19220
J. Phys. Chem. C 2010, 114, 19220–19226
Constructive Nanolithography by Chemically Modified Tips: Nanoelectrochemical Patterning on SAMs/Au Zhikun Zheng,† Menglong Yang,‡ and Bailin Zhang* State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry and Graduate School of Chinese Academy of Sciences, Chinese Academy of Sciences, Changchun 130022, P. R. China ReceiVed: July 7, 2010; ReVised Manuscript ReceiVed: October 7, 2010
Nanoelectrochemical patterning of 1-hexadecanethiol (HDT) monolayer on Au(111) was realized by thiolmodified conductive atomic force microscopy (AFM) tips in a fashion of mild oxidation of top methyl groups of the monolayers. Pt-coated tips modified by a hydrophobic and appropriate thick monolayer of thiol (e.g., HDT) can significantly increase the bias threshold and extend the bias range suitable for the constructive nanolithography (CNL) in comparison with those tips unmodified or modified by hydrophilic monolayers. Furthermore, the switchover from the CNL to the deep oxidation can be governed by the reductive desorption of thiol monolayers from the tip surface when appropriate thiol-modified tips are used. A hydrophobic HDT/ tip can exhibit a CNL bias range with an extent of 5.3 V, much wider than the reported extents of the CNL bias ranges on octadecyl trichlorosilane (OTS)/silicon surfaces by unmodified tips. The resulting CNL patterns were shown to have potential as multifunctional templates by guiding the self-assembly of OTS and the site-defined immobilization of aminopropyltriethoxysilane-modified R-Fe2O3 nanoparticles. Introduction Patterning the self-assembly monolayers (SAMs) of thiols on gold by various scanning probe lithography (SPL) techniques has been widely used to create functional nanostructures.1-4 In these patterning processes, various SAMs of alkanethiols and their functional derivates are generally used either to form the primordial scaffolds for subsequent functional nanolithographies such as nanoshaving/nanografting2 and bias-assisted nanolithography5 or to backfill the unpatterned areas as surrounding matrix after dip-pen nanolithography (DPN).4 The functionalization of patterned monolayers normally needs multistep self-assemblies of different thiols, and thus, the molecule exchange always occurs between the adsorbed thiols in the patterned monolayers and the free thiols in the self-assembly solution.6-9 Such a molecule exchange of thiols can greatly blur the pattern boundaries and degrade the efficiency of goal templates.10 It is possible to avoid the above disadvantages if a one-step patterning approach can change the local chemical functionalities of the top surface of thiol monolayers without damaging the monolayer framework and the bottom S-Au linkage layer. Recently, catalytic probe nanolithography was demonstrated to comply with the above idea; however, these techniques are only suitable to the layers with catalytic top groups.11-14 Constructive nanolithography (CNL),15 as a mild fashion of tip-induced local anodic oxidation (LAO) nanolithography, will be a promising candidate for above idea in SPL techniques because of its reliability and general applicability. CNL was first proposed as a bottom-up strategy for the creation of nanostructures, beginning from the tip-induced mild oxidation of the surface end groups of well-defined SAMs of silanes on * To whom correspondence should be addressed. Phone/fax: +86 431 85262430. E-mail:
[email protected]. † Present address: Technische Universitaet Berlin, Institut fuer Chemie, Strasse des Juni 17, 124, 10623 Berlin, Germany. ‡ Present address: Opening Public Laboratory of Bioenergy and Bioprocess, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, P. R. China.
silicon wafers.15 In CNL, the operational parameters (e.g., oxidization bias and duration) of conductive atomic force microscope (c-AFM) were strictly controlled only to oxidize the top methyl or vinyl groups of monolayers into the carboxylic acid groups, meanwhile maintaining the undamaged monolayer framework and the underlying substrate. As a result, the local surfaces in the oxidized areas converse from hydrophobic, neutral, and inert to hydrophilic, negative-charged, and chemically active, forming the multifunctional templates for the wetting-driven patterning,16,17 immobilization of positive-charged materials,18 bonding active molecules to create two- or threedimensional nanostructures,19-21 and electrochemical pattern transfer.22 However, an obvious disadvantage of CNL by unimproved commercial AFM tips is that the bias range suitable for the monolayer surface oxidization was very narrow.15,19 For example, the extents of CNL bias ranges were reported from 0.5 to 1.2 V for the methyl-terminated surface of octadecylsilated SiOx/Si (i.e., OTS/silicon) samples, depending on different bias durations and ambient humidity.23 Narrow bias ranges make the CNL difficult to be reproducibly realized and widely applied. CNL is an electrochemical process in nature which is driven by the electric field between the conductive AFM tip and the substrate sample. The influences of the tip on the tip/substrate electric field will affect the results of CNL to a great degree. The conductivity of commercial AFM tips is normally due to their outside conductive coatings (materials of Pt, Au, TiN, W2C, etc.). It has been reported that the tips with different conductive coatings may present different bias ranges for CNL.23,24 Much more than the influences due to the different work functions of the materials of the tip’s conductive coatings, the organic layers modified on tip surfaces have a great ability to tune the surface wettability and electric characters (e.g., resistances on electron transfer) of conductive tips25 and thus certainly affect the tip/ substrate electric field on both the field distribution and the effective field intensity. The organic layer-modified conductive tips have not been used in CNL by far, even though the chemically modified tips have been well used in the chemical
10.1021/jp106296x 2010 American Chemical Society Published on Web 10/27/2010
Nanoelectrochemical Patterning on SAMs/Au
J. Phys. Chem. C, Vol. 114, No. 45, 2010 19221
TABLE 1: Comparison of SAMs-Modified and Unmodified Tips for Tip-Induced Local Oxidation tip
HDT/tip
MHA/tip
MUA/tip
MUD/tip
SAM-removed Pt/tip
contaminated Pt/tip
cleaned Pt/tip
contact angle on probe base (deg) tip wettability coating layer thickness bias range (V) of constructive nanolithography extent of CNL range (V) Vth for deep oxidation (V)
101 ( 3 none C16 4.5-9.7 5.3 9.8
23 ( 3 good C16 3.6-4.8 1.3 4.9
22 ( 3 good C11 3.6-4.6 1.1 4.7
20 ( 3 good C11 3.6-4.5 1.0 4.6
N/Aa N/Aa N/Aa 3.0-3.1 0.2 3.2
75 ( 5 poor N/Aa 3.1-3.4 0.4 3.5
12 ( 2 good none 3.0 0.1 3.1
a
Not applicable.
force microscopy for chemically sensitive imaging, force titration and molecular recognition,26 and the lithography as catalytic probe nanolithography12,14 and redox probe lithography.27 Furthermore, although CNL has been realized on different surface end groups like methyl,19 vinyl,15 sulfhydryl (mercapto),28 aminophenyl,29 N-hydroxysuccinimide,24 oligo(ethylene glycol) (OEG),30 and R,R-dimethyl-3,5-dimethoxybenzyloxycarbonyl (DDZ)-protected amine31 or thiol,32 all of these were based on the SAMs on silicon substrates. These studies reported that the bias ranges for CNL vary not only with the characters of SAMs (e.g., end groups, molecular structure, thickness, packing density) but also with the characters of substrates (e.g., conductivity, oxidizability) as well as the linkages between SAMs and substrates.24 CNL on SAMs on gold remains a great challenge because the easy destruction of the Au-S linkage of the SAMs and oxidation of the gold substrate.5 Given the possibility that the chemically modified conductive tips can realize CNL on the surface of thiol SAMs on gold by tuning the surface wettability and dielectric resistance of conductive tips, a detailed investigation on the influences of organic coating layers on the tip performances for LAO is essentially necessary in order to get more insights on LAO and further to optimize the conditions for the realization of CNL. In this paper, conductive Pt/Cr-coated tips modified with various thiol SAMs were used to realize CNL on the surface of 1-hexadecanethiol monolayer on Au(111) (HDT/Au(111)). The influences of thiol SAM-modified tips on the results of LAO were investigated, and a wider bias range suitable for CNL was obtained with a hydrophobic HDT-modified tip. In addition, the resulting CNL nanopatterns were demonstrated as active templates for the self-assembly of n-octadecyltrichlorosilane (OTS) and the selective adsorption of aminopropyltriethoxysilane (APTS) modified R-Fe2O3 nanoparticles. Results and Discussion 1. Chemically Modified Tips for CNL. Platinum-coated silicon tips (Pt/tip) are suitable for AFM tip-induced LAO nanolithography for their excellent conductivity and chemical stability. Here, the platinum surfaces of Pt/tips were modified with various thiol SAMs in a chemically well-defined manner.33,34 The SAMs of ω-functionalized thiols can flexibly and reproducibly tailor the surface wettability and surface dielectric characters of the supports.25 Four thiols (HDT, MHA, MUA, and MUD) were chosen to modify Pt/tips, resulting in HDT/ tip, MHA/tip, MUA/tip, and MUD/tip, respectively, with the opposite surface wettability and the different thickness of the dielectric monolayers outside tips. In detail, HDT/tip and MHA/ tip have a similar dielectric thickness (thiols of C16 molecular length) but a contrary wettability of surfaces terminated by the hydrophobic methyl vs the hydrophilic carboxyl groups, respectively; MUA/tip and MUD/tip have hydrophilic surfaces (terminated with carboxyl and hydroxyl, respectively) similar to MHA/tip but a decreased dielectric thickness (thiols of C11
molecular length).25 These thiol-modified tips were used to carry out tip-induced LAO on HDT/Au(111) surface. The wettability of tips was roughly classed, according to the water contact angle (θ) on the surface of probe base, as nonwetting (when θ > 90°), poor wetting (when 45° < θ < 90°), and good wetting (when 0° < θ < 45°). The results are listed in Table 1. Hydrophobic HDT-Modified Tip. A series of line arrays was fabricated by a HDT/tip in Figure 1. Two typical oxidation results can be observed after positive biases greater than 4.5 V were applied on the HDT/Au(111) substrate. One result is the “mild oxidation” during a bias range of 4.5-9.7 V in which the fabricated lines can be identified by the combination of the increased friction signals in friction images with the undetectable topographic changes in topography images both before (Figure 1) and after the ethanol immersion (Figure S1, Supporting Information). These characters can be attributed to the tipinduced local oxidation of the top end groups of the monolayer from the hydrophobic methyl (-CH3) into the hydrophilic oxygenic end groups (typically as -COOH), without the noticeable decomposition of the alkyl chain in the monolayer and oxidation of the gold substrate.15,35,36 This kind of tipinduced oxidative conversion of the end groups in the monolayer surface cannot apparently alter the real height of the oxidized areas, but the great change of local surface wettability from hydrophobic to hydrophilic can strongly increase the local friction force when the AFM tip moves over the oxidized areas, in line with the higher friction signal of oxidized lines than that of the surrounding HDT monolayer in friction images. Furthermore, the friction signal of oxidized line increases with the applied biases (Figure 1, bottom), revealing the increasing conversion ratio of the top groups from the hydrophobic methyl into the hydrophilic oxygenic groups, in line with the fact that a higher bias could oxidize more methyl end groups into oxygenic end groups during the same oxidization time.15,35,36 Significantly, this tip-induced local conversion of surface terminal groups on HDT/Au(111) is in nature the same as that reported in the constructive nanolithography (CNL) on the surface of OTS/silicon.19 The slight friction contrast for the 4.5 V line indicates only traces of methyl end group were oxidized. HDT/tip with biases smaller than 4.5 V cannot reliably result in any detectable changes in both AFM topography and friction images. In contrast, another oxidation result is the “deep oxidation” (DO) by biases greater than 9.8 V in which the protuberant lines exhibit the positive heights in the AFM topography image combined with the increased friction signals in the friction image (Figure 1e). In contrast to no height changes after the “mild oxidation”, the appearance of the protruded patterns after the “deep oxidation” reveals that the application of high positive biases has induced the local oxidation of the underlying gold (mainly into gold oxide) to such a great degree that the volume expansion of the resulting gold oxide can lead to much more height increase to exceed the height loss caused by the
19222
J. Phys. Chem. C, Vol. 114, No. 45, 2010
Zheng et al.
Figure 1. AFM topography (top) and friction (bottom) images of lines fabricated by a HDT/tip with biases of 4.5-9.9 V (step-up by 0.1 V) on HDT/Au(111). The profiles below the friction images are the section profiles of the lower row of lines in the friction images. Images a-e were obtained immediately after fabrication; arrows in e indicate the opposite writing direction of the lines. Image f was obtained after the 2-h immersion of the sample (e) in ethanol. The full contrast scales for height and friction images are 1.65 nm and 20 mV, respectively.
accompanying oxidative desorption or serious decomposition of the upper monolayer.5 The increased friction signals on the surface of the oxidized lines are also in agreement with the more hydrophilic surface of gold oxide factures. The opinion that the protuberant lines are mainly composed of gold oxide is confirmed by the ethanol immersion test. Figure 1f clearly shows that these protuberant lines became the negative grooves in the height image while the high friction remained in the friction image after the 2-h immersion in ethanol because the gold oxide was reduced into gold by ethanol, resulting in the volume shrinkage until the reversion of gold substrate with a hydrophilic surface and height loss due to the destroyed monolayer in the fabricated domain.5 A bias threshold for the deep oxidation (Vth for DO) of 9.8 V is evidenced by the appearance of a partly protuberant line (Figure 1e) and its corresponding incomplete negative groove after ethanol immersion in topography images, in combination with the strong friction signals in both friction images (Figure 1f). Remarkably, the protuberant topography appears from the middle part instead of the beginning part of the 9.8 V line; meanwhile, a turning point can be observed in the 9.9 V line before which the topography of the beginning part of the 9.9 V line is more like the end part of the previous 9.8 V line. These topographic characters suggest that the HDT/tip might have undergone a significant change. To reveal the tip’s change, this freshly used HDT/tip was used for further nanolithography in Figure S2 (Supporting Information). The line arrays fabricated by such a “HDT/tip” that have freshly undergone high biases have characters between those produced by a freshly cleaned Pt/tip and those produced by a contaminated Pt/tip (vide post), more like those by the cleaned Pt/tip (see Table 1). The fact that this freshly used “HDT/tip” could work quite like an unmodified Pt/tip implies that a higher bias like the DO bias thresholds has caused the reductive desorption of the HDT layer from the tip surface. Hydrophilic Thiol-Modified Tips. Analogous “mild oxidation” and “deep oxidation” were also realized by those hydrophilic thiol-modified tips like MHA/tip (Figure 2), MUA/tip (Figure S3, Supporting Information), and MUD/tip (Figure S4, Supporting Information), of course, in different bias ranges (Table 1). As shown in Figure 2, MHA/tip-fabricated lines clearly exhibit a bias range of 3.6-4.8 V relating to the local oxidation
Figure 2. AFM topography (top) and friction (bottom) images of lines fabricated by a MHA/tip with biases of 3.5-6.0 V (step-up by 0.1 V) on HDT/Au(111). (a and b) Obtained immediately after fabrication; (c) obtained after the 2-h immersion of the sample (b) in ethanol. The full contrast scales for height and friction in images are 1.65 nm and 20 mV, respectively.
of the methyl end groups on the HDT monolayer surface (i.e., realization of CNL), as confirmed by the combination of the increasing friction signals in friction images and the undetectable topographic changes in topographic images both before and after ethanol immersion. The Vth (threshold voltage) for DO of 4.9 V for MHA/tip is indicated by the first appearance of a protuberant gold oxide line and its corresponding negative groove after ethanol immersion in AFM topography images, in combination with the strong friction signals in friction images. In comparison with MHA/tip, MUA/tip (Figure S3, Supporting Information) and MUD/tip (Figure S4, Supporting Information) exhibit similar CNL bias ranges, starting from the same bias of 3.6 V but ending at a little lower biases (4.6 and 4.5 V, respectively) and thus a little lower DO bias thresholds (4.7 and 4.6 V, respectively), in line with their similar tip wettability but the decreased thickness of tip coating layers. All these hydrophilic SAM/tips have DO bias thresholds much lower than the hydrophobic HDT/tip but still significantly higher than the unmodified Pt/tips. Furthermore, the DO bias thresholds for these hydrophilic SAM/tips are quite close to the critical bias for the Pt/tip-induced reductive desorption of HDT monolayer
Nanoelectrochemical Patterning on SAMs/Au
J. Phys. Chem. C, Vol. 114, No. 45, 2010 19223
Figure 3. AFM topography (top) and friction (bottom) images of lines fabricated by a freshly piranha-cleaned Pt/tip with biases of 3.8-2.6 V (a) and by a contaminated Pt/tip with biases of 4.2-3.0 V (c) (step-down by 0.1 V) on HDT/Au(111) before (a, c) and after (b, d) the 2-h immersion in ethanol. The full contrast scales for height and friction in images are 1.65 nm and 20 mV, respectively.
on gold substrate,5 which implies that these hydrophilic SAMs could be removed from the tip surfaces by reductive desorption under the DO bias thresholds. Therefore, at least for the above SAM/tips, either the HDT/tip or the MHA (MUA, MUD)/tips, a bias to initiate the DO of HDT/gold substrate needs to be high enough to reductively desorb the tip coating layers from tip surfaces. However, the current evidence is still insufficient to tell whether the switch from CNL to DO on thiol SAMs/Au can be always controlled by the reductive desorption of SAM/ tips since different thiol SAMs’ resistance to either oxidative destruction or reductive desorption depends strongly on their structure and quality. 2. Unmodified Tips: Comparison between Cleaned and Contaminated Pt/Tips. To highlight the functions of tip coating layers, unmodified Pt/tips were used to carry out the LAO nanolithography on HDT/Au(111) as control experiments. Freshly cleaned conductive tips are generally considered optimal for such bias-assisted processes to ensure a good electric contact between the tip and the sample surface. The cleaning procedure, for example, immersion in piranha solution, can remove the most of organic contaminants from the tip surface and thus produce a very hydrophilic tip surface. However, this hydrophilic tip surface can easily become more hydrophobic after storing a freshly cleaned probe on a cured silicone cushion in a commercial probe box due to the spontaneous adsorption of hydrophobic airborne and package-released polydimethylsiloxane (PDMS)-like organic contaminants on the tip surface,37-39 though the hydrophobic degree of contaminated tips seems difficult to control exactly. A freshly piranha-cleaned Pt/tip fabricated the oxidation patterns in Figure 3a, and it was used again in Figure 3c after being stored in a probe box for 2 h. Comparison of Figure 3a with Figure 3b confirms that the observed lines fabricated by a freshly cleaned Pt/tip all belong to the DO of gold substrate (when biases are greater than 3.0 V). No changes could be detected by AFM in either topography or friction images when biases were lower than 3.0 V, which can be defined as “no oxidation”. A bias of 3.0 V in the current case could be a critical bias which was just able to initiate oxidation of the underlying gold in parts. The hydrophilic and nonuniformly oxidized surface on the 3.0 V line could be clearly identified in the friction image, while growth of the gold oxide
in parts was confirmed by the appearance of a weak but detectable part of a groove after the 2-h immersion in ethanol. However, no obvious topographic changes were observed for the 3.0 V line in the height image, probably because the height increase caused by a small quantity of gold oxide was just able to remedy the thickness decrease of the decomposed HDT SAMs in the DO domains.36 In contrast, comparison of Figure 3c with Figure 3d indicates that this contaminated tip increased the Vth for DO to 3.5 V and realized a narrow bias range of 3.1-3.4 V for the CNL between the DO (when biases are >3.4 V) and the no oxidation (when biases are
