Switchable Silver Nanostructures Controlled with an Atomic Force

Article pubs.acs.org/JPCC

Switchable Silver Nanostructures Controlled with an Atomic Force Microscope S. Bakhti, S. Biswas,∥,⊥ C. Hubert, S. Reynaud, F. Vocanson, and N. Destouches* University of Lyon, F-42023 Saint-Etienne, France; CNRS, UMR 5516, Laboratoire Hubert Curien, 18 Rue Pr. Lauras, F-42000 Saint-Etienne, France; University of Saint-Etienne, Jean Monnet, F-42000 Saint-Etienne, France ABSTRACT: A process of creating controlled silver nanostructures using atomic force microscope technique is reported. The nanostructures are grown on both the silica surface and silica−indium-tin-oxide interface of a silver salt impregnated mesoporous silica film, deposited on indium tin oxide coated glass substrate. Voltage is applied between a conductive atomic force microscope tip and the indium tin oxide substrate to create the silver nanostructures. The possibility of switching the positions of the nanostructures between the silica film surface and the film−indium-tin-oxide interface, by reversing the electrical polarity of the tip, is demonstrated. It is also shown that the conductive channels can be created through the silica layer provided that the created metallic nanostructures are large enough to grow inside the film volume from one side to the other. The electrical conductivity of the film can be locally changed in a reversible manner by applying successively negative and positive voltages to the tip.

1. INTRODUCTION Under electromagnetic excitation, metal nanoparticles (NPs) exhibit localized surface plasmon resonances1 (LSPRs) that are due to a collective resonant oscillation of conduction electrons.2 The electric field of NPs at resonance is characterized by exalted radiative (scattering) and nonradiative (absorption) components,3 and by a near-field enhancement confined at the nanoscale.4,5 Noble metal NPs such as gold or silver are wellknown to exhibit LSPRs at optical frequencies,6 leading to a wide range of applications in biological and chemical sensing,7−9 surface-enhanced Raman scattering,10 or photovoltaics.11 Their spectral response strongly depends on the nature of the metal, the size, shape, and spatial arrangement of the NPs and on their dielectric environment.12,13 In many applications, precise control of these parameters is a major challenge for tuning the optical properties. Different scanning probe microscopy based methods have been developed to monitor the spatial arrangement of variously shaped metal NPs. Dip-pen nanolithography14,15 consists of depositing an ink containing already formed NPs or a metal salt on various surfaces through an atomic force microscope (AFM) tip, leading to nanosized metal patterns after post-treatment.16 Other approaches, without the use of ink, have been reported, where an ionic metal salt is directly included into a thin film and where NPs are created through an electrochemical process by applying an electrical potential between electrodes constituted by a conductive AFM tip and the substrate.17−20 Metal NPs have been created on hybrid sol−gel SiO2/TiO2 films,17 mesoporous silica films,18 and polymer matrix,19 where the size and shape have been tuned from circular particles of a © 2014 American Chemical Society

few nanometers in diameter to dendrites of few hundreds of nanometers wide by controlling the applied voltage and time of application. A controlled growth of silver and gold NPs has also recently been achieved on the surface of conductive Nb-doped TiO2 substrates.20 Scanning tunneling microscope (STM) tips are also used on mixed electronic-ionic conductors21,22 and on superionic conductors.23,24 Application of a voltage between these two electrodes leads, through a solid-state electrochemistry driven process, to silver nanocluster formation 24 and surface patterning.23 Special attention is paid to these materials because of the reversible formation of a nanometer sized silver bridge reported between the sample surface and the STM tip, opening a new approach to control electrical switches at atomic scale.21,24 In this paper, we demonstrate, for the first time to our knowledge, the possibility to create metallic nanostructures on both the TOP side (on silica surface) and BOTTOM side (at silica−indium-tin-oxide (ITO) interface) of a mesoporous silica film with an electrically biased AFM tip as depicted in Figure 1. We also show that the location of formation of the metallic nanostructures can be switched between the TOP and BOTTOM positions by reversing the polarity of the bias applied to the tip. This Article further demonstrates that conductive channels can be created through the silica layer provided that the created metallic nanostructures are large Received: February 5, 2014 Revised: March 13, 2014 Published: March 17, 2014 7494

dx.doi.org/10.1021/jp501268y | J. Phys. Chem. C 2014, 118, 7494−7500

The Journal of Physical Chemistry C

Article

Figure 1. Experimental setup: (a) negative and (b) positive voltage applied to the tip leads to the formation of metallic nanostructures on the TOP and BOTTOM positions, respectively.

Figure 2. Silver nanostructures grown in intermittent contact mode. (a) AFM topography image of the silver nanostructures grown with various voltages (mentioned in rows at the left-hand side of the image) and time of application (mentioned in columns at the top of the image). (b) SEM image of a silver dendrite obtained by applying −10 V for 4 s. (c) AFM topography image of aligned particles grown by applying −3 V for 200 ms. (d) Topography profile, measured along the red line, gives the full width (in nanometers) of each particle. (e) Example of a dendritic structure generated numerically using the DLA algorithm with 5000 particles. Horizontal scale bars correspond to (a) 1 μm, (b) 2 μm, and (c) 100 nm.

enough to grow in the film volume from one side to the other. The electrical properties of the film can be locally tuned and the film can be reversibly switched from conductive to insulator by applying negative and positive voltages to the tip, respectively.

then rinsed with MilliQ (MQ) grade water to remove the excess silver salt from the surface and dried under N2. The samples are finally stored in a high humidity environment (higher than 50% RH). Silver nanostructure inscriptions are performed by applying a bias voltage E between a conductive PtIr coated tip (ANSCM-PT-20, radius of curvature < 30 nm, spring constant of the cantilever ≈ 1−5 N.m−1) and the ITO film (Figure 1). The AFM (Agilent 5500) is used in both intermittent contact and contact modes. In intermittent contact mode, the bias is applied to the tip, keeping the ITO layer at the ground potential. In contact mode, voltages are applied to the ITO layer and the tip is grounded. For the sake of clarity, the electrical voltages of the tip relative to the substrate are indicated. All experiments are carried out at (65 ± 5)% relative humidity (RH) to ensure the presence of an adsorbed water layer on the film surface.27

2. EXPERIMENTAL SECTION Samples are composed of a silver nitrate impregnated mesoporous silica layer deposited on an ITO layer supported by a glass substrate. Mesoporous silica films are prepared through a room temperature sol−gel process described in ref 25. These films have a thickness of 200 ± 20 nm (measured using a Veeco Dektak 3 ST surface profiler), a surface roughness of 7 ± 1 nm measured by AFM on a 25 μm2 area, and a mesopore size of 7.5 ± 2.5 nm deduced from top view scanning electron microscopy (SEM) pictures. The mesopores are connected together by a microporosity in the silica walls.26 The film can thus be used as a reservoir for the metal precursor solution. The films are impregnated with an ionic silver nitrate solution 0.5 M (water/EtOH 1:1) by soaking the samples into the solution for 30 min under ultrasonication. The samples are

3. RESULTS AND DISCUSSION 3.1. AFM Induced Inscription of Nanostructures. In AFM intermittent contact mode, voltages are applied after 7495



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Figure 3. (a) Optical image of the dots obtained by applying 10 V for 2 s to the tip. (b) SEM image of one of these dots with a zoom on the film mesostructure (inset) and (c) transmission optical spectroscopy of an array of similar dots. Scale bars correspond to (a) 5 μm and (b) 400 nm.

growth kinetics is thus mainly imposed by the silver ions migration through the mesoporous film following decreasing potential. The dendritic growth is governed by a process of random agglomeration of silver nanoclusters. The SEM (FEI Nova nanoSEM 200) image (Figure 2b) shows that the emerged parts of the dendrites (brighter parts in the image) are not always connected together. We, therefore, may suppose that the dendrites also grow within the film. The dendritic structures grown in contact mode are much larger than in intermittent contact mode when using equivalent experimental parameters. This is probably due to the presence of a larger water meniscus (potentially hundreds of nanometers in width at working humidity32) between the tip and the sample surface while using contact mode. Since the meniscus size also varies with contact time,37 the inscription of smaller silver structures (less than 100 nm) are poorly reproducible in contact mode and can only be achieved in intermittent contact mode. In this mode, the narrower water meniscus permits to obtain particles as small as 40 nm in diameter as can be seen in Figure 2c and d. When the tip is positively polarized, the reduction of silver ions occurs at the silica−ITO interface that plays the role of anode. In this case, the growth of nanostructures in both contact and intermittent contact modes does not induce any topographic changes on the film surface for voltage and time of application as large as 10 V and 30s respectively. However, bright dots, obtained by applying 10 V for 2 s to the tip, can be observed under optical microscope (Leica FTM 200 optical microscope) as can be seen in Figure 3a. SEM characterizations of these dots (Figure 3b) clearly show the mesoporosity of the film on the sample surface (shown in the inset of Figure 3b), in the form of a contrast variation, and the presence of agglomerated wide particles under the film surface. Transmission optical spectroscopy performed on arrays of such dots shows an absorption band centered at 480 nm characteristic of the localized surface plasmon resonance of silver nanoparticles in silica (Figure 3c). Therefore the bright dots observed under optical microscope (Figure 3a) are silver nanoparticle ensembles located near the ITO layer as depicted in Figure 1b. The nanostructures look different from dendrites. They are made of close but independent islets whose size decreases as the distance of the islets increases from the center position where the growth process starts initially. The overall size of the dots increases with the applied voltage and the time of application. Unlike silver reduction on the top surface, many metal nuclei are initially formed at the silica−ITO interface (where the negative potential is uniformly applied), provided that the electric field between the tip and the ITO is high enough to allow the anodic reduction to start. The

reducing the oscillation amplitude of the tip to a thirtieth of the free amplitude unless otherwise stated. On the application of a negative voltage to the conductive tip, silver nanostructures grow on the film surface (as shown in Figures 1a and 2) through local anodic reduction of silver ions. These dendritic shaped patterns have a lateral size that increases with the amplitude of the applied voltage and the time of application; however, the height remains constant around 6−8 nm irrespective of the inscription parameters as can be clearly seen in Figure 2a). The ambient humidity plays a key role on the formation of metal nanostructures. Different experiments have been performed with humidity rates ranging from 0% RH (dry atmosphere) to 80% RH. It appears that no metal NP is created below 50% RH even with the application of −10 V bias (maximum magnitude of the voltage that can be applied to the tip/sample with the AFM used for our experiments) to the tip for 30 s. Above this threshold RH, the structure size increases with the moisture level. The role of ambient humidity in redox reactions28−31 has already been underlined in literature. The absorbed water is mainly involved in two critical steps of the electrochemical process. The water oxidization first acts as a counter reaction essential to the metal reduction31 and second, the metal ion mobility increases with the amount of absorbed water.28,29 Moreover, the adsorbed water on the sample surface and its interaction with the AFM tip also appears as critical for the nanostructure formation. During the voltage application, a field-induced water bridge is likely to be created between the oscillating tip and the surface.32 According to literature, the lateral and vertical sizes of this water meniscus depend on the applied voltage, the time of application, the RH, the tip−sample distance, the nature of the surface and the tip shape.33−35 This meniscus acts as an electrolytic medium which permits the electrochemical process to start even if the tip is not directly in contact with the sample surface. As shown in ref 35, the water meniscus only forms above a threshold RH that depends on the nature of the surface. The absence of meniscus may limit the formation of nanostructures (below 50% RH in our case). On the application of negative voltage, a silver nucleus is initially formed under the tip, which later corresponds to the central higher point of the induced structures (Figure 2a). It acts as a local anode where silver particles agglomerate to form the final pattern. The dendritic structure is attributed to a diffusion limited aggregation (DLA) growth mechanism.36 Simulation (using the DLA algorithm with 5000 particles) of such mechanism in a two-dimensional lattice leads (after successive agglomeration to the cluster of thousands randomly walking particles) to very similar structures to those obtained on the sample TOP surface as shown in Figure 2e. In this case, the 7496



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nanostructures then grow from these nuclei as metallic ions diffuse toward the interface, leading to a distribution of buried silver particles that are heterogeneous in size and shape. These nanostructures are dense enough to create a visible optical contrast with a characteristic spectral response. 3.2. Electrical Switching. Interestingly, the silver nanostructures inscribed on the sample surface when applying a negative voltage to the AFM tip can be switched to the silica− ITO interface by applying a positive voltage on the previously created silver nanostructures. This is a two step mechanism. The top metallic nanostructure is first reoxidized in form of silver ions. These ions then migrate with those remaining in the film pores toward the silica-ITO interface. At the silica−ITO interface, these ions are reduced to form a silver pattern similar to that of a positively biased tip on a pristine film as described earlier. Above the threshold positive voltage (depending on the time of application and RH), silver dendrites can totally be erased from the top surface. Figure 4a−c shows the multistep erasure of a 300 nm wide silver nanostructure. First application

of positive voltage (2 V for 300 ms) below the erasure threshold (3 V in this case) only removes a disc-shaped part of the nanostructure at the position of the application of voltage (Figure 4b), whereas a larger positive voltage applied to the tip (4 V for 300 ms) results in a complete erasure of the nanostructure. This process appears to be reversible; silver nanostructures in the BOTTOM position can be switched back to the top surface by applying a negative voltage to the tip, and several cycles of TOP ↔ BOTTOM switching can be performed. Figure 4d−h shows consecutive switching steps while applying successively negative (−3 V for 1 s) and positive (5 V for 1 s) voltages to the tip to create nanostructures on the TOP and BOTTOM positions, respectively. These AFM topography images show that dendrites appear on the film surface after each application of a negative voltage and disappear after each application of a positive voltage. The size of the dendrites, formed on the TOP position, however, clearly decreases with the number of switches despite the fact that the magnitude of the negative voltage and time of application are kept constants for each step. Optical observations of the nanostructures after each switching step (not the same ones shown in Figure 4d−h) at different locations on the sample are shown in Figure 4i (four nanostructures are created using the same conditions each time). These steps are repeated separately at different locations on the sample using the same parameters that are used for the switching steps shown in Figures 4d−h. The first square (referred as 1 in Figure 4i) shows the optical image of the area just after creating four dendrites using −3 V for 1 s. Similarly, the second one (referred as 2 in Figure 4i) shows the optical image of another area just after the first TOP → BOTTOM switching step (i.e., first creating four dendrites using −3 V for 1 s and then erasing the dendrites using 5 V for 1 s) and so on and so forth. It is important to mention here that optical images (Figure 4i) are taken after each switching step at different locations on the sample to visualize the changes occurring inside the sample volume that can not be observed using AFM. Dendrites created at the TOP position do not produce any optical contrast (area 1) whereas nanostructures created at the BOTTOM position, which are always at least two times larger than the TOP dendrites, give a bright contrast (area 2). The optical images show that the nanostructures at the BOTTOM position remain in the layer after the BOTTOM→TOP switches (areas 3 and 5). This confirms that the negative voltage and the time of application used are not high enough to totally oxidize the nanostructure on the BOTTOM position and to form a dendrite as large as the first one (shown in Figure 4d) created on the TOP position. The use of same electrical parameters for all the BOTTOM → TOP or TOP → BOTTOM switching steps, therefore, results in a limited number of consecutive switches. This decrease in the switching efficiency may originate from the large amount of metallic silver structures at the silicaITO interface that is not totally dissolved by applying negative bias to the AFM tip with the used parameters. The switching process could be improved by applying higher negative bias after the first step to ensure an efficient silver reoxidization rate. However, the problem probably results from the fact that new silver ions, not reduced during the first bias applications, contribute to the growth of nanostructures buried at the silicaITO interface during the oxidation of the TOP side ones. This is likely to lead to larger structures more difficult to completely oxidize using the same bias, as we did. In the perspective of applications for memory devices, a better reproducibility could

Figure 4. AFM topographic images. (a−c) Single silver nanostructure (a) inscribed by applying −2 V for 300 ms, (b) partially erased by applying 2 V for 300 ms, and (c) totally erased by applying 4 V for 300 ms. (d−h) Silver nanostructure switching steps: silver dendrites are successively formed (d,f,h) and erased (e,g) at the same place on the film surface by applying −3 V to the tip for 1s (d,f,h) and 5 V for 1 s (e,g). (i) Optical images of the different areas on the sample after each switching step. The steps are repeated using the same experimental parameters as the ones used for (d−h) switching steps: area 1 corresponds to the formation of 4 identical dendrites (as in (d)) on the TOP position, area 2 shows the sample after the first TOP → BOTTOM switch (as in (e)), ... and area 5 after the second BOTTOM → TOP switch (as in (h)). Scale bars correspond to (a−c) 500 nm, (d−h) 300 nm, and (i) 3 μm. 7497



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compared to such cells, except that the reactive metallic ions are directly embedded into the insulating film. The TOP → BOTTOM switching carried out by applying a positive voltage on the silver dendrite leads to disappearance of the conductive channel (Figure 5b). The positively polarized tip is likely to oxidize the top part of the structure and to remove the conductive metal contact on the top surface.42 As shown in Figure 5a−e, successive switching steps (alternative application of negative and positive voltage to the tip) lead to reversible changes in the conductive state of the surface. Recordings of current flowing through the film during the voltage application are shown for each step in Figure 5f−j. During the first application of a negative voltage (Figure 5f), the current significantly increases during the first 0.5 s and then nearly stabilizes. During the next applications of negative voltage steps (Figure 5h, j), the current versus time curves show the similar trend except that the time during which the current increases is longer. This can be explained by the fact that the availability of ionic silver is less inside the film and silver must first be oxidized from the nanostructure presents on the BOTTOM position. During the TOP → BOTTOM switching steps (Figures 5g, i), the current decreases in the first hundreds of milliseconds, mainly due to the total or partial dissolution of the conductive nanochannel and then saturates to a constant value. These observations are consistent with the previously proposed switch mechanism and confirm that the reversible surface conductivity originates from the successive creation and dissolution of a conductive nanochannel through the silica layer.

maybe be obtained by forming all the needed nanostructures on one interface then removing the remaining silver ions from the mesoporous film (by soaking in a sodium thiosulfate solution for instance38) before performing the switching cycles. 3.3. Conductive Channel Formation. Conductive nanochannels between the TOP and BOTTOM positions of the silica film can also be created provided that the formed dendrites are large enough; that is, the applied voltage and the time of application are high enough. Conductive AFM (CAFM) characterizations have been conducted on dendrites wider than 500 nm created at the TOP position using contact mode AFM. Figure 5a shows that the electrical conductivity of

4. CONCLUSIONS In summary, we have shown that silver nanostructures can be grown on both sides of a mesoporous silica film using a properly biased AFM tip. We have also shown that the positions of the nanostructures, either embedded in the film or at the outer surface, can be switched by reversing the bias. A negative bias forms a dendritic structure on the top surface of the film starting from the position where the voltage is applied on the surface through the AFM tip. A positive bias, however, creates a dense assembly of silver particles at the silica ITO interface corresponding to the influence area of the tip located at the top surface. The size of the NPs can therefore be controlled more precisely if the growth process starts on the top surface. Successive switching of the positions (between film surface and silica ITO interface) of the nanostructure can be achieved by reversing the polarity of the bias applied to the tip. Switching mechanism consists of an oxidation of the nanostructures located at the cathode, followed by the diffusion of ions from one interface to the other and finally rereduction of the ions at the anode. When the dendritic structures are large enough, conductive channels are generated through the silica film from the top surface to the silica−ITO layer. The electrical conductivity of the film can thus be locally and reversibly switched from conductive to insulator by applying negative and positive voltages to the tip respectively. Such properties, observed for the first time in case of silver salt impregnated silica film using AFM, may find applications in reversible optical and electrical data storage.

Figure 5. (a−e) CAFM electrical mapping after successive applications of (a,c,e) −10 V and (b,d) 10 V for 2 s, and (f−j) the corresponding current recorded during the voltage application. Scale bars correspond to 600 nm.

the nanostructures increase compared to the rest of the film, confirming their metallic nature and proving the creation of a conductive channel between both sides of the silica film. It, therefore, proves that the silver dendrites not only grow on the top surface, but also toward the counter electrode. The increase in the conductivity however only occurs when the dendrite size typically exceeds 500 nm, which is twice the film thickness. The spatial extent of the dendrite on the surface plane is therefore assumed to be at least twice larger than its depth. The redoxbased formation of conductive filaments is well-known in resistive switching cells with an active electrode providing metallic ions.39−41 The tip/silica-Ag+/ITO configuration can be 7498



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AUTHOR INFORMATION

Corresponding Author

*Mailing address: Laboratoire Hubert Curien, 18 rue Pr. B. Lauras, 42000 St Etienne, France. Phone: +33477915823. Email: [email protected]. Present Address

⊥ S. Biswas: Laboratory for Bio- and Nano-Instrumentation (LBNI), Ecole Polytechnique Fédérale de Lausanne (EPFL), Building BM Station 17, CH-1015 Lausanne, Switzerland.

Author Contributions ∥

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the French national research agency (ANR) for its financial support in the framework of projects UPCOLOR No. JCJC 2010 1002 1, the Rhône Alpes region for the thesis grant of S. Bakhti. and Jean Monnet University for the postdoc grant of S. Biswas.

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Switchable Silver Nanostructures Controlled with an Atomic Force

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