Free-Standing Monolayered Metallic Nanoparticle Networks as

Letter www.acsami.org

Free-Standing Monolayered Metallic Nanoparticle Networks as Building Blocks for Plasmonic Nanoelectronic Junctions Haoxi Wu,†,‡,§ Chuanping Li,†,‡,§ Zhenlu Zhao,‡,§ Haijuan Li,‡ and Yongdong Jin*,‡ ‡

State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 130022 Jilin, China § University of Chinese Academy of Sciences, 100049 Beijing, China S Supporting Information *

ABSTRACT: The effective coupling of optical surface plasmons (SPs) and electron transport in a plasmonic-electronic device is one of the fundamental issues in nanoelectronics and the emerging field of plasmonics, and offer promise in providing a solution to next generation nanocircuits in which all coupling is in the near field. Attempts toward this end, however, are limited because of the integration challenge to compatible nanodevices. To date, direct electrical detection of SP-electron coupling from metallic nanostructures alone are not reported, and thus it remains a great experimental challenge. In this paper, we succeed in preparing a new suspended-filmtype nanoelectronic junction, in which free-standing 2D fractal nanoparticle networks act as plasmonically active nanocomponents. Direct electrical detection of optical collective SPs was evidenced by photocurrent response of the junction upon illumination. Room-temperature I−V characteristics, differing from nonlinear to Ohmic behaviors, are found to be sensitive to the nanometer-scale morphology changes of the nanomembranes. The finding and approach may enable the development of advanced plasmonic nanocircuits and new nanoelectronics, nanophotonics, and (solar) energy applications. KEYWORDS: free-standing, plasmonics, nanoelectronics, junction, photoresponse

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transport properties of such membranes are rather rare21 because measurement of the substrate-free transport properties of free-standing 2D nanomembranes requires innovative techniques, which remains a great experimental challenge, to manipulate them and integrate them into active solid-state devices. Very recently, Liao and co-workers21 reported first electrical measurements of monolayered free-standing nanoparticle sheets suspended over trenches, and found the scaling exponent to be significantly different from that of substratesupported nanoparticle arrays prepared by the same technique. Unfortunately, in their studies the I−V characteristic of the nanomembranes showed only ohmic behavior and no any optical plasmonic effects were examined at room temperature. Here, we report on preparing a new suspended-film-type nanoelectronic junction, in which free-standing 2D fractal nanoparticle networks act as plasmonically active nanocomponents. Very interestingly, room-temperature I−V characteristics of the junctions, differing from nonlinear to Ohmic behaviors, are found sensitive to the nanometer-scale morphology changes of the nanomembranes. And direct electrical detection of optical collective SPs was evidenced by photocurrent response of the junction upon light illumination.

ntegrating plasmonics with current-carrying nano/molecular electronic devices offer great promise and is a key step toward “dark” optoplasmonic nanocircuits in which all coupling is in the near field.1−4 Attempts toward this end, however, are limited 1,2,4−10 because of the integration challenge to compatible nanodevices, as well as a lack of fundamental understandings of plasmonic-electronic coupling.11,12 To date, surface plasmons (SPs) were used to control the transport properties of molecular junctions/devices,5−8 and electrical detection of plasmons in a photodiode,1 nanogap9,10 or nanowire cross junction,2 with built-in electro-optical transducers/photodetector, has been demonstrated. However, direct nanometer-scale electrical detection of confined optical SPs from metallic nanostructures alone does not show any measurable photoresponse,10 and thus it remains a great experimental challenge. Among other plasmonic nanostructures, free-standing twodimensional ultrathin nanoparticle arrays or membranes (nanomembranes) exhibit excellent mechanical stability, flexibility, functionality, and customizability, making them ideal for a broad spectrum of applications ranging from sensing, separation, nanoelectronics/-photonics, to energy and catalysis.13 Hitherto, free-standing 2D nanomembranes have been successfully fabricated by various bottom-up assembly approaches.13−22 Although free-standing 2D nanomembranes are an ideal material that enabling the study of substrate-free optical and electrical properties,13 electrical studies on the © XXXX American Chemical Society

Received: December 3, 2015 Accepted: January 7, 2016

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DOI: 10.1021/acsami.5b11805 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Letter

ACS Applied Materials & Interfaces

mainly determined by the dimensions of colloid Ag precursors (Figure S1). The resultant snowflake-like, free-standing nanomembranes, typically on the millimeter to centimeter scale, could be closely seen by the naked eye. The quality and morphology of the resulting particles and nanomembranes can be easily controlled by variables reaction (boiling) time. The suspended-film-type junction structures were then completed by facile transfer of the floating nanomembranes from the “soft” air−water interface onto two microgapped Au trench electrodes. The as-prepared junction configuration and the corresponding suspended nanomembranes were characterized by optical microscopy and scanning electron microscopy (SEM). The optical photograph (Figure 2a) and low-magnification SEM image (Figure 2b) showed that the membrane is continuous, uniform and open-structured, with snowflake-like fractal patterns, bridging over Au trench electrodes with ∼180 μm gap. Figure 2c showed that the membranes were networked by close-packed nanoparticles with a low packing density, which is typical for nanoparticle monolayer derived from interfacial selfassembly via diffusion-limited aggregation.14 The shell-type structure nature of the nanoparticle units was also clearly revealed by dark-field TEM since atoms of a higher atomic number (Ag/Au) in the shells appear brighter in the image. In addition to shell-type structure nature of the particles, high magnification TEM images in Figure S2, showed that surfaceroughened nanoshells, with an overall diameter of approximately 60−80 nm and shell thickness of ca. 10−15 nm, linked together through narrow metallic “junctions”, which is spontaneously formed via a proximity-induced electroless soldering process during the sample preparation at the presence of HAuCl4 and NH2OH14 to form a robust and flexible networks. A tilted SEM image in Figure 2d (and cf. SEM image in Figure 2b) showed that the free-standing nanomembrane remained intact after floating transfer and is mechanically stiff enough to suspend over the microscale trench for electrical measurements. As shown in Figure 2e−h, the alloyed Ag/Au bimetallic nature of the shell-type nanoparticles of the membrane was evidenced by both TEM elemental mapping and cross-sectional compositional line-scan energy dispersive Xray spectroscopy (EDS) analysis, which are two typically accepted characterizations revealing the core−shell structures and elemental distributions of nanostructures.23 The SP band of plasmonic nanostructures has been found to be sensitive to various factors, including particle size, shape, surrounding media, and interparticle interactions.24 We therefore thoroughly characterized the unique optical properties of the resulting plasmonic nanostructures, both in solution and in dry monolayer form. The optical properties of the dry nanomembranes were found distinct from that of parent nanostructures in solution. Figure 2i shows the extinction spectra of the typical colloidal solution of shell-type Ag/Au bimetallic nanoparticles and their monolayered dry films (on a clean glass slide) corresponding to the optical/SEM images shown in Figure 2a, b. Suspended colloidal Ag/Au bimetallic nanoparticles display shell-type NIR absorption25 with a SP band centered at ∼690 nm. Interestingly, the broadened SP band of the monolayered dry nanomembrane, unlike that of aggregates or cross-linked thin films of solid nanoparticles where red-shift often occurs, blue-shifted to ∼600 nm. The optical, SEM and TEM images of a typical device (device a) and nanomembrane are shown in Figure 3 (a1, a2, and a3 inset), of which the nanomembranes are prepared with

Previously we succeeded in preparing free-standing 2D fractal nanoparticle films (networks) composed of cross-linked, hollowed silver/gold bimetallic nanoshells by diffusion-limited aggregation at the air−water interface.14 With the aim of integrating plasmonics with current-carrying nanoelectronic devices, we explore in this letter a facile solution-processed approach for the preparation of a new type of plasmonic nanoelectronic junctions. Specifically, freshly prepared, freestanding 2D nanoparticle networks, acting as plasmonically active components, were assembled simply by floating transfer of them gently from the “soft” air−water interface onto two microgapped gold electrodes to form a suspended-film-type junction, as schematically illustrated in Figure 1. Details of the sample preparation and device fabrication are described in the Materials and Methods in the Supporting Information.

Figure 1. (a) Schematic of free-standing nanoparticle networks (nanomembranes) transfer processes. Macroscopic free-standing 2D fractal nanoparticle networks were spontaneously formed at the air− water interface via a process of diffusion-limited aggregation during sample preparation, and then transferred gently by floating transfer from the “soft” air−water interface onto two Au electrodes. (b) Scheme of the suspended-film-type junction, in which the suspended 2D nanoparticle networks acting as plasmonically active components for current transport. This figure is not drawn to scale.

The advantages of the junction configuration over previous ones are obvious. First, our method is based on a simple solution-based approach, allowing the fabrication of flexible nanocircuits with no substrate restrictions. Second, the freestanding monolayered nanoparticle networks bridging over microscale trenches enable the study of substrate-free optical and electrical properties. Finally, because the bimetallic nanoparticle networks are formed by soldering together of surface-roughened individual nanoparticles through metallic “junctions” during sample preparation14 without the addition of additional molecular ligands/linkers, it is possible to direct probe plasmonic-electronic coupling/effects from metallic nanostructures alone (without molecule effects). Free-standing nanomembranes were prepared according to our method described previously14 with minor modifications. In a typical synthesis, 500 μL of 0.1% HAuCl4 aqueous solution was added in reaction solution to produce nanomembranes. The as-prepared nanomembranes consist of close-packed hollowed nanoshells, of which the core diameter (void size) B


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Figure 2. (a) Typical optical microscope image, (b, c) low-magnification SEM image, and (d) a tilted SEM image of the free-standing nanomembrane and device configuration; (e) a dark-field TEM image of the networked shell-type nanoparticles and corresponding nanoscale TEM elemental mappings of (f) Ag and (g) Au, respectively; (h) elemental distribution along a single shell-type nanoparticle indicated by the scan line in part e; (i) typical UV−vis spectra of shell-type bimetallic nanoparticles in solution and when assembled into monolayer networks and transfer them onto a glass substrate (dry film).

Figure 3. Typical successive “zoom in” optical (a1), SEM (a2), and TEM (a3, inset) images of device a made from the nanomembrane prepared with 8 min of boiling time, and the corresponding room-temperature raw I−V curves (a3) measured at ambient conditions over a ± 1 V bias range. (b1− b3, c1−c3, and d1−d3) Typical successive “zoom in” optical, SEM and TEM images and the corresponding room-temperature raw I−V curves of devices b, c, and d, prepared with 12 min (b1−b3), 20 min (c1−c3), and 30 min (d1−d3) of reaction boiling time, respectively. Pronounced nanoscale morphology changes of the three samples were clearly revealed by close SEM/TEM observations, and its strong effects on I−V characteristics were manifested by one-to-one measurements.

researchers,21,26 room-temperature observation of Coulomb blockade is impossible in such big nanoparticles (or membranes) and devices like ours, which were fabricated by using either Au nanoparticle arrays26 or monolayered freestanding nanoparticle sheets.21 The nonlinear I−V character-

8 min of reaction boiling time. Figure 3a3 shows corresponding room-temperature current−voltage (I−V) curves of the junction, measured at ambient conditions over a ± 1 V bias range. Interestingly, the I−V curves show observable nonlinear characteristic and weak “steplike” wiggles. As reported by many C


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the same preparation conditions. Although a steplike increase of I was not observed, the nonlinear I−V characteristic and similar photoresponse of the junction (device b) were maintained (Figure 3b1), if a few-nanometer-scale metallic “junctions” between individual nanoparticles still exist (Figure 3b3, inset). As clearly shown in Figure 3 (c1, c2, d1, and d2), the longer the reaction boiling time (device c ∼ 20 min and device d ∼ 30 min) the denser monolayer and poorer quality of the nanomembranes become, and the individual shell-type nanoparticles are almost merging together into a dense, fiberlike networks (Figure 3c3-inset and Figure 3d3, inset). In this case, only metallic Ohmic behavior of the I−V curves was observed (Figure 3c3 and Figure 3d3). Because the only difference in fabricating our nanomembranes was the boiling time that varied to result in different fine morphology of the resulting nanomembranes, this nonlinear/linear transition is therefore a reflection of the changes in electron transport pathways of the resulting nanomembranes. As the case shown in Figure 3 (a and b, especially a1 and a3), because of low packing density and thereby limited current transport paths of the nanoparticle networks, the nonlinear I−V characteristics were obviously observed as tunneling effect occurs if current transported through only one or a few “nano-junctions” (acting as the tunneling gaps) that created between adjacent nanoparticles; while overgrowth of the nanomembranes will ruin up the nanojunction feature (Figure 3c, d), resulting in bulk-like metallic films and Ohmic I−V responses. Direct electrical detection of optical collective SPs of the suspended nanomembranes is demonstrated by illuminating the aforementioned robust suspended-film-type junction and recording simultaneously the coupled/enhanced photocurrent flowing through it. The nanomembranes of stable device a (Figure S3) and device b (Figure 4) present photoresponse in junction current. Upon irradiation with white light, the steadystate junction (device b) current increases from approximately −16.3 to −25.2 pA at a −1 V applied bias (Figure 4a). Once the light was blocked, the current decayed immediately to its original dark value. The system can be cycled between these two states by alternating turn on/off light (Figure 4b). The photocurrent response was found to be related to the existence of nanoplasmonic properties of the nanomembranes, as confirmed by the microscopy-based selected area dark-field scattering images and spectra of the corresponding free-

istics of our junction presumably result from both low packing density (and then very limited current transport paths) of the free-standing fractal nanoparticle networks and the tunneling barrier created by the narrow metallic “junctions” between individual shell-type nanoparticles. The observed weak “steplike” wiggles might be a reflection of tunneling through such narrow metallic “nano-junctions” and with asymmetric coupling strength to the two electrodes, rather than the Coulomb blockade effect. Measurements on the same devices through successive potential cycling show very reproducible I−V characteristics and indicate reliable stable contact formation. The asymmetric I−V curves presumably result from somewhat asymmetric random electrical contacts of the nanomembranes with the two microgapped Au trench electrodes. The contact effect was further manifested by the asymmetric current enhancement of the junctions upon irradiation (cf. Figure 4a)

Figure 4. (a) I−V curves of device b recorded in the dark and then with white light, (b) photocurrent response curves when the light is switched on and off at the voltage of −1 V.

since the asymmetry increases when they irradiated. Very interestingly, room temperature I−V characteristics, differing from nonlinear to Ohmic behaviors, were found sensitive to the nanometer scale morphology changes of the resultant nanomembranes during the one-to-one measurements. The quality and morphology of the resulting particles and nanomembranes can be effectively controlled by simply varying reaction (boiling) time, while keeping the others constant during sample preparations. And all measured devices fabricated with

Figure 5. (a, c) Optical images and (b, d) the corresponding dark-field images of nanomembranes which prepared with 8 min (device a) and 20 min (device d) of reacting boiling time. (e) Microscopy-based selected area dark-field scattering spectra of the suspended networked nanoparticles in devices a−d. D


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may still have a small, but nonignorable effect. Last but not least, the plasmonically induced LSPR field enhancement (upon light illumination), which could facilitate current transport via narrowing the tunneling gaps of the “nanojunctions” between the nanoparticles, may also have a nonignorable effect. Further investigations are needed to confirm such assumptions and finally ascertain the underlying physical mechanisms. We note that a few of previous reports on nanometer-scale electrical detection of optical SPs, e.g. in an optical nanoantenna-diode,1 a nanogap9,10 or a nanowire cross junction2 were conducted with built-in electro-optical transducers or photodetector. And direct electrical plasmon detection between gold electrodes separated by a subnm gap−results may varies in this case as the metal electrodes rearranged themselves during electrical measurements− was reported very recently,12 showing that nonlinear tunneling conduction between the gap leads to optical rectification and producing a d.c. photocurrent when the gap is illuminated. However, reliable direct electrical plasmon detection, for example from sub-10 nm scale Au break junctions without graphene (used as a localized photodetector), do not produce any measurable photocurrent response,10 and thus it remains a great experimental challenge. So this is an important report on direct electrical detection of optical collective SPs from freestanding 2D metallic nanomembranes alone in a junction configuration, without any additional built-in transducers or photodetector. In summary, we have developed a facile solution route to the preparation of monolayered, free-standing fractal nanoparticle networks, and measure their intrinsic transport properties in a suspended-film-type junction configuration. Our integrated device structure allows us to probe nm-scale morphologydependent transport behaviors and to direct electrical detection of optical collective SPs of the junctions at ambient conditions, which has not been achieved previously. This integration of plasmonics with current-carrying nanoelectronic devices offers great promise and enables the development of advanced plasmonic nanocircuits and new nanoelectronics and nanophotonics applications, ranging from photosensing and (solar) energy harvesting to optoplasmonic nanocircuits.1−3

standing nanomembranes (Figure 5). As clearly revealed in Figure 5e, the dark-field scattering spectra of the suspended networked nanoparticles in devices a&b that display nonlinear I−V characteristic (cf. Figure 3a, b) and photocurrent responses (cf. Figure 4) showed a pronounced SP band centered at ∼600−650 nm; while the suspended nanomembranes in devices c&d that display Ohmic I−V characteristic (cf. Figure 3c, d) and lacking of photoresponses (cf. Figure S4) showed very weak dark-field scattering spectra, due to the gradual loss of nanoplasmonic nature of the resulting nanomembranes (with the loss of small nanojunctions between adjacent nanoparticles along with the formation of dense and bulklike nanoparticle networks due to overgrowth). Therefore, both the fine nanomorphology and their corresponding extinction spectra of the resulting nanomembranes are believed to influence the photocurrents. Plasmonic effect of the photocurrent responses was further confirmed since remarkable difference was observed in the photocurrent of device b responsed as a function of excitation wavelength. As seen from Figure S5, illumination with green (λ ∼ 550 nm) and blue (λ ∼ 490 nm) light with comparable light intensity causes an increase in the conductivity over that of the dark current, and the current upon irradiation with green light (more spectral overlap with nanomembrane’s plasmon resonance wavelengths in the case) is detectably larger than that with blue light. It is therefore believed that the photoenhancement behavior that is observed in our device a and device b can be mainly attributed to the effective coupling/ conversion of optical collective SPs into electrical currents. Simultaneously, we have two satisfactory evidence to rule out the radiation heating effect caused by the laser source. First, in our I−V studies we used a light source with relatively low power density (100 mW/cm2) to illuminate the sample, and placed a heat filter before it to effectively absorb the heat radiation from the light (preventing the devices from getting hot during measurement). Second, the fact that no detectable photoresponse appears with both the blank electrodes (Figure S6) and linear junctions (cf. Figure S4, device c and d), as the disappearance of nanoplasmonic property in these cases (Figure 5), suggests that the effect of radiation heating from the light source might be negligible. The observed photocurrent enhancement is therefore attributed mainly to the underlying plasmonic effects of the nonlinear junctions. The underlying physical mechanism (origin) of the plasmonically induced photocurrent enhancement was further investigated by the illumination intensity-dependent I−V measurement. As shown in Figure S5, whereas the wavelength has an obvious influence on the IV characteristics, the irradiation power shows only (negligible) very small effect on photocurrent of the typical active device. It may seem surprisingly that the nominally dominant contribution of the photoinduced “hot electron”,27 whose effect on photocurrent is irradiation power-dependent, to the observed (photo)current enhancement in our system is weak. This is understandable, given that the plasmonically generated “hot electrons” (usually be excited optically), which can also be electrically excited by electron current28 in the current-carrying device, will contribute “hot electrons” photocurrent and directly overlay onto the (dark) junction current,29 making it separately undetectable. Although temperature increase on nanoparticles’ surfaces is roughly estimated to be well below 1 °C in our system (see details in the Supporting Information),30 plasmonically induced local thermal effect on junction current upon light illumination

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b11805. Experimental details and additional figures. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: 86-431-8526 2661. Tel: 86-4318526 2661. Author Contributions †

H.W. and C.L. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been financially supported by the National Natural Science Foundation of China (Grants 21475125, 21175125) and the Hundred Talents Program of the Chinese Academy of Sciences. E


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(21) Liao, J. H.; Zhou, Y. Z.; Huang, C. L.; Wang, Y.; Peng, L. M. Fabrication, Transfer, and Transport Properties of Monolayered Freestanding Nanoparticle Sheets. Small 2011, 7, 583−587. (22) Wu, H. X.; He, H. L.; Zhai, Y. J.; Li, H. J.; Lai, J. P.; Jin, Y. D. A Facile and Interfacial General Preparation of High-Performance Noble-Metal-Based Freestanding Nanomembranes by A Reagentless Self-Assembly Strategy. Nanoscale 2012, 4, 6974−6980. (23) Lim, B.; Wang, J.; Camargo, P. H. C.; Jiang, M.; Kim, M. J.; Xia, Y. N. Facile Synthesis of Bimetallic Nanoplates Consisting of Pd Cores and Pt Shells Through Seeded Epitaxial Growth. Nano Lett. 2008, 8, 2535−2540. (24) El-Sayed, M. A. Some Interesting Properties of Metals Confined in Time and Nanometer Space of Different Shapes. Acc. Chem. Res. 2001, 34, 257−264. (25) Oldenburg, S. J.; Averitt, R. D.; Westcott, S. L.; Halas, N. J. Nanoengineering of Optical Resonances. Chem. Phys. Lett. 1998, 288, 243−247. (26) Wessels, J. M.; Nothofer, H. G.; Ford, W. E.; von Wrochem, F.; Scholz, F.; Vossmeyer, T.; Schroedter, A.; Weller, H.; Yasuda, A. Optical and Electrical Properties of Three-Dimensional Interlinked Gold Nanoparticle Assemblies. J. Am. Chem. Soc. 2004, 126, 3349− 3356. (27) Baffou, G.; Quidant, R. Nanoplasmonics for Chemistry. Chem. Soc. Rev. 2014, 43, 3898−3907. (28) Schull, G.; Becker, M.; Berndt, R. Imaging Confined Electrons with Plasmonic Light. Phys. Rev. Lett. 2008, 101, 136801. (29) Conklin, D.; Nanayakkara, S.; Park, T. H.; Lagadec, M. F.; Stecher, J. T.; Chen, X.; Therien, M. J.; Bonnell, D. A. Exploiting Plasmon-Induced Hot Electrons in Molecular Electronic Devices. ACS Nano 2013, 7, 4479−4486. (30) Qin, Z. P.; Bischof, J. C. Thermophysical and Biological Responses of Gold Nanoparticle Laser Heating. Chem. Soc. Rev. 2012, 41, 1191−1217.

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Free-Standing Monolayered Metallic Nanoparticle Networks as

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