Microelectronic Engineering 115 (2014) 6–12

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Microelectronic Engineering

journal homepage: www.elsevier .com/locate /mee

Design and fabrication of diverse three-dimensional shell-likenano-structures

0167-9317/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.mee.2013.10.013

⇑ Corresponding authors. Address: Department of Physics, Sichuan University,No. 24 South Section 1, Yihuan Road, Chengdu 610065, China. Tel./fax: +86 (028)85412983.

E-mail addresses: jian-hun.123@163.com (Y. Hou), dujl@scu.edu.cn (J. Du).

Yarong Su a, Song Ye b, Yidong Hou a,⇑, Sha shi c, Shuhong Li a, Fuhua Gao a, Jinglei Du a,⇑a Department of Physics, Sichuan University, Chengdu 610065, Chinab Department of Physics and Electronic Science, Chaohu College, Chaohu 238000, Chinac Department of Electrical and Computer Engineering, University of Minnesota, Minneapolis, MN 55455, USA

a r t i c l e i n f o

Article history:Received 24 August 2013Accepted 21 October 2013Available online 7 November 2013

Keywords:Shell-like nano-structureMicrosphere self-assembly technologySemi-shell

a b s t r a c t

The shell-like nano-structures (SLSs) own special ability to manipulate electromagnetic waves, and canbe used as three-dimensional (3D) nano-antenna, highly efficient generator of second harmonic light,chiral metamaterials, and so on. However, to achieve this complicated geometry shape still remainschallenging, which limits the related applications. In this paper, we propose a new method based onthe microsphere self-assembly technology (MAT) to fabricate this type nano-structures, which is theuse of basic SLSs (such as nano-cups or fragmented shells) as building blocks (BBs) to construct desiredSLSs, just like that in architecture. Through establishing and using the related 3D geometric model, wedesign a great many of new SLSs, include some interesting structures, such as T-shape, ‘maple leaf’-shape,and boat-shape structures. In theory, the number is infinite. In experiment, some new SLSs have beenachieved, and their geometrical morphologies agree well with the designed. These new SLSs will providea foundation for the related researches and applications.

� 2013 Elsevier B.V. All rights reserved.

Introduction

The shell-like nano-structures (SLSs) include spherical shell,ellipsoid shell (nano-eggs), semi-shell (nano-cup and nano-bowl)and fragmented shells, where the fragmented shells is defined asthe spherical shells with internal surface area smaller than that ofthe semishells. Due to their special physicochemical propertiesand the broad application prospects in various fields, these shellshave attracted many attentions in recent years. In fact, such a struc-ture of these shells can be used as carrier for materials or biologicalscaffold [1], and also can attach to the surface of colloidal spheres topattern the surface of these particles or form ‘Janus particles’ or‘Patchy particles’ [2,3]. The material depended physicochemicalproperties of these SLSs are another popular issue [4–12]. The SLSsarranged on target substrate or dispersed in solutions can be usedto enhance the fluorescence or Raman spectroscopy intensities[13,14], treat diseases [15,16], detect biomacromolecules [17],and so on. Particularly, the Au nano-cups can create optical fre-quency ‘magnetic plasmon’ responses of comparable magnitudeto the ‘electric plasmon’ response, and redirect scattered light in adirection dependent on cup orientation [10,12]. And the harmoniclight also can be generated by these Au nano-cups, which provides

a promising approach for the design and fabrication of stable, syn-thetic second-order nonlinear optical materials [15]. However, thefabrication of this type nano-structure still remains challengingdue to their complicated geometrical morphology, especially forthe desired structures described in Refs. [7,18].

In the previous work, some simple shell-like nano-structureshave been achieved, such as the semi-shell, nano-eggs and half-shell strings. And the wet chemistry reduction [19,20] and themicrosphere self-assembly technology (MAT) [10,21] are the mostpopular fabrication methods. But, the wet chemistry reductionmethod is just suitable for the fabrication of several simple nano-shells with specific materials, and the MAT is also just reportedin the fabrication of nano-cups and a few simple fragmented shells[22]. In this paper, we propose a new method based on the MAT tostudy and fabricate this type SLSs systematically. The simple SLSsobtained through the single material-deposition are used as build-ing blocks (BBs) to construct new SLSs. Through the establishedthree-dimensional (3D) geometrical model, we achieve the designand simulation of these new structures. In experiment, these de-signed structures agree well with the fabrications, indicating theeffectiveness of the method proposed here.

The 3D geometrical model and design principle of new SLSs

The MAT is a low-cost, material independent and simple oper-ation method for fabricating micro/nano-structures. When the

Y. Su et al. / Microelectronic Engineering 115 (2014) 6–12 7

desired material with suitable thickness is deposited on the PS ar-rays supported by a substrate, two kinds of structures will beformed: one is the 2D structures, such as triangle particle array,binary array and multiplex quasi-3D grids (in this case, the MATalso has another popular name ‘colloid sphere lithography techno-logy’) [23–26]; the other is SLSs when peeled off from the topsurface of microspheres, such as shell fragments and nano-cups[22,27]. The geometrical morphology of these SLSs is very complex,and cannot be imagined before they have been achieved in exper-iment. In this section, we establish a 3D geometrical model toachieve the simulation and design of these SLSs, and improve theconvenience and high-efficiency of the method proposed in thispaper.

Fig. 1(a) is the schematic of material vapor on a hexagonallyclose-packed sphere monolayer, h and u; are the deposition angles.In this paper, h is defined as the angle of the incident material va-por beam measured from the substrate side, and u is the angle todescribe the PSs array monolayer orientation. Through the singlematerial deposition on the top surface of micro-sphere array, a ser-ies of simple SLSs can be obtained, and some of them are showingin Fig. 1(b). While the multiple material deposition on the samemicro-sphere array will lead to the formation of more SLSs, i.e.complex SLSs, just like the schematic and the simulated structuresshowing in Fig. 1(c). From the simulation and design point of view,the complex SLSs can be saw as the combination of some simple

Fig. 1. (a) Schematic of material vapor on a hexagonally close-packed sphere monolayer.(c) Schematic of multiple material deposition and simple SLS combination. (d) The simulaFig. 1(c).

SLSs, i.e. basic BBs. In this case, the design process of new SLSscan be divided into two parts: one is to choose appropriate simpleSLSs as BBs; the other is to design the combination modes of theseBBs. Obviously, a great number of new SLSs can be obtainedthrough this combination design, due to the permutation and com-bination theory. For example, if two identical simple shell-likestructures has been chosen as BBs, three more new SLSs can be ob-tained through this combination design, as showed in Fig. 1(d).

The geometrical morphologies of the obtained new SLSs aredepended by two main parameters:

One is the parameters related to the micro-sphere array,

f1ð~r; d; RÞ ¼ ½~r; d; R�

where ~r is defined as the monolayer orientation of PSs array. And~r ¼~r4 for the micro-sphere array with quartet symmetry, ~r ¼ ~r6

for the micro-sphere array with hexagonal symmetry; d and R (unit:nm) are the distance between two neighboring particles and thediameter of the micro-spheres, respectively.

The other is the parameters related to the multiple materialdeposition process,

f2ðn; h; u; kÞ ¼ k1½h1; u1� þ k2½h2; u2� þ . . .þ kn½hn; un�

¼Xn

i¼1

ki½hi; ui�

h is the incident angle, and u is the orientation angle. (b) The simulated simple SLSs.ted complex SLSs through the combination of two identical simple SLSs as showed in

8 Y. Su et al. / Microelectronic Engineering 115 (2014) 6–12

where, n is the number of the BBs used to construct new structures(i.e. the deposition number); ki (unit: nm) is the deposition thick-ness on the plane perpendicular to the vapor beam.; hi and ui arethe related deposition angles.

Thus, the obtained new SLSs can be described as below:

Hðf1; f 2Þ / Fðf1ð~r; d; RÞ; f 2ðn; h; u; kÞÞ

In theory, the value domains of these parameters in H(f1, f2) ared > 0, R > 0, n = 1, 2, 3, . . . , 0 < h 5 90 deg, 0 < u 5 360 deg and k > 0,respectively. From these value domains, we can get that the valuenumber of H(f1, f2) is infinite, i.e. the number of new SLSs is infinite.

The fabrication of new SLSs

The fabrication process of SLSs can be divided into two mainparts: one is the use of micro-sphere assembly technology toachieve the monolayer micro-sphere array with parameter off1ð~r; d; RÞ ¼ ½~r6; 0; 750� ; the other is the use of material depositiontechnology to achieve the SLSs, and the deposition parameters aredepended on the designed parameter f2(n, h, u, k). According tothese two main technologies, the fabrication process is showingin Fig. 2:

Firstly, a monolayer of PSs array with hexagonal symmetry onglass substrate is obtained through the method reported in Ref.[28]. The monolayer orientation is confirmed by the scanning elec-tron microscope (SEM). The orientation along line 1 showed inFig. 2(a) is set as a reference monolayer orientation, i.e. u = 0�,and for all the other orientations, u is measured through the anti-clockwise rotation of the domain with respect to the referenceorientation.

Secondly, the silver deposition on the closely packed PSs arraymonolayer is performed inside a vacuum thermal evaporation sys-tem, which is improved with a collimation component as reportedin Ref. [29]. The base pressure, the deposition rate and temperature

Fig. 2. Schematic illustration of the technological process: (a) self-assembly of PSs monoPSs from glass substrate to PDMS stamping; (e) the wet chemical etching of PSs. The upf1 ¼ ½~R6; 0; 750� and f2 ¼ 30½45�; 0�� þ 30½45�; 180��.

are 4 ⁄ 10�4 pa, 0.1 nm/s and 30 �C, respectively. For a single depo-sition, we just need to control the deposition time (i.e. depositionthickness) and the angle h and u, which can be easily adjustedby tilting and rotating the samples in the vacuum chamber, respec-tively. For stepwise deposition, the operational process is therepeat of the single deposition, as showed in Fig. 2(b).

Thirdly, the silver coated PSs array monolayer is placed on thetop (flat) surface of a polydimethylsiloxane (PDMS) stamp with auniform pressure of about 500g=cm2 (Fig. 2(c)). After lifted up, thisparticle monolayer is transferred to the surface of the PDMS stamp,due to the larger van der Walls force with the PDMS stamp com-pared with the silicon substrate (Fig. 2(d)).

Finally, the PDMS stamp together with the silver coated PSs ar-ray monolayer is immersed in tetrahydrofuran for 3 min. Then thisstamp is dried in fume hood at room temperature. Six hours later, anew nano-particle array is obtained on the PDMS stamp as showedin Fig. 2(e). The upper-right image in Fig. 2 is the obtained newnano-shell array with parameters of f1 ¼ ½~r6; 0; 750� andf2 = 30[45�, 0�] + 30[45�, 180�], which agrees well with thedesigned structures.

Certainly, these arrays also can be fabricated on any other de-sired substrate through an appropriate transferring method asindicated in Ref. [12].

The parameter analysis of SLSs and experiment discussions

In fact, the design process of new SLSs is also the parameter-chosen process. As described above, there two key processes inthe design of new SLSs, i.e. choose basic BBs and design the relatedcombination modes. The morphologies of the basic BBs, i.e. simpleSLSs, are depended on the parameters f1ð~r; d; RÞ and f2ð1; h; u; kÞ,and the combination modes are depended on the parameters off2ðn; h; u; kÞ. In this section, we will achieve the related analysisand discussion of these parameters with the experimental results.

layer on glass substrate; (b) stepwise material deposition; (c) and (d) the transfer ofper-right image is the obtained new nano-shell array with designed parameters as

Fig. 3. (a) Schematic illustration of the hexagonally close-packed sphere monolayer. The line 1, 2, 3 and 4 are the projections of material vapor beam on the substrate, and thearrows on these lines indicate the deposition direction. (b) The simulated basic structures with h = 45� and u = 0�, 15�, 30� and 45�.

Fig. 4. The SEM images of the obtained simple SLSs with parameters of: (a) f2 ¼ 80½30�; 0��; (b) f2 ¼ 80½30�; 30��; (c) f2 ¼ 80½45�; 0��; (d) f2 ¼ 80½45�; 30��; (e) f2 ¼ 80½60�; 0��;(f) f2 ¼ 80½60�; 30��. The insets are the related simulated results.

Y. Su et al. / Microelectronic Engineering 115 (2014) 6–12 9

The influence of shadow effect on the basic BBs

In this paper, the parameter f1 is chosen as f1ð~r; d; RÞ ¼½~r6; 0; 750�, as showed in the experimental section, and k is the

thickness of SLSs, which has little effect on the morphologies ofthe basic BBs. Thus, u and k are the two main parameters, whichneed to be discussed. In fact, the morphologies of the basic BBsare depended on the shadow effect from the neighboring spheres

Fig. 5. The SEM images of the obtained complex SLSs with parameters of: (a) f2 ¼ 80½75� ; 30�� þ 80½75�; 210��; (b) f2 ¼ 80½75� ; 45�� þ 80½75�; 225��; (c) f2 ¼ 80½60�; 0��þ80½60�; 180��; (d) f2 ¼ 80½75� ; 45�� þ 80½75�; 165�� þ 80½75�; 285��; (e) f2 ¼ 80½30�; 30�� þ 80½30�; 210��; (f) f2 ¼ 80½60�; 30�� þ 80½60�; 210��; (g) f2 ¼ 80½75� ; 30��þ80½75� ; 150��; (h) f2 ¼ 80½75� ; 15�� þ 80½75�; 30��; (i) f2 ¼ 80½75�; 15�� þ 80½75�; 90��. The insets are the related simulated results.

Fig. 6. The images of the designed and fabricated SLSs, where the complex SLSs is achieved through the combination of the identical BBs.

10 Y. Su et al. / Microelectronic Engineering 115 (2014) 6–12

in the material deposition process. And this shadow effect dependsmainly on the parameters h and u. So the relationship between themorphologies of the basic BBs and the parameters of u and kshould be discussed with the introduction of shadow effect.

Fig. 3(a) is schematic diagram of PSs array with hexagonal sym-metry, where the line 1, 2, 3 and 4 are the projections of material va-por beam on the substrate, and the arrows on these lines indicatethe deposition direction. When the angle h is fixed, the hexagonalsymmetry will lead to the repeat of the obtained basic structures

with period of Tu = 60� (i.e., the basic structures with u = a is thesame as that with u = a + 60�, and the structures with0� 5 u < 60� are the mirror images of each other around u = 30�(i.e., Fig. 3(b) shows that the structure with parameters of [45�,15�] and [45�, 45�] are the mirror images of each other). Thus, wejust need to discuss the shadow effect with u; of half period, i.e.0� 5 u 5 30�. Here, the shadow effect with u = 0�, 15� and 30� is dis-cussed, which will lead to the formation of basic SLSs with signifi-cant difference with each other. And the detail is showing as below:

Fiw

Y. Su et al. / Microelectronic Engineering 115 (2014) 6–12 11

u

g. 7. The SEMith f2 ¼ 80½1

h

images of the ob5�; 0�� þ 80½15�; 18

The identifier of the particles whichwill create shadow effect on the SLSsobtained on the blue particle

u = 0�

0� < h < 55� 1 and 2 55� < h < 90� 1, 2 and 6

u = 15�

0� < h < 20� 1 and 2 20� 5 h < 68� 1, 2 and 3 68� 5 h < 84� 1, 2, and 6 84� 5 h < 90� 2, 6 and 7

u = 30�

0� < h < 90� 1, 2 and 3 u = 45� 0� < h < 20� 2 and 3

20� 5 h < 68�

1, 2 and 3 68� 5 h < 84� 2, 3 and 4 84� 5 h < 90� 2, 4 and 8

Fig. 3(b) shows some simple shell-like structrues with differentmorphologies, which is simulated with h and u of different values,and indicates the influence of different shadow effects on the mor-phology of simple SLSs. Fig. 4 shows some simple SLSs withu ¼ 0� or 30� and h = 30�, 45� or 60� and the related experimentalstructures. These simulated simple SLSs agree well with theexperimental structures, indicating the validity of our geometricmodel and the related discussions. Obviously, when h ¼ 0�,there is no shadow effect which will affect the nano-shells on theblue particle, and lead to the formation of semi-shells (i.e.nano-cups).

The analysis of combination modes

The combination of the simple SLSs (as showed in Fig. 4) shouldbe properly performed with the consideration of the restrictions infabrication. In fact, this combination design is completely de-pended on the hexagonal repeat symmetry of the PSs array, i.e.the periodicity of the angle u (as showed in Fig. 3(a)). This symme-try limits the number of the identical BBs used in the design of onenew structure (i.e., for the hexagonal repeat symmetry (Tu = 60�),the maximum number of the same BBs used in the design of onenew structure is six (6 = 360�/60�)). However, we also can developa great deal of new structures as showed in Figs. 4 and 5. Particu-larly, through optimizing the related design parameters, manyinteresting SLSs achieved, such as T-shape (showed in Fig. 5(g)),‘maple leaf’-shape (showed in Fig. 4(e)), and boat-shape structures(showed in Fig. 5(a, c and f)).

In order to facilitate the related analysis and discussion, thecombination modes are divided into two categories: one is thecombination of the identical BBs; the other is the combination ofthe different BBs. For the combination modes with the use of theidentical BBs, the combination parameter f2(n, h, u, k) should

tained structures: (a) the SLS as showed in Fig. 5(d) whic0�� and the deposition temperature at 90 �C.

satisfy the following conditions: 1, n = 2, 3, 4, 5 or 6, due to theperiodicity of the angle u (Tu = 60�, as showed in Fig. 3(a)); 2, h1

= h2 = . . .= hn and ui = uj ± 60� ⁄ k, i – j, i = 1, 2, . . . , n, j = 1, 2, . . . , n,k = 1, 2, . . . , n � 1 (these conditions are used to ensure that all ofthese BBs are identical). Fig. 5 shows some designed and fabricatedSLSs with the use of this type combination modes, which showsthat a lot of new SLSs have been achieved. We also make a sum-mary of the combination of some identical simple SLSs withu = 0� or 30� and h = 30�, 45� or 60�, and the obtained SLSs areshowing Fig. 6, where through the combination of two or threeidentical BBs, 16 new SLSs have been designed and fabricated. Cer-tainly, to analysis this combination modes is easy when comparedwith the combination of different BBs, and the number of the ob-tained new complex SLSs is relatively small.

For the combination modes with the use of the different BBs,the combination parameter f2(n, h, u, k) should satisfy the follow-ing conditions: 1, n = 2, 3, . . . ; 2, hi – hj or ui – uj ± 60� ⁄ k, i – j,i = 1, 2, . . . , n, j = 1, 2, . . . , n, k = 1, 2, . . . , n � 1 (these conditions areused to ensure that not all of these BBs are identical). In this case,the value number of n is infinite, and a great number of new SLSscan be achieved. Fig. 5(h) and (i) show two complex SLSs throughthe combination of different BBs. Due the complex of the relatedanalysis, we just give two images of the complex SLSs designedand fabricated through this type combination mode, which areshowing in Fig. 5(h) and (i). These obtained SLSs indicate the fabri-cation power of this method.

The experimental discussions

The SLS fabricated conditions in experiment should be takeninto account in the theoretical discussions. In fact, in order to ob-tain the designed structures, the value of the parameters inHðf1ð~r; d; RÞ; f 2ðn; h; /; kÞÞ should meet some restrictions in exper-iment. For example, the thickness, k, should be larger enough toavoid the structural damage from gravity, interfacial tension, orother forces; the modification of~r and d still remains challenging,although this modification has been achieved through the mechan-ical stretching method reported in Ref. [30]. However, a great num-ber of new structures also have been designed in theory andachieved in experiment, as indicated in the previous discussions.

In experiment, the number of new SLSs can be further enlargedthrough postprocessing the obtained SLSs. For example, if wechange the deposition temperature from 30� to 90 �C, a honeycombstructures can be achieved, as showed in Fig. 7(b); Fig. 7(a) is theSEM image of the nano-shell as showed in Fig. 5(d) which has beenannealed under 300 �C for 30 min. In addition, the further treat-ment of these obtained structures with physical or chemical meth-ods also can modify these obtained structures and enable theformation of new structures [31,32]. From the point to developmore new nano-structures, this postprocessing method is veryeffective.

h has been annealed under 300 �C for 30 min; (b) a honeycomb structures obtained

12 Y. Su et al. / Microelectronic Engineering 115 (2014) 6–12

Conclusion

In summary, an accurate method for the fabrication of diverseSLSs has been proposed, which is the use of simple nano-structuresas BBs to construct new structures, and a great number of newshell-like structures have been designed and fabricated throughthis method. In theory, we establish a 3D geometric model, analysethe shadow effect from the neighbouring spheres and the combi-nation modes, and achieve the design of a great number of newSLSs, include some interesting structures, such as T-shape, ‘mapleleaf’-shape, and boat-shape structures. In experiment, the micro-sphere assembly technology and the material deposition technologyare employed to achieve the fabrication of the monolayer micro-sphere array and the SLSs. The experimental results agree wellwith the designed results, indicating the validity of the establishedgeometric model. In addition, more new SLSs have been achievedthrough the related postprocessing method. These developednew SLSs will provide more options for the relevant applications,especially for the development of 3D chiral metamaterials.

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