Electrochemical AFM "dip-pen" nanolithography.

In recent years, SPM-based lithography has attracted great attention because of its simplicity and precise control of the structure and location. Many SPM lithography techniques based on mechanical scratching,1 anodization of Si surfaces,2 electrochemical decomposition of self-assembled monolayers,3 electric field-induced chemical reactions,4 electrochemical reactions in solution using electrochemical STM tips5 have been developed in the past decade. Comprehensive reviews of SPM-related lithography can be found in the literature.6 More recently, a “dippen” nanolithography (DPN) method has been invented that uses an atomic force microscope (AFM) tip as a “nib” to directly deliver organic molecules onto suitable substrate surfaces, such as Au.7 By using this technique, organic monolayers can be directly written on the surface with no additional steps, and multiple inks can be used to write different molecules on the same surface. However, the current “dip-pen” method can only be used to deliver organic molecules to the surface. The long-term stability of the created structures is a potential problem. Here we report a new electrochemical “dip-pen” lithography technique that can be used to directly fabricate metal and semiconductor nanostructures on surfaces. This technique has all the advantages of the previous “dip-pen” technique and improves the thermal stability and chemical diversity of the structures because they now could be made of various inorganic materials. Furthermore, the ability to directly fabricate metal or semiconductor nanostructures on surfaces with a high degree of control over location and geometry is of significant interest in nanotechnology. Potentially, one could use this method to fabricate nanodevices with multiple metal and semiconductor components. When AFM is used in air to image a surface, the narrow gap between the tip and surface behaves as a tiny capillary that condenses water from the air. This tiny water meniscus is actually an important factor that has limited the resolution of AFM in air. “Dip-pen” AFM lithography uses the water meniscus to transport organic molecules from tip to surface.7 In our new technique, we also use the tiny water meniscus on the AFM tip as the transfer medium. However, unlike in the previous AFM “dip-pen” method where water is only used as a solvent for the molecules, we have used this tiny water meniscus as a nanometer-sized electrochemical cell in which metal salts can be dissolved, reduced into metals electrochemically, and deposited on the surface (Figure 1). Although electric field-induced chemical reactions,4 electrochemical reactions in solutions using electrochemical STM,5 and electrochemical deposition using self-assembled monolayer as resist3c have been previously used to create metallic nanostructures, our method is the first that combines the versatility of electrochemistry with the simplicity and power of the DPN method to produce nanostructures with high resolution. Electrochemical STM-based methods require that the substrates be metallic, but substrates used in our method do not have to be metallic since the control feedback of the AFM does not rely on the current between the tip and surface. Si wafers coated with native oxide provides enough conductivity for the reduction of the precursor ions. This development significantly expands the scope of DPN lithography, making it a more general nanofabrication technique that not only can be used to deliver organic molecules to surfaces but is also capable of fabricating metallic and semiconducting structures with precise control over location and geometry. Because of the electrochemical nature of this new approach, we call this technique electrochemical “dip-pen” nanolithography (E-DPN). We have investigated the deposition of several metals and semiconductors on Si surfaces at room temperature using the E-DPN technique. Here we show the deposition of Pt metal as an example.8 The experiments were performed using a Nanoscape IIIa AFM (Digital Instruments). In a typical experiment, an ultrasharp silicon cantilever coated with H2PtCl6 is scanned on a cleaned P-type Si (100) surface with a positive DC bias applied on the tip. During this lithographic process, H2PtCl6 dissolved in the water meniscus is electrochemically reduced from Pt(IV) to Pt(0) metal at the cathodic silicon surface and deposits as Pt nanofeatures according to the following equation: