Self-Assembled Three-Dimensional Graphene

Subscriber access provided by UB + Fachbibliothek Chemie | (FU-Bibliothekssystem)

Communication

Self-Assembled Three-Dimensional Graphene-Based Polyhedrons Inducing Volumetric Light Confinement Daeha Joung, Andrei Nemilentsau, Kriti Agarwal, Chunhui Dai, Chao Liu, Qun Su, Jing Li, Tony Low, Steven J. Koester, and Jeong-Hyun Cho Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b05412 • Publication Date (Web): 01 Feb 2017 Downloaded from http://pubs.acs.org on February 3, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Nano Letters is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Figure 1. Schematic illustration of converting (a) 2D graphene sheets and (b) patterned graphene sheets into (c) 3D structures and (d) further modifying (functionalizing) the 3D graphene-based structures by surface patterning and encapsulating chemicals and biomaterials. Figure 1 181x51mm (283 x 283 DPI)

ACS Paragon Plus Environment

Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2. Schematic illustration and their corresponding optical images of the self-folding fabrication process of 3D integrated graphene. (a) Patterning 2D net protection layer, (b) transferring graphene onto the substrate, (c) surface patterning on graphene, (d) patterning an SU-8 frame and an SPR 220 polymer hinge, (e) releasing structure from the substrate and self-assembly driven by the reflow of the polymer hinge in heated water, (f) 3D graphene-based cube after self-assembly and removal of the protection layer, and (g) 3D graphene with surface patterning. Optical images of (f) and (g) are shown in Figure 3.The scale bars are 200 µm. Figure 2 89x233mm (283 x 283 DPI)


Page 2 of 29

Page 3 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Figure 3. Optical images of the 2D nets and 3D assembled graphene and graphene oxide (GO)-based cubes with and without surface patterns. (a-c) 2D and 3D structure with three layers of graphene membranes: (a) lithographically patterned 2D graphene membranes, (b) self-folded 3D graphene-based structure, and (c) zoomed-in image of top surface of the 3D graphene-based cube; (d-f) 2D and 3D structure with metal patterns on the graphene membranes: (d) 20 nm thick Ti patterns on patterned 2D graphene membranes, (e) self-folded 3D graphene-based structure with the Ti patterns, and (f) a zoomed-in image of top surface of the 3D graphene-based cube with the Ti patterns; (g-i) 2D and 3D structure with ten layers of GO membranes: (g) patterned 2D GO membranes after a lift-off process, (h) self-folded 3D GO-based structure, and (i) a zoomed-in image of top surface of the 3D GO-based cube; (j-l) 2D and 3D structure with metal patterns on the GO membranes, (j) 20 nm thick Ti patterns on patterned 2D GO membranes, (k) self-folded 3D GO-based cube, and (l) zoomed-in image of top surface of the 3D GO-based cube with the Ti patterns. Scale bars are 100 µm. Figure 3 174x164mm (283 x 283 DPI)


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 4. Schematic illustrations and their corresponding scanning electron microscope (SEM) images and Raman mapping of a 500 nm scale 3D single layer graphene-based structure (a-c) before and (d-f) after self-assembly. (a) 2D nets contain 5 nm thick Al2O3/ monolayer graphene/ 20 nm thick Al2O3 sandwich panels and PMMA hinges. (b) SEM image of 2D nets before self-assembly. The size of the panels was defined as 500 (width) × 500 nm (length) and the gaps between the panels were 50 nm. (c) A result of Raman mapping of the 2D nets based on the intensity of graphene G band (~ 1580 cm-1), which shows the clear shape of the 2D nets. (d) A schematic of the in-situ monitored self-assembled 3D nanoscale graphene-based structure. (e) A SEM image of the self-assembled 3D graphene-based structure. (f) A Raman mapping of the graphene G band of the self-assembled 3D structure, which demonstrates that the properties of graphene are conserved during the self-assembly process due to the sandwiched structures. Figure 4 136x84mm (283 x 283 DPI)


Page 4 of 29

Page 5 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Figure 5. Transmission spectra for 2D and 3D graphene-based structures as a function of wavenumber (in cm-1). In all simulations, the dimension, L, is 500 nm. The polarization of the electric field is indicated by red arrows. Transmission spectra for (a) a graphene square, an infinite graphene ribbon, and a 2D, infinite, four-faced graphene hollow box without graphene on two faces (electric field distribution is shown in Supporting Information Figures S10); (b) a finite ribbon of length 4L and a 3D open box formed by folding up the 4L ribbon; (c) a six-faced continuous graphene before and after assembly; (d) a six-faced discontinuous graphene (gaps between graphene patterns are 0.1L) before and after assembly. The 3D graphene-based box shows the superior single resonance peak overcoming the multiple non-uniform coupled modes of a 2D structure. Figure 5 181x134mm (283 x 283 DPI)


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 6. Simulated electric field distribution for various graphene resonators (considered in Figure 5) at the frequency of a fundamental resonance. (a)-(b) Electric field distribution for a ribbon of length 4L. (c)-(f) Transformation of the distribution from the uniform 2D ribbons when patterned into non-symmetric structures and the non-uniform electric field distribution in the case of discontinuous closely spaced patterned arrays. (g)-(j) Variation in the plasmon coupling and electric field in the case of (g) 3D four-faced open box under TM mode, (h) 3D six-faced closed box of continuous graphene, and 3D closed boxes of discontinuous graphene separated by a gap of (h) 0 nm, (i) 50 nm, and (j) 150 nm. Figure 6 156x88mm (283 x 283 DPI)


Page 6 of 29

Page 7 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Figure 7. Variation in the simulated electric field enhancement (where, Eg is the electric field in the presence of graphene and E0 is the incident electric field) as a function of distance (d) along with an imaginary line drawn perpendicular to the graphene surface. In 2D ribbon case (black curve), the line passes through the geometrical center of graphene ribbon. The distances are measured below (- 250 nm) and above (+ 250 nm) the graphene surface. For the 3D open (blue curve) box, the line is perpendicular to the direction of polarization of incident electric field and passes through the center of the graphene faces on the bottom of the box. For the 3D closed (red curve) box, the line is parallel to the direction of polarization of incident electric field and passes through the hotspot created by the plasmon hybridization. The distances are measured from the outside the box (- 250 nm) to the center of the box cavity (+ 250 nm). The illustrations depict the orientation of the line with respect to the field. The uniformly coupled plasmons in the 3D structure reduce the electric field decay with distance as compared to 2D graphene (ribbon), leading to the strong electric field that exists inside the 3D open box (from 0 to + 250 nm) and inside/outside of the 3D closed box (from 0 to +/- 250 nm). Figure 7 136x88mm (283 x 283 DPI)


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ToC figure ToC figure 87x39mm (283 x 283 DPI)


Page 8 of 29

Page 9 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Self-Assembled Three-Dimensional Graphene-Based Polyhedrons Inducing Volumetric Light Confinement Daeha Joung, Andrei Nemilentsau, Kriti Agarwal, Chunhui Dai, Chao Liu, Qun Su, Jing Li, Tony Low, Steven J. Koester, Jeong-Hyun Cho* Department of Electrical and Computer Engineering, University of Minnesota Minneapolis, MN 55455, USA E-mail: [email protected]

Abstract The ability to transform two-dimensional (2D) materials into a three-dimensional (3D) structure while preserving their unique inherent properties might offer great enticing opportunities in the development of diverse applications for next generation micro/nano devices. Here a self-assembly process is introduced for building free-standing 3D, micro/nanoscale, hollow, polyhedral structures configured with a few layers of graphenebased materials: graphene and graphene oxide. The 3D structures have been further modified with surface patternings; realized through the inclusion of metal patterns on their 3D surfaces. The 3D geometry leads to a non-trivial spatial distribution of strong electric fields (volumetric light confinement) induced by 3D plasmon hybridization on the surface of the graphene forming the 3D structures. Due to coupling in all directions, resulting in 3D plasmon hybridization, the 3D closed box graphene generates a highly confined electric field within as well as outside of the cubes. Moreover, since the uniform coupling reduces the decay of the field enhancement away from the surface, the confined electric field inside of the 3D structure shows two orders of magnitude higher than that of 2D graphene before transformation into the 3D structure. Therefore, these structures might be used for detection of target substances (not limited to only the graphene surfaces, but using the entire volume formed by the 3D graphene-based structure) in sensor applications.

Keywords: 3D structures, 2D materials, graphene, self-assembly, plasmon 1 ACS Paragon Plus Environment

Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Owing to its extraordinary optical, electronic, and mechanical properties, twodimensional (2D) graphene sheets (Figure 1a) have the potential to be an ideal platform for the observation of novel quantum phenomena and to serve as building blocks for future electronics, optoelectronics, and plasmonics.1-8 In particular, tailoring the shapes and architectures of graphene have been studied to obtain new physical phenomena. For example, lithographically-patterned graphene (Figure 1b) into quantum dots and nanoribbons produce a finite energy gap, leading to new properties such as quantum confinement effects, magnetism, spin-polarized edge states, and localized electron distributions.9-11 Beyond changing the surface morphology and layered structure of 2D graphene, additional dimensionality, such as the three-dimensional (3D) graphene-based structure illustrated in Figure 1c, can also impart new physical effects. Such 3D graphene-based structure might offer an excellent platform for investigating or tailoring exotic phenomena and for exploiting the unusual behavior (or distinct properties) in devices.

Figure 1. Schematic illustration of converting (a) 2D graphene sheets and (b) patterned graphene sheets into (c) 3D structures and (d) further modifying (functionalizing) the 3D graphene-based structures by surface patterning and encapsulating chemicals and biomaterials. An area of particular interest pertains to graphene’s interactions with light when it is configured in a 3D geometry. Taking a cue from nature, forming multiple facets with a crystal substantially can affect its brilliance, which leads to various spectral colors and luminousness like cut diamonds used for cosmetic jewelry.12 Transforming 2D graphene into a 3D 2 ACS Paragon Plus Environment

Page 10 of 29

Page 11 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

polyhedral structure would similarly introduce light scattering, multiple optical paths, and interferences. In addition, it is well known that plasmonic effects in graphene can arise when it is patterned into micro and nanostructures.5 Hence, new electrical and optical phenomena which could not be observed in 2D materials defined on planar substrates can be expected in 3D architectures configured with 2D materials. In addition to new electrical and optical properties, 3D graphene might be beneficial for a variety of other applications (Figure 1d). Examples include: (i) gas/water protected containers or barriers for environmentally sensitive chemical/biological materials, and molecular storage (due to graphene’s gas-impermeability),13-16 (ii) targeted delivery systems,17 (iii) enclosed environmental cells for application in transmission electron microscopy (TEM) providing in-situ observation of fuel cells, batteries, catalysts, and biomedical materials,15,18 (iv) functional (or tunable) devices achieved through surface modification and encapsulation of nanomaterials such as colloidal nanoparticles, quantum dots, DNA, or segments,6,15,19 and (v) electronic devices and metamaterials (i.e. hollow microcube resonators)20,21 created by metal patterning on graphene surfaces. Hence, 3D graphene can potentially lead to a new generation of 3D micro/nano devices. Although there are a few known methods in constructing a 3D structure, there is a technical limitation of producing a 3D, hollow, enclosed structure without losing the intrinsic properties of the graphene sheets. Conventional 3D fabrications have typically involved layerby-layer (LBL) assembly and self-aligned membrane projection lithography.22-25 However, since these traditional methods are top-down strategies, integration of 3D structures with 2D layered materials has been a challenge. To overcome this limitation, a 3D printing method has been studied.26,27 However, the technique fails to preserve the graphene’s unique inherent properties. Moreover, the technique suffers from a low throughput. In addition, a number of chemical routes for the production of 3D graphene-based structure have been introduced via template-assisted assembly, flow-directed assembly, leavening assembly, and conformal 3 ACS Paragon Plus Environment

Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

growth methods (or yolk-shell nanoarchitecture).28-31 Nevertheless, these methods require strong chemical reactions with surfactants or nanoparticles which affect the intrinsic properties of graphene or form amorphous carbon. Recently, in order to realize a 3D graphene-based structure without sacrificing intrinsic graphene properties, origami-inspired folding graphene cages have been suggested;32 yet the transformation of 2D graphene-based structures without a chemical functionalization into highly pure and well-defined shapes such as a polyhedral structure has not been realized because of technical challenges. Here, we report a methodology for realizing multi-faced 3D micro- and nanocubes with 2D materials, graphene and graphene oxide (GO), by using self-folding, which overcomes the foremost challenges in the construction of multi-faced, free-standing, hollow, enclosed, 3D, polyhedral, graphene-based materials, with the use of polymer SU-8 or aluminum oxide (Al2O3) frames. The self-folding approach allows heterogeneous integrations with control of size and shape, and various materials which can produce free-standing, 3D, multifunctional devices as illustrated in Figure 1b-c.20,33 Also, the approach described below allows us to create surface modifications by realizing metal patterning on their 3D faces. In addition, we elucidate through modeling its novel optical properties in 3D graphene-based materials. In particular, we found a high degree of volumetric light confinement induced by rich 3D plasmonic hybridization behavior arising from the coupling between interfacial plasmonic modes in graphene. Hence, this dimensional extension from 2D to 3D offers a new way to sculpt light-on-demand. The overall fabrication process of the 3D graphene-based structures and their corresponding optical images are shown in Figure 2 (the detailed process is included in the Supporting Information Figures S1-S7). First, 2D patterned (or nets of) graphene is defined on a copper (Cu) sacrificial layer on a silicon (Si) substrate with six square Al2O3/chromium (Cr) protection layers (the dimension of each square is 150 µm × 150 µm) (Figure 2a). The protection layers allow retention of physical and chemical properties of graphene that could 4 ACS Paragon Plus Environment

Page 12 of 29

Page 13 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Figure 2. Schematic illustration and their corresponding optical images of the self-folding fabrication process of 3D integrated graphene. (a) Patterning 2D net protection layer, (b) transferring graphene onto the substrate, (c) surface patterning on graphene, (d) patterning an SU-8 frame and an SPR 220 polymer hinge, (e) releasing structure from the substrate and self-assembly driven by the reflow of the polymer hinge in heated water, (f) 3D graphene-based cube after self-assembly and removal of the protection layer, and (g) 3D graphene with surface patterning. Optical images of (f) and (g) are shown in Figure 3.The scale bars are 200 µm. be modified during the fabrication process, including self-folding. After self-folding, the protection layers are selectively removed by wet chemical etching. Next, three separate membranes of single-layer graphene are transferred on top of the patterned Al2O3/Cr 5 ACS Paragon Plus Environment

Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

protection layer (Figure 2b). Then, the unwanted graphene is removed by a combination of photolithography and oxygen plasma treatment. Since the designed structure for the 3D graphene has a width of ~ 150 µm, one sheet of single-layer of graphene is not strong enough to obtain parallel production of few-hundred-micrometer-sized 3D graphene. Thus, in this study, single-layer graphene is transferred three separate times using multiple polymethyl methacrylate (PMMA) coating/removal steps. For the structures made with GO instead of graphene, the 2D nets are defined by spinning 10 nm thick GO on photoresist patterned on the prefabricated Al2O3/Cr protection layer. Then the unwanted portions of the GO layer are removed by a lift-off process with flood exposure.34 Since these approaches are compatible with a conventional photolithography process, surface modification of the 3D graphene or 3D GO can be realized by applying metal patterning on the graphene or GO surface with any desired patterns. On top of the graphene or GO membranes, 20 nm thick titanium (Ti) metal patterns are deposited by a lithography and lift-off process (Figure 2c). Then, a photodefinable epoxy (SU-8, 5 µm thick) is used as a frame to support the graphene membranes. The advantages of using frames are that they allow: (i) realization of the 3D structure with pristine CVD graphene or GO without a chemical reaction;34 (ii) realization of surface patterning with metals or semiconductors to induce a new physical property;20 (iii) easy control of size (nm to mm) and shapes (any polyhedron) of the 3D structures, and easy control of folding angle of 3D structure for the realization of diverse 3D structures including semi-3D structures;33,34 (iv) realization of multi-faced polyhedral structures for multiple optical reflections, resulting in an optical switching behavior;34 (v) control of the precise position of the structures (or devices), resulting in a well-aligned array for optical characterization;35 and (vi) realization of well-defined four- or five-faced open cubic (or void) structures which can be used for fluidic sensors as described in later section of this Letter. Each SU-8 frame is connected by a polymer hinge (SPR 220) at the intersections (Figure 2d). The 2D nets are placed in water and heated up to the melting point of the polymer hinge which is about 100 oC 6 ACS Paragon Plus Environment

Page 14 of 29

Page 15 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

(Figure 2e).36 As a result of the polymer melting (or reflow), a surface tension force is generated, inducing a self-assembly process transforming the 2D nets into 3D structures (a video capture sequence is shown in Figure S6 in Supporting Information). Finally, the Al2O3/Cr protection layers are removed by Cr etchant after self-folding (Figure 2f, g). The completed overall sizes of the self-assembled cubes are 200 µm, and the dimensions of the free-standing graphene or GO membrane on each face is 150 µm × 150 µm (Figure 3). Figure 3a displays a representative 2D net of graphene after patterning by an oxygen plasma treatment and before the self-assembly process. The 3D graphene-based cube image presents a highly transparent, free-standing, enclosed architecture (Figure 3b). From the zoomed-in images (Figure 3c), a buckled graphene membrane is observed that might be caused by ripples from during the transfer of graphene onto the substrate and/or by the freestanding nature of the graphene after self-assembly (crumpling is an inherent property of freestanding graphene because of static wrinkles).37,38 However, no noticeable cracks, holes, or other damaged areas are observed on the 3D hollow graphene-based structure. The same overall approach has also been applied to the 3D GO-based structures, allowing for a successful demonstration with membranes comprised of ~ 10 layers (~ 10 nm thick) of GO sheets (Figure 3g-i). An important advantage of the described approaches is the 3D modification of graphene and GO membranes by 3D patterning. To demonstrate the patterning versatility on the surface of the 3D graphene/GO-based cubes, different features and “UMN” lettering as metal patterns with 20 nm thick Ti are defined on each face of the membranes (Figure 3d-f for graphene and Figure 3j-l for GO). As a result, the self-assembly process shown here not only offers control of size and shape, allowing for fabrication of free-standing, hollow, polyhedral structures, but also allows for patterning with a different combination of metal, semiconductor, and insulator materials on the 2D materials. Thus, a complex optical and electronic integration in a 3D architecture can be realized for applications in next generation optoelectronic devices. 7 ACS Paragon Plus Environment

Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3. Optical images of the 2D nets and 3D assembled graphene and graphene oxide (GO)-based cubes with and without surface patterns. (a-c) 2D and 3D structure with three layers of graphene membranes: (a) lithographically patterned 2D graphene membranes, (b) self-folded 3D graphene-based structure, and (c) zoomed-in image of top surface of the 3D graphene-based cube; (d-f) 2D and 3D structure with metal patterns on the graphene membranes: (d) 20 nm thick Ti patterns on patterned 2D graphene membranes, (e) selffolded 3D graphene-based structure with the Ti patterns, and (f) a zoomed-in image of top surface of the 3D graphene-based cube with the Ti patterns; (g-i) 2D and 3D structure with ten layers of GO membranes: (g) patterned 2D GO membranes after a lift-off process, (h) self-folded 3D GO-based structure, and (i) a zoomed-in image of top surface of the 3D GObased cube; (j-l) 2D and 3D structure with metal patterns on the GO membranes, (j) 20 nm thick Ti patterns on patterned 2D GO membranes, (k) self-folded 3D GO-based cube, and (l) zoomed-in image of top surface of the 3D GO-based cube with the Ti patterns. Scale bars are 100 µm. Before and after self-assembly, material properties of the graphene-based structure are characterized using a Raman spectra to evaluate changes in the graphene properties39,40 that occur as a result of the self-assembly. The result of Raman spectroscopy of 2D and 3D graphene and GO structures indicates the 3D graphene and GO structures retain their intrinsic properties, demonstrating the robustness of the proposed method (detail analysis of the Raman spectra is included in the Supporting Information). 8 ACS Paragon Plus Environment

Page 16 of 29

Page 17 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

The size of the fabricated 3D polyhedron structures is 100’s of µm. However, it is possible to reduce the size down to 500 nm by combining the fabrication process shown above with our previous reported an in-situ monitored self-assembly process using a focused ion-beam microscopy.35 Unlike the use of the SU-8 frames and SPR 220 hinges for microscale fabrication, Al2O3/graphene/Al2O3 panels and PMMA hinges were used for building nanoscale 3D structures (Figure 4).

Figure 4. Schematic illustrations and their corresponding scanning electron microscope (SEM) images and Raman mapping of a 500 nm scale 3D single layer graphene-based structure (a-c) before and (d-f) after self-assembly. (a) 2D nets contain 5 nm thick Al2O3/ monolayer graphene/ 20 nm thick Al2O3 sandwich panels and PMMA hinges. (b) SEM image of 2D nets before self-assembly. The size of the panels was defined as 500 (width) × 500 nm (length) and the gaps between the panels were 50 nm. (c) A result of Raman mapping of the 2D nets based on the intensity of graphene G band (~ 1580 cm-1), which shows the clear shape of the 2D nets. (d) A schematic of the in-situ monitored selfassembled 3D nanoscale graphene-based structure. (e) A SEM image of the self-assembled 3D graphene-based structure. (f) A Raman mapping of the graphene G band of the selfassembled 3D structure, which demonstrates that the properties of graphene are conserved during the self-assembly process due to the sandwiched structures. It is expected that 3D graphene-based structures possess unique physical properties owing to the geometric effect. To study the influence of the geometric shape (i.e., 2D and 3D) on graphene’s optical properties, the transmission spectra of the 2D and 3D graphene-based structures are simulated using COMSOL (RF module, ver 5.2). In all simulations, the 9 ACS Paragon Plus Environment

Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

dimension, L, of 500 nm of 3D graphene was used. For modeling analysis, the graphene conductivity was calculated using the Kubo formula41,42 assuming the graphene relaxation time is 0.35 ps and the graphene doping is 0.4 eV. Figure 5a contains transmission spectra for the graphene ribbons and 2D graphene boxes, assuming that boxes are open (i.e., box lacks graphene sheets at two opposite faces, forming a long square tube), the length of the ribbons and the boxes is infinite. As shown in Figure 5a, pronounced dips in the transmission spectra

Figure 5. Transmission spectra for 2D and 3D graphene-based structures as a function of wavenumber (in cm-1). In all simulations, the dimension, L, is 500 nm. The polarization of the electric field is indicated by red arrows. Transmission spectra for (a) a graphene square, an infinite graphene ribbon, and a 2D, infinite, four-faced graphene hollow box without graphene on two faces (electric field distribution is shown in Supporting Information Figures S10); (b) a finite ribbon of length 4L and a 3D open box formed by folding up the 4L ribbon; (c) a six-faced continuous graphene before and after assembly; (d) a six-faced discontinuous graphene (gaps between graphene patterns are 0.1L) before and after assembly. The 3D graphene-based box shows the superior single resonance peak overcoming the multiple non-uniform coupled modes of a 2D structure.

10 ACS Paragon Plus Environment

Page 18 of 29

Page 19 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

are clearly seen which correspond to the geometrical resonances of the surface plasmons in the graphene. It is well known that the frequency of the geometrical resonances in graphene ribbons scales as 1/ L .43 This explains the blue shift of the plasmon resonance in the case of transverse electric (TE) excitation of the ribbon (Figure 5b) compared to the transverse magnetic (TM) case (Figure 5a,b for ribbon cases). By folding a graphene ribbon into a 3D open box (Figure 5b), plasmons are effectively limited to the faces of the box. The geometric plasmon resonances at each of the faces in 3D are similar to that of the 2D graphene square (Figure 5a). However, the coupling between plasmons (plasmon hybridization) at each of the faces in the 3D open box leads to the splitting of the geometric resonances, which reveals itself as a blue- or red-shift of the resonances in the transmission spectra (Figure 5b). The transmission spectra for six-faced continuous and discontinuous graphene ribbons/ 3D boxes are presented in Figure 5c and 5d, respectively. In the continuous ribbon case before self-assembly (Figure 5c), plasmon resonances are on the whole length of the ribbon (i.e., 3L for horizontal or 4L for vertical), depending on the wave polarization. The response of the continuous closed 3D box (six faces, Figure 5c) is similar to that of the 3D open box (four faces, Figure 5b), except for the plasmon in all six faces coupled to etch other (for open box, only 4 faces are coupled). When the discontinuous ribbon is formed with graphene squares separated by 50 nm (0.1L) gaps (Figure 5d), the plasmon resonance modes change due to the non-uniform hybridization that exists between the individual squares along the 3L and 4L directions. The non-uniform hybridization leads to multiple closely spaced resonances with small dips in both TE and TM excitations (Figure 5d). It should also be noted that the assembled graphene cube generates only a single resonance in transmission spectra since the geometrical symmetry of the cube causes the uniform plasmon hybridization at each face.


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The electric field distributions in these 2D and 3D structures are of particular interest and provide further insight into plasmon hybridizations (Figure 6). For a 2D graphene ribbon (Figure 6a,b), the distribution is that of conventional plasmon dipolar resonances with fields concentrated at the edges of the ribbon. The continuous graphene cross ribbon pattern (Figure 6c-f) continues to demonstrate an electric field similar to that of the 2D graphene ribbon (Figure 6a,b). However, under TE excitation (Figure 6c), the fundamental resonance frequency and the field at resonance correspond to the resonance of the ribbon of length 4L (north and south edges), while the two squares in the 3L direction (east and west edges) do not demonstrate a high field due to their resonance frequency being higher; the reverse phenomenon takes place for the TM excitation (Figure 6d). For the discontinuous 2D

Figure 6. Simulated electric field distribution for various graphene resonators (considered in Figure 5) at the frequency of a fundamental resonance. (a)-(b) Electric field distribution for a ribbon of length 4L. (c)-(f) Transformation of the distribution from the uniform 2D ribbons when patterned into non-symmetric structures and the non-uniform electric field distribution in the case of discontinuous closely spaced patterned arrays. (g)-(j) Variation in the plasmon coupling and electric field in the case of (g) 3D four-faced open box under TM mode, (h) 3D six-faced closed box of continuous graphene, and 3D closed boxes of discontinuous graphene separated by a gap of (h) 0 nm, (i) 50 nm, and (j) 150 nm.


Page 20 of 29

Page 21 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

graphene patterns (Figure 6e,f), the electric field distribution further supports the theory of multiple resonances in the transmission spectrum. The non-uniform coupling within adjoining graphene strips enhances the electric field within the gaps of the graphene. This enhancement of the field within a given gap is highly dependent on the number of graphene surfaces surrounding it and the polarization direction of the electric field. For the 3D open box with 4 faces, the electric field pattern under TM excitation (Figure 6g, for TE excitation, see Figure S11 in Supporting Information) differs from that of the 2D graphene ribbons (Figure 6b) to a great extent. Instead of the electric field rapidly decaying away from the 2D graphene surface (Figure 6b), the electric field in the 3D open box extends into the void between the graphene faces (Figure 6g). The 3D distribution shows the uniform reduction in the field as it moves from the edges to the center of a face without any graphene, creating circular electric field spots due to the symmetry of the structure. The distribution of the plasmon electric field in the case of 3D six-faced cube (3D closed box) provides further insight in the hybridization of the plasmons in 3D structures. To explore the effect of the packing density, the 3D closed box is studied with three different gap sizes between the faces: no gap (Figure 6h), 50 nm gap (Figure 6i), and 150 nm gap (Figure 6j). One can clearly see circular interference patterns on the cube faces which are orthogonal to the polarization of the electric field (Figure 6h-j). As the faces of 3D cubes are orthogonal to the electric field polarization, the surface plasmons cannot be excited directly by the incident wave. Rather, they are generated by the fields of the surface plasmons excited on the neighboring faces. The uniform constructive and destructive interferences of the plasmon produce the above-mentioned circular interference patterns (Figure 6h-j). The electric field intensity and the diameter of the maxima at the center of the face increase as the gap decreases due to increased interaction between plasmons generated at the neighboring faces. For a continuous graphene cube in Figure 6h, the diameter is slightly bigger than the size of the face of the cube which leads to the interference maxima occupying nearly the entire face. 13 ACS Paragon Plus Environment

Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The unique optical properties of 3D closed and open box structures indicate the potential of these devices for the fabrication of ultra-sensitive, compact molecular sensors. Exposing a graphene-based sensor to a foreign substance(s) (e.g., particles and molecules) changes the optical response (resonance frequency in the transmission spectrum) of the sensor depending upon the characteristics of the target substances.44,45 A problem is the multiple closely spaced resonances seen in the transmission spectra of the 2D graphene arrays (Figure 5d) could mask the frequency shift induced by the target substances. However, using the 3D structure (with a single resonance in transmission spectrum) as a sensor allows the detection of the foreign substances without complications arising from closely spaced resonances. The detection of a target by an optical sensor is to a large degree determined by its spatial overlap with the excitation optical fields. Particularly, for high sensitivity, it is necessary to increase the sensing area to the entire volume of the targeted substances, in order to detect very minute concentrations of the target that may be far away from the sensor surface. Thus, a very strong electric field extending into the bulk of the substances containing the target is required for ultra-sensitive detection. This concept can be realized by transforming 2D graphene into 3D graphene-based structures, which generates volumetric light confinement induced by 3D plasmon hybridization in 3D graphene-based structure (Figure 7). The sensing area of 2D planar graphene is limited to the region close to the graphene face because the electric field is localized only on the surface and rapidly decays as one moves away from the 2D graphene face (Figure 7, black curve). However, the 3D open box structure induces a non-trivial spatial distribution of strong electric fields, resulting from the 3D plasmon hybridization in graphene (Figure 7, blue curve). The electric field enhancement at the surface of 3D graphene is ~ 4 times higher than that of the 2D ribbon as well as the minimum field enhancement at d = + 250 nm in the middle of inside 3D open box is close to the maximum enhancement obtained from a 2D ribbon surface (d = 0 nm). This result shows the sensing area can be further extended into the void within the open box, 14 ACS Paragon Plus Environment

Page 22 of 29

Page 23 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Figure 7. Variation in the simulated electric field enhancement (where, Eg is the electric field in the presence of graphene and E0 is the incident electric field) as a function of distance (d) along with an imaginary line drawn perpendicular to the graphene surface. In 2D ribbon case (black curve), the line passes through the geometrical center of graphene ribbon. The distances are measured below (- 250 nm) and above (+ 250 nm) the graphene surface. For the 3D open (blue curve) box, the line is perpendicular to the direction of polarization of incident electric field and passes through the center of the graphene faces on the bottom of the box. For the 3D closed (red curve) box, the line is parallel to the direction of polarization of incident electric field and passes through the hotspot created by the plasmon hybridization. The distances are measured from the outside the box (- 250 nm) to the center of the box cavity (+ 250 nm). The illustrations depict the orientation of the line with respect to the field. The uniformly coupled plasmons in the 3D structure reduce the electric field decay with distance as compared to 2D graphene (ribbon), leading to the strong electric field that exists inside the 3D open box (from 0 to + 250 nm) and inside/outside of the 3D closed box (from 0 to +/- 250 nm). creating a high sensitivity optical scanner. For instance, a graphene-based, 3D, (blood-) vessel-like tube with a strong electric field at the inner/outer surfaces and within the void (or cavity) can be used to sense, with sensitivity higher than that of 2D planar sensors, a substance in the fluid as it flows through the inside of the 3D open box by monitoring for any change in this field. The circular interference patterns at the faces of 3D closed box graphene cubes demonstrate the importance of the cubic structures for the development of plasmonic devices that allows for efficient manipulation of the electric field and for the creation of focused hotspots, which may also lead to high sensitivity. Unlike the 3D open box, the 3D closed box graphene generates a highly confined electric field within as well as outside of the 15 ACS Paragon Plus Environment

Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

cubes due to coupling in all directions (Figure 7, red curve). When looking at the electric field enhancement inside and outside the cube, the uniform plasmon coupling from all directions creates strong electric field enhancement (~ 230) at the surface of the graphene (Figure 7a, red curve at d = 0). The uniform coupling reduces the decay of the field enhancement with a minimum value of ~100 (inside cube at d = +250 nm) and ~80 (outside cube at d = -250 nm) at a distance of 250 nm from the surface; which is more than two orders of magnitude higher than that of the 2D ribbon (Figure 7a black curve, ~1 at d = ±250 nm). This result implies that by utilizing the highly confined electric field, the 3D closed box graphene can be used as a sensor with high sensitivity to detect and/or secure the targeted substances while maintaining their integrity due to the impermeability of the graphene membranes. In conclusion, a strategy has been developed for building free-standing, hollow, 3D, polyhedral graphene and GO micro/nanostructures with 3D surface patterning that could functionalize the 3D structures without sacrificing the intrinsic properties of the 2D materials. The 3D graphene-based structure induces uniform plasmon-plasmon couplings at each of the faces in the 3D that features spectrally isolated resonance modes and non-trivial spatial distribution of the electric field of this mode. The ability to sculpt the interaction of graphene with light through 3D structural engineering reveals the new degree of freedom to design desirable optical properties of materials for physics, chemistry, and devices as illustrated in Figure 1. Furthermore, 3D cubes can be constructed with the use of other 2D materials such as transition metal dichalcogenides and black phosphorus with various surface patterning with metal, semiconductor, and insulator patterns. Thus, the knowledge put forth in this paper lays the foundation for the development of next-generation 3D reincarnations of 2D materials.


Page 24 of 29

Page 25 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxxxxx. The fabrication process, Raman measurement & analysis, transmission spectra simulation process of the 2D and 3D graphene-based structures, and field distributions for 2D and 3D graphene-based structures are shown. (PDF)

ACKNOWLEDGMENT This material is based upon work supported by a start-up fund at the University of Minnesota, Twin Cities, and an NSF CAREER Award (CMMI-1454293). A.N. and T.L. acknowledge financial support by DARPA grant award FA8650-16-2-7640. J.L., Q.S. and S.J.K. were supported in part by the National Science Foundation (NSF) under award ECCS-1124831. C.D. acknowledges support by the 3M Science and Technology Fellowship. Parts of this work were carried out in the Characterization Facility, University of Minnesota, a member of the NSF-funded Materials Research Facilities Network (www.mrfn.org) via the MRSEC program under award DMR-1420013. A portion of this work was also carried out in the Minnesota Nano Center which receives partial support from the NSF through the NNCI program.

Abbreviations TEM, transmission electron microscopy; DNA, deoxyribonucleic aci; LBL, layer-by-layer; GO, graphene oxide; Cu, copper; Si, silicon; Al2O3, aluminum oxide; Cr, chromium; PMMA, polymethyl methacrylate; Ti, titanium; TE, transverse electric; TM, transverse magnetic; SEM, scanning electron microscope

References (1)

Geim, A. K.; Novoselov, K. S. Nature Mater. 2007, 6, 183. 17 ACS Paragon Plus Environment

Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(2)

Bonaccorso, F.; Sun, Z.; Hasan, T.; Ferrari, A. Nature Photon. 2010, 4, 611.

(3)

Novoselov, K. S.; Fal, V.; Colombo, L.; Gellert, P.; Schwab, M.; Kim, K. Nature 2012, 490, 192.

(4)

Avouris, P. Nano Lett. 2010, 10, 4285.

(5)

Low, T.; Avouris, P. ACS Nano 2014, 8, 1086.

(6)

Singh, V.; Joung, D.; Zhai, L.; Das, S.; Khondaker, S. I.; Seal, S. Prog. Mater, Sci. 2011, 56, 1178.

(7)

Areshkin, D. A.; Gunlycke, D.; White, C. T. Nano Lett. 2007, 7, 204.

(8)

Chen, J.; Badioli, M.; Alonso-González, P.; Thongrattanasiri, S.; Huth, F.; Osmond, J.; Spasenović, M.; Centeno, A.; Pesquera, A.; Godignon, P. Nature 2012, 487, 77.

(9)

Fei, Z.; Rodin, A.; Andreev, G.; Bao, W.; McLeod, A.; Wagner, M.; Zhang, L.; Zhao, Z.; Thiemens, M.; Dominguez, G. Nature 2012, 487, 82.

(10)

Fernández-Rossier, J.; Palacios, J. J. Phys. Rev. Lett. 2007, 99, 177204.

(11)

Son, Y.-W.; Cohen, M. L.; Louie, S. G. Nature 2006, 444, 347.

(12)

Hemphill, T. S.; Reinitz, I. M.; Johnson, M. L.; Shigley, J. E. Gems & Gemology 1998, 34, 158.

(13)

Choi, K.; Nam, S.; Lee, Y.; Lee, M.; Jang, J.; Kim, S. J.; Jeong, Y. J.; Kim, H.; Bae, S.; Yoo, J.-B. ACS Nano 2015, 9, 5818.

(14)

Bunch, J. S.; Verbridge, S. S.; Alden, J. S.; Van Der Zande, A. M.; Parpia, J. M.; Craighead, H. G.; McEuen, P. L. Nano Lett. 2008, 8, 2458.

(15)

Berry, V. Carbon 2013, 62, 1.

(16)

Zhu, S.; Li, T. ACS Nano 2014, 8, 2864.

(17)

Nature Mater. 2016, 15, 485.

(18)

Yuk, J. M.; Park, J.; Ercius, P.; Kim, K.; Hellebusch, D. J.; Crommie, M. F.; Lee, J. Y.; Zettl, A.; Alivisatos, A. P. Science 2012, 336, 61.


Page 26 of 29

Page 27 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

(19)

Shin, S. R.; Li, Y.-C.; Jang, H. L.; Khoshakhlagh, P.; Akbari, M.; Nasajpour, A.; Zhang, Y. S.; Tamayol, A.; Khademhosseini, A. Adv. Drug Delivery Rev. 2016, 105, 255.

(20)

Joung, D.; Agarwal, K.; Park, H.-R.; Liu, C.; Oh, S.-H.; Cho, J. H. Adv. Electron. Mater. 2016, 2, 1500459.

(21)

Cho, J. H.; Gracias, D. H. Nano Lett. 2009, 9, 4049.

(22)

Andres, C. M.; Larraza, I.; Corrales, T.; Kotov, N. A. Adv. Mater. 2012, 24, 4597.

(23)

Kiryukhin, M. V.; Man, S. M.; Tonoyan, A.; Low, H. Y.; Sukhorukov, G. B. Langmuir 2012, 28, 5678.

(24)

Burckel, D. B.; Wendt, J. R.; Ten Eyck, G. A.; Ellis, A. R.; Brener, I.; Sinclair, M. B. Adv. Mater. 2010, 22, 3171.

(25)

Soukoulis, C. M.; Wegener, M. Nature Photon. 2011, 5, 523.

(26)

Kim, J. H.; Chang, W. S.; Kim, D.; Yang, J. R.; Han, J. T.; Lee, G. W.; Kim, J. T.; Seol, S. K. Adv. Mater. 2015, 27, 157.

(27)

García‐Tuñon, E.; Barg, S.; Franco, J.; Bell, R.; Eslava, S.; D'Elia, E.; Maher, R. C.; Guitian, F.; Saiz, E. Adv. Mater. 2015, 27, 1688.

(28)

Vickery, J. L.; Patil, A. J.; Mann, S. Adv. Mater. 2009, 21, 2180

(29)

Lv, Y.; Fang, Y.; Wu, Z.; Qian, X.; Song, Y.; Che, R.; Asiri, A.M.; Xia, Y.; Tu, B.; Zhao, D., Small 2015, 11, 1003.

(30)

Yang, X.; Zhu, J.; Qiu, L.; Li, D. Adv. Mater. 2011, 23, 2833.

(31)

Niu, Z.; Chen, J.; Hng, H. H.; Ma, J.; Chen, X. Adv. Mater. 2012, 24, 4144.

(32)

Shenoy, V. B.; Gracias, D. H. Mrs Bulletin 2012, 37, 847.

(33)

Cho, J. H.; Azam, A.; Gracias, D. H. Langmuir 2010, 26, 16534.

(34)

Joung, D.; Gu, T.; Cho, J. H. ACS Nano 2016, 10, 9586.

(35)

Dai, C.; Cho, J. H. Nano Lett. 2016, 16, 3655.

(36)

Agarwal, K.; Liu, C.; Joung, D.; Park, H.-R.; Oh, S.-H.; Cho, J. H. submitted 2016. 19 ACS Paragon Plus Environment

Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(37)

Nicholl, R. J.; Conley, H. J.; Lavrik, N. V.; Vlassiouk, I.; Puzyrev, Y. S.; Sreenivas, V. P.; Pantelides, S. T.; Bolotin, K. I. Nature Comm. 2015, 6.

(38)

Deng, S.; Berry, V. Mater. Today 2016, 19, 197.

(39)

Polsen, E. S.; McNerny, D. Q.; Viswanath, B.; Pattinson, S. W.; John Hart, A. Sci. Rep. 2015, 5, 10257.

(40)

Xie, X.; Ju, L.; Feng, X.; Sun, Y.; Zhou, R.; Liu, K.; Fan, S.; Li, Q.; Jiang, K. Nano Lett. 2009, 9, 2565.

(41)

Gusynin, V.; Sharapov, S.; Carbotte, J. J. Phys.: Condens. Matter 2006, 19, 026222.

(42)

Hanson, G. W. J. Appl. Phys. 2008, 103, 064302.

(43)

Ju, L.; Geng, B.; Horng, J.; Girit, C.; Martin, M.; Hao, Z.; Bechtel, H. A.; Liang, X.; Zettl, A.; Shen, Y. R. Nature Nanotech. 2011, 6, 630.

(44)

Rodrigo, D.; Limaj, O.; Janner, D.; Etezadi, D.; de Abajo, F. J. G.; Pruneri, V.; Altug, H. Science 2015, 349, 165.

(45)

Li, Y.; Yan, H.; Farmer, D. B.; Meng, X.; Zhu, W.; Osgood, R. M.; Heinz, T. F.; Avouris, P. Nano Lett. 2014, 14, 1573.


Page 28 of 29

Page 29 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

ToC figure


Self-Assembled Three-Dimensional Graphene

Recommend Documents