Molecular printing

Abstract

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.

Figure 1. Nanoscale printing

a, The number of papers with the terms ‘nanolithography’, ‘printing’ and ‘patterning’ in the title published each year since 1988 (searched using Thomson Scientific Web of Knowledge). Red columns correspond to the total number of papers with any of the terms ‘nanolithography’, ‘printing’ and ‘patterning’ in their titles published in all science and engineering journals. Blue columns indicate the number of papers with the terms ‘nanolithograhpy’, ‘printing’ and ‘patterning’ in their titles published in chemistry journals. The green line shows that the ratio of those papers published in chemistry journals is increasing yearly. b, Diagram dividing molecular printing by constructive and destructive methods.

Figure 2. Timeline of the historical development of molecular printing or direct ink transfer to a surface

Historically, methods of transferring ink directly to a substrate have evolved along two separate paths: serial writing and parallel printing. Approximately 2000 bc, the quill pen — a serial writing tool that can create patterns with high registration and high resolution — was invented. However, the low throughput and poor reproducibility of serial writing tools could not keep abreast with the rapid development of human society and the demand for knowledge, resulting in the evolution of parallel printing. With the invention of woodblock printing in ancient China (~200 ad), and later, a more sophisticated printing press in Europe (1439) by Gutenberg, parallel printing emerged as an art that rapidly and reproducibly forms patterns. Serial writing and parallel printing complemented each other to help people communicate, record information and create art. A similar, dramatic transition is currently underway in the field of nanoscale printing. Microcontact printing — a parallel nanoprinting technique — provides high-throughput, large-area patterning; however, µCp can only duplicate patterns that are predefined by a template and thus cannot arbitrarily generate different patterns. In contrast, DPN — a serial writing technique — creates patterns with high resolution and registration but is limited in throughput. Recently, the two branches of nanoscale printing merged with the invention of massively parallel DPN and subsequently PPL. PPL combines many advantages of µCp with the attributes of DPN to provide high resolution, registration and throughput, soft-matter compatibility and affordability.

Figure 3. Advances in DPN throughput

The time needed to pattern 1 cm² with 1 billion 100-nm dot features. Using single-pen DPN, this process requires several years, which renders single-pen DPN unsuitable for commercial lithographic applications. The parallelization of DPN has increased throughput by several orders of magnitude over the past decade as a result of 1D and 2D pen arrays. PPL can be used to print a billion nanoscale features over 1 cm² of substrate in 30 minutes. To complete the same task, µCp requires less than 1 minute. However, to fabricate a different pattern, soft lithography requires more than 1 day to produce a new mould and stamp, a process that is not required for parallel DPN and PPL. Figure reprinted with permission from ref. ; © 2008 AAAS.

Figure 4. Polymer-pen lithography

a, Schematic of a PPL pen array. The array is glued to a piezoelectric scanner that is software-controlled. When the tips of the polymer pens are brought in contact with a substrate, ink is delivered at the points of contact. b, Three-dimensional representation demonstrates how the massively parallel nature of the pen array results in replication of a digitized pattern into nanoscale features over large lengthscales. c, Optical image of a representative region of ~15,000 miniaturized duplicates of the 2008 Beijing Olympic logo made by PPL. d, A magnified scanning electron microscope image of the logo made by PPL. The letters and numbers ‘Beijing 2008’ were generated from ~20,000 90-nm dots, whereas the picture and Olympic rings were made from ~4,000 600-nm dots. e, A representative optical microscope image of 100 gold circuits in 1 cm² fabricated in a single writing stage by PPL, with features ranging from 100 µm to 500 nm. The inset shows a magnified SEM image of the circuit centre (red dashed square). f, A fluorescence microscopy image of an anti-mouse IgG array fabricated by PPL. Parts a, c, e and f reprinted with permission from ref. , © 2008 AAAS.