Molecular printing
Molecular printing techniques, which involve the direct transfer of molecules to a substrate with submicrometre resolution, have been extensively developed over the past decade and have enabled many applications. Arrays of features on this scale have been used to direct materials assembly, in nanoelectronics, and as tools for genetic analysis and disease detection. The past decade has witnessed the maturation of molecular printing led by two synergistic technologies: dip-pen nanolithography and soft lithography. Both are characterized by material and substrate flexibility, but dip-pen nanolithography has unlimited pattern design whereas soft lithography has limited pattern flexibility but is low in cost and has high throughput. Advances in DPN tip arrays and inking methods have increased the throughput and enabled applications such as multiplexed arrays. A new approach to molecular printing, polymer-pen lithography, achieves low-cost, high-throughput and pattern flexibility. This Perspective discusses the evolution and future directions of molecular printing.
The past decade has witnessed the genesis and evolution of printing technologies capable of patterning surfaces with features smaller than 100nm. These capabilities are a result of simultaneous advances in physics, chemistry, materials science and nanotechnology. The development of tools for reducing feature size is motivated primarily by (1) the semiconductor industry's desire to continue increasing the number of transistors in a given area; (2) the central dogma of nanotechnology, which states that as feature sizes approach the nanoscale, new properties emerge that are not observed in bulk materials; and (3) biological studies and applications made possible by high-density bioarrays. As feature size approach the single-molecule limit, molecular transport, assembly and intermolecular interactions become dominant considerations and have shifted the science of patterning to the chemists' domain.
Solutions to the problem of nanoscale patterning being explored have traditionally included destructive, radiative techniques such as extreme UV lithography, soft X-ray lithography, electron-beam lithography and focused ion-beam writing, as well as methods such as nanoimprint lithography and certain types of scanning-probe lithography (SPL). As nanopatterning techniques have become increasingly important to chemists, materials scientists and biologists, molecular printing capabilities, defined as processes where molecules or materials are directly transferred to a substrate of interest in the form of submicrometre features with at least one dimension on the molecular scale, have also become increasingly important. In this regard, two technologies -- soft lithography and dip-pen nanolithography (DPN) -- have emerged as the most widely variety of substrates. This perspective discusses the history and applications of molecular printing, focusing primarily on DPN and soft lithography as the first and most widely used techniques, and the recent development of polymer-en lithography (PPL), a molecular printing method that combines the advantages of soft lithography and DPN into a single lithographic platform.
Emergence of dip-pen nanolithography
In the early 1980s, new methods for shrinking feature sizes of patterns on silicon substrates were being actively pursued by the semiconductor industry as it became apparent that photolithography could not continue to indefinitely reduce the sizes of features as predicted by Moore's Law and desired by electronics manufacturers. The brisk rate of feature-size reduction in CMOS devices demanded new ways to address and image these nanoscale structures as the existing technologies were no longer compatible with the smaller features. During this period, scanning probe microscopes (SPMs), specially the scanning tunnelling microscope (STM) and atomic force microscope (AFM), emerged as imaging and spectroscopic tools that probe the topology of surfaces using nanosized tips that raster across them by piezoelectric actuation. SPMs have revolutionized surface science and even enabled the manipulation of surfaces at the single-atom level. For the development of the SPM, Binnig and Rohrer were awarded the Nobel Prize in Physics in 1986.
Immediately following the invention of SPMs, researchers began using these machines for patterning surfaces, marking the birth of SPL. A major milestone in SPL development occurred when Eigler and co-workers used an STM to pattern individual atoms on a surface, suggesting that SPMs could indeed be used for molecular printing or perhaps even manufacturing. In this experiment, the researchers repositioned individual Xe atoms on a single-crystal Ni surface to form the letters "IBM" using an STM tip at 4 K. At this temperature, the van der Waals forces and electrostatic interactions between the atoms and the surface are greater than the interactions between the tip and the atom, thereby keeping the atoms anchored to the surface while they are dragged to their intended sites with the tip. Subsequently, the IBM group showed that this patterning technique could be used to study fundamental surface properties as a result of the atomic resolution aorded by the STM. By arranging 48 Fe atoms in a circular structure on a Cu(111) surface, they were able to observe standing electron waves within the circular corral at 4 K. STM repositioning was extended to molecules as well, and a molecular counting device was fabricated by sequentially moving a row of 10 fullerenes on a Cu(111) surface.
Although these impressive studies demonstrate the potential of SPMs as patterning tools, they also emphasize their limitations and impracticality. Specially, this approach (1) requires controlled environments and low temperatures; (2) is painstakingly slow and indirect in nature, as it requires the picking up and subsequent movement of atoms; and (3) is not easily scaled. However, the work by Eigler, no matter how impractical, demonstrated the ultimate resolution of SPL techniques -- atom-by-atom construction of nanopatterns -- and established a challenge for the research community to develop rapid ways of printing atom- and molecule-based structures on a surface with nanoscale resolution.
For the next decade, scientists followed the model of the semi-conductor industry and developed a series of indirect SPL methods, which focus on the delivery of energy rather than molecules to a surface to create functional patterns, typically with the aid of a resist material. Taking advantage of the registration enabled by piezo-actuation and the anoscale radii of the tips, scientists were able to generate sub-50-nm features by scratching, etching and oxidizing surfaces. For example, dynamic plough lithography can be used to scratch polymer-coated silicon surfaces, which can be subsequently processed using conventional silicon wet etching. A process known as nanoshaving or nanografting uses the tip of an AFM and an applied force to remove a molecular monolayer on gold in a site-specific fashion. Anodic oxidation of silicon was developed by Quate for patterning silicon substrates. Sagiv et al. pioneered a related approach, which uses an applied bias between a conductive AFM tip and an n-octadecyltrichlorosilane monolayer, to electro-chemically convert methyl groups to acids. The reactive acid groups are used for subsequent chemistry and molecular immobilization. Such techniques have important applications but are limited by their indirect nature and challenges associated with parallelization.
A problem in developing any SPL or scanning probe imaging technique that functions in air, is dealing with the consequences of the capillary effect and meniscus formation. These factors convolute measurements taken with scanning probe instruments, and therefore many scientists rely on ultrahigh vacuum for their experiments. Surprisingly, the controlled direct transfer of material to a surface with nanoscale precision was not realized until 1999 when alkanethiols were deposited on gold substrates, with the use of humidity and the meniscus to facilitate transport and generate stable chemisorbed nanostructures, marking the invention of DPN. Interestingly, a few years earlier, others had attempted what seemed to be a nearly identical experiment and concluded that transport did not occur. The stronger interactions between the thiols and gold at room temperature, compared with the weak interactions between the tip and the ink, were key to forming stable patterns on the surface. The versatility of this method, using direct molecular transport from a tip to a surface to form patterns, was immediately apparent, and in the decade since its invention, DPN has been used to generate structures made of organometallic molecules, polymers, DNA, proteins, peptides and affinity templates, which allow for the subsequent immobilization of viruses, colloidal nano-particles, metal ions and single-walled carbon nanotubes on many types of surfaces. Furthermore, DPN has enabled fundamental studies related to molecular transport, and the fabrication of technological tools such as photomasks, gas sensors and biological screening devices, including an assay for human immunodeficiency HIV-1 virus p24 antigen in serum samples and gene chips.
DPN relies on the spontaneous formation of a meniscus between the tip and surface, which serves as a conduit for ink transport. The deposition rate is a function of tip-substrate contact time, the ink diffusion coefficient, and the ink coverage on the pen. Under appropriate conditions, feature sizes can be controlled on the sub-50-nm to many-micrometre length scale. Patterns are formed by moving the tip across the surface at a controlled velocity, and piezo-actuation on three axes provides near-perfect registration. Because the piezo-actuators are computer controlled, arbitrary patterns can be formed by the movement of the tip, which cannot be done by so lithography, where the pattern is predetermined by a mould. By increasing the rastering speed of the tip across the surface, the transport of molecules is halted, and the same probe tip that is used for writing, acts instead as an imaging tool rather than a pen to provide feedback on the pattern quality. Finally, materials that are resistant to transport in a conventional DPN experiment can be dispersed in a hydrophilic or lipophilic carrier matrix that assists their movement through the meniscus41. Importantly, there is no need to expose the substrate to harsh ultraviolet, ion- or electron-beam radiation, characteristic of indirect patterning techniques, and therefore DPN can be used to print fragile or reactive organic and biological materials.
These advantages of DPN are responsible for its rapid development and dissemination over the past decade, however, in the early stages of its development, DPN faced many of the same drawbacks as surface-destructive indirect SPLs, primarily poor throughput, a concern that soon would become a major focus of DPN research.
High-throughput molecular printing with soft lithography
So lithography is a complementary molecular printing technique that overcomes the throughput problem associated with the early incarnations of DPN. Specially, microcontact printing (Cp), a form of so lithography, uses an elastomer stamp, typically fabricated by conventional lithographic methods, to directly transfer materials to a surface of interest. The Cp approach has flourished as a research-grade molecular printing technology because of its ease of use, low cost, and high throughput. It has become the poor man's replacement for photolithography, allowing researchers in many fields to use micro- and nanofabrication. In Cp, elastomeric stamps are prepared by pouring an elastomer, including poly(dimethylsiloxane) (PDMS), polyurethanes, polyimides and natural polymers such as agarose, in a mould prepared by conventional photolithographic methods. The polymer is cured in the mould, thereby generating a relief structure in the elastomeric stamp. The stamp can then be used to pattern proteins, DNA, cells, alkanethiols, silanes, colloids and salts on a variety of at as well as curved surfaces.
The Cp technique, however, has several limitations. Although the stamps can print over large areas, they can only deposit a single pattern that is predetermined by the stamp. Therefore, a new mould must be fabricated each time a new pattern is desired. Feature size control is affected and limited by stamp swelling and shrinking, both during curing and stamp inking. The mechanical properties of the stamps can also limit design flexibility. For example, the softness of the elastomer limits the aspect ratio, height/length (h/l), of the features. If the feature aspect ratio on the stamp is not between 0.2 and 2, bending of the stamp can cause pattern defects, and, for features that are too widely separated (>20h), sagging of the stamp causes defects. Indeed, the fabrication of alkanethiol features smaller than 150 nm is a significant challenge using conventional methods. By using DPN to introduce chemical modications on a flat PDMS stamp and by passivating the surrounding areas with a perfluorinated monolayer, we have helped overcome this deficiency, and shown that features as small as 80nm could be easily prepared with aspect ratios as low as 0.01. is study underscores the synergistic aspects of so lithography and DPN.