Single-Layer Graphene as a Barrier Layer for Intense UV Laser

Subscriber access provided by CMU Libraries - http://library.cmich.edu

Article

Single Layer Graphene as a Barrier Layer for Intense UV Laser Induced Damages for Silver Nanowire Network Suprem R. Das, Qiong Nian, Mojib Saei, Jin Shengyu, Doosan Back, Prashant Kumar, David B. Janes, Muhammad A. Alam, and Gary J. Cheng ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.5b04628 • Publication Date (Web): 08 Oct 2015 Downloaded from http://pubs.acs.org on October 10, 2015

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.

ACS Nano 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 38

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

ACS Nano

Single Layer Graphene as a Barrier Layer for Intense UV Laser Induced Damages for Silver Nanowire Network Suprem R. Das1,2,a,*, +, Qiong Nian2,3,a, Mojib Saei2,3, Shengyu Jin2,3, Doosan Back1,2, Prashant Kumar2,3,#, David B. Janes1,2, Muhammad A. Alam1,2, and Gary J. Cheng2,3,* 1. School of Electrical and Computer Engineering, Purdue University, West Lafayette, IN 47907 2. Birck Nanotechnology Center, Purdue University, West Lafayette, IN 47907 3. School of Industrial Engineering, Purdue University, West Lafayette, IN 47907 a.

These authors contribute equally to this work

*Correspondence: [email protected]; [email protected] #

Present address: Department of Physics, Indian Institute of Technology Patna, Patna 800013,

India +

Present address: Microelectronics Research Center, Applied Sciences Complex I, Iowa State

University, Ames, IA 50011, USA; Mechanical Engineering Department, 2025 Black Engineering, Iowa State University, Ames, IA 50011, USA

KEYWORDS: Silver nanowire network, Hybrid nanowire graphene network, Thermal barrier, Pulsed laser annealing, Raleigh instability

ACS Paragon Plus Environment

ACS Nano

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

ABSTRACT Single layer Graphene (SLG) has been proposed as the thinnest protective/barrier layer for wide applications involving resistance

to

oxidation,

atomic/molecular

corrosion, diffusion,

electromagnetic interference, and bacterial contamination. nanostructures

Functional have

lower

metallic thermal-

stability than their bulk forms and thereby susceptible to high energy photons. Here we demonstrate that SLG can shield metallic nanostructures from intense laser-radiation that would otherwise ablate them. By irradiating UV laser beam with nanosecond pulse-width and a range of laser intensities (in millions of watt per cm2), onto a silver nanowire network, and conformally-wrapped by SLG on top of the nanowire network, we demonstrate that graphene “extracts and spreads” most of the thermal energy away from nanowire, thereby keeping it damage-free. Without graphene wrapping, the radiation would fragment the wires into smaller pieces, and even decompose them into droplets. A systematic molecular dynamics simulation confirms the mechanism of SLG shielding. Consequently, particular damage-free and ablation-free laser based nano-manufacturing of hybrid nanostructures might be sparked off, applying SLG on functional surfaces and nano-features.


Page 2 of 38

Page 3 of 38

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

ACS Nano

Since their discovery almost two decades ago, various allotropes of carbon, such as fullerence,1 nanotubes,2 and single layer graphene (SLG) have demonstrated extraordinary physical properties with wide range of applications. In particular, graphene has shown unique electrical, optical, mechanical and thermal properties pertaining to its linear band structure.3-7 For example, single and bilayer of graphene have been utilized to sense the transport of a single electron and to detect very low level of electronic noise, leading to their applications in sensitive electronic and chemical sensors.8,9 Graphene may also have applications in practical, large scale and scalable systems such as non-ITO based flexible and transparent conducting electrodes (TCEs), super capacitors, and composite materials.10-12 In addition to its application in active devices, graphene also appears to be a remarkable passive/protectivelayer. Despite its single atomic layer thickness (of 0.35 nm), graphene has been proposed and demonstrated as a barrier layer for wide range of applicationsrequiring

resistance

to

oxidation,

corrosion,

atomic/molecular

diffusion,

electromagnetic interference, etc.13,14-18 Recently, graphene has been proposed as a protective layer for biological samples, with applications in nanobiotechnology.19 Graphene has also been proposed as a protection layer for plasmonic enhancements from metals such as copper and silver.20 The active and passive functionalities of SLG, collectively, may lead to a number of multifunctional applications that do not have any classical analog.21,

22, 23

For example, hybrid

nanowire-graphene networksstructures have shown robust TCE performance (electrical sheet resistance of 22Ω/ at 88 % optical transmission at 550 nm wavelength), with electronic conduction via co-percolation through both materials and long-term stability attributed to the graphene providing a diffusion barrier for oxidizing species, thus inhibiting oxidation in the underlying silver nanowire network.21 Direct experimental evidence of reduction of microscopic


ACS Nano

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

Page 4 of 38

self-heating at single nanowire-nanowire junction in such a structure has been shown to occur due to intimate wrapping of graphene.24 Graphene has recently been grown on Ta/Cu thin interconnect lines and has been shown to demonstrate two fold benefits: protecting from ambient corrosion and joining the grains – leading to improved properties.23 While a number of studies have demonstratedSLG for surface-coating and passivation technologies, graphene’s usage in harsh radiation environments (especially to high energy density photons) remains unexplored, particularly in terms of protection of underlying layers of nanostructures. Graphene’s potential as saturated absorber using ultrafast pulsed laser and as electromagnetic interference shielding (EMI) have been shown previously.25,

26

Pulsed laser

beams have been used to induce lattice damage in graphene and for patterning.27, 28 However, if graphene could be configured such that it serves as a protective layer for radiation induced thermal damages, it could enable a number of applications. In particular, damage-free and ablation-free laser-based nano-manufacturing, laser annealing29 and recrystallization30,

31

of

hybrid nanostructures might be achieved using graphene on functional surfaces and nanostructures. Use of graphene as a top layer/membrane in laser annealing process might exert large pressure/shock to improve material integration at nanoscale and nanoscale junction resistance and it might eventually enhance electrical transport properties. For example, the nanowire-nanowire junctions in a nanowire network, that act as the transport bottlenecks, could significantly be improved using graphene as protection layer and post annealing of the stack.24. UV annealing near plasmonic resonance of silver NW-NW junctions has recently been shown by using a simple UV light source and pulsed laser.32,33 UV pulsed laser radiation of high power density, can easily ablate nanomaterial.34 The primary reason ascribes to the excessive activity or instability of nanomaterials, especially high aspect


Page 5 of 38

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

ACS Nano

ratio nanowires. For instance, it has been demonstrated that metal nanowire could be fragmented when exposed to temperatures as low as 300°C, and even room temperature for thinner nanowires.35,

36

What’s more, low frequency resistance fluctuations and moderate UV

exposurewould also cause the degradation of metallic nanowires.37,38 Thereby, construction of an anti-radiation damaging surface of functional materials and metallic nanowire building blocks, would be of most sought for developments of solid state detectors those are intended to operate at those specific extreme conditions. In this report, by irradiating UV laser pulses with high laser fluence, we demonstrate that graphene retains its sp2 hybridizing bonding properties and yet shields the nanowires beneath it from radiation -induced thermaldamage, whereas the regions where graphene is absent, the metal nanowire geometry transforms intosmaller segments and then to droplets. Such technique could use graphene-wrapping both as an effective thermal management route as well as protection barrier material. RESULTS AND DISCUSSION Figure 1 (a) shows the schematic diagram of the experimental set-up, including the focusing lens and beam shaper. Prior to any exposure to the sample, the laser waspowered-on for ~ 15 minutes to avoid any fluctuation in the output power. As shown in the figure, the sample consists of two regions: the right half is Ag-NW network covered with SLG [henceforward called ‘hybrid NW network’], while the left half is a nanowire network without SLG covering [henceforward called ‘NW network’]. This side-by-side configurationallows us to compare the effectiveness of radiation-shielding of SLG, as the laser beam raster over the samples. The SLG as well as the hybrid NW network structure were studied using field emission scanning electron microscopy (FESEM) and Raman spectroscopy and are shown in supplementary information S1. Figure 1 (b) and (c) show FESEM images illustrating the effect of intense pulsed laser radiation with a power


ACS Nano

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

density of 2.6 MW cm-2on NW network region and hybrid network counterpart.Initial visual inspection showsthat the nanowires within the hybrid network region are perfectly protected in shape and size as long as beneath SLG, whereas the nanowires within the NW network region are severely damaged. Figures 2 and 3 show the FESEM images of the surface nanostructure of the laser beam irradiated TCE for different laser power densities ranging from 0.8-2.6 MW cm-2. This particular energy range is used due to similar typical laser energy that has been used before for soft metal evaporation such as silver nanowires.33, 42 Figure 2 presents images following the lowest laser power density of 0.8 MW cm-2.Figure 2a shows a distinct contrast between the two regions within the sample, namely the hybrid network (upper left) and the NW network (bottom right). Figures 2b and 2c show higher resolution images of the hybrid network imaged at two different spots randomly chosen. The original shape and structure of the nanowires (diameters and lengths) is maintained after laser annealing.Figure 2d shows the representative high resolution FESEM image taken at a random spot on NW network region. Significant structural changes are observed in case of laser treatment on NW network. Well defined cut marks are observed after irradiation, resulting in the fragmentation of nanowires into segments of varying lengths. These segments, with lengths ranging from several hundred nanometers to several microns long, are much shorter than the original nanowires (average length ~ 40µm). The surface nanostructure comparison between figures 2a, 2b, 2c and 2d clearly demonstrates SLG as protection layer for pulse-laser induced thermal barrier to the underlying nanowire network. To further evaluate the effect, higher energy densitieswere employed. Figure 3 (a) – (f) show the network structure after laser irradiating with power densities of 1.4 MW cm-2, 2 MW cm-2 and 2.6 MW cm-2. Figure 3 (a) – (b) illustrates a hybrid network and a NW network, respectively,


Page 6 of 38

Page 7 of 38

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

ACS Nano

under 1.4 MW cm-2 power density. In the hybrid network, almost all nanowires survive during intense radiation due to SLG passivation. On the other hand, nanowires within the NW network fragment into smaller segments or small metal beads. While closer observation of the hybrid sample reveals the presence of torn portion of graphene especially near crowded junctions of several wires (indicated by the arrow marks), the wires themselves retain the continuous structures. However, the shapes of the wires at the damaged graphene sites become distorted like ribbons. While for an ideal hybrid TCE, deposition process of the nanowires must be optimized so that the torn regions are minimized, local damages are hardly detrimental in a co-percolating network in which undamaged section of the TCE will help bypass the bottleneck of the damaged sections. To evaluate the effects at higher power densities, we increased the laser intensity to 2 MW cm-2 and 2.6 MW cm-2. Laser intensity at or above 2 MW cm-2 have been used for photo-etching of substrates,39, 40 indicating the level of material removal that can be induced. Figure 3 (c) – (e) present images of hybrid network and NW network area of the TCE following 2 MW cm-2 and 2.6 MW cm-2 exposure, respectively. At 2 MW cm-2 (Figure 3 (c)), though the graphene starts developing weak spots marked by bright dots on its surface, there is still well retention of overall integrity of the hybrid structure. The development of weak spots (microscopic defects) may originate from the defective grain boundaries of CVD graphene upon laser bombardment at higher intensities. Graphene grain boundaries contain defective hexagonal sp2 carbon (particularly pentane and heptane carbon chains) and might be initial damage sites under intense laser energies. Indeed in our graphene the average grain size is 1µm and the bright spots develop with average ~1µm separation.21 At 2.6 MW cm-2 (figure 3 (e)), the average length of the nanowires remains mostly unaffected but a few marked differences were detected in the


ACS Nano

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

graphene, including breaks at crowded junction area as well as sharp cut marks along the nanowire edges of the wires. In contrast, within the NW network regions, at 2 MW cm-2 (Figure 3 (d)), the wires are broken into small beads (diameter ~ hundreds of nanometer) decreasing significantly the mass density of silver on surface of the substrate. It is clearly seen that rather than fragmentations the nanowires are broken into chains of beads, primarily along the length of the original wires. At 2.6 MW cm-2 (figure 3f), the nanowires have been fragmented to tiny sections and they further decay to chains of beads along the original length of the wires as shown in Figure 3g. In order to see if graphene only protects the metallic nanostructure or any other type of material (for example, the substrate material), we replaced the (thermally robust) quartz substrate with PET plastic substrate (Dupont, Tejin film), fabricated the NW network as well as hybrid NW network structure with similar density of wires and conducted laser shinning experiment with 0.8 MW cm-2 and 2 MW cm-2 power densities. Figure 4 (a) and (c) show hybrid network on PET following laser bombard with 0.8 MW cm-2 and 2 MW cm-2 power densities. Both the underlying nanowire network as well as the PET substrate gets protection due to graphene shield. However, for the NW network shown in figure 4 (b) and (d), there was significant damage to the PET substrate in addition to the nanowires. This confirms the use of SLG as single-atomic thin barrier for pulsed-laser induced thermal damage. The effect of laser annealing and the comparison between hybrid structure and the bare nanowire network are further investigated by transmission electron microscopy (TEM) imaging. Figure 5 shows systematic representative TEM images captured on the hybrid NW network and NW network without and with laser treatment (images with lowest and highest laser power (LP) studied have been shown). Figure 5 (a) and (b) show the TEM images of the as- prepared


Page 8 of 38

Page 9 of 38

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

ACS Nano

(without any laser treatment) hybrid NW network and NW network respectively. As is seen in figure 5 (a), there is no damage initially in the graphene after its transfer onto the nanowire network on TEM grid. Figure 5 (c) and (d) show the structures after pulsed laser irradiation with LP of 0.8 MW cm-2. This is the lowest LP we used in all of our experiments performed in this study. Consistent with FESEM study, the Ag NWs residing underneath graphene in hybrid NW network remain unaffected with this laser power as seen from figure 5 (c). On other hand, for the NW network sample (figure 5 (d)), clear cuts and silver nanowire segments were identified. Figure 5 (e) – (g) show three different representative locations on hybrid and NW network structure after treating them with LP of 2.6 MW cm-2. Figure 5 (e) shows SLG protected Ag NW structure even at the highest LP. The NWs retain their original shape and structure. Significant heating at one end of the wires, apparently where the graphene has its boundary forms the droplet structure. Figure 5 (f) shows an area on hybrid structure where both the graphene as well as the NWs have been damaged. Such locations were discussed at/near to the junctions in FESEM pictures discussed earlier. Furthermore, the silver beads formation along the wire direction could be realized from figure 5 (f) [shown by arrow marks]. Figure 5 (g) shows the silver droplets formation on bare NW network structure, where the entire wire has undergone the laser induced heating, evaporation, and droplet formation. The TEM results thus clearly demonstrate the role of SLG as thinnest barrier to high energy radiation induced thermal damages to metallic nanostructures. Next, we conducted electrical sheet resistance and optical transmittance measurements on the laser irradiated samples to evaluate the impact on the hybrid network properties. Figure 6 (a) shows the four terminal sheet resistance schematic of the hybrid network (as well as the nanowire network sample). Thesheet resistances before the laser irradiation were 539 Ω/ and


ACS Nano

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

Page 10 of 38

2.064 kΩ/ respectively. The samples use a nanowire density D1 corresponding to approximately 2x106 NWs cm-2. Figure 6 (b) plots the average sheet resistance values taken at four different spots on both the networks after irradiation versus the laser power density. For the hybrid network, the sheet resistance reduces dramatically to 199.75 Ω/ and 152.33 Ω/ with 0.8 MW cm-2 and 1.4 MW cm-2 power densities respectively, after which it goes up rapidly to ~ kΩ/and ~ MΩ/ values corresponding to 2 MW cm-2 and 2.6 MW cm-2 respectively. The sheet resistance at the locations of NW network, however, did not decrease with 0.8 MW/cm2 power density, rather it climbed up to 3.441 kΩ/ and 15.692 kΩ/ respectively for 0.8 MW/cm2 and 1.4 MW/cm2 power densities. (Even for 1.4 MW/cm2 the sample at some places exhibited overloaded sheet resistance due to discontinuous nanowire segments). At 2 MW/cm2 and 2.6 MW/cm2 power densities the sample sheet resistance become completely open circuited. This is consistent with our earlier observations of physical damage formation on nanowires and graphene as the power density goes beyond 1.4 MW cm-2. In order to study and compare the optical transmission spectra at the two areas on the sample, the monochromator light was positioned at respective spots on the sample, within the NW network region and the hybrid network region – both before and after the laser annealing. Figure 7 (a) and (b) show the optical transmission of the NW network and hybrid network, respectively, before and after 0.8 MW cm-2 pulsed energy radiations [since 0.8 MW cm-2 still preserves the hybrid TCE without much damage to the graphene]. An increase in the transmittance of the NW network after laser exposure is observed,butwith a minimal increase in case of hybrid network. This is believed to be due to material (i.e., silver) loss in case of the NW network due to pulsed laser radiation (consistent with FESEM images of figure 3 (b)). The nearly unchanged transmittance of hybrid NW network demonstrates the robustness of graphene and its sp2 bonding for protecting material


Page 11 of 38

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

ACS Nano

loss from high power radiation. The transmittance results thus supportthe FESEM result (Figure 3). Plasmonic pulse welding of NWs has been shown for silver NW network wherepulsed laser treatment for the nanowire-nanowire junctionshave been investigated and an optimized laser energy density of 20-45 mJ cm-2 has been proposed (Ti:sapphire 800nm laser excitation) beyond which the nanowires loose material from its body and for a Nd:YVO4 355nm laser, 35-50 mj cm2

optimal energy density has been proposed.41, 42 All of these energy densities are in similar range

that we have obtained in this study (see supplementary information S3 for power density per µm2 generated by the Excimer laser); however, the graphene barrier in our case is unique and provides robustness in manufacturing the hybrid NW network TCEs without any physical damages. Figure 7 (c) shows the optical transmittance of SLG (on quartz substrate) after pulsed laser irradiation with various energy densities. The transmittance remains unchanged upto 0.8 MW cm-2, but for 2 MW cm-2 and beyond the transmittance drastically decreases. This decrease in transmittance is due to the formation of wrinkles and folding of the graphene as well as possible surface modifications of the silver nano-beads. These structures are visibly evident from the TEM images (figure 5). High laser power renders the hybrid structure with more disorder and defective leading to a decrease in the optical transmission. The differences in structural changes in nanowires within NW networks versus those in hybrid networks motivated us to explore the mechanism of the laser induced physical processes.In order to obtain the atomic scale insight of structural evolution underlying laser annealing processes, molecular dynamics (MD) simulation was performed. Details of the technique are described in methods section (MD simulation subsection) as well as in the supplementary information S2. In particular, MD simulation was performed to understand (i) the laser and nanowire as well as the laser and hybrid networkinteraction; (ii) the difference in annealing between silver nanowire


ACS Nano

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

Page 12 of 38

network and hybrid NW network; (iii) laser energy density dependence of the stability of NW network and hybrid NW network. MD simulation has been done in two steps: First, the NVT ensemble is applied to reach the equilibrium structure of the network corresponding with room temperature; and then the laser irradiation is initiated to induce heating in silver nanowires (see methods section and supplementary information S2). Initial structures with presence and absence of graphene were compared to ascertain the role of graphene. Figure 8 (a) depicts the initial structure of a bare silver nanowire on quartz substrate. To decrease computational time, silver NW chosen for the simulation setup was made smaller (50nm long and 10 nm in diameter and consisted of 2, 21,264 silver atoms) than experimental situation. Figure 8 (b) shows the NW after laser irradiation with an input heating rate of 200 µJ/s and heating duration of 250 ps, respectively. The choice of heating rate parameters was to achieve power equivalent with half of a laser pulse (0.4MW cm-2) but 100 times faster. In this way, it is possible to decrease the computational time significantly. Figure 8 (c) shows representative of initial structure of silver nanowire and single layer graphene sheet. The self-wrapping process after the release of SLG on nanowire network and the following effect of laser heating are shown in figure 8 (d) and (e) respectively, in which the heating rate (in simulation input) was kept solely for bare NW network (no heating input was provided to graphene). We observe that SLG is thoroughly wrapped onto silver NW surface (see supplementary information S2 video). Next, to match the experimental conditions, the edge of the graphene was fixed with the substrate as shown in figure 8 (f). Figure 8 (g) and (h) show spontaneous wrapping process and post laser irradiation structure related to initial structure and boundary condition (set in figure 8 (f)). Comparing these three different cases shown in figure 8 (b), (e), and (h), we find that graphene protects the NWs from intense irradiation ablation. As observed in figure 8 (b), in the case of


Page 13 of 38

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

ACS Nano

heating of bare nanowire, above a critical energy (discussed in figure 9 and figure 10) the surface atoms are ejected from nanowire lattice, leading to fragmentation of the nanoscale cylinder. The ‘evaporation’ of silver atoms is due to extremely high localized kinetic energy generated by laser pulses, leading to “necking” on the body of the nanowire. This scenario compares well with the deformation and fragmentation observed in the surface image captured by SEM and TEM, demonstrating the effect may be interpreted through Raleigh Instability. However, with graphene’s presence, attractive interactions leading to intact wrapping between graphene surface and silver atoms decreases the tendency of silver nanowire toward deformation and fragmentation (EAM potential43,

44

). Most importantly, the shielding effect attributing to the 45

theoretical basis of graphene with outstanding thermal conductivity

helps nanostructure

beneath to instantly spread the heat out. In figure 8 (e), and (h), tightly wrapped graphene layer not only enable the phonon-transfer from silver NW to graphene, but also could serve as physical confinement against potential ablation of silver atoms under heavy radiation dose. This confinement by atomically thin membrane prevents silver atoms from escaping nanowire surface and forces them to recrystallize back to the silver lattice. Four different laser power densities (0.8 MW cm-2, 1.4 MW cm-2, 2 MW cm-2 and 2.6 MW cm-2) corresponding to the experimental conditions were simulated and will be further discussed. Figure 9 plots the kinetic energy of silver atoms and graphene lattice as a function of heating time (input heating time provided in the simulation). We assumed that graphene is semitransparent to laser irradiation, so that in both types of samples, only Ag NWs absorb the incident energy. Figure 9 (a) demonstrates the kinetic energy of the silver atoms located in the central axis of the wire (shown in insert). The heating due to the irradiation excites silver atoms and enables intense lattice vibration, thereby increases the surface kinetic energy (KE).The input


ACS Nano

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

Page 14 of 38

heating is only given to the NWs in both the cases, in Figure 9 (a). However, it is observed that the KE of silver atoms are different for the two cases beyond 100 ps heating time. Silver atom’s KE from bare NW network sample surpasses the case of hybrid NW sample. We might ascribe the difference to lack of .heat spreading within the NW network. Figure 9 (b) depicts the KE of the top surface (shown in insert) silver atoms increasing along heating time, as well. Typically the simulation is done by constructing mesh structure within simulation box and the similar setting for the two cases obtained asserts that the graphene wrapping decreases the KE of the silver atoms. Figure 9 (c) depicts the distinctly observed increase in the kinetic energy of the graphene lattice as a function of simulation run time. The excitation of graphene lattice with no initial heating added to the simulation demonstrates phonon transferring from the NWs to wrapped- graphene. Furthermore, ultra-high thermal conductivity of graphene would help to extract and spread heat away. In reality, CVD grown macro-scale graphene with sp2 bonding characteristics and highly anisotropic thermal conductivity with at least 100 times higher inplane thermal conductivity than out-of-plane spreads out the heat far away from nanowires and junctions.46 The cooling of electrically induced self-heated silver nanowires and nanoscale wirewire junctions have been recently reported with the hybrid NW network structure.24 Figure 9 (d) shows the statistical analysis of number of silver atoms loss (evaporated from nanowire surface) during laser irradiation, in which it is observed at least 10 times higher loss of silver atoms for silver NW network compared to the hybrid network case. The conservation of silver atoms in the nanowire geometry and the protection of the wires by graphene shielding for the nanowire network is central to our present work and confirms graphene as high energy density radiation barrier. The video provided in supplementary information file S2 shows a comparison between


Page 15 of 38

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

ACS Nano

the two cases of the complete sample simulation process, confirming our claim of using graphene as the thinnest protection layer for intense high energy density irradiation. Finally, the MD simulation was performed on both the NW network and hybrid NW network to address the stability of the TCEs with various energy densities of the laser, particularly those we used in our experiment. Figure 10 shows the corresponding simulation results, in which red color atoms imply evaporated ones from nanowire. As it is observed from figure 10 (a) – (d), the NW network sample remains mostly unaffected for 0.8 MW cm-2 except the loss of few atoms, leading to the appearance of random fragmentation (experiment). But most of the structure retains its cylindrical shape and size. Moreover, above 1.4 MW cm-2 and higher power densities, drastic changes are observed with much of the silver atoms evaporation from the nanowire surface. This matches with the experimental observations from FESEM imaging, TEM imaging as well as electrical measurements that, for the hybrid NW network the nanowires well maintain their geometry up to 2.6 MW cm-2 as shown in figure 10 (e) – (h). Nevertheless, CVD graphene (apparently due to presence of imperfections such as grain boundaries) starts developing damages which also causes the silver nanowires to evaporate more and more from the surface as shown in figure 10 (g) and (h). In figure 10 (i) we compare the extracted statistical silver atoms loss from NW network sample and hybrid NW network sample. In all of our laser energy values it was found that the average loss of silver atoms is at least one order of magnitude larger in NW network sample as compared to the values in hybrid NW samples. Co-percolation of phonon transport: The robustness of SLG wrapping to protect the radiationinduced thermal damages could conceptually be understood by a co-percolation phonon transport in the hybrid structure, an analogous phenomena that previously we have explored but for


ACS Nano

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

Page 16 of 38

electronic transport.21 In the steady state, the temperature change in the network structure can be expressed as the following simple equation ∆ =

(1)

With fixed input power for both the nanowire and hybrid networks (Pin = Elaser/(Area*number of laser pulse*pulse width)), there are two ways that the induced temperature in the network could be reduced: by shielding the input power by graphene or by decreasing the thermal resistance across the boundaries. Since graphene is broadband transmitter to electromagnetic radiation, a very small fraction of the input power gets shielded in reaching the underneath nanowire network. Therefore, the only way the two networks get the thermal excitation is through the thermal resistance involved in the structure. Assuming constant Pin, therefore, we can model ∆T1, NW

and ∆T2, hybrid as follows

∆ , = , = ( , + , + , )

(2)

∆, = , = ( , + , + , + , + ,

)

(3)

Where, GB stands for graphene grain boundaries. Comparing above two expressions for laser induced temperatures on the network structure, the fact ∆, < ∆ , , and the experimental evidences of graphene acting as thermal barrier, we anticipate the presence of a low thermal resistance involved between the NWs and the graphene GBs, leading to funneling of thermal energy/phonons from the NWs to graphene lattice and vice versa. CONCLUSIONS


Page 17 of 38

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

ACS Nano

In summary, we have demonstrated that a hybrid network consisting of single-layer graphene over a nanowire network far superior laser-robustness compared to those observed in a nanowire-only network. By irradiating intense 248nm KrF excimer laser beams with nanosecond pulse-widths and with varied laser intensities corresponding to millions of Watt cm-2 power densities, we demonstrate that a conformably-wrapped single layer grapheneon top of nanostructure (in this study, a network of silver nanowires) “extracts and spreads” most of the thermal energy from the nanostructure to the graphenelattice, thereby keeping it damage-free. The interfacial contacts between graphene and nanowire network are suitable to enable fast phonon transferring from hot nanowires to cool graphene lattice.Upon exposure to the increasing laser energies, the nanowire network exhibits deformed nanowires, nano-segments of silver, and eventually nano-beads along the length of the nanowires, indicative of Raleigh instability. Molecular dynamics simulations indicate a significantly smaller NW mass-loss in the hybrid network, in part due to the efficient heat spreading through the graphene. This finding paves the way to use graphene as atomic layer thin, robust protective layer for nanoscale materials from intense radiation-induced thermal damages and thermal managements. METHODS EXPERIMENTAL: Silver nanowire networks (which essentially form similar structure to a thin filmnetwork structures) were prepared by drop casting the nanowires (90 nm average diameter; 40 µm average length) from a precursor solution (Blue Nano Inc., NC) on pre-cleaned SPI quartz substrates21. Areal densities of nanowires with a number density of ~ 2.2 x 106was uniformly sprinkled by multistep coating and then thoroughly dried and cleaned with hot acetone (at 700C for 30 minutes), then by methanol rinsing for another 30 minutes and dried overnight in nitrogen in a glove box. A single layer graphene (CVD grown) was then wet transferred on to


ACS Nano

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

Page 18 of 38

thesubstrates containing the nanowire networks such that half of the area is covered by graphene and other half contains the bare nanowire network.21 Then PMMA polymer was stripped off followed by cleaning of any residue by hot acetone (set at 700C for 3 hours). The Raman spectrum of the hybrid TCE was acquired at various spots, i.e., over the surface of a nanowire (covered by graphene) as well as away from the nanowires (purely on graphene lattice) using a Horiba Jobin Yvon xplora Raman spectrometer and confocal microscope in backscattered geometry. 530 nm wavelength was used to acquire the spectra with 100X objective lens (N.A. 0.9). For the high energy density radiation experiment, the fabricated sampleswere placed in a vacuum chamber (with base pressure of ~ 1 x 106 torr) pumped overnight and purged with nitrogen to reduce the partial pressure of oxygen. A KrF UV excimer pulsed laser (Lambda Physik LPX2000; λ= 248 nm and ∆τ (pulse width) of 25 ns) with repetition rate (RR) of 10 Hz was used to irradiate the pulsed radiation normal to the sample surface. A top-hat profile (8 × 8 mm square) of the laser beam was formed using the optics attached to the laser output. In order to process multiple areas rapidly with different laser energies and total number of pulses, the sample was placed on a motorized stage which enables translations along both X and Y axes. Laser intensities used in the exposure experiments ranged from 0.8 to 2.6 MW cm-2. The laser pulse number (N) used ranged from 20 to 100, corresponding to a total laser irradiation time of 0.5 – 2.5 µs. Post UV laser exposure, the samples were characterized as follows and were compared with the samples before laser treatment: Field emission scanning electron microscopy (FESEM) was used to observe the surface nanostructure for each of the laser intensities. For TEM measurements, several Ag NW network and hybrid NW network samples were fabricated on TEM grids following similar procedures described earlier.21 The as-fabricated samples were imaged using a FEI Titan TEM microscope with 80 kV accelerating voltage applied to the


Page 19 of 38

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

ACS Nano

electron beam. Then the NW network and well as the hybrid NW network samples (on TEM grids) were pulsed laser annealed with 0.8 MW cm-2 and 2.6 MW cm-2 laser power densities (corresponding to the lowest and highest laser power used in this study). Samples were laser treated in a vacuum chamber with ~ 10-5 torr pressure. Finally, all laser annealed samples were further TEM imaged with similar measurement conditions. Electrical sheet resistance was measured using a four-probe measurement set up with two far end leads measuring the current flow and middle two leads measuring the voltage. Optical transmittance spectra and scattering were measured by a Perkin Elmer Lambda 950 ultraviolet-visible and infrared (UV-Vis-IR) spectrophotometer. MD SIMULATION: To make it simple and to avoid complexity of involvement of large number of atoms, a small subset of the entire sample was considered. The MD simulation by Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS)

47

was run on Radon

cluster computer at Purdue University. Our simulation consisted of three materials: substrate (quartz), silver nanowire consisted of silver atoms with face centered cubic (FCC) crystal structure, and graphene with honeycomb lattice structure with carbon atoms. For nanowire and graphene, the initial structures were created using MATLAB program. The adaptive intermolecular reactive empirical bond order (AIREBO)48 potential was used for interactions of carbon atoms in single layer of graphene. To suppress spuriously long atomic interactions of carbon atoms, the cutoff distance is set to be 2.0 Å. EAM potential43, 44 produced from Williams et al.’s work49 extracted from Interatomic Potentials Repository Project50 has been used for silver. The interactions of carbon-silver, carbon-quartz, and silver-quartz have been defined by utilizing the standard Lennard-Jones (LJ) potential. LJ parameters are presented in Table 1. All


ACS Nano

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 values have been extracted using Lorenz-Berthelot (LB) combination rule and LJ parameters used for uniform phase: 1 "# = $ ' (" + "# ); +# = √+ +# 2 Where index denotes the parameter involving two different atom types of i and j, while i/j indicates a parameter for single atom type of i or j. σ and ε are parameters for LJ potential: " " 4 - = 4+[0 2 − 0 2 ] 1 1 Where ε is the depth of the potential well, r is the inter-atomic separation and σ is the finite distance at which the inter-atomic potential becomes zero. The reason behind the use of LJ pair potential for non-uniform contacts lies in the ease of access, saving simulation time, and sufficiently enough precision for the present purpose. Graphene was fixed onto the surface of the substrate and away from nanowire surface (as part of the initial condition set up as illustrated in figure 8f, g and h). Also periodic boundary condition in planar directions was applied for all the simulations. Starting with a randomly distributed initial velocity proﬁle, equivalent to 300 K, canonical ensemble (NVT) was implemented for 50 ps with a time step of 0.5 fs to allow the system to reach its equilibrium conﬁguration. Then kinetic energy equivalent to 200 µJ/s heating rate has been added to silver atoms with microcanonical ensemble (NVE). Graphene was assumed to be transparent to the laser irradiation and so no initial heating was assigned to graphene in case of hybrid nanowire sample. A prescribed loading force of 0.0001 eV/Angstrom vertical to the substrate plane was applied to graphene atoms in equilibrium phase of simulation, to prevent the structure from flying off due to lack of contact force in initial configuration. This assumption matches with experimental


Page 20 of 38

Page 21 of 38

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

ACS Nano

characteristics in which graphene completely attached to substrate before laser irradiation. Also for the last set of simulations (figure 10), to consider cooling effect during irradiation, a viscous function (acting as a drag force opposing the speed of moving atoms) with a parameter of 0.0001 eV/picosecond has been implemented.


ACS Nano

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

Page 22 of 38

TABLES: Table 1: LJ potential parameters used in the simulation of the hybrid NW network structure

Interaction C-Ag C-Si C-C Ag-Si

ε(eV) 0.031 0.007 0.0035 0.074

σ(Ȧ) 2.3 3.652 3.5 3.177

Conflict of interest: The authors declare no competing financial interest. Acknowledgement This work is supported by National Science Foundation (NSF) award number ECCS-1408346 (D.B.J. and M.A.A.), National Research Council Senior Research Associateship (G.J.C.), and NSF grants CMMI-0547636, CMMI 0928752(G.J.C.). We thank Doosan Back and Shengyu Jin for helping in TEM experiments. We thank Dr. Yong P. Chen (Department of Physics and Astronomy, Purdue University) for providing Raman spectroscopy measurement facility for measurements on hybrid graphene – silver nanowire network. Author contributions S.R.D. conceived the idea and discussed with D.B.J, M.A.A and G.J.C; S.R.D., Q. N. and P. K. designed and conducted the experiments and acquired the experimental data, M. S. performed the MD simulations and acquired the data. S.R.D and Q. N. took part in the discussions and manuscript writing; G.J.C. oversaw the project.


Page 23 of 38

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

ACS Nano

Figure captions Figure 1 (a) Schematic diagram of the experimental set-up for laser annealing using a KrF pulsed-laser system at 248nm. Focusing lens and a pulse shaper were used to focus the beam on the sample surface as well as to shape the ultra-short pulses. Two regions on the sample with similar nanowire densities are studied: a network of silver nanowires and a hybrid structure consisting of a silver nanowire network covered by single layer graphene. Fabrication of both a nanowire network and a hybrid network on the same substrate (quartz) is essential to evaluate the role of graphene in high density laser irradiation of metal nanostructures. (b)Scanning Electron Micrographs of nanowire network following pulsed laser irradiation with 2.6 MW/cm2power density;(c) FESEM images of hybrid network taken after pulsed laser treatment with 2.6 MW/cm2powerdensity. Figure 2 FESEM of silver nanowires after laser irradiation with intensity of 0.8 MW cm-2: (a) shows comparison between NW network region and hybrid NW network region (note the difference in the image contrast between the two regions). (b) and (c) indicate high magnification FESEM view of two randomly locatedhybrid network regions. (d) indicates high magnification FESEM view of a representativerandomly locatednanowire networkregion. Figure 3 FESEM images ofnetwork structures after laser irradiation at indicated power densities, showingthe effect ofgraphene barrier;(a) – (b) hybrid network and NW network, respectively, at 1.4 MW cm-2 ; (c) – (d) hybrid network and NW network, respectively, at2 MW cm-2; and (e) – (f) hybrid network and NW network, respectively,at2.6 MW cm-2; (g) Schematic sketch of the bare silver nanowires developing necking and fragmentation leading to shorter segments and finally decayedto a chain of silver-beads during intense laser beam irradiation. Broken graphene sheet in (a), (c), and (e) due to either the presence of crowded junctions underneath or due to very high energy induced cutting of graphene at nanowire edges are shown by the arrow marks. Figure 4 Effect of graphene barrier on silver nanowire network fabricated on flexible PET substrate (DupontTejin film): (a) and (b) SEM Micrographsfor hybrid network and NW network, respectively, after laser irradiation with power density of 0.8 MW cm-2; (c) and (d) are images of hybrid and NW networks, respectively, after laser irradiation with power density of 2 MW cm-2. Figure 5 Representative Transmission Electron Microscope (TEM) study of the pulsed laser irradiation induced thermal damages of Ag NW network and hybrid NW network: (a) – (b) TEM images of as-prepared hybrid and NW


ACS Nano

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

Page 24 of 38

network samples respectively. SLG shown is damage-free after the standard transfer procedure on to a TEM grid. The hybrid and NW network have been prepared on TEM grids following a similar procedure to the TCE samples; (c) – (d) Corresponding TEM images of hybrid and NW network samples respectively after laser irradiation with laser power density of 0.8 MW cm-2; (e) – (f) Representative TEM images of hybrid network and (g) NW network after laser irradiation with laser power density of 2.6 MW cm-2. Both damage free and laser induced thermal damages are seen in figure (e) and (f) respectively. In the absence of graphene the entire NW turns in to linear chain of beads as shown in figure (g). Figure 6 Comparison of the sheet resistances of the hybrid NW network sample and NW network sample after laser irradiation, measured using four probe method. Nanowire areal density of ~ 2x106 NWs cm-2 was used for theseparticular samples. The sheet resistance of the hybrid and NW samples measured before the laser exposure are539 Ω/ and 2.064 kΩ/ respectively. The laser energy dependence on sheet resistances shows the optimal power densityat 1.4 MW cm-2for the hybrid and beyond 2 MW cm-2 the resistance increases drastically. For the NW network sample, however, the sheet resistance increases and beyond 1.4 MW cm-2the resistance becomes completely open. This clearly supports the results from FESEM imaging. This is further supported by the discontinuous droplet structures of the NWs as observed in FESEM images. Figure 7 (a) UV-Vis-IR transmittance measurement of bare silver nanowire network before and after laser irradiation at power density of0.8 MW cm-2. The increased transmittance after treatment might be ascribed to nanowire material loss during irradiation; (b) UV-Vis-IR transmittance measurement of hybrid network before and after irradiation, indicating no material loss;(c) shows optical transmittance of SLG (on quartz substrate) after subjected to laser irradiation with various power densities. The transmittance remains unchanged up to 0.8 MW cm2

, while 2 MW cm-2treated CVD SLG drastically reduces the transmittance implying severe damage and/or defect

generation at high energy density of laser treatment. Figure 8 MD simulation of irradiation on silver nanowires in three cases: (i) bare AgNW(a) initial bare silver nanowire on quartz, (b) NW after laser irradiation with an input heating rate of 200 µJ/s and heating duration of 250 ps; (ii) AgNW with graphene wrapping freely(c) Single layer graphene andAgNWbefore the graphene set for wrapping (initial status), (d) after equilibrium self-wrapping, (e) graphene-wrapped silver nanowire after pulsedlaser heating; (iii) AgNW with graphene wrapping and with graphene boundary fixed to the substrate:(f) initial


Page 25 of 38

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

ACS Nano

configuration, (g) after equilibrium wrapping of graphene over AgNW, (h) graphene-wrapped silver nanowire after pulsed-laser heating. Figure 9 Silver atoms’average kinetic energy as a function of irradiation heating timein (a)NW network and hybrid network cases where silver atoms are considered at the center of nanowires; (b) NW network and hybrid network cases where silver atoms are considered at thetop surface of the nanowires; inserts show schematic target position of silver atoms analyzed; (c) graphene lattice kinetic energy as a function of simulation run time, the heating of graphene was achieved by phonon transferring from metal atoms to graphene lattice; (d) histogram of lost silver atoms at the end of simulation. This occurs due to evaporation of silver atoms observed in a simulation run time of 250 ps. Figure 10 Power density dependence of AgNWs and hybrid networksunder pulsed laser irradiation. Bare AgNWs under laser power density of (a) 0.8 MW cm-2, (b) 1.4 MW cm-2, (c) 2 MW cm-2, (d) 2.6 MW cm-2 were shown. Very small evaporation occurs at 0.8 MW cm-2 for bare nanowires; while increasing in laser power density, gradual increase in evaporation of silver atoms is clearly visible. Hybrid NWs network under laser power density of (e) 0.8 MW cm-2, (f) 1.4 MW cm-2, (g) 2 MW cm-2, (h) 2.6 MW cm-2 were shown. Graphene, owing to its larger surface area with exceptionally high in-plane thermal conductivity, helps extracting the NW thermal energy, leaving the system unperturbed; the hybrid structure remains almost intact up to2.6 MW cm-2, with NWs underneath graphene retaining their shape and size.partial damage of graphene causes the silver atom evaporation and thus forming damages in the structure at 2 MW cm-2and2.6 MW cm-2. (i) Comparison of average silver atoms loss in both cases (bare AgNWs and hybrid NW network) as a function of laser power density.


ACS Nano

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 1


Page 26 of 38

Page 27 of 38

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

ACS Nano

Figure 2


ACS Nano

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


Page 28 of 38

Page 29 of 38

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

ACS Nano

Figure 4


ACS Nano

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 5


Page 30 of 38

Page 31 of 38

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

ACS Nano

Figure 6


ACS Nano

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 7


Page 32 of 38

Page 33 of 38

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

ACS Nano

Figure 8


ACS Nano

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 9


Page 34 of 38

Page 35 of 38

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

ACS Nano

Figure 10


ACS Nano

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

Page 36 of 38

REFERENCES AND NOTES

1 2 3 4

5 6 7 8 9 10

11 12

13

14 15 16 17

18 19

20

Kroto, H. W.; Heath, J. R.; O'Brien, S. C.; Curl, R. F.; Smalley, R. E. C60: Buckminsterfullerene. Nature 1985, 318, 162-163. Iijima, S. Helical Microtubules of Graphitic Carbon. Nature 1991, 354, 56-58. Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666-669. Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Katsnelson, M. I.; Grigorieva, I. V.; Dubonos, S. V.; Firsov, A. A. Two-Dimensional Gas of Massless Dirac Fermions in Graphene. Nature 2005, 438, 197-200. Zhang, Y.; Tan, Y.-W.; Stormer, H. L.; Kim, P. Experimental Observation of the Quantum Hall Effect and Berry's Phase in Graphene. Nature 2005, 438, 201-204. Geim, A. K.; Novoselov, K. S. The Rise of Graphene. Nat. Mater. 2007, 6, 183-191. Castro Neto, A. H.; Guinea, F.; Peres, N. M. R.; Novoselov, K. S.; Geim, A. K. The Electronic Properties of Graphene. Rev. Mod. Phys. 2009, 81, 109-162. Schedin, F.; Geim, A. K.; Morozov, S. V.; Hill, E. W.; Blake, P.; Katsnelson, M. I.; Novoselov, K. S. Detection of Individual Gas Molecules Adsorbed on Graphene. Nat. Mater. 2007, 6, 652-655. Lin, Y. -M.; Avouris, P. Strong Suppression of Electrical Noise in Bilayer Graphene Nanodevices. Nano Lett. 2008, 8, 2119-2125. Bae, S.; Kim, H.; Lee, Y.; Xu, X.; Park, J-S.; Zheng, Y.; Balakrishnan, J.; Lei, T.; Kim, H. R.; Song, Y. I. et al. Roll-To-Roll Production of 30-Inch Graphene Films for Transparent Electrodes. Nat. Nanotechnol. 2010, 5, 574-578. Liu, C.; Yu, Z.; Neff, D.; Zhamu, A.; Jang, B. Z. Graphene-Based Supercapacitor with an Ultrahigh Energy Density. Nano Lett. 2010, 10, 4863-4868. Stankovich, S.; Dikin, D. A.; Dommett, G. H. B.; Kohlhaas, K. M.; Zimney, E. J.; Stach, E. A.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Graphene-based Composite Materials. Nature 2006, 442, 282286. Zhao, Y.; Xie, Y.; Hui, Y. Y.; Tang, L.; Jie, W.; Jiang, Y.; Xu, L.; Lau, S. P.; Chai, Y. Highly Impermeable and Transparent Graphene as an Ultra-Thin Protection Barrier for Ag Thin Films. J. Mater. Chem. C 2013, 1, 4956-4961. Topsakal, M.; Şahin, H.; Ciraci, S. Graphene Coatings: an Efficient Protection from Oxidation. Phys. Rev. B 2012, 85, 155445. Sutter, E.; Albrecht, P.; Camino, F. E.; Sutter, P. Monolayer Graphene as Ultimate Chemical Passivation Layer for Arbitrarily Shaped Metal Surfaces. Carbon 2010, 48, 4414-4420. Prasai, D.; Tuberquia, J. C.; Harl, R. R.; Jennings, G. K.; Bolotin, K. I. Graphene: CorrosionInhibiting Coating. ACS Nano 2012, 6, 1102-1108. Li, J.; Wang, G.; Zhu, H.; Zhang, M.; Zheng, X.; Di, Z.; Liu, X.; Wang, X. Antibacterial Activity of Large-Area Monolayer Graphene Film Manipulated by Charge Transfer. Sci. Rep. 2014, 4, article number: 4359. doi: 10.1038/srep04359 Yoon, T.; Mun, J. H.; Cho, B. J.; Kim, T. -S. Penetration and Lateral Diffusion Characteristics of Polycrystalline Graphene Barriers. Nanoscale 2014, 6, 151-156. Zhang, W.; Lee, S.; McNear, K. L.; Chung, T. F.; Lee, S.; Lee, K.; Crist, S. A.; Ratliff, T. L.; Zhong, Z.; Chen, Y. P.; Yang, C. Use of Graphene as Protection Film in Biological Environments. Sci. Rep. 2014, 4, article number: 4097; doi: 10.1038/srep04097 Kravets, V. G.; Jalil, R.; Kim, Y. –J.; Ansell, D.; Aznakayeva, D. E.; Thackray, B.; Britnell, L.; Belle, B. D.; Withers, F.; Radko, I. P. et al. Graphene-Protected Copper and Silver Plasmonics. Sci. Rep. 2014, 4, article number: 5517; doi: 10.1038/srep05517


Page 37 of 38

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

ACS Nano

21

22

23 24

25

26 27 28 29

30

31

32

33

34

35

36 37 38

Chen, R.; Das, S. R.; Jeong, C.; Khan, M. R.; Janes, D. B.; Alam, M. A. Co-Percolating GrapheneWrapped Silver Nanowire Network for High Performance, Highly Stable, Transparent Conducting Electrodes. Adv. Funct. Mater. 2013, 23, 5150-5158. Lee, M. -S.; Lee, K.; Kim, S. Y.; Lee, H.; Park, J.; Choi, K. –H.; Kim, H. –K.; Kim, D. –G.; Lee, D. –Y.; Nam, S.; Park, J. –U. High Performance, Transparent and Stretchable Electrodes Using GrapheneMetal Nanowire Hybrid Structures. Nano Lett. 2013, 13, 2814-2821 Mehta, R.; Chugh, S.; Chen, Z. Enhanced Electrical and Thermal Conduction in GrapheneEncapsulated Copper Nanowires. Nano Lett. 2015, 15, 2024-2030. Maize, K.; Das, S. R.; Sadeque, S.; Mohammed. A. M. S.; Shakouri, A.; Janes, D. B.; Alam, M. A. Super-Joule Heating in Graphene and Silver Nanowire Network. Appl. Phys. Lett. 2015, 106, 143104. Bao, Q.; Zhang, H.; Wang, Y.; Ni, Z.; Yan, Y.; Shen, Z. X.; Loh, K. P.; Tang, D. Y. Atomic-Layer Graphene as a Saturable Absorber for Ultrafast Pulsed Lasers. Adv. Funct. Mater. 2009, 19, 3077-3083. Hong, S. K.; Kim, K. Y.; Kim, T. Y.; Kim, J. H.; Park, S. W.; Kim, J. H.; Cho, B. J. Electromagnetic Interference Shielding Effectiveness of Monolayer Graphene. Nanotechnology 2012, 23, 455704. Currie, M.; Caldwell, J. D.; Bezares, F. J.; Robinson, J.; Anderson, T.; Chun, H.; Tadjer, M. Quantifying Pulsed Laser Induced Damage to Graphene. Appl. Phys. Lett. 2011, 99, 211909. Sahin, R.; Simsek, E.; Akturk, S. Nanoscale Patterning of Graphene Through Femtosecond Laser Ablation. Appl. Phys. Lett. 2014, 104, 053118. Nian, Q.; Zhang, M. Y.; Wang, Y.; Das, S. R.; Bhat, V. S.; Huang, F.; Cheng, G. J. Charge Carrier Transport and Collection Enhancement of Copper Indium Diselenide Photoactive NanoparticleInk by Laser Crystallization. Appl. Phys. Lett. 2014, 105, 111909. Nian, Q.; Callahan, M.; Look, D.; Efstathiadis, H.; Bailey, J.; Cheng, G. J. Highly Transparent Conductive Electrode with Ultra-Low Haze by Grain Boundary Modification of Aqueous Solution Fabricated Alumina-Doped Zinc Oxide Nanocrystals. APL Mater. 2015, 3, 062803. Nian, Q., Zhang, M. Y., Schwartz, B. D. & Cheng, G. J. Ultraviolet Laser Crystallized Zno:Al Films on Sapphire with High Hall Mobility for Simultaneous Enhancement of Conductivity and Transparency. Appl. Phys. Lett. 2014, 104, 201907. Garnett, E. C.; Cai, W.; Cha, J. J.; Mahmood, F.; Connor, S. T.; Christoforo, M. G.; Cui, Y.; McGehee, M. D.; Brongersma, M. Self-limited Plasmonic Welding of Silver Nanowire Junctions. Nat. Mater. 2012, 11, 241-249. C. B. Arnold. Plasmonic-Enhanced Pulsed-Laser Welding of Metal Nanowire Networks for Transparent Conducting Layers. International Photonics and Optoelectronics Meetings, OSA Technical Digest (online) (Optical Society of America, 2012), paper MTh3C.1. Nian, Q.; Wang, Y.; Yang, Y.; Li, J.; Zhang, M. Y.; Shao, J.; Tang, L., Cheng, G. J. Direct Laser Writing of Nanodiamond Films from Graphite under Ambient Conditions. Sci. Rep.2014, 4, article # 6612; doi:10.1038/srep06612. Karim, S.; Toimil-Molares, M. E.; Balogh, A. G.; Ensinger, W.; Cornelius, T. W.; Khan, E. U.; Neumann, R. Morphological Evolution of Au Nanowires Controlled by Rayleigh Instability. Nanotechnology 2006, 17, 5954-5959. Sun, Y.; Mayers, B.; Xia, Y. Transformation of Silver Nanospheres into Nanobelts and Triangular Nanoplates through a Thermal Process. Nano Lett. 2003, 3, 675-679. Bid, A.; Bora A.; Raychaudhuri, A. K. Observation of Large Low-Frequency Resistance Fluxtuations in Metallic Nanowires: Implications on Its Stability. Phys. Rev. B 2005, 72, 113415. Mansky, P.; Allemand, P. Silver Nanostrcture-Based Optical Stacks and Touch Sensors with Uv Protection. US Patent WO2015048293 A1 (2014)


ACS Nano

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

39 40 41

42 43 44 45 46 47 48 49 50

Page 38 of 38

Srinivasan, R.; Mayne -Banton, V. Self-Developing Photoetching of Poly(Ethylene Terephthalate) Films by Far-Ultraviolet Excimer Laser Radiation. Appl. Phys. Lett. 1982, 41, 576-578. Ilcisin, K. J.; Fedosejevs, R. Direct Production of Gratings on Plastic Substrates Using 248-Nm Krf Laser Radiation. Appl. Opt. 1987, 26, 396-400. Hu, A.; Deng, G. L.; Courvoisier, S.; Reshef. O.; Evans, C. C.; Mazur, E.; Zhou, Y. Femtosecond Laser Induced Surface Melting and Nanojoining for Plasmonic Circuits. Proc. SPIE 2013, 8809, Plasmonics: Metallic Nanostructures and Their Optical Properties XI, 880907 doi: 10.1117/12.2022482 Spechler, J.; Arnold, C. Direct-Write Pulsed Laser Processed Silver Nanowire Networks for Transparent Conducting Electrodes. Appl. Phys. A 2012, 108, 25-28. Foiles, S. M.; Baskes, M. I.; Daw, M. S. Embedded-Atom-Method Functions for the Fcc Metals Cu, Ag, Au, Ni, Pd, Pt, and Their Alloys. Phys. Rev. B 1986, 33, 7983-7991. Daw, M. S.; Baskes, M. I. Embedded-Atom Method: Derivation and Application to Impurities, Surfaces, and Other Defects in Metals. Phys.Rev. B 1984, 29, 6443-6453. Balandin, A. A.; Ghosh, S.; Bao, W.; Calizo, I.; Teweldebrhan, D.; Miao, F.; Lau, C. N. Superior Thermal Conductivity of Single-Layer Graphene. Nano Lett. 2008, 8, 902-907. Pop, E.; Varshney, V.; Roy, A. K. Thermal Properties of Graphene: Fundamentals and Applications. MRS Bull. 2012, 37, 1273-1281. Plimpton, S. Fast Parallel Algorithms for Short-Range Molecular Dynamics. J. Comput. Phys. 1995, 117, 1-19. Stuart, S. J.; Tutein, A. B.; Harrison, J. A. A Reactive Potential for Hydrocarbons with Intermolecular Interactions. J. Chem. Phys. 2000, 112, 6472-6486. Williams, P. L.; Mishin, Y.; Hamilton, J. C. An Embedded-Atom Potential for the Cu–Ag System. Modell. Simul. Mater. Sci. Eng. 2006, 14, 817. Becker, C. A.; Tavazza, F.; Trautt, Z. T.; Buarque de Macedo, R. A. Considerations for Choosing and Using Force Fields and Interatomic Potentials in Materials Science and Engineering. Curr. Opin. Solid State Mater. Sci. 2013, 17, 277-283.

Supporting Information Available: SLG and hybrid NW network structural characterization, MD Simulation approach, Laser power density estimation. This material is available free of charge via the Internet at http://pubs.acs.org


Single-Layer Graphene as a Barrier Layer for Intense UV Laser

Recommend Documents