Quantifying Liquid Transport and Patterning Using Atomic Force

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Quantifying Liquid Transport and Patterning using Atomic Force Microscopy Nikolaos Farmakidis, and Keith A Brown Langmuir, Just Accepted Manuscript • Publication Date (Web): 10 May 2017 Downloaded from http://pubs.acs.org on May 11, 2017

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Quantifying Liquid Transport and Patterning using Atomic Force Microscopy Nikolaos Farmakidis1,2 and Keith A. Brown1,3,* 1

Department of Mechanical Engineering, Boston University, Boston, MA, 02215, USA

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Present address: Department of Mechanical Engineering, Columbia University, New York, NY, 10027, USA 3

Division of Materials Science & Engineering and Physics Department, Boston University, Boston, MA, 02215, USA *

Correspondence to be addressed to [email protected]

Abstract:

Atomic force microscopy (AFM) provides unique insight into the nanoscale properties of materials. It has been challenging, however, to use AFM to study soft materials such as liquids or gels because of their tendency to flow in response to stress. Here, we propose an AFM-based technique for quantitatively analyzing the transport of soft materials from an AFM probe to a surface. Specifically, we present a method for loading an AFM probe with a single 0.3 to 30 pL droplet of liquid, and subsequently measuring the mass of this liquid by observing the change in the vibrational resonance frequency of the cantilever. Using this approach, the mass of this liquid was detected with pg-scale precision by a commercial AFM system. Additionally, sub-fL droplets of liquid were transferred from the probe to a surface with agreement found between the real-time change in mass of the liquid-loaded probe and the volume of the feature written on the surface. To demonstrate the utility of this approach in studying nanoscale capillary and transport phenomena, we experimentally determine that the quantity of liquid transported from the tip to a surface in a given patterning operation scales as the mass of liquid on the probe to the 1.35 power. In addition to providing new avenues for studying the dynamics of soft materials on the

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nanoscale, this method can improve nanopatterning of soft materials by providing in situ feedback.

Introduction

Atomic force microscopy (AFM) is a powerful tool for imaging properties on the nanoscale.[1] By choosing an appropriate probe, AFM has also been used to map the topography and viscoelasticity of soft materials such as gels.[2-4] Simple liquids have also been studied using AFM through measurements of the forces exerted by capillary bridges,[5-9] which can provide measurements of surface tension.[10] In addition to these measurements of static properties, a great deal of insight into the dynamics of nanoscale liquid flows comes from a community focused on using scanning probes for lithography.[7,11-14] Specifically, dip-pen nanolithography (DPN) comprises a patterning technique wherein a scanning probe is coated with a material of interest, then used to locally transfer this material to a surface.[15] This approach has attracted attention because of its ability to additively pattern soft and biological materials with sub 100 nm resolution.[16,17] There has been a sustained effort to establish mechanistic models of this patterning process by studying the size of written features as a function of patterning parameters.[16] For example, small water-soluble molecules that selfassemble into monolayers are known to diffusively transport through the water meniscus that condenses when a probe touches a surface in a humid environment.[18,19] When patterning liquids or gels, a complex set of scaling relationships has been observed that has not coalesced into a unified picture of transport.[7,11-13,20] One major challenge stymying the development of a unified model of transport originates from the lack of a method to quantify the amount of

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liquid on a probe in situ, despite it being acknowledged that transport dynamics are affected by how much liquid is on the probe.[14]

Here, we present a method for selectively depositing and quantifying liquid on the tip of an AFM probe. Specifically, we find that by dip-coating the tip of a hydrophobic probe in a liquid bath, a controlled quantity of liquid can be withdrawn in a discrete pendent droplet at the tip of the probe. The mass of this droplet is sufficiently large that it produces an appreciable (~1 kHz) shift in the vibrational resonance frequency of the cantilever. This frequency shift, combined with the localized nature of the liquid droplet, allows for the determination of droplet mass. Importantly, once loaded on the tip, the droplet mass can be monitored in real time, an attribute that enables us to quantitatively study liquid transfer from the tip to a surface. These experiments allow us to verify that the quantity of liquid transferred matches the volume deposited on the surface. Finally, we perform first of their kind experiments in which liquid transfer is studied from probes loaded with different quantities of liquid. Importantly, these experiments reveal that the volume transferred in a given patterning operation increases with the mass of liquid on the probe to the 1.35 power. In addition to noting how this relationship could be useful for patterning liquids using a scanning probe, we explore how this power law provides a connection to the broader literature related to liquid transfer from surface tension-driven flows. Thus, our results provide a new path for studying nanoscale liquid transport and an avenue for performing closed-loop DPN in a manner that can transform the reliability and reproducibility of the technique.

Experimental

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Atomic force microscope probes (MikroMasch – HQ:CSC17/No Al) were used as received unless otherwise noted. These probes consist of 50 µm wide cantilevers with ~14 µm tall conical tips that converge to an 8 nm radius of curvature. The use of a single model of AFM probe allowed us to hold the tip- and cantilever-geometry constant across all experiments. Fluorescein isothiocyanate (FITC) and glycerol were purchased from Sigma Aldrich and used as received. Silicon chips (University Wafer) for patterning experiments were rendered hydrophobic by treating them with O2 plasma for 5 min and subsequently placing in a vacuum chamber containing a vial with 1 mL perfluorodecyltrichlorosilane (FDTS) and 4 mL toluene for 90 min. After this vapor coating process, the silicon chips were heated on a hot plate for 10 min at 100 oC, then sonicated in toluene for 5 min, sonicated in ethanol for 5 min, and finally dried under a stream of N2. All liquid transfer, weighing, and patterning experiments were performed using an Asylum MFP Infinity AFM. All patterning experiments were performed transferring glycerol onto fluorosilanized silicon surfaces with a 20 pN repulsive set point. Dwell time was not studied here because prior work has shown that it plays a minor role when studying transfer onto hydrophobic surfaces with fast withdrawal speeds (>10 µm/s).[7] Dark field and fluorescence microscopy was performed with an Olympus BX43 microscope.

Results and Discussion

The addition of a liquid droplet to an AFM probe leads to a perceptible shift of the vibrational resonance frequency f0 of the cantilever. More generally, cantilever-based microelectromechanical systems (MEMS) have been used extensively for mass detection by monitoring changes in f0. Typically, AFM probes are analyzed in a lumped element framework

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as a spring-mass-dashpot system (Figure 1a) where the cantilever defines a spring constant k and an effective mass M that is smaller than the total mass of the system.[21] In a typical experiment to explore how liquid loading affected the mechanics of an AFM probe, k and f0 were found for a dry probe using a standard protocol that comprised observing the thermal power spectral density (PSD) of cantilever motion and subsequently measuring cantilever deflection while moving the probe towards a rigid silicon surface.[22] Fitting the thermal PSD to a Lorentzian plus a constant background yielded k = 0.200 ± 0.002 N/m, f0 = 13.1044 kHz ± 0.0005 Hz, M = 29.5 ± 0.2 ng, and a quality factor Q = 52.9 ± 0.2 (Figure 1a). Importantly, the same probe can, without removing it from the AFM system, be dipped into a bath of glycerol. Since the position of the laser spot does not change during this dipping process, it is not necessary to bring the probe back into contact with a solid surface for re-calibration – only the non-contact observation of the thermal PSD is required. Interestingly, after the probe tip was lowered 5 µm into glycerol and then withdrawn, f0 was observed to decrease by nearly 20%, with k = 0.201 ± 0.004 N/m, f0 = 11.707 kHz ± 1 Hz, M = 37.1 ± 0.8 ng, and Q = 56.9 ± 0.6 for the liquid-loaded probe (Figure 1b). While this measurement clearly demonstrated that the mass shift from added liquid is perceptible, the analysis of this shift depends upon the spatial distribution of the liquid.

In order to quantify the mass of liquid on an AFM probe, it is important to explore the spatial distribution of that liquid. Initially, we coated a scanning probe with liquid using a protocol that is widely used to prepare probes for nanopatterning experiments.[23,24] Specifically, an AFM probe was rendered hydrophilic by exposing it to an O2 plasma for 3 min. Following plasma treatment, the probe was immersed in a 10 mM bath of the fluorescent dye fluorescein isothiocyanate (FITC) in glycerol. After removing the probe from the solution, optical microscopy revealed that liquid coated the entire surface of the probe (Figure 2a-c).

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While one could estimate the distribution of liquid and calculate a relationship between the frequency shift and the mass of the liquid, it would require a number of assumptions about the details of the coating. Furthermore, it is not clear what, if any, of the liquid present on the probe will participate in transfer as the majority is located on the cantilever far from the probe tip. While studying the tip-sample force during transfer could provide a means of elucidating the liquid configuration,[7] we hypothesized that confining the liquid to a drop at the tip of the probe would provide more controlled and simpler experiments to analyze. As a means of having a more controlled distribution of ink on the probe, we utilized probes that had not been plasma cleaned, and therefore were non-wetting with a contact angle of ~45º. When these hydrophobic probes were dip-coated into a glycerol bath, the result was a single microscale pendant droplet at the tip of the AFM probe (Figure 2d-e).

Having recognized that it is possible to deposit liquid exclusively on the tip of a probe by utilizing non-wetting probes, we developed a protocol for transferring a controlled quantity of liquid to the probe (Figure 3a). Specifically, an AFM probe was controllably lowered into a bath of glycerol while under feedback control. The AFM was programmed to lower the probe until reaching an attractive set point force of 38 nN. This method is in contrast with most forcedistance measurements that lower the probe until a particular repulsive force is reached. Choosing an attractive set point force is critical because contact with the liquid surface results in an attractive capillary force. After locating the surface of the bath, the AFM was programmed to travel a fixed insertion depth D beyond this initial contact point. Subsequently, the probe was retracted from the liquid. It is worth noting that even when using an attractive set point to limit how deeply the AFM probe extended into the glycerol drop, hydrophilic probes that underwent

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this process were completely coated with glycerol as the liquid wicked up the entire probe before they could be withdrawn.

Measurements of the force exerted on the probe while immersing the probe in liquid provide insight into the behavior of the liquid during transfer (Figures 3b,c). Upon contact with the glycerol bath, a capillary bridge formed between the tip and the bath. This wetting process results in a strong attractive force pulling the probe down into the liquid, giving a clear force signature to the exact point at which the probe comes into contact with the liquid. The attractive force increased as the probe was lowered into the glycerol and as it was withdrawn from the drop. Interestingly, the attractive force persisted even after the probe had raised numerous micrometers above the initial contact point, indicating that some combination of wetting up the probe and capillary bridge stretching were taking place. Importantly, when the capillary bridge finally ruptured, a stable liquid droplet remained on the tip, as observed both optically and through the change in f0. Furthermore, immersing the probe in DI water after this transfer process resulted in f0 returning to within 0.05% of the original value (Figure S1), reflecting that (1) the change in frequency was due to a reversible process, and (2) in the case of this water-miscible liquid, probes could be cleaned and reused.

Having developed a reversible procedure for loading a liquid droplet at the tip of the AFM probe, we sought to determine the factors that dictate the quantity of liquid transferred to a probe. Specifically, we hypothesized that D is a key determinant of liquid transfer. In order to study the effect of varying D, we performed a series of liquid transfer experiments in which a probe was cleaned with DI water and then inserted into a bath of glycerol with increasing D. Following every measurement, ∆M was quantified by measuring f0. Interestingly, we found two

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qualitatively different regimes of D, for D < 400 nm, the liquid was able to dewet the probe resulting in ∆M ~ 0. However, for D > 400 nm, we find ∆M > 200 pg with greater D corresponding to more liquid transfer (Figure 3e). As expected, these results show a monotonic increase in ∆M with increasing D. We hypothesize that the range in D in which no transfer takes place is related to the geometry and surface energy of the AFM probe. It is worth elaborating on how one can estimate changes in M with pg precision. Using this commercial AFM, standard protocols for collecting thermal PSD, and conventional non-linear least squares fitting, we are able to find f0 with error approaching one part in ten thousand because of the precision inherent to these methods. Computing changes in mass also requires knowing k, which we may only estimate within a few percent from a single measurement, limiting measurements of M to precisions of ~200 pg. However, if we assume that k is not changing as M changes, a good assumption given that the compliance of the probe is dictated by the cantilever which remains dry during this process, then uncertainty in ∆M is ~5 pg as it is dictated by the uncertainty in f0 alone, which is smaller than one part in 10,000.

While we assume that the shift in f0 is due to the addition of mass at the tip of the probe, this is challenging to verify through measurements of the static state of the probe. Instead, we hypothesize that it is possible to quantify liquid transport from the probe to a surface as a means of verifying that the frequency shift is reflective of liquid loading. In particular, we performed experiments in which a probe was loaded with a glycerol solution of FITC, and then brought into contact with a dry hydrophobic silicon surface to allow liquid to flow from the tip into discrete drop features on the surface. In a typical liquid transfer experiment, the probe was moved towards the substrate at 10 µm/s until it reached a 20 pN repulsive force. The probe was then held in contact for 100 ms, and finally withdrawn at 100 µm/s. Importantly, quantifying the mass

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of liquid on the probe before (∆M) and after (∆M – δm) transfer of liquid onto a surface (i.e. patterning) allowed for an estimate of the mass lost during the transfer event δm (Figure 4a). Following patterning, the written features were measured using fluorescence microscopy as a means of computing their area (Figure 4b). By assuming that the contact angle of these microscopic drops match the contact angle for glycerol on this surface (found to be 105º), the feature volume V was estimated from optical methods alone. By plotting δm vs. V, we find a clear correlation (Figure 4c) in which the data fall along a line defined by the density of glycerol, further confirming the quantitative link between frequency shift and liquid on the AFM probe.

Detailed knowledge of the state of the liquid on the probe, as well as real-time measurement of features as they are transferred, provides access to new types of experiments to determine the factors that govern capillary-driven flows at the nanoscale.[25] For example, we hypothesized that the quantity of liquid on the probe will directly determine the quantity of liquid transferred in a given patterning event. This hypothesis is motivated by studies in which liquid drops were confined between two plates with different surface energies, and the volume remaining on each plate was quantified after pulling the plates apart.[26-29] Importantly, the fraction of the total liquid remaining on each plate was found to be scale independent. This similarity suggests that, even at nanoscopic scales, the volume of transferred liquid would be proportional to the volume of liquid on a probe.

In order to test the hypothesized connection between liquid loading and transfer, we performed a series of experiments wherein a probe was loaded with total liquid ∆M, and the average volume V of four features transferred under otherwise identical conditions were recorded (Figure 4d). As expected, a positive correlation was observed between ∆M and V. A consistent

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power law scaling was found between ∆M and V that persisted over more than two decades in both ∆M and V. Interestingly, we find V ∝ ∆M1.35 ± 0.07, which is different than V ∝ ∆M, as found in the classical two plate result. While this 1.35 power law scaling is different from the value of 1.0 found in plate-plate transfer experiments,[26-29] a non-linear relationship between initial liquid volume and volume of liquid transferred has been observed in other processes with nontrivial geometries, such as gravure printing.[30] Thus, we attribute this difference in power law scaling to the geometrical difference between the cone-plate transfer experiment presented here and the classical two plate transfer experiment. To test this hypothesis, this empirical power law can be found for different tip architectures including sphere-tipped probes and blunt-tipped probes. Additionally, the apparent contact angle can change with droplet radius,[7,31] further complicating interpretation of this power law at sub-micrometer length scales.

Summary and Conclusions

In summary, we have developed a method to place liquid on the tip of an AFM probe, quantify the amount of liquid present, and then track the mass of liquid remaining on the probe as it is transferred onto a substrate. Importantly, these capabilities have enabled the first experiments in which the transfer rate of liquid from an AFM probe to a surface is quantified as a function of the total quantity of liquid on the probe. The observation that V ∝ ∆M1.35 has profound implications for patterning experiments as it provides important insight relevant to choosing patterning conditions for generating features of consistent size despite a depleting liquid supply. Further, this work opens the door for future experiments that quantify nanoscopic liquid flows in tightly controlled systems. These types of studies could include rheological

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measurements that shed light on the scale-dependent rheological properties of liquids, including those that are non-Newtonian. Perhaps most importantly, this work provides an avenue for controlling the patterning of liquids from a tip to a surface. Indeed, the quantitative information regarding liquid loading, and the in situ observation of liquid patterned, could be used to provide real time feedback during the patterning process, paving a way for closed-loop lithography. While here, the concept of quantifying liquid transfer was demonstrated using thermal tunes that require prolonged observation of passive cantilever motion, active tuning can be used to dramatically speed up this process. In fact, techniques such as dual AC resonant tracking allow one to continuously measure the resonant frequency of a cantilever.[32] It is important to note that while the mass resolution in this study was found to be ~ 5 pg, reports of ~10 fg resolution are common in cantilever-based mass sensing in air through the use of higher eigenmodes and integrated sensing.[33-35] Thus, through further exploration of novel probe architectures and active sensing schemes, it is in principle possible to detect individual sub-micrometer features as they are written without compromising the speed of the process.

Acknowledgements This work was supported by start-up funds from Boston University. We thank Professor Kamil Ekinci for helpful discussions.

Supporting Information

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Figure S1 Describes additional experiments showing the reversibility of the liquid loading process.

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Figure 1. Liquid loading significantly shifts the resonance frequency of atomic force microscope (AFM) probes. (a) Schematic of an AFM probe along with the lumped element mechanical model that parametrizes such a probe in terms of the effective mass M, spring constant k, and dampening coefficient c. The vibrational resonance of this underdamped oscillator is evident the power spectral density (PSD) plotted vs. frequency f. (b) The addition of a small mass ∆M of liquid at the tip of the cantilever results in a considerable decrease in the natural frequency of the probe.

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Figure 2. Optical microscopy of the liquid distribution on an (a-c) hydrophilic and (d-f) hydrophobic AFM probe after immersing the probe in a glycerol solution of fluorescein isothiocyanate (FITC). Bright field (a, d) and fluorescence (b, c, e, f) microscopy images are presented.

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Figure 3. (a) Protocol for the localized liquid transfer to the tip of an AFM probe. An initially dry probe (i) is brought in contact with the surface of the liquid bath (ii) and a liquid meniscus forms. The probe is then lowered to an insertion depth D. Once reaching the maximum depth, the probe is retracted from the liquid, resulting in a stretching (iii) and ultimately rupture (iv) of the capillary bridge. Force F measured by the AFM probe vs. (b) time t and (c) tip-sample distance Z during the liquid loading process. The red curve indicates when the probe is moving towards the drop and the blue curve denotes that the probe is moving away from the drop. While evaluating F vs. t shows a nearly continuous increase in F until rupture of the meniscus, examining F vs. Z reveals that F is increasing as the probe is lifted out of the liquid, indicating that the meniscus is stretching. (c) Repeated liquid transfer experiments with increasing D revealed that a minimum depth ~400 nm is required for the transfer of appreciable liquid.

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Langmuir

Figure 4. Quantifying transfer of microscale liquid features from a tip onto a surface. (a) Monitoring the changing mass of liquid on the tip allows for the quantification of the mass change from writing a single feature δm while optical microscopy allows for the determination of the volume V of that feature. (b) Fluorescence micrograph showing a series of liquid drops each written by bringing a liquid-coated AFM probe into contact with a surface and subsequently withdrawing it. (c) Comparing the AFM-derived δm with fluorescence microscopy-derived V verifies the interpretation of the change in f0 as being due to liquid on the tip. Importantly, the

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Langmuir

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line represents the density of glycerol, indicating agreement between optical and AFM measurements. The inset shows the image processing procedure used to extract feature area using fluorescence microscopy. Error bars include uncertainty from a ~0.3 pg/s drift which is presumed to be due to evaporation of glycerol. (d) A series of experiments were performed in which a probe was loaded with a mass ∆M of glycerol and used to pattern a series of four features, the V of which are averaged together. The line denotes power law fitting.

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Quantifying Liquid Transport and Patterning Using Atomic Force

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