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Site-selective assembly of quantum dots on patterned self-assembled monolayers fabricated
by laser direct-writing
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2012 Nanotechnology 23 235302
(http://iopscience.iop.org/0957-4484/23/23/235302)
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Nanotechnology 23 (2012) 235302 (6pp) doi:10.1088/0957-4484/23/23/235302
Site-selective assembly of quantum dotson patterned self-assembled monolayersfabricated by laser direct-writing
Chong Wu1,4, Yongsheng Wang1,2,4, Xuemingyue Han1, Xinming Hu1,Qianyi Cheng1, Baohang Han1, Qian Liu1, Tianling Ren2, Yonghong He3,Shuqing Sun3,5 and Hui Ma3
1 National Center for Nanoscience and Technology, Graduate School of Chinese Academy of Sciences,11 Beiyitiao, Zhongguancun, Beijing 100190, People’s Republic of China2 Institute of Microelectronics, Tsinghua University, Beijing 100084, People’s Republic of China3 Laboratory of Optical Imaging and Sensing, Graduate School at Shenzhen, Tsinghua University,Shenzhen 518055, People’s Republic of China
E-mail: [email protected] and [email protected]
Received 28 December 2011, in final form 1 April 2012Published 17 May 2012Online at stacks.iop.org/Nano/23/235302
AbstractA simple and efficient route for quantum dot (QDs) patterning using self-assembledmonolayers (SAMs) as templates is described. By means of a laser direct-writing (LDW)technique, SAMs of octadecylphosphonic acid formed by adsorption on native oxide layer oftitanium film were patterned through laser-induced ablation of the SAM molecules. Thistechnique allows the creation of chemical-specific patterns accompanied by slight change inthe topography. Using atomic force microscopy and friction force microscopy, the dependenceof feature size and characteristics on the irradiation dose was demonstrated. Upon immersionof a substrate with patterned SAMs bearing thiol as the terminal group into a dispersion ofQDs resulted in the assembly of QDs on the specific thiol-terminated areas. Patterns of QDswith different photoluminescent wavelength were generated. The LDW technique, which isconvenient and flexible due to its path-directed and maskless fabrication process, provided anew powerful approach for patterning materials on surfaces for various applications.
S Online supplementary data available from stacks.iop.org/Nano/23/235302/mmedia
(Some figures may appear in colour only in the online journal)
1. Introduction
Quantum dots (QDs), as one kind of the most popularmaterial in scientific research, exhibited inherent luminescentproperties, such as high quantum yields, broad absorption,narrow and tunable emission, and high photostability, whichare superior to organic luminescent materials [1–3]. With allof these excellent features, QDs show their potential in avariety of applications including biological imaging [4, 5],light-emitting diodes (LEDs) [6, 7] and solar cells [8, 9].
4 These authors contributed equally to this work.5 Author to whom any correspondence should be addressed.
While most of the applications for imaging and labelling arebased on colloidal bioconjugates of QDs, those for LEDs andsolar cells required the deposition or assembly of QDs toorganized arrays in many cases to improve the performance,e.g. arrayed nanostructures enhanced the light adsorption forsolar cells. Several methods of surface modification for QDself-assembly have been reported [10–12]. Based on the self-assembly, micro/nanopatterning of QDs have been achievedin different ways which combine top-down and bottom-upapproaches. For instance, conventional photolithography wasutilized by Zhang’s group for the preparation of top-emittingQD-LEDs on silicon [13], and electron-beam lithography was
10957-4484/12/235302+06$33.00 c© 2012 IOP Publishing Ltd Printed in the UK & the USA
Nanotechnology 23 (2012) 235302 C Wu et al
used by Subramani and co-workers to generate nanostructuresfor QD deposition, through the supramolecular interactionbetween the structure and surface-modified QDs [14]. Softlithography was another method for the fabrication ofQD patterns. Junkin and co-workers developed a methodof plasma lithography to provide the template for QDassembly [15]. In addition, researchers from SamsungElectronics manufactured a full-colour display by solvent-freetransfer printing films of diverse QDs to the displaysubstrates [7].
As an alternative to the variety of lithography meth-ods [16–18], the laser direct-writing (LDW) technique hasbeen widely used for the fabrication of photonic crystals [19,20], the patterning of graphene structures [21] and the printingof complex materials [22]. Compared with the techniquesmentioned above, LDW is much more convenient andflexible because it is a path-directed and maskless fabricationprocess. Self-assembled monolayers (SAMs), which playedan important role in QD assembly behaviour [12], is onetype of typical material that can be patterned utilizingLDW [23, 24]. Laser-induced thermal desorption of SAMs oncertain substrates, mainly thiols on the gold films, providedsites available for the adsorption of a different kind ofmolecule. The obtained multicomponent surfaces can beused as templates for further functionalization, includingsite-selective deposition of QDs.
As complements to thiols on gold films, SAMs oforganophosphonic acid on the native oxide of titaniumfilms have been proved to be promising for the fabricationof structures with multi-functionality [25]. Upon heatedby a localized laser beam, different from using goldfilms as substrates, titanium films can be oxidized withelevated features rather than ablated, thus providing acontamination-free fabrication process in a wide variety oflaser dose and avoided the necessity to carry out LDW inliquid, which increased its complexity. In this paper, weutilized LDW as a tool for the fabrication of multicomponentchemical patterns using SAMs formed by adsorption ofoctadecylphosphonic acid (ODPA) on a native oxide oftitanium film. By backfilling of the laser-irradiated areas with10-mercaptodecylphosphonic acid (MDPA), active reactionsites of ω-substituted thiol group were introduced for furtherconjugation and QD patterns were obtained through a simplechemical bath deposition method. Multi-coloured fluorescentpatterns were achieved by repeating the LDW process and thedeposition of QDs with diverse emitting wavelengths.
2. Experimental section
2.1. Materials
Titanium slugs (99.995%) and ODPA (crystalline) werepurchased from Alfa Aesar. Chloroform was purchased fromBeijing Chemical Works, China. All the chemicals were usedas received except where specially mentioned. MDPA wassynthesized through a four-step reaction. Its synthesis andcharacterization were described in the supplementary data(available at stacks.iop.org/Nano/23/235302/mmedia).
2.2. SAM preparation
Glass coverslips were soaked in piranha solution (concen-trated sulfuric acid and 30% hydrogen peroxide mixed in thevolume ratio of 7:3), rinsed with ultrapure water for severaltimes and dried under a nitrogen stream (CAUTION! Piranhasolution is aggressively oxidizing and should avoid contactwith organics). ODPA SAMs were prepared according to aprevious report [25]. SAMs were prepared by first evaporatingabout 50 nm titanium film onto cleaned glass coverslipsand followed by immersion of the substrate into a 1 mMisopropanol solution of ODPA for at least 48 h.
2.3. Laser direct-writing
A home-built LDW system adopted a frequency-doubledNd:YAG 532 nm laser (Spectra Physics, Millennia Pro 2i)was used to write the ODPA SAMs on titanium films bya raster scan with a typical scan speed of 50 µm min−1
and a repetition rate of 250 Hz. The sample was placedon an X–Y–Z sample stage and the focus point (about350 nm diameter) of the objective lens in the system couldautomatically focus on the surface of the sample in the writingprocess. The writing path of the laser beam was controlled bya computer, while the writing power and the pulse width couldbe tuned by an acousto-optic modulator. In the laser-writingprocess, the writing power was changed from 1 to 10 mWwith a pulse width of 1 ms, corresponding to the laser energydensity ranging from 10 to 100 J mm−2.
2.4. Refunctionalization and QD attachment
Quantum dots were synthesized using a reported method [26],in which the synthesis was accomplished in a simple one-stepreaction. According to the synthesis procedure, the QDs werecapped with oleic acid, which would be easily replaced bythiol-terminated ligand [2]. The LDW-patterned SAMs wereimmersed into a solution of 1 mM MDPA in chloroform forthiol functionalization for over 12 h, rinsed three times withchloroform and followed by immersion in the dispersion ofQDs in chloroform for the QD attachment.
2.5. Characterization
Atomic force microscopy (AFM) was performed on a DigitalInstrument Dimension 3100 atomic force microscope incontact mode with Nanoworld CONTR-10 contact probes at ascanning angle of 90◦. The Raman spectrum of LDW-inducedtitanium oxide was recorded with a Renishaw inVia PlusRaman microscope (Renishaw, UK) using the 514.5 nm laseras an excitation wavelength. Scanning electron microscopy(SEM) was performed on a Hitachi S-4800 scanning electronmicroscope (Japan) at an acceleration voltage of 5 kV. Opticaland fluorescent microscopy images were captured using aLeica DMI 3000B inverted fluorescent microscope in differentmodes and channels for specific requirements. A fluorescentimage of multi-coloured patterns was recorded on a Nikon-Tifluorescent microscope with an UltraVIEW VoX confocalsystem.
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Scheme 1. The overall procedure for the LDW and generation offluorescent QD patterns. (a) LDW of SAMs; (b) formation of SAMsof second molecules on the laser-patterned regions; (c) attachmentof QDs to the patterned areas; and (d) fluorescent image of QDpattern of the word ‘NANO’ generated following the aboveprocedure.
3. Results and discussion
The overall patterning process of QDs is illustrated inscheme 1. A laser beam with a wavelength of 532 nm wasfocused on the ODPA SAMs on the titanium film with anative oxide layer. While the laser was scanning across thesurface, SAM molecules desorbed or decomposed at the areaswhere the laser-induced local heating of the substrate to athreshold temperature (scheme 1(a)). Subsequent immersionof the laser-patterned substrate into a solution of MDPAresulted in a compositional chemical pattern composed ofthiol-terminated groups in a background of primary ODPASAMs (scheme 1(b)). Benefiting from the high affinity ofthe thiol group to the II–VI QDs, a ligand exchange reactionoccurred, resulted in the immobilization of QDs throughchemical-specific interaction (scheme 1(c)) and led to thecreation of fluorescent patterns (scheme 1(d)).
To establish the experimental conditions for thepatterning of SAMs using LDW, the laser power wasgradually decreased from 7 to 1 mW with a decrease of 1 mWto write parallel lines with a separation of 5 µm and theresulting substrate was analysed by AFM. Figure 1 showsthe AFM image and corresponding section analysis image ofthe parallel lines written by different powers of the laser. Theline on the right was induced by the laser of 10 mW, definedby a computer-controlled system as registration, showed anelevation in topography. The first three lines from the left,written with laser powers of 7, 6 and 5 mW, respectively, alsoshowed a similar height of about 10 nm as the registration linebut with slightly narrower width. The fourth line, written witha laser power of 4 mW, was lower in height and narrower inwidth. For all these four lines, while elevated in the centrearea, the topography in the perimeter area were lower. It isalso clear that for line 5 and line 6 from the left, whichwere written with laser powers of 3 and 2 mW, respectively,
Figure 1. AFM image of a pattern fabricated by LDW in SAMs onnative oxide of titanium (a) and cross-section analysis (b). From leftto right the laser power was decreased from 7 to 1 mW with 1 mWdecrement, and the rightmost registration line was written at apower of 10 mW. Image size: 50× 50 µm2.
no obvious height elevation was observed, but reversed toa groove about 1.3 nm in depth, with an elevated featurein line 5 just recordable. While for the line written with1 mW, it is hard to see any change in topography. A graphof line height as a function of the laser power was shownin figure S2 in the supplementary data (available at stacks.iop.org/Nano/23/235302/mmedia) to illustrate the effect oflaser power on the patterned feature sizes. We thereforeestablished the threshold parameter for SAM patterning inour system. The minimum laser power required to induceany changes in SAMs was about 2 mW, which may eitherascribed to the partial removal of organic molecules or therearrangement of grains in titanium film. With the increasingof laser power, the oxidation of titanium under a thin nativeoxide layer occurred and resulted in the elevation of theirradiated regions. The oxidation of the titanium film wasfurther shown by micro-Raman measurements. Two Ramanpeaks appeared around 442 and 612 cm−1 (figure S1 inthe supplementary data available at stacks.iop.org/Nano/23/235302/mmedia), which were associated with the rutile TiO2structure [27]. The threshold of laser power for oxidation wasaround 3 mW, which was consistent with the AFM data.
Friction force microscopy (FFM) has always been apowerful tool to characterize the surface composition in thenanometre scale. The AFM and FFM images were recordedsimultaneously for a patterned structure fabricated by LDWwith a laser power of 4 mW. As shown in figure 2, parallellines induced by laser illumination can be clearly seen inboth images. In contrast to the topography changes shownin the AFM image (figure 2(a)), the bright contrast shownin the FFM image (figure 2(b)) indicated desorption of the
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Figure 2. AFM (a) and FFM (b) image of parallel lines fabricated by LDW using laser power of 4 mW. Image size: 30× 30 µm2, scale:30 nm and 0–0.5 V, respectively.
Figure 3. FFM images of LDW patterns before (a) and after (b) ODPA backfilling. Image size: 30× 30 µm2, scale: 0–0.5 V, for bothimages.
SAM molecules as the interaction between SAM moleculesand the AFM tip is weaker and resulting in dark contrastin the background areas. The linewidth is about 450 nm,which is consistent with that shown in figure 1 under identicalconditions.
To verify the desorption of the SAM molecules and thecapability of functionalization of the laser-irradiated areas, thepatterned SAMs was immersed in an ODPA solution againunder the same condition for primary SAM formation tobackfill the pattern generated by LDW and characterized byFFM. As shown in figure 3, the contrast of parallel lines inFFM changed essentially before and after ODPA backfilling.In figure 3(a), the friction of the pattern was obviously higherthan the background, as discussed above. After backfillingwith ODPA, the same molecules as those composing theprimary SAM, the friction contrast was significantly reduced,as shown in figure 3(b), indicating the success of SAMformation in the patterned regions.
Instead of using ODPA as secondary SAM molecules, thepatterns were functionalized by MDPA, which formed SAMsbearing a thiol group. Upon immersion in a dispersion ofsurfactant-stabilized CdSe/ZnS QDs, site-specific attachmentoccurred at the regions presenting thiol functionality. Thefluorescent spectrum of the QD solution was shown infigure S3 in the supplementary data (available at stacks.iop.
org/Nano/23/235302/mmedia) to illustrate the luminescentproperties of the QDs. Optical microscopy images of samplesat different preparation stages were captured in differentmodes for comparison, as shown in figure 4. Figure 4(a)showed a bright-field image of a sample before QDattachment in the transmission path, in which the brightdots and lines represented the regions of titanium oxidewith high transmittance induced by laser and the darkbackground represented the unwritten titanium substrate.After the attachment of QDs with red-light emission, thefluorescent image of the same pattern shown in figure 4(a)was captured using the N21 channel, as shown in figure 4(b).The N21 channel, with a filter of long pass 590, only allowedlight with a wavelength longer than 590 nm to pass through.Figure 4(c) showed an enlarged area in figure 4(b), Thebright fluorescence emitted from the patterned areas, eitherfrom parallel lines or dot arrays, is obvious. To confirmthe origin of the fluorescence is from QDs rather thansomething else, fluorescent microscopy measurement of thesame pattern was also performed using the GFP channel,and no fluorescent image was obtained. This is because theGFP channel, which uses a filter that only allowed greenfluorescence with wavelength at 525±50 nm to pass through,blocked the red fluorescence from the QDs. The absence ofa fluorescent pattern in figure 4(d) provided further evidence
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Figure 4. Microscopy images of LDW patterns before and after QD attachment: (a) bright-field image before QD attachment. (b) and (c)Fluorescent image captured in N21 channel after QD attachment. (d) Fluorescent image in GFP channel after QD attachment.
Figure 5. Microscopy images of fluorescent patterns using LDWfollowed by green-light emission QD attachment: (a) parallel linesand (b) the word ‘NANO’.
that the patterned image from figures 4(b) and (c) originatedfrom the immobilized red-emission QDs. Some fluorescentdots appeared in the background, which might be caused bynon-specific deposition of QD clusters. It should be notedthat the colour in figures 4(b) and (c) was pseudo, as themicroscope could not recognize the wavelength but only lightintensity. Nevertheless, the contrast between the pattern andbackground was excellent in general.
Identical to the immobilization of red-light-emissionQDs, smaller QDs that emit green light were also attachedto the patterns generated by LDW. The fluorescent patternof parallel lines and the word ‘NANO’ were successfully
Figure 6. QD distribution in and out of LDW areas: the fluorescentimage of QD patterns of parallel wide stripes (a), the AFM imagesat the edge before (b) and after (c) QD attachment (image size:10× 10 µm2) and the corresponding cross-section profiles (d) and(e), and SEM images of QDs out of (f) and in (g) the stripes.
fabricated, as shown in figures 5(a) and (b), respectively,recorded by using the GFP channel. It is worth noting thatthe linewidth presented in figure 5(b) is well below 1 µm,
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demonstrating the powerfulness and the flexibility of thecurrent approach.
In order to further understand the obtained fluorescentpatterns, we investigated the topographical feature of the QDpatterns. 5 µm wide stripes were created and used for QDattachment and figure 6(a) showed the QD patterns of thewide stripes obtained using the GFP channel, in which greenQDs were used for attachment. Topographical changes of thepattern were investigated using AFM and SEM. Figures 6(b)and (c) showed the AFM images of a single stripe before andafter QD attachment. The only visible difference of figures6(c) from (b) is that there were some scattered particlesaround the stripe, which may be due to the non-specificadsorbed QDs. While the comparison of the height profileof the two images, the significant increase in height in thelatter image could be easily seen. An average differenceof ∼5 nm in vertical direction indicated the deposition ofQDs onto the stripes, as marked in figures 6(d) and (e).However, the QDs were not closely packed even on thestripes. RMS (root-mean-square) roughness was measured tobe 3.57 nm and 1.67 nm on and off the stripes, respectively,resulting from the different grain size between the depositedQDs and the substrate. Further evidence on the coverageof the QDs in the patterned areas was shown in figures6(f) and (g), for laser-irradiated regions and backgroundareas, respectively, clearly showed the distribution of QDson different areas due to the difference in electron scatteringcapability between the QDs and the substrate. On the stripe,which were written by laser and functionalized by MDPA,the QDs deposited selectively to a sub-monolayer, while offthe stripe, in the background composed of ODPA SAMs, asmall amount of QDs randomly attached to the surface dueto the non-specific adsorption, which is in accordance withthe morphologies shown in figure 6(c). In the simple method,the fabrication of multi-coloured fluorescent QD pattern hasalso been demonstrated following a serial process (shownin figure S4 available at stacks.iop.org/Nano/23/235302/mmedia). The parallel lines with green fluorescence remainedafter the second orthogonal immobilization of the red QDswhich gave rise to distinct contrast. The uniform monolayerof QDs is important for the application in array-baseddevices and our method for the surface modification andQD attachment provide a competitive approach for patterningclosely packed QD arrays.
4. Conclusion
The application of LDW on SAMs of organophosphonicacid adsorbed on the native oxide of titanium films provideda powerful and flexible route for patterning of SAMs onsubstrates with various feature sizes. Chemical patterningof SAMs over a wide range of doses without ablation ofthe substrate under ambient conditions were demonstrated,which facilitates a contamination-free, easy and reliableprocess. Refunctionalization of the patterned area with MDPAfollowed by immersion of the substrate in a dispersion offluorescent QDs allowed the occurrence of ligand exchangereactions and resulted in the site-selective assembly of QDs
in the patterned area, exhibiting well-defined fluorescentpatterns upon excitation. In addition, with the unique physicalproperties of the rutile structure formed under the chemicalspecific feature, we believe this process will enable awide variety of patterning tasks for hybrid electronic andoptics-based sensor applications.
Acknowledgments
This work was supported financially by the NationalScience Foundation of China (NSFC) (grants 20971030,90923007 and 10974037) and MOST (grants 2009CB930700,2010CB934102 and 2010DFA51970).
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