force varied from 0.3 to 1.0 nN.16 The lines were patterned vertically (slow scan direction) with a single stroke at predetermined speeds. Immediately after each patterning attempt, the surface was imaged with the same AFM tip, using horizontal scans (fast scan direction, left to right) in lateral force microscopy mode (LFM, see Figure 2, low friction is shown as dark). Dot dwell times and line writing speeds are labeled on the LFM image. The average LFM signal (shown below each LFM image in Figure 2) was calculated from all the horizontal scans that traversed the patterned lines (indicated by the boxed region of the LFM images in Figure 2). ODT is lower friction than gold while MHA is higher friction than gold. All LFM images were flattened such that the signal corresponding to bare gold was zero and were otherwise left unedited. Calculation of Molecular Coverage. ODT and MHA bond to the gold surface through the sulfur-gold interaction, forming ordered monolayers with a molecular surface density of 4.64 molecules/nm2.17,18 The molecular dose to the surface is then simply the product of the surface area of the patterned line and the molecular density. However, at high AFM tip speeds, full monolayer coverage is not always achieved. In the resulting thinner lines, the ODT molecules self-assemble in isolated patches5 and the LFM signal does not reach the same (low friction) level attained for the wider, full coverage lines patterned at lower AFM tip speeds (Figure 2). The molecular coverage of these sparser lines is not simply proportional to the line width. Thus, there is a need to define an “effective line width”, which represents the reduced width the line would have if the molecules assembled at full monolayer density. To estimate the effective line width, we presume that the LFM signal for less-than-full coverage lines corresponds to the weighted average of the areas of bare gold and islands of ODT. This measurement is thus proportional to the monolayer island coverage and is therefore proportional to the number density of thiol molecules. Effective line width was calculated by dividing the integrated area of the peak on the averaged LFM cross section by the LFM signal corresponding to full coverage. The integrated LFM signal appears as a stepped line in Figure 2 below each image. This method yields the width of the line that would have resulted if all the molecules had formed a straight, uniform line of full density self-assembled monolayer (SAM). Throughout the paper, the term “line width” refers to the effective line width as described above, allowing the calculation of flux to be straightforward (see Discussion). (13) Dust-Off, Falcon Safety Products, Inc., Branchburg, NJ, 08876. (14) 30% H2O2 and concentrated H2SO4. Danger! Reacts violently with organic substances. (15) These conditions resulted in AFM tips that were well-coated with ODT. As vapor pressure is acutely dependent on the bulk heat of vaporization, we suspect that this vapor deposition process is only applicable to molecules with bulk heats of vaporization that lie within a very narrow range. ODT has a vapor pressure of about 2 × 10-5 Torr at 45 °C, whereas MHA requires a temperature of about 80 °C to achieve the same vapor pressure (estimated from: Yaws, C. L.; Lin, X.; Bu, L. Coefficients for Vapor Pressure Equation; Lamar University: Beaumont, TX). (16) Previous work has indicated that patterning behavior is independent of contact force. Also see ref 5. (17) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991. (18) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481.

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Figure 2. (a) Lines generated on a gold surface via the DPN of ODT (octadecanethiol, vapor deposited on the AFM tip, 26% relative humidity, ∼27 °C, 0.3 nN contact force) appear dark (lower friction than the gold surface) on LFM (lateral force microscopy). (b) Lines generated via the DPN of MHA (mercaptohexadecanoic acid, solution deposited on the AFM tip, 0% relative humidity, ∼27 °C, 1.0 nN contact force) are higher friction than the gold surface and appear light. Averaging all the horizontally scanned lines in the boxed area of each image yields an average LFM cross section (below each image) and an integrated peak area (the “stepped line” in the cross section) from which normalized line widths are calculated. The size of the vertical step is taken to be the integrated monolayer coverage over the width of the line (see section Calculation of Molecular Coverage). Under these low humidity conditions (less than 50% relative humidity), patterns produced from solution deposited ODT and vapor-deposited ODT are indistinguishable. Reproducibility. The molecular flux varies greatly between experiments, even if the tip-coating protocol and environmental conditions are kept constant. In fact, both solution and vapor coating protocols often resulted in an AFM tip that did not pattern at all. Therefore, we cannot assume that the molecular flux will be the same for two tips that have undergone the same tipcoating protocols. Another result of this variability is that none of the three diffusion processes investigated (solution-deposited MHA, and vapor- and solution-deposited ODT) can be identified as being faster than any other. During a single experiment the diffusion rate varied significantly, as is evident from the scatter in the experimental data.

Molecular Transport from an AFM Tip

Figure 3. Line widths as a function of AFM tip speed, collected from six patterning attempts at three different temperatures: two attempts at 13 °C (circles), two attempts at 26 °C (triangles), and two attempts at 36 °C (squares). These data resulted from measurements of images represented by Figure 2, produced from DPN (ODT solution deposited on the AFM tip). The data nicely fit a model (see text: Mathematical Diffusion Model).

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Figure 5. Line width is plotted as a function of relative humidity for DPN of octadecanethiol (ODT) vapor deposited on an AFM tip. Data for three AFM tip speeds are shown: 0.03 µm/s (squares), 0.06 µm/s (crosses), and 0.30 µm/s (triangles). Lines were DPN generated and LFM imaged under various relative humidity (temperature remained constant). Line widths were calculated from LFM images as represented in Figure 2.

Figure 6. Same as Figure 5 except that octadecanethiol (ODT) was solution deposited on AFM tip. Data for two AFM tip speeds are shown: 0.012 µm/s (squares), and 0.03 µm/s (crosses).

Figure 4. Temperature dependence of flux of ODT (solution deposited on an AFM tip). ODT lines were DPN generated and LFM imaged at different temperatures at 0% relative humidity at AFM tip speeds ranging from 0.03 to 1.2 µm/s. Line widths were measured as in Figure 2, and molecular flux was calculated from fitting data, as in Figure 3 (see Results). An Arrhenius plot of molecular flux yields an activation barrier to diffusion of about 0.74 eV. The diffusion rate changed over longer periods of time (whether or not the AFM tip was patterning) or after a temperature cycle. Variations in diffusion rate over time made a precise flux measurement difficult, as each measurement took 5-10 min, and many measurements are required to determine mean values for the diffusion rate with reasonable certainty.

Results Line width is plotted as a function of reciprocal writing speed in Figure 3 for six data sets patterned at three different temperatures with the same AFM tip. If the rate of molecular transport was the same for all AFM tip speeds, there would be a linear relationship between line width and inverse writing speed. The less-than-linear dependence of line width on inverse writing speed indicates a decrease in the rate of molecular transport from the AFM tip with increased line width (lower writing speeds). A mathematical diffusion model, developed in the Discussion to address this phenomenon, is fit to the data. Temperature Dependence. Molecular transport rates were calculated from data represented in Figure 3. The molecular flux of both ODT and MHA exponentially increase with temperature. Figure 4 is an Arrhenius plot of the molecular flux of ODT. A linear curve fit yields an energy barrier to diffusion of about 0.74 eV. Because full

Figure 7. Same as Figure 5 except that the patterning substance was mercaptohexadecanoic acid (MHA) solution deposited on AFM tip. Data for three AFM tip speeds are shown: 0.06 µm/s (squares), 0.12 µm/s (crosses), and 0.60 µm/s (bars). Best-fit lines are shown.

monolayer coverage may not have been achieved for the cold temperature (13 °C) depositions, molecular fluxes for these conditions may be slightly lower than indicated. The data imply an activation barrier to diffusion for MHA that is about the same as that for ODT. The uncertainty in the measurement is greater than the perceived difference, so no comparison with confidence can be made. Humidity Dependence of Molecular Diffusion Rate of ODT and MHA. The results of humiditydependent DPN patterning of ODT are shown in Figure 5 (vapor deposited on the AFM tip) and Figure 6 (solution deposited on the AFM tip). While no humidity dependence is shown from 0% relative humidity (RH) to 100% RH for ODT, the diffusive transport of MHA (solution deposited on an AFM tip, Figure 7) exhibits a molecular diffusion rate that has a small dependence on humidity. Patterning of MHA: Dependence on Method of Coating AFM Tip. Where MHA exhibited diffusive transport immediately after solution deposition, MHA

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Figure 8. Effect of vapors on line profile. (a) Lines of ODT (solution deposited on AFM tip) were DPN generated and LFM imaged in a dry nitrogen environment. (b) Lines of ODT (vapor deposited on AFM tip) were DPN generated and LFM imaged25 in an environment high in water vapor. (c) Lines of ODT (solution deposited on AFM tip) were DPN generated and LFM imaged in an environment of ethanol vapors. All experiments were done at room temperature. Below each LFM profile is a representation of the corresponding ODT molecular distribution that is consistent with both the corresponding LFM profile as well as the bulk behavior of ODT (see Vapor Dependence).

could no longer be patterned after the AFM tip was left under vacuum for 2 h. Vapor deposition of MHA also did not produce diffusive patterning of MHA. The mobility of MHA may be enhanced by the coadsorbed acetonitrile or the difluoroethane13 used in the solution-deposition process. Qualitative Effect of Water and Ethanol Vapors. Experiments were also conducted with ODT in an atmosphere containing varying amounts of ethanol vapor. Ethanol vapor, like water vapor, did not measurably affect the rate of ODT molecular transport. However, the presence of each vapor had significant effects on the molecular distribution in the DPN patterns. While high RH often produced “double lines” with little deposition of ODT in the middle of the line (Figure 8), high concentration of vapors of ethanol (in which ODT is very soluble) produced an ODT surface distribution that is very peaked in the middle and has very poor edge resolution. Discussion Recent studies19 using sum-frequency generation (SFG) and scanning polarization force microscopy (SPFM) have shown that below 20% RH, less than a single layer of water molecules exists on a mica surface. The surface of mica, due to the presence of exposed potassium ions, is more hydrophilic than gold. Therefore, at relative humidities less than 20%, the surfaces of the gold and the AFM tip should have a surface population of water molecules that is even less than that of mica. Thus, it is (19) Miranda, P. B.; Xu, L.; Shen, Y. R.; Salmeron, M. Phys. Rev. Lett. 1998, 81, 5876.

reasonable to presume that the DPN transport of MHA and ODT at less than 1% RH took place in the absence of a water meniscus. Although earlier publications1-6 propose that DPN uses a water meniscus as the transport medium, our results indicate that molecular transport can occur in the absence of a water meniscus. Thus, the water meniscus cannot be universally responsible for DPN molecular transport and is specifically not responsible for the transport of either ODT or MHA. Furthermore, under conditions of high RH, the presence of a water meniscus can actually inhibit the hydrophobic ODT molecules from contacting the gold substrate. These observations are consistent with early hexadecanethiol microcontact printing studies, where edge resolution was improved if the process was conducted underwater.20 The exponential temperature dependence of the diffusion rate, indicating that ODT and MHA molecules individually diffuse across the surface, is also inconsistent with a model of transport requiring a water meniscus. Although the modulation of temperature can control the DPN process, the maximum flux will be limited due to the vapor pressure of the molecular ink. We suspect that we approached this limit by patterning ODT at 36 °C, only 9 °C below the temperature for the vapor deposition of ODT on the AFM tip. Our tip-coating protocols yielded a maximum molecular flux of about 105 molecules/s, consistent with previous reports,10 and corresponds to a patterning time of about 100 years/cm2. Physical Diffusion Model. We present here two different, but compatible, models to explain the molecular (20) Xia, Y.; Whitesides, G. M. J. Am. Chem. Soc. 1995, 117, 3274.

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transport constituting the DPN process. The degree to which each model is applicable depends on the characteristics of the patterning molecule: (1) Nonpolar Molecules Transport via Simple Molecular Surface Diffusion (Reactive Spreading). The transport behavior of the less-polar molecules is not consistent with a water-meniscus-driven transport model. The process is consistent with simple molecular diffusion and Arrhenius temperature dependence. This diffusive transport is responsible for the success of microcontact printing,21 where the diffusion and surface bonding process is referred to as “controlled reactive spreading”.22 (2) Small Ions Transport as a Solution in Adsorbed Waters of Hydration. Recent reports6 of the deposition of gold onto a silicon substrate via the DPN of a gold salt (HAuCl4) require a second model to explain ionic transport. The transport behavior of salts should be more consistent with the water-meniscus-driven molecular transport originally attributed to the DPN of ODT and MHA,1-5 since ions would be expected to be essentially immobile unless dissolved in water. This model is supported by the reported humidity dependence of the patterning rate of the above-mentioned gold deposition process.6 Also consistent with this model is our observation that the patterning of DNA (an ionic polymer) with an AFM tip depends crucially on humidity.23,24 The DNA remained immobile on the AFM tip until the RH was greater than 90%, at which point large drops (1-100 µm) of bulk DNA solution transferred onto the gold surface. The degree of polarity of the patterning molecule should determine where, between the extremes of these two models, the patterning behavior falls. Increased RH may weaken the dipole interactions of more polar molecules through insertion of waters of hydration. MHA and ODT are almost identical molecules, except that ODT is largely nonpolar, whereas MHA has a polar carboxylic group, resulting in higher intermolecular attraction. We have shown that increased RH increases the molecular flux for MHA, and not for ODT. If thermal molecular mobility rather than the previously proposed water meniscus “conveyor-belt” is responsible for DPN transport, then each molecule will have unique mobility and DPN cannot be thought of as a universal lithography for all molecular inks. We have shown that even the very similar molecules of MHA and ODT have different transport behavior: MHA must be solutiondeposited to be mobile at room temperatures and has a diffusion rate that is affected by humidity. Mathematical Diffusion Model. Given that the AFMbased patterning of ODT and MHA is the result of diffusive molecular transport, we can now develop a mathematical diffusion model that fits the data of Figure 3. The rate at which the gold surface is covered with monolayer can be found from the ratio of the y value to the x value in Figure 3:

y w∆l ∆area ) wv ) ) x ∆t ∆t

(1)

where w is the line width, v is the speed of the AFM tip during the patterning process, and l is the distance that (21) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1998, 37, 550. (22) Bain, C. D.; Whitesides, G. M. Langmuir 1989, 5, 1370. (23) Despite claims that DNA has been successfully patterned via DPN (see refs 3 and 4), our efforts to reproduce these results (data not shown) indicate that the resulting patterns are not DNA. (24) Schwartz, P. V. In preparation. (25) Humidity can affect friction and may thus affect the resulting LFM image contrast.

Figure 9. An AFM tip forms a monolayer via DPN. The cross sectional view is of an AFM tip as it moves across a gold substrate, out of the paper toward the reader. A line of width, w, forms under the tip as the ink molecules attach to the gold surface at the edge of the monolayer. The area where there is a concentration gradient of the mobile patterning molecules is between the dotted line (where the molecular concentration approaches that of the average coverage of the cantilever) and the edge of the monolayer where the concentration is zero.

the AFM tip has moved. The flux of molecules to the surface, F, is found by multiplying eq 1 by the monolayer surface density, F ) 4.64 molecules/nm2

F ) F∆A/∆t ) Fwv

(2)

Adopting Fick’s first law to the two-dimensional case of a point source (the point of a coated AFM tip) on a monolayer surface, the molecular flux to the surface is

F ) 2πrD

dC dr

(3)

where r is a radius on the monolayer surface around the AFM tip, D is the diffusion constant, and dC/dr is the concentration gradient at radius r. Time-dependent surface diffusion experiments done via microcontact printing20 indicate that the width of the line made by reactive spreading of hexadecanethiol on gold obeys the surface diffusion equation

w ) (2Dt)1/2

(4)

where w is the width of the line, D is the diffusion constant, and t is time. We can establish a similar relationship for our data, where the inverse speed of the AFM tip is substituted for time. Below, we present a rough model that will allow us to estimate molecular flux from the data as well as explain the shape of the graphs in Figure 3. Figure 9 illustrates the DPN process from an AFM tip that is moving outward, toward the reader. The molecules diffuse down the AFM tip, across the monolayer, and anchor onto the gold surface where the molecular concentration is zero. We approximate the concentration gradient of molecular ink as the quotient of the molecular concentration at the source divided by the total distance the molecules must travel. We place the “molecular source” on the AFM tip some distance, l0, from the contact point (Figure 9), so the total distance that the molecules must travel is w/2 + l0. Substituting this quotient for the concentration gradient in eq 3 and equating the flux to eq 2 we find

F ) 2πrD

C dC ) 2πrD ) Fwv dr (w/2) + l0

(5)

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Rearranging terms, we find

G ≡ [4πrDC/F] ) vw2 + 2vwl0

(6)

where G is defined as the product of quantities that should remain constant throughout a single measurement (the patterning of several lines as depicted in Figure 2). The physically relevant solution to this quadratic equation is

w ) -l0 +

(Gv + l ) 2

0

1/2

(7)

Equation 7 nicely fits the data in Figure 3, where l0 ) 148 nm for all experiments conducted with the same tip. Line widths less than l0 scale rather linearly with inverse writing speed. However, l0 does not represent any physical length on the AFM tip, as some geometric considerations (such as the AFM tip radius and contact area) were neglected. At high writing speeds, v, eq 7 reduces to

w ≈ G/2vl0

(8)

consistent with the linear behavior we see in Figure 3 near the origin. At low writing speeds

w ≈ (G/v)1/2

(9)

consistent with the square root dependence observed in Figure 3 at low AFM tip speeds. The molecular flux, as well as the fitting parameter, l0, varied greatly between experiments done with different tips, despite apparently identical tip coating protocols. In particular, we observed the linear regime (eq 8) to persist for line widths greater than a micrometer, and we have observed the molecular diffusion rate to vary by over an order of magnitude (compare Figures 5 and 6). Because molecular flux decreases with increasing line width, it was necessary to define a distinct molecular flux for each set of patterned lines in order to compare fluxes for different temperatures (Figure 4). Equation 2 was used to find this flux in the regime of high writing speeds (eq 8), where line width is still proportional to inverse writing speed (near the origin in Figure 3) and flux is independent of the AFM tip speed, v. Vapor Dependence. Several studies7,8 have shown that the surface contact of an AFM tip on a mica substrate results in an increased surface water concentration through capillary action. Our data (see Figure 8b) indicate that at higher RH, the water meniscus in the vicinity of the AFM tip is thick enough to prevent the attachment of ODT onto the gold surface. This “surface protection” effect provides a measurement of the spatial extent of the “capillary meniscus thickening” as a function of RH and AFM writing speed. Because nonpolar ODT is not soluble in water, the lowest energy state would be for the ODT molecules to be at the air/water interface of the water meniscus. Figure 8b at the bottom depicts this nanoscopic analogue to bulk behavior, consistent with the double line produced by ODT under high humidity. However, ODT is

very soluble in ethanol, and the molecule appears to readily diffuse through a meniscus of ethanol, as illustrated in Figure 8c. The highly peaked line profile that increases in depth (molecular number density) with decreasing writing speed, as well as the poor edge definition, is an observation consistent with a meniscus that readily dissolves the patterning molecule. Figure 8c at the bottom is a nanoscopic representation of the patterning of ODT dissolved in ethanol. Correlation between Patterning Behavior and Macroscopic Characteristics. The nanoscopic patterning behavior of each molecular ink correlates well with the macroscopic behavior of the substance. Because ODT melts at about room temperature, ODT molecules might be expected to be diffusively mobile near room temperature. As room temperature is about 40 °C below the 65 °C melting point of MHA, it is likely that vapor-deposited MHA would not be patternable at room temperature. However, we might expect MHA to be diffusively mobile at higher temperatures such as 70 °C. The effect of vapors on the resulting line profiles of ODT depicted in parts b and c of Figure 8 is consistent with observations of oil mixing with either water or ethanol, respectively. The apparent inability of thiol-modified DNA to pattern by a diffusive process from an AFM tip23,24 also correlates well with characteristics of the bulk substance: DNA solution becomes progressively thicker and more viscous as it dries. Macroscopic observations such as melting, solubility, viscosity, and adhesion are useful in predicting the nanoscopic patterning behavior of a substance. Conclusion We have studied the effect on DPN patterning of different parameters including temperature, relative humidity, ethanol vapors, and tip-coating protocol. Each variation yields interesting information about intermolecular attraction and molecular dynamics. Thus DPN can be used to investigate molecular properties. We have been able to make a preliminary assessment of how the DPN process works for two distinct molecular inks. Octadecanethiol (ODT) and mercaptohexadecanoic acid (MHA) can be readily patterned on a gold substrate at 0.0% relative humidity. Thus, the water meniscus is not responsible for their molecular transport to the substrate. The exponential temperature dependence of the molecular flux of MHA and ODT indicates that the DPN transport of these two molecules is a surface diffusion process dependent on thermal energy. Acknowledgment. P.V.S. gratefully acknowledges careful reading of the manuscript by Christopher Chidsey (Stanford), Miquel Salmeron (UC, Berkeley), Ly-Lan Lofgren, and from Cal Poly Kathy Chen, Chance Hoelworth, Dane Jones, Brooke Silvers, Linda Vanasupa, and John Walkup and machining by Thang Bui, Northwestern. Data were collected at Northwestern University in the laboratory of Chad A. Mirkin. LA011652J