Barrier-Guided Growth of Micro- and Nano-Structured Graphene

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    Barrier-Guided Growth of MicGraphene

    typically been achieved by first exfoliating or growing graphene

    micro2eg), phene of the ular to rystalws an

    n a Cu N. S. Safron, M. Kim, Prof. P. Gopalan, Prof. M. S. ArnoldDepartment of Materials Science and Engineering University of Wisconsin-Madison, USA E-mail: msarnold@wisc.edu

    DOI: 10.1002/adma.201104195

    as a continuous membrane and then patterning it via top-down porary polymer support and etching of the Cu and oxide.We have successfully implemented BGCVD to create

    nscale boxes (Figure 2ac), arbitrary patterns (Figure nanoribbon arrays (Figure 2h), and nanoperforated gra(Figure 3b), showing the versatility and scalability method. We have employed the (60 m)2 boxes in particbetter understand the function of the barriers and the clinity of the resulting patterned graphene. Figure 2a shooptical micrograph of a (60 m)2 region of oxide barrier oNathaniel S. Safron, Myungwoong Kim, Pad

    The prospects of exploiting graphenes exceptional electronic,[13] mechanical,[4,5] and thermal properties[6] in widespread applications has recently been advanced by rapid progress in the chemical vapor deposition of continuous membranes of the material on metal surfaces.[5,7,8] For many applications, however, it is not continuous graphene that is desired but rather graphene that is patterned on the micron and nanometer scales. Here, we report on a new strategy for the rational synthesis of lowdefect density, patterned graphene from the bottomup, called barrierguided chemical vapor deposition (BGCVD). In BGCVD, graphene growth is laterally restricted on planar metal surfaces by selectively passivating the catalytic activity of the metal with patterned barrier templates designed to (i) locally limit the generation of atomic C species, and (ii) confine their migration. We have successfully implemented BGCVD using aluminum oxide barriers on Cu substrates to fabricate highly crystalline, singlelayered structures including channels, nanoribbon arrays, and nanoperforated membranes, over largeareas. We show here that the barriers can restrict the nucleation of graphene to the exposed Cu and then guide its growth, remarkably, with 1 nm lateral precision. We also show that by avoiding damaging topdown etching, the direct synthesis of micro and nanostructured graphene by BGCVD produces superior edges with less disorder.

    The patterning of graphene is a powerful approach for tuning its physical and electronic structure and for deviceintegration. Graphene patterned on the micronscale has been employed to create ultrahigh frequency analog amplifiers,[1] electrical interconnects,[9] conduits for heat dissipation,[6] and mechanical resonator membranes.[4] At the nanometerscale, the patterning of graphene opens up a bandgap, making it intriguing for semiconductor electronics and sensing.[1018] Nanostructured graphene materials are furthermore attractive for energy storage because of their ultrahigh surface area; and, nanoperforated graphene membranes with high pore density have been proposed as ultrafiltration membranes.[19,20]

    A critical advantage of graphene over other highperformance carbon materials such as nanotubes, is that its twodimensional form factor lends itself to patterning via scalable and standardized planar processing tools. Patterned graphene has 2012 WILEY-VCH Verlag GAdv. Mater. 2012, 24, 10411045ma Gopalan, and Michael S. Arnold*

    ro- and Nano-Structured

    subtractive etching. Topdown processing, however, is limited in fidelity by the etching tools that are available, resulting in structural and chemical disorder, at edges and extending several nanometers away from edges, that degrade graphenes exceptional properties.[1013,17]

    The challenges with topdown processing have motivated the exploration of superior bottomup synthetic methods to avoid etching and to achieve more abrupt edges with fewer defects. Bottomup approaches have been investigated but thus far limited by difficulties in controlling growth orientation or assembly.[16,21] BGCVD overcomes these challenges, relying on selfterminating growth processes rather than harsh chemical etchants to abruptly define edges, while preserving the capability for creating rationallydesigned patterns and compatibility with planar processing. By patterning the catalytic activity of the metal using barriers (rather than the metal itself), BGCVD maintains the planarity of the metal growth substrate and ultimately achieves superior fidelity, especially on the nanoscale, by averting growth on the sidewalls of patterned metal features and avoiding the deterioration and restructuring of patterned metal templates at hightemperature.[22]

    The BGCVD process is schematically depicted in Figure 1 for atmospheric pressure methane CVD on the aluminum oxide barrier/Cu system. Prior to growth, 10 nm of aluminum oxide is deposited on the Cu and patterned. We have chosen aluminum oxide due to its high temperature stability, low C solubility, and relative chemical inertness with respect to Cu and C.[23] During growth, methane decomposes selectively on the exposed Cu to produce C (Figure 1(i)), which does not appreciably dissolve into the bulk Cu due to low solubility.[24] The C instead laterally diffuses and accumulates on the exposed Cu until a supersaturation is reached and graphene nucleates (Figure 1(ii)). The preferential deposition and accumulation of C on Cu is driven by the relative inertness of the oxide and the strong CuC bonding energy 5 eV.[25] Following nucleation, the graphene crystallites grow up to the Cu/barrier edgeboundary (Figure 1(iii)). At this stage, the supply of C locally depletes, and after the entire Cu surface is passivated by either graphene or the barrier, the catalytic decomposition of methane ceases, terminating growth (Figure 1(iv)). Following BGCVD, the patterned graphene is transferred to arbitrary substrates via a temmbH & Co. KGaA, Weinheim 1041wileyonlinelibrary.com

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    foil immediately following CVD at 1000 C. Both the resulting graphene on the exposed Cu and the oxide are nearly transparent. However, the exposed regions of the Cu are smoother after growth indicating a restructuring of the foil. In contrast,

    the Cu underneath the barrier retains its initial coarse morphology, suggesting that the oxide is wellbonded to the Cu, frustrating restructuring. After transferring the graphene and oxide to a Si/SiO2(89 nm) substrate for enhanced optical contrast (Figure 2b), single and fewlayered graphene are observed on the previously unmasked regions contiguous to the barrier. After chemical etching of the barrier, a sharp boundary is observed between the graphene and the previously masked Cu, observed as bare Si/SiO2 in Figure 2c.

    The graphene on the unmasked Cu is highly crystalline with low defectdensity, evidenced by a Raman 2Dband approximately twice the intensity of the Gband and the absence of a measurable Dband (Figure 2d). In contrast, nongraphitic carbon is detected in the barrier region following growth. The frequency and width of the Dband and absence of 2Dband in this region is consistent with that of small clusters of C or poly

    aromatic hydrocarbons[26] rather than graphene. These small clusters may arise from C species that are unable to appreciably crystallize, kinetically trapped on the oxide upon cooling, or form in pinholes of the oxide. Regardless, this C is removed

    Figure 1. Schematic of BG-CVD: (i) methane decomposes into C, which (ii) diffuses and nucle-ates graphene, (iii) growing until (iv) the entire Cu surface is covered.wileyonlinelibrary.com 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Wein

    Figure 2. Demonstration of BG-CVD: a-c) optical micrograph (top) and schematic (bottom) of Bon Cu a) immediately following BG-CVD, b) after Cu etch and transfer to Si/SiO2, and c) after Cspectroscopy of (i) graphene, (ii) the barrier region after growth, and (iii) the barrier region afteC. Escher pattern, f) Maze, g) Bucky the Badger, and h) Nanoribbon array, scale bars = 20, 20, 10corresponds to mono- and bi-layered graphene, respectively, while light grey regions are where nothere, which were subsequently etched for imaging. In (h) white lines are wrinkles.G-CVD with a 60 m square aluminum oxide barrier u and barrier etch and transfer to Si/SiO2. d) Raman r growth and oxide etching. SEM of BG-CVD: e) M. , 0.1 m, respectively. In (e-g), medium to dark grey graphene growth occurred due to barriers deposited heim Adv. Mater. 2012, 24, 10411045

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    concurrently with the etching of the oxide, as verified by Raman

    Figure 3. BG-CVD of nanoperforated graphene. a) SEM of aluminum oxide transfer to SiO2/Si (scale bars = 100 nm). c) Histogram of oxide dot (blu(d, f) and optical micrographs (e, g) of graphene crystallites on pristine CBG-CVD nanoperforated graphene with 800 nm selective-area aperture, i) C 2012 WILEY-VCH Verlag Gm

    (Figure 2c(iii)). Importantly, the latter two observations demonstrate that BGCVD is fundamentally distinct from previously reported liftoff mechanisms.[27] Overall, Figure 2 shows the effectiveness of oxide barriers at terminating the growth of graphene crystallites, thereby preventing their continuation over barriers and at inhibiting the nucleation and growth of graphene under and on top of the barriers.

    In addition to the (60 m)2 box pattern, BGCVD can be implemented using barrier templates that incorporate many discrete and disconnected regions of exposed Cu, in which case the nucleation of graphene in each region will be stochastic (e.g. some regions will nucleate and grow before others). However, one advantage of implementing BGCVD on Cu, in particular, is that it is selfpassivating. Once graphene nucleates and grows to completion in one discrete region, the growth will selfterminate in that region. Nucleation and growth will then proceed in other regions, until all have finished. Figure 2e, for example, demonstrates the effectiveness of BGCVD at reproducing a template with hundreds of discrete regions.

    To explore the resolution limits of the BGCVD method, we have fabricated graphene nanoribbons (Figure 2h) and nanoperforated graphene (also semiconducting[11,12]) using electronbeam and block copolymer (BCP) lithography, respectively, to create nanopatterned aluminum oxide barrier templates on Cu (Figure 3b). For the latter, we adopted BCP lithography to create a hexagonal array of 16 nm aluminum oxide dots with a periodicity of 41 nm (Figure 3a) on Cu. Remarkably, these dots guide the growth of graphene through the network of 25 nm channels of exposed Cu (Figure 3b), resulting in largearea,

    Adv. Mater. 2012, 24, 10411045bottomup graphene perforated by arrays of holes (see also

    anodot array on Cu and b) nanoperforated graphene grown from (a), after , forward-slash) and perforation (red, back-slash) diameters. Schematics

    u (d, e) and Cu with oxide nanodots (f, g), scale bars = 2 m. h) TEM of orresponding electron-diffraction.1043wileyonlinelibrary.combH & Co. KGaA, Weinheim

    Supporting Figures S1S3 for additional characterization via SEM, optical microscopy and AFM topography). The dotsize and holesize distributions differ only by 1 nm (Figure 3c), revealing that the oxide barriers are stable and terminate the growth of graphene with nearly atomic exactness. These data suggest that the BGCVD strategy could be used to template nanostructured graphene materials with sub5 nm features and

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    2 2

    r eofinanoperforated graphene sheet, using 800 nm (Figure 3h) and 4 m (Supporting Information, Figure S4) selectedarea apertures. The hexagonal diffraction patterns from the 2.13 and 1.23 lattice periodicities of graphene (Figures 3i and S5) correspond to one domain of highly crystalline and orientated graphene inside the aperture area. These data indicate a nominal grain size >4 m, in quantitative agreement with optical microscopy, confirming that nucleation is largely unaffected by the barriers and that the graphene maintains its crystallographic orientation as it grows around the barriers.

    We have characterized the edge and defectrelated D and D Raman bands[8,26] of the nanostructured graphene to compare the quality of edges produced by topdown etching (via oxygen reactive ion etching) and by BGCVD (Figure 4a). We have specifically quantified the integrated intensity of the D and D bands, normalized to the integrated intensity of the Gband, referred as Raman defect ratio, for nanoribbon arrays and nanoperforated graphene structures, with various edge densities (Figure 4b). Both the D and D Raman defect

    Figure 4. Characterization of BG-CVD nanostructures: a) Raman spectraetching (bottom, red), b) D-band defect ratio (left-axis, nanoribbons = squaaxis, nanoribbons = circles and nanoperforated sheets = hexagons) versus(open, red). Dotted lines show linear fit. c) Square sheet conductance fgraphene at T = 295K (blue), 150K (green), 58 K (red). Dotted lines show wileyonlinelibrary.com 2012 WILEY-VCH Verlag G

    ratios increase linearly with edge density indicating that the ratios are related to disorder at edges. On average, the topdown etched samples have 1.5x and 2x larger D and D Raman defect ratios, respectively, than the more pristine BGCVD samples. Furthermore, the BGCVD materials are also substantially better than topdown samples from literature as well,[12,28,29] which have 510x higher Dband Raman defect ratios (see Supporting Information, Figure S7). We hypothesize that the more substantial defect bands in the topdown samples arise from increased edge roughness (e.g. more edge atoms) and increased structural and chemical disorder and defects near edges that result from limitations inherent to topdown etching. For example, oxygen reactive ion etching through a polymer mask is not perfectly abrupt and therefore is expected to result in a region of disorder and defects in the interior of etched structures, adjacent to the edges.[29] In contrast, the superiority of the edges created by BGCVD results from its selflimited nature in which growth abruptly terminates at the barrier boundary, circumventing etchinduced disorder near edges.We have quantified charge transport through BGCVD graphene materials in order to further characterize their structure and continuity (Figure 4c). The materials are highly conductive, for example with a square sheet conductance of 0.4 and 0.1 mS at a charge density of 1.5 1013 cm2, for the nanoribbons and nanoperforated structures, respectivelyagain showing that the BGCVD structures are highly continuous. The roomtemperature ON/OFF conductance modulation of the nanoperforated graphene is 3.9 0.6 (14 devices), improving marginally to 10 at 58 K, as expected for the w = 25 nm dimensions and small band gap 15 meV produced in this study (see Supporting Figures S9S12). The grap...

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