Thermal Dip-pen Nanolithography Research Articles

Nanometer-scale capillary-driven flow and molecular weight govern polymer nanostructure deposition from a heated tip

This paper investigates the physical process of polymer nanostructure deposition from a heated atomic force microscope (AFM) tip and focuses on the role of capillary-driven flow on deposited feature sizes during thermal dip-pen nanolithography. We used a heated AFM tip to deposit 50–350 nm wide poly(methyl methacrylate) nanoribbons by varying tip temperature, tip speed, and polymer molecular weight. For polymers of different molecular weights, the width of the deposited polymer nanoribbons decreases with capillary number (Ca), independent of tip temperature, tip speed, and polymer molecular weight. These results indicate that the capillary-driven flow governs polymer nanostructure deposition from a heated tip. For high molecular weight polymers deposited with feature size close to the polymer radius of gyration Rg, the molecular weight also influences the size of the deposited polymer ribbon. Using scaling arguments, we show that the feature size can be predicted by Ca and Rg. Uniform and continuous deposition occurs only when Ca << 1, confirming that the polymer flow is driven by the capillary force. The results of this study enable better control of speed and resolution at which polymer nanostructures can be fabricated using AFM.

Nanometer-scale flow of molten polyethylene from a heated atomic force microscope tip

We investigate the nanometer-scale flow of molten polyethylene from a heated atomic force microscope (AFM) cantilever tip during thermal dip-pen nanolithography (tDPN). Polymer nanostructures were written for cantilever tip temperatures and substrate temperatures controlled over the range 100–260 °C and while the tip was either moving with speed 0.5–2.0 µm s−1 or stationary and heated for 0.1–100 s. We find that polymer flow depends on surface capillary forces and not on shear between tip and substrate. The polymer mass flow rate is sensitive to the temperature-dependent polymer viscosity. The polymer flow is governed by thermal Marangoni forces and non-equilibrium wetting dynamics caused by a solidification front within the feature.

Temperature-dependence of ink transport during thermal dip-pen nanolithography

We investigate the control of tip temperature on feature size during dip-pen nanolithography (DPN) of mercaptohexadecanoic acid (MHA) on Au. Heated atomic force microscopy (AFM) probes operated between 25 °C and 50 °C wrote nanostructures of MHA for various dwell times and tip speeds. The feature size exhibited an exponential dependence on tip temperature with an apparent activation barrier of 165 kJ/mol. Analysis of the ink transfer process shows that, while ∼1/3 of the barrier is from ink dissolution into the meniscus, the rest reflects the barrier to adsorption onto the growing feature, a process that has been ignored in previous DPN models.

The Effects of Temperature and Scan Velocity on Direct-write Dip-pen Nanolithography Using Molecular Dynamics Simulation

The two important controllable parameters in the dip-pen nanolithography (DPN) process, such as writing temperature and velocity, are used to investigate the related effects on mechanisms of transference of alkanethiol self-assembled monolayer (SAM), transfer number, gasification number, and nanowire formation using molecular dynamics simulations. The simulated results show that the molecular transport ability during the direct-write process from the tip to the substrate is dependent on writing temperature and velocity, because the molecules have high kinetic energy and undergo fast diffusion when the temperature is increased; high transfer ability occurs at a slow writing velocity. The nanowire thickness and length increase significantly with increasing writing temperature, and its length increases much faster than its thickness (height). When the writing temperature is increased, transfer number and gasification number of the molecules become dramatically larger. The transfer number of ink molecules increases with decreasing writing velocity. Keywords: Thermal dip-pen nanolithography, DPN, temperature, velocity, nanowire, SAM, transfer number, gasification number, molecular dynamics, MD, transport ability, diffusion, ink, direct-write, adsorption

Reversible electron-induced conductance in polymer nanostructures

We report a mechanism for controlling conductance in polymer nanostructures. Poly(3-dodecylthiophene-2,5-diyl) (PDDT) nanostructures were directly written between gold electrodes using thermal dip pen nanolithography and then characterized in UHV. We find that the conductivity of a PDDT nanostructure can be increased by more than five orders of magnitude (from <10−4 to 10 S cm−1) by exposure to energetic electrons, and then repeatedly returned to a semi-insulating state by subsequent exposure to hydrogen. Based on systematic measurements complemented by calculations of electronic structure and electron transport in PDDT, we conclude that the conductance modulation is caused by H desorption and reabsorption. The phenomenon has potential applications in hydrogen sensing and molecular electronics.

The nanopatterning of a stimulus-responsive polymer by thermal dip-pen nanolithography

We demonstrate that poly(N-isopropylacrylamide) (PNIPAAm) nanostructures can be directly and reproducibly written from the melt using thermal dip-pen nanolithography. Whereas previous research examined polymer brushes where the strands were oriented normal to the surface, here the polymer strands are aligned parallel to the surface. Alignment parallel to the surface allows ultrathin films to be formed while still completely masking the substrate from the solution. Moreover, the aligned PNIPAAm retains its unique ability to reversibly bind and release proteins when driven through a hydrophobic–hydrophilic phase transition by a change in temperature. Unlike the conventional brush form, the phase transition in the aligned form is not associated with a change in height, retaining its dimensional stability.

Direct Writing of a Conducting Polymer with Molecular-Level Control of Physical Dimensions and Orientation

Polymer nanostructures composed of poly(3-dodecylthiophene) (PDDT) have been directly written with control of polymer strand alignment and monolayer-by-monolayer thickness down to a single molecular monolayer (2.6 nm). The molecularly ordered nanostructures were written on silicon oxide surfaces using thermal dip-pen nanolithography, where an atomic force microscope cantilever with integrated tip heater was precoated with solid PDDT. The PDDT was precisely deposited onto the surface when the tip temperature was set close to PDDT's melting temperature.

Direct deposition of continuous metal nanostructures by thermal dip-pen nanolithography

We describe the deposition of continuous metal nanostructures onto glass and silicon using a heated atomic force microscope cantilever. Like a miniature soldering iron, the cantilever tip is coated with indium metal, which can be deposited onto a surface forming lines of a width less than 80 nm. Deposition is controlled using a heater integrated into the cantilever. When the cantilever is unheated, no metal is deposited from the tip, allowing the writing to be registered to existing features on the surface. We demonstrate direct-write circuit repair by writing an electrical connection between two metal electrodes separated by a submicron gap.

Nanoscale deposition of solid inks via thermal dip pen nanolithography

We demonstrate that nanolithography can be performed using a heated atomic force microscope (AFM) cantilever tip to control the deposition of a solid organic “ink.” The ink, octadecylphosphonic acid (OPA), has a melting temperature near 100°C and can self-assemble on mica. Postdeposition analysis shows that deposition occurs only when the cantilever tip is heated above OPA’s melting temperature, that the deposited structure does not spread significantly while cooling, and that a cool tip coated with OPA does not contaminate the substrate during subsequent imaging. Single lines were written with a width of 100nm. This approach greatly expands the potential of dip pen nanolithography, allowing local control of deposition and deposition of materials typically immobile at room temperature, while avoiding potential problems arising from inadvertent deposition and postdeposition diffusion.