Electrical Double Layer Catalyzed Wet-Etching of Silicon Dioxide Haitao Liu, Michael L. Steigerwald,* and Colin Nuckolls* Department of Chemistry and the Columbia UniVersity Center for Electronics of Molecular Nanostructures, Columbia UniVersity, 3000 Broadway, New York, New York 10027 Received April 24, 2009; E-mail: mls2064@columbia.edu; cn37@columbia.edu We show here that the electrical double layer can be used to pattern nanoscale trenches in silicon dioxide. The ability to spatially modulate reaction kinetics lies at the heart of the photolithography process, in which a photochemically patterned film of photoresist is used to mask part of the wafer from reacting with the etchant. The resolution of the photolithography process is diffraction-limited. State-of-the-art lithography processes use 193 nm light to produce features as small as 45 nm; processes that use even shorter wavelengths face significant technological and economic challenges. 1 Here we show an alternate type of lithog- raphy where the electrical double layer around a carbon nanotube (CNT) can be used to create ultrasmall trenches in silicon dioxide. The electrical double layer exists at the electrode-water interface. It plays an important role in many electrochemical and electrokinetic processes. 2 Within the electrical double layer, one polar (positive or negative) component of the electrolyte is preferentially ac- cumulated both at the solid surface and in the diffuse layer, while the other polar component of the electrolyte is accordingly depleted in the same region. Such variations in the ion concentrations in principle can be used to spatially modulate the rate of chemical reactions. Thus, the electrical double layer can be used as a “mask” to transfer an existing pattern to a substrate. The ultimate resolution of this process is limited by the dimension of the electrical double layer, which is characterized by the Debye length. For an aqueous solution of a 1:1-type strong electrolyte at 25 °C, the Debye length can be estimated as where 1/κ is the Debye length in nanometers and I is the concentration of the electrolyte in moles per liter. 3 The thickness of the electrical double layer is smaller than 1 nm in a moderately concentrated (∼1 M) solution of a strong electrolyte. The electrical double layer exists not only on the surface of conducting materials but also on insulating ones. The adsorption of hydroxide ions on hydrophobic surfaces is of particular relevance to our current study. It has been known for decades that a hydrophobic surface acquires negative charges in pure water. 4 Recent studies have shown that this phenomenon is due to the spontaneous adsorption of hydroxide at the hydrophobic-water interface. 5 The exact mechanism is not fully understood. Molecular dynamics simulations have shown that water molecules restructure near the hydrophobic surface with the hydrogen atoms pointing toward the hydrophobic surface and the oxygen atoms pointing into the aqueous phase. 6 This ordered structure creates a huge dipole moment near the hydrophobic surface and presumably allows hydroxide ions to adsorb via dipole interactions and hydrogen bonding. Indeed, a recent electrochemical study suggested that the adsorbed OH - forms a more ordered structure at the graphene -water interface than H 3 O + does. 7 Because of the spontaneous hydroxide adsorption, pure hexa- decane droplets acquire a  potential of -120 mV in a pH 9 solution. 5 The density of hydroxide at the hexadecane-water interface is ∼0.3 molecule/nm 2 . 5 The average hydroxide-to- hydroxide distance on the hexadecane surface is comparable to that in a ∼0.3 M hydroxide solution (i.e., the effective pH on the surface of hexadecane is ∼14, in contrast to the pH of 9 in the solution). All of this information clearly suggests that the hydrophobic surface may significantly accelerate chemical reactions that involve hydroxide. To experimentally verify this hypothesis, we studied the effect of the electrical double layer on the wet etching of silicon dioxide in a strong alkaline solution: CNTs were chosen as the hydrophobic material. According to our hypothesis, the SiO 2 near the electrical double layer of the CNTs should experience a higher effective concentration of hydroxide and be preferentially etched away to produce a nanoscale trench (Figure 1). Ultralong, aligned CNTs on a Si wafer (300 nm of thermal oxide) were grown using the chemical vapor deposition (CVD) method. 8 Our growth conditions produced both single-walled and multiwalled CNTs with diameters in the range 1-4 nm [Figure S1 in the Supporting Information (SI)]. After the growth, marks (50 nm of thermally evaporated Au with 5 nm of Cr or Ti as an adhesion layer) were patterned onto the wafer with electron-beam lithography. The CNTs were located relative to the mark by using a scanning electron micro- scope (SEM). The wafer was then treated with an aqueous solution of tetramethylammonium hydroxide, (CH 3 ) 4 N + OH - (TMAH). In a typical reaction, the wafer was immersed in a 0.26 M solution of TMAH at 50 °C for 10 h (see the SI). The wafer was rinsed with deionized water after the etching and dried with a stream of nitrogen. We then used an atomic force microscope (AFM) to scan the area where the CNTs were known to exist before the etching. In most cases (see below), we observed nanoscale trenches instead of the CNTs. The shapes of the trenches closely resembled those of the original CNTs (Figure 2). The trenches survived an oxygen plasma treatment (250 mTorr O 2 , 50 W, 45 s), showing that they were not an artifact due to the CNTs 1 κ ≈ 0.30 √ I (1) SiO 2 + 2OH - f SiO 2 (OH) 2 2- (2) Figure 1. Adsorption of hydroxide ions at the CNT-water interface locally increases the rate of SiO 2 etching. Published on Web 06/02/2009 10.1021/ja903333s CCC: $40.75  2009 American Chemical Society 17034 9 J. AM. CHEM. SOC. 2009, 131, 17034–17035