Template assisted synthesis and assembly of nanoparticles 6

Chapter Chapter Template-assisted assembly of Ag2S/CuXS (x = 1.75) nanoparticles As introduced in Chapter 1, assembly as one of the most efficient methods is used to order small particles on surfaces Further growth of these ordered structures into 2D or 3D well-defined and sufficiently large colloidal structures have potential application in photonics The use of physical template to assemble colloidal particles (e.g SiO2, ZrO2) into aggregates with long-range order has proven to be a versatile approach for the fabrication of more efficient light sources, detectors etc.1, Generally, this approach is called template-assisted assembly In template-assisted assembly process, a topographically patterned (formed by assembly of polymer beads/copolymer3, photolithography, electron beam lithography4 etc.) or chemically patterned surface (produced by flexible aliphatic molecules as linking groups)5 is normally used as template However, templates of patterned topography offer more accurate positioning of particles compared with a chemically patterned surface They are used to create a well-defined spatial distribution of forces that direct the motion of particles towards specific areas of the substrate The use of wet colloidal self-assemblies as template to define the structure for further nanoparticles assembly has been demonstrated.6, Wang et al have co- crystallized Au@SiO2 nanospheres together with PS latex spheres on quartz slides.8 They also investigated the relocalization of silica colloidal spheres using 2D patterned 193 Chapter substrate as the templates through stepwise spin-coating technique.6 Kitaev et al reported the formation of well-ordered self-assembled binary colloidal crystal (silica & PS spheres) films in the scale of a few square centimeters, using microspheres with a large disparity of sedimentation rates through accelerated evaporation induced coassembly.9 Another commonly used technique to produce templates is lithography such as nanoimprint lithographic technique (NIL)10 Xia have demonstrated the capability of template-assisted assembly in producing a rich variety of polygonal, polyhedral, spiral11, and hybrid aggregate of spherical PS spheres or silica colloids on physical template made by conventional microlithographic techniques.3, 12 The structure of the assemblies could be conveniently controlled by simply changing the shape and dimensions of the template Template-assisted assembly method typically combines physical templating and capillary forces to assemble colloidal particles into uniform aggregates and structures The assembly of colloidal particles relies on the interaction between particle and/or particles and surfaces to drive the formation of ordered arrangements Depending on the nature of the interaction between the particles themselves and the template surface, adequate driving forces such as gravitational sedimentation by solvent evaporation13, fluid flow14, electric field, or centrifugal force due to spinning15 are employed to facilitate the assembly process Two templates were investigated in this chapter, namely the spontaneous selforganization of colloidal PS beads and PS line patterns generated via NIL technique These templates were employed to direct the assembly of semiconductor 194 Chapter nanoparticles In this work, we have studied two specific nanoparticles, i.e faceted Ag2S nanoparticles and CuxS nanodisks Assembled Ag2S nanoparticles could be used as optical filter, emitter while regular assembled nanodisks could give rise to technologically useful properties, such as anisotropic electrical transport and optical properties 6.1 Assembly of Ag2S nanoparticles on pre-assembled polystyrene beads 6.1.1 Estimation of the size of PS beads needed Before using polystyrene (PS) beads as templates, calculations were carried out to determine the size of PS beads needed for the assembly of specific sizes of nanoparticles Figure 6.1 illustrated the calculations and Table 6.1 gave the estimated sizes of the cavities for certain diameter of the PS beads used in the template Ag2S nanoparticles were prepared using our reported procedures detailed in Sections 2.3.4 and 2.3.5 Faceted nanoparticles were prepared with an average size of about 40-50 nm Based on Table 6.1, the minimum size of PS beads that could be used as template is 400 nm Taking into consideration the lower estimation and also availability of commercial PS beads in the laboratory, beads with average diameter of 1.053 µm were used as template in this study Details about the assembly of PS colloidal solution were described in Section 2.7.4 Some preliminary trials and comparisons were made to decide which method and concentrations would be optimum for the pre-assembled template of PS beads 195 Chapter rps L rss D rps H 45° H L 2rss (A) (B) Diameter of each PS bead = D Radius of each PS bead, rps = D/2 Height of green triangle, H = D2 rps Area of void between three PS beads, AV = HD – 3( rps2 2 ( = rad.) Radius of small sphere, rss = (2/3) H - rps Area of small sphere, AS = (rss)2 Width of cube, L = 2rss sin 45° Area of cube, AC = L2 (C) Figure 6.1 (A) Diagram illustrating the void in between three PS beads and a small sphere which is in grey that can fit into the void; (B) Size of a cube that can fit into the void can be estimated based on the size of the small sphere; (C) Calculations steps to estimate the area of the void; and the maximum size of the small sphere, AS, and cube, AC Table 6.1 Estimation of maximum sizes of the small sphere and cube for certain diameter of PS beads (Refer to Figure 6.1 for symbols) D (nm) H (nm) AV (nm2) rss (nm) AS (nm2) L (nm) AC (nm2) 100 87 403 188 11 120 200 173 1613 15 752 22 479 300 260 3628 23 1692 33 1077 400 346 6450 31 3007 44 1915 500 433 10078 39 4699 55 2992 600 520 14513 46 6767 66 4308 700 606 19754 54 9210 77 5863 800 693 25801 62 12030 88 7658 900 779 32654 70 15225 98 9693 1000 866 40314 77 18796 109 11966 1053 912 44700 81 20842 115 13268 196 Chapter 6.1.2 Assembly of Ag2S nanoparticles on PS beads pre-assembled patterns First, to study the interactions between Ag2S nanoparticles, the self-assembly of Ag2S on bare silicon wafer was investigated by solvent evaporation or dipping & interface method as detailed in Sections 2.7.2 and 2.7.3 respectively Analysis under SEM showed that Ag2S nanoparticles formed clusters instead of monolayers when assembled by these two methods (Figure 6.2) It thus seems that strong interactions existed among the Ag2S nanoparticles and resulted in aggregation In the following, we attempted to influence the assembly using template-assisted method, i.e using pre-assembled PS beads to define the location for nanoparticles aggregation a b Figure 6.2 SEM images showing direct assembly of Ag2S on silicon substrates through: (a) solvent evaporation method, (b) dipping & interface method As reported by Kitaev9 and Kim7, regular PS beads pattern can be easily obtained through convective vertical evaporation Upon solvent evaporation, convective mass flow and capillary forces cause the PS microspheres to assemble at the air-solventsubstrate interface to form a well-ordered hexagonal close-packed monolayer Subramanian16 also found that on slowly evaporating the water, the monodispersed 197 Chapter PS particles will organize themselves in an ordered pattern due to a gradual increase in their concentration SEM images in Figure 6.3 showed that large area of regular self-assembled PS beads pattern can be achieved through slow water evaporation when the tilt angle of the Si substrate was set at 30° The simplicity and the reproducibility of this method were proven by many trials of assemblies This regular PS close-packed pattern has been used as template for the assembly of nanoparticles.17, 18 a b Figure 6.3 SEM images showing the assembly of PS beads (1.053 µm) at different concentrations through solvent evaporation method: (a) 0.26%, (b) 0.65% The surface of the close-packed PS microsphere presents two types of cavities suitable for the arrangement of nanoparticles: interstitial sites between three adjoining spheres and channels bridging these interstices When this PS pre-assembled pattern was dipped into Ag2S dispersion, the Ag2S nanoparticles supposed to pack into these cavities SEM images in Figure 6.4 showed clearly that the Ag2S nanoparticles had aggregated into these cavities of the PS templates Dipping & interface method seems to give better assembly of Ag2S into these cavities although the area of assembly into the cavities was not uniform 198 Chapter a b c d Figure 6.4 SEM images showing the assembly of Ag2S onto pre-assembled template of PS beads by using (a, b) dipping & interface method, and (c, d) solvent evaporation method Capillary forces due to surface tension between the nanoparticles and solvent have been shown to play a major role for the ordering during assembly.13, 19 Capillary forces allow the nanoparticles assembled together and accumulate into the cavities of PS pattern In the solvent evaporation method, the surface tension and convective mass flow would act to pull the nanoparticles together In the dipping & interface method, the controlled movement of the template against the solvent provided the pulling and capillary interaction Nevertheless, it seems that the strength of the surface tension between Ag2S nanoparticles and solvent was not strong enough to overcome van der Waals interactions between the Ag2S nanoparticles, thus 199 Chapter multilayers and aggregations of Ag2S particles were covering the PS beads pattern in most area a b c d e f Figure 6.5 SEM images showing assembly of Ag2S nanoparticles on PS beads template with different amount of nanoparticles: (a) 10 µL, (b) 15 µL, (c) 20 µL, (d) 30 µL, (e) 50 µL, and (f) 100 µL SEM images in Figure 6.5 showed the assembly patterns using varying amount of 200 Chapter Ag2S nanoparticles While larger areas of assembly would be expected using larger amount of nanoparticles, the distribution of coverage is not even and the surface of the PS beads was almost completely covered in some cases We have also attempted to remove the PS beads after assembly in order to expose and examine the assembly of Ag2S nanoparticles clearly Since PS beads were much larger than the Ag2S nanoparticles, we achieved the removal by softly touching the surface using a piece of adhesive tape SEM images in Figure 6.6 confirmed that most of the PS beads could be removed from the surface using this simple method In some regions of the assembly (marked with rectangular box in Figure 6.6), the closed-packed pattern of the PS beads was clearly evident However, some patterns were removed together with PS beads (marked with circle), which indicated further trial should be done to obtain optimized Ag2S pattern: sitting in the interstices of PS pattern a b Figure 6.6 Ag2S pattern after the removal of PS beads from the template 6.1.3 Assembly of CuxS (x = 1.75) nanodisks on the PS spheres pattern It is known that the shape and morphology of nanoparticles have great effect on their assembly behavior Non-spherical nanoparticles show different types of self201 Chapter assembly.20 For example, raft-like aggregates have been observed for nanorods assembly.21-23 Thus, we have also investigated the assembly of copper sulfide (CuxS) nanodisks on the PS beads pattern using the same method CuxS nanoparticles with regular disk shape (diameter ~ 100 nm; thickness ~ 15 nm) were prepared using hot injection method developed in our laboratory (Section 2.3.5)24 As shown in Figure 6.7(a), the size dispersity of the CuxS nanodisks was found to be better than that of Ag2S nanoparticles Since the average diameter of the disks is slightly larger than the estimated cavity size (Table 6.1) of the ~1 µm PS beads, we would expect these nanodisks can only assemble into the cavity by sitting on their sides, which proved by SEM images shown in Figure 6.7 It is clear that although some CuxS nanoparticles can enter into the interstices, most of them were randomly dispersed on the top of PS pattern After PS pattern was removed, there is no regular patterned CuxS nanoparticles can be found Thus, no further investigations were done on its assembly a b Figure 6.7 SEM images showing the assembly of CuxS nanodisks on different substrate: (a) 30 µL on bare Si substrate, and (b) 30 µL on PS pattern In conclusion, Ag2S nanoparticles can be assembled into the interstitial and channel cavities of PS close-packed pattern while only partial CuxS nanodisks can 202 Chapter The arrangement of nanoparticles depends on space geometry as well as on the nanoparticles shape The pre-assembled PS beads template, however, was found to be too soft and may be destroyed by the action of dipping or withdrawing from the interface In the following section, we prepare a harder template using nanoimprint lithography (NIL) to further investigate the assembly of CuxS nanodisks 6.2 Assembly of CuxS (x = 1.75) on PS line-pattern prepared by Nanoimprint Lithography PS line-patterns were fabricated through the combined use of NIL and ATRP as presented in Section 2.7.5 The width of the line pattern, and hence the spacing between channels, can be controlled by varying the time of ATRP reaction As shown in Figure 6.8, the channel spacing changed from 250 nm to 200 nm after ATRP treatment for hours Although the depth of the channel may also be changed with the ATRP process because the residual layer in the channel was not removed, this dimension would not affect the assembly behavior of our nanodisks in this study a b Figure 6.8 SEM images showing (a) the original PS line-pattern with 250 nm channel spacing prepared by NIL, and (b) pattern after ATRP treatment giving channel spacing of 200 nm 203 Chapter The detailed assembly procedure was discussed in Section 2.7.5 The amount of CuxS nanodisks was varied and a suitable concentration ~ ×10-4 mol/L was chosen for the best assembly results All our attempts showed that after participating (of CuxS nanodisks) step and washing (by solvent) step, CuxS nanodisks would accumulate inside the channels of the PS line-pattern, rather than on top of the line pattern The assembly is believed to be similar to a sedimentation process combining with physical restriction of PS channels and the arrangement of nanodisks inside the channels will be influenced by physical or spatial constraint and surface property of PS channels First, we investigated the effect of channel spacing to the assembly of CuxS nanodisks SEM images in Figure 6.9 showed the assemblies of ~190 nm CuxS nanodisks on PS line-pattern with 180 nm and 210 nm channel spacing It is obvious that when the size of the nanodisks is bigger than the channel spacing (Figure 6.9a), the nanodisks were forced to stand on their sides Whereas when the channel spacing is bigger, the nanodisks sit on their faces inside the channels (Figure 6.9b) a b Figure 6.9 Effect of channel spacing on the assembly of CuxS nanodisks (particle size: ~ 190 nm): (a) channel spacing = 180 nm, and (b) channel spacing = 210 nm Next, we varied the size of CuxS nanodisks and investigated their assembly behavior in the same channel spacing of 200 nm From SEM images in Figure 6.10, 204 Chapter it can be seen that while all the CuxS nanodisks can be assembled into the larger PS channels, the assembled morphology become different depending on disk sizes When the diameter of the nanodisks is ~ 100 nm, which is much smaller than the channel spacing, CuxS nanodisks were found to assemble randomly (Figure 6.10a) As the diameter was increased to ~ 130 nm, a better assembled pattern was obtained with most of the nanodisks sitting on their faces When the diameter was increased to comparable with the channel spacing ~ 200 nm, very regular assembly with nearly all the CuxS nanodisks sitting on their faces was obtained a b c Figure 6.10 SEM images showing assembly of CuxS nanodisks of different sizes into PS line-pattern with 200 nm channel spacing: (a) 100 nm, (b) 130 nm, and (c) 200 nm These observations confirmed that the interplay between the sizes of nanodisks and the channel spacing is a critical parameter to the formation of good assembly 205 Chapter morphology By carefully controlling this parameter, we believe it is possible to regularly arrange CuxS nanodisks into PS channel in a well-ordered pattern In order to verify the surface effect of PS line-pattern to the assembly, PS linepattern without ATRP treatment was used as template for a trial assembly The corresponding SEM image in Figure 6.11 showed that scarce amount of CuxS nanodisks were randomly dispersed in the channel This suggested that indeed the ATRP modification has assisted the assembly of CuxS nanodisks in the PS linepattern Figure 6.11 SEM image showing CuxS nanodisks assemble on PS line-pattern without ATRP modification After precipitation, CuxS particles will locate both on the top and inside the PS channels The PS chains on the surface of PS channel are grown by ATRP They are much easier to swell into organic solvent (toluene) compared with cross-linked PS chains During washing step, the PS chains inside the channel will swell into solvent (Gaussian chains) and wrap around CuxS nanodisks These swollen chains block the movement of CuxS nanodisks and thus hold these particles inside the channel The PS chains on top of the PS channels will push the CuxS nanodisks off the surface accompanying with swelling into solvent Thus CuxS nanodisks which located on the 206 Chapter top of channels are washed away As for the PS channels without ATRP treatment, the cross-linked PS chains on the surface can only swollen a little and will not affect the movement of CuxS nanodisks in solvent In conclusion, ATRP treatment of PS channels is prerequisite to pattern CuxS nanodisks Organic ligand is often added to assist or direct the assembly of nanoparticles into regular assembled pattern Thus, we attempted to add 20 µL DDT to mL CuxS dispersion before the assembly in the PS-line pattern Because DDT is used as capping agent in the preparation of CuxS nanodisks, thus precipitated CuxS were washed thoroughly with ethanol before redispersed in toluene for assembly SEM images shown in Figure 6.12 presented a comparison between assemblies with and without the addition of DDT It thus seems that DDT will affect the density of CuxS nanodisks inside PS channels, probably due to the interactions between DDT on CuxS and swollen PS chains Further investigations such as vary DDT to long chain amine or dithiol molecules, are required to draw a conclusion if the morphology of the assembly is also affected by organic ligands a b Figure 6.12 SEM images showing the effect of adding DDT to the assembly of CuxS nanodisks (a) with DDT added, (b) without DDT 207 Chapter 6.3 Conclusions In summary, two kinds of templates have been employed to assist the assembly of faceted Ag2S nanoparticles and CuxS nanodisks The driving force for assembly is capillary force and physical (spatial) constraint Ag2S can be assembled into the cavities of pre-assembled hexagonally closed-packed PS beads and the effect of concentration on the assembled morphology was investigated and optimized In the second system, CuxS nanodisks can be assembled into the channels of PS line pattern The assembly of CuxS nanodisks was found to be mainly controlled by spatial constraints (PS channels) and sizes of the nanodisks CuxS nanodisks were found to regularly pack inside the channel when the size of CuxS nanodisks fit well with the channel spacing of the PS line pattern 208 Chapter References Xia, Y N.; Gates, B.; Yin, Y D.; Lu, Y Advanced Materials 2000, 12, (10), 693-713 Arduini, M.; Rampazzo, E.; Mancin, F.; Tecilla, P.; Tonellato, U Inorganica Chimica Acta 2007, 360, (3), 721-727 Xia, Y N.; Yin, Y D.; Lu, Y.; McLellan, J Advanced Functional Materials 2003, 13, (12), 907-918 Wang, D Y.; Mohwald, H Journal of 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W P.; Wong, C T.; Ang, S L.; Low, H Y.; Chin, W S Chemistry of Materials 2006, 18, (26), 6170-6177 211 ... 260 362 8 23 169 2 33 1077 400 3 46 6450 31 3007 44 1915 500 433 10078 39 469 9 55 2992 60 0 520 14513 46 6 767 66 4308 700 60 6 19754 54 9210 77 5 863 800 69 3 25801 62 12030 88 765 8 900 779 3 265 4 70... 62 12030 88 765 8 900 779 3 265 4 70 15225 98 969 3 1000 866 40314 77 187 96 109 11 966 1053 912 44700 81 20842 115 13 268 1 96 Chapter 6. 1.2 Assembly of Ag2S nanoparticles on PS beads pre-assembled patterns... size of PS beads needed for the assembly of specific sizes of nanoparticles Figure 6. 1 illustrated the calculations and Table 6. 1 gave the estimated sizes of the cavities for certain diameter of