Article pubs.acs.org/Langmuir
Self-Assembly of Gold Nanoparticles on Gallium Droplets: Controlling Charge Transport through Microscopic Devices Kan Du,†,‡ Elizabeth Glogowski,§,∥ Mark T. Tuominen,† Todd Emrick,§ Thomas P. Russell,§ and A. D. Dinsmore*,† †
Department of Physics, University of MassachusettsAmherst, Amherst, Massachusetts 01003, United States Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States § Department of Polymer Science and Engineering, University of MassachusettsAmherst, Amherst, Massachusetts 01003-9263, United States ∥ Department of Materials Science, University of WisconsinEau Claire, Eau Claire, Wisconsin 54702, United States ‡
ABSTRACT: We describe the spontaneous assembly of ligandstabilized gold nanoparticles on the surfaces of gallium droplets in suspension. By subsequent deposition of these coated droplets onto substrates with patterned electrodes, we form devices that have controlled architecture on the nanometer scale, which allows control of electron transport. In particular, we show that microscopic droplets can be brought into contact with one another with a monolayer of nanoparticles between them, resulting in a junction where electron transport is limited by the Coulomb blockade effect. We characterize the gallium surfaces by optical and electron microscopy and measurement of the interfacial tension. We measure the current−voltage characteristics of devices consisting of one or more Ga droplets and nanoparticle layers in series. The results agree well with the conventional theory of the Coulomb blockade and show how this approach could be used to form hierarchically structured electronic devices. abricating and assembling microscopic devices by fluidbased methods has the potential to allow new structures, conformal coatings on flexible substrates, and three-dimensional device architectures using scalable, low-cost processes. In general, patterning and manufacturing at low cost and high volume are expected to allow new applications in large-area control and sensing, electronic paper and flexible displays, thinfilm solar cells, electronic skins, and other macroelectronic applications.1−3 An advantage of fluid-based assembly is that capillary, gravitational, electrostatic, shear flow, and steric (shape recognition) forces may be utilized so that the desired arrangement of the components occurs spontaneously. For example, fluid-based assembly allows for controlled deposition of nanoparticles onto solid substrates in dense monolayers4−6 or multilayers;7,8 electrodes may then be lithographically patterned on top. At larger scales, microscopic electronic components such as field effect transistors (FETs) can be batch-microfabricated using lithographic processes, released into suspension, and then assembled into larger hierarchical structures by shape-controlled capillary forces (e.g., by use of droplets of low-melting point alloys), hydrophobic forces, and electrostatic forces.9−12 Preformed devices can also be directed at high yield onto a substrate by a combination of capillary and gravitational forces that are tuned by patterning shape or hydrophobicity of the substrate.13−19 After the components assemble, low-resistance electrical contacts are formed by wetting of liquid solder droplets15,17−19 or by electroplating.20
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© 2013 American Chemical Society
These examples demonstrate that suspension-based fabrication and assembly methods can be an alternative to high-vacuum fabrication methods, offering new device architectures at potentially low cost. Here we describe a straightforward self-assembly technique that yields microscopic constructs with controlled architecture at the nanometer-scale, using only fluid-based assembly and no lithography. These microscopic constructs are then directed onto solid substrates that have prepatterned electrodes, thereby providing electrical contact without further processing. While the earlier reports of assembly with solder droplets15,17−19 yielded conducting junctions, our approach incorporates a layer of polymers or functionalized nanoparticles in the junctions. This leads to non-Ohmic contacts, which expands the range of potential devices that can be made by fluid-based assembly. Our method begins with self-assembly of ligand-stabilized gold nanoparticles on the surfaces of liquid-metal gallium (Ga) droplets. These droplets serve as electrodes and make controlled and reproducible electrical contact with the nanoparticles. We found that the electron transport from one droplet to anotheror from a droplet to a solid electrodeis limited by the Coulomb blockade, owing to the presence of Received: August 27, 2013 Revised: October 6, 2013 Published: October 8, 2013 13640
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Figure 1. Images of Ga droplets in HCl solution (pH 1.0). (a) In the absence of gold nanoparticles, Ga droplets rapidly coalesce into a single droplet. (b) In the presence of gold nanoparticles (Au−TEG), the droplets are stabilized by an adsorbed layer. The droplets remain shiny for several hours, which indicates that the surface remains smooth. The diameter of the glass vial is approximately 1 cm.
spectroscopy and scanning-electron microscopy, as described previously.21 Further evidence comes from the electrontransport and interfacial tension measurements, as described below. Adsorption of the nanoparticles is presumably driven by the large interfacial tension between the Ga and the aqueous suspension. We measured the interfacial tension, γ, of Ga in the aqueous solution both before and after adsorption of the nanoparticles. The value of γ was measured from the shape of a sessile droplet on a glass substrate in the presence of the aqueous solution using the method described previously25,26 and assuming a mass density of Ga equal to 6.095 g/cm3. In brief, the measurement method consists of parametrizing the shape of the droplet, which is determined by a balance of interfacial tension (tending to make the droplet spherical) and gravity (tending to make the droplet flat). At pH 1.4 and in the absence of nanoparticles, we obtain γ = 560 ± 10 mN/m. (Previously, the surface tension of Ga in vacuum was found to be 766 ± 17 mN/m using light scattering from capillary waves.27) With such a large interfacial tension, there should be a strong tendency for particles to adsorb and thereby remove some of the Ga−water contact area, as described by the model of Pieranksi28 and Koretsky et al.29 When gold nanoparticles were present, γ decreased sharply to 210 mN/m (for 0.5 mg/ mL Au−TEG) or 65 mN/m (for 0.1 mg/mL Au−cit). Diluting the nanoparticle suspension by a factor of 10 caused γ to increase to 221 mN/m for Au−TEG and to 147 mN/m for Au−cit. The initial reduction of γ indicates the adsorption of the nanoparticles, though it is possible that excess ligands in solution may account for some of the decrease in γ. We attribute the increase of γ upon dilution to the spontaneous desorption of particles, which in turn indicates that the bound particles are in equilibrium with the bulk suspension. To make emulsions with micrometer-scale droplets, we sonicated the Ga suspension for 10 min using a benchtop water-bath ultrasonicator. The resulting micrometer-scale droplets remained stable against coalescence. Figure 2a shows an optical microscope image of Ga droplets stabilized by Au− TEG nanoparticles suspended in the acidic solution (pH 1.0). In this sample, the Ga droplets ranged in size from less than 1 μm to more than 10 μm. Figure 2b shows a scanning electron microscope (SEM) image of the emulsion after it was deposited on a silicon wafer and dried for a few hours at room temperature in air. The image shows that Ga droplets remained stable after water evaporation, indicating that Au−TEG nanoparticles were still adsorbed on the droplet surface and prevented coalescence. Figure 2c shows Ga droplets with much smaller size, which were formed by extending the time of ultrasonication to 3 h. In this case we found droplets as small as 30 nm, which could be used to form very small devices at a very high density on a substrate. We found that smaller Ga droplets
nanoparticles at the junctions. The device conductance increases sharply above a threshold voltage that ranges from approximately 0.2 to 5 V and that scales with the number of nanoparticle layers in series, as expected. The results show the potential for using this simple emulsion approach to fabricate microscopic electronic devices with nanometer-scale architectural control. These devices can then be arranged on large-area substrates, where the leads can be prepatterned with micrometer-scale resolution at comparatively low cost. In previous work, we demonstrated this concept with millimeter-scale liquid-Ga droplets as well as with 120-μm solidified Woods metal particles and 20-μm solid Al particles.21 In the present paper, we use liquid-Ga droplets ranging in size from a few tens of nanometers to micrometers and show how large numbers of microscopic devices can be readily formed and interfaced with a solid substrate.
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INTERFACIAL ASSEMBLY, INTERFACIAL TENSION, AND EMULSIFICATION We chose gallium (Ga) for these experiments because it is a liquid metal near room temperature; it has a melting temperature of 29.8 °C and can be supercooled below room temperature. It has a low electrical resistivity of 210 nΩ·m at 20 °C. Gold nanoparticles with (1-mercaptoundec-11-yl)tetra(ethylene glycol) ligands (Au−TEG) were synthesized21,22 or purchased and used as received (Sigma-Aldrich, St. Louis, MO, item # 687863). The radius of the nanoparticles (R) is 2.3 ± 0.5 nm according to Sigma-Aldrich. Gold nanoparticles stabilized by citrate (Au−cit) were also purchased and used as received (Sigma-Aldrich, item #G1402). The radius of these nanoparticles was 2.5 nm according to Sigma-Aldrich. The typical procedure for the formation of Ga emulsion was as follows. First, hydrochloric acid (HCl) was added to aqueous suspension of Au−TEG nanoparticles (0.2 mg/mL, a volume fraction of approximately 10−5) to set the pH to 1.0 or 1.4. (All of the charge-transport measurements were made with devices formed at pH 1.0.) The temperature of the suspension was controlled at ∼33 °C, which is above the melting temperature of Ga. Then, a droplet of liquid Ga (10 μL) was added into the suspension. (Caution: the suspension of HCl and Ga may lead to toxic Ga(III)Cl3; appropriate safety precautions should be used.) After the suspension is shaken by hand, the single droplet was broken into multiple stable droplets as shown in Figure 1. In the absence of gold nanoparticles, these droplets coalesced again within seconds, leading to a single droplet. The stability of the droplets in the presence of the gold nanoparticles is evidence of the adsorption of a nanoparticle layer onto the water−Ga interface; these particles provide a steric and/or electrostatic barrier against coalescence of the Ga droplets.23,24 Direct evidence of a monolayer or submonolayer of adsorbed gold nanoparticles was provided by X-ray photoelectron 13641
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containing four parallel gold microelectrodes on an insulating SiN4-coated glass chip were purchased (IAME 0504.3 from ABTECH Scientific, Inc., Richmond, VA). The microelectrodes are 3 mm in length and 5 μm in width and are separated by 5μm-wide gaps. Nanoparticle-coated Ga droplets were deposited on the patterned substrates by drying a suspension while taking advantage of convection and capillary forces to direct the Ga droplets on or near the electrodes. The typical procedure was as follows: A droplet of HCl (20 μL, pH 1; no Ga droplets or nanoparticles) was deposited on the substrate. By addition or removal of solution with a micropipet, the contact line was manipulated so that it crossed the microelectrodes. Then, a small volume (∼1 μL) of Ga emulsion was added into the aqueous droplet using a micropipet. Owing to the convection caused by evaporation, some of the micrometer-sized Ga droplets were dragged to the contact line and trapped there.30 Some of these droplets were isolated; others formed chain-like Ga-droplet aggregates. These Ga droplets formed contacts with the microelectrodes underneath them. After drying for a few hours, the devices were ready for current−voltage measurements. Figure 3a shows an SEM image of a microscopic electronic device formed using the above procedure with the Au−TEG
Figure 2. (a) Optical microscope image of Ga−water emulsion stabilized by 5-nm Au−TEG nanoparticles. The scale bar is 10 μm. (b) Scanning electron microscope (SEM) image of dried Ga droplets that were formed by sonication with Au−TEG nanoparticles for approximately 10 min. The scale bar is 10 μm. (c) SEM image of Ga droplets formed by sonication for approximately 3 h. Note that this scale bar is 1 μm.
could also be obtained by increasing the concentration of nanoparticles in the starting suspension. We also formed a stable Ga−water emulsion using gold nanoparticles functionalized with citrate ligands (Au−cit; R = 2.5 nm). While we were able to form devices with these nanoparticles, the results were less consistent owing to aggregation of the nanoparticles in the low-pH suspension. We therefore do not report on these devices here. Finally, we formed stable Ga−water emulsion with only ligands (no nanoparticles). For this purpose, we used (1-mercaptoundec11-yl)tetra(ethylene glycol) (TEG) which was dispersed in water. The stable Ga emulsion was indistinguishable in appearance from the one with Au−TEG nanoparticles. We show below that the TEG ligands form a tunnel barrier between the Ga droplets.
Figure 3. (a) An SEM image of a dried microscopic electronic device consisting of one Ga droplet coated with a layer of Au nanoparticles. A schematic of the measurement circuit is superimposed near the top of the image. (b) Schematic illustration shows a side view of the sample. The inset is an enlarged schematic illustration of a single nanoparticle layer.
nanoparticles. This device contains a single Ga droplet, approximately 10 μm in diameter, that connects two microelectrodes. We note that the SEM image was taken after the transport measurements (described below) and after a few hours of exposure to air. The irregularity of the droplet’s shape may therefore arise from oxidation of the exposed surface. Using this same procedure, we assembled devices with a variety of droplet configurations. These will be described in detail below.
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DEVICE ASSEMBLY To prepare devices that were suitable for transport measurements, we deposited the nanoparticle-coated Ga droplets onto substrates that had prepatterned electrodes. Substrates 13642
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ELECTRON-TRANSPORT MEASUREMENTS To measure the transport of electrons through the Ga droplets and the nanoparticle layers, we connected a voltage source and ammeter across the electrodes. A schematic of the circuit is shown in Figure 3a, superimposed on the SEM image. A picoammeter with built-in voltage source (model 6487, Keithley Instruments, Inc., Cleveland, OH) was connected in series with the two electrodes. A Labview program (National Instruments, Austin, TX) was used to control the picoammeter and step through a series of voltage settings and current measurements. To test the time response, we measured the current as a function of time for 100 s after the voltage was changed from 0 to 0.1 V; we obtained a steady current (approximately 8 pA) with no observable time dependence. In particular, we could detect no transient ionic current, suggesting that the samples were sufficiently dry that a negligible number of ions remained in the junctions. In our previous experiments with millimeter-scale droplets,21 we observed transient ionic currents with a decay time of ∼2 s; the lack of ionic current in the present microdevices might be because the smaller drop size allows more effective drying of the aqueous solvent. For the data reported here, the voltage was stepped in intervals of 3 mV and held for 10 s before the current was measured. The applied voltage was started at zero, increased in the positive direction and then reset to zero and scanned in the negative direction. Figure 4 shows the measured current (I) as a function of applied voltage (V) for the device that was shown in Figure 3
surfaces (see Figure 3b). The width of the gap between the metal surfaces is controlled by the size of the nanoparticles and is expected to be approximately equal to the diameter of the nanoparticle plus the ligands, ≈7−8 nm. The Coulombblockade effect can be approximately summarized in the following way: electrons tunnel from one metal surface (electrode or Ga droplets) across the ligands onto the nanoparticle and then tunnel again from the nanoparticle to the other metal surface. Each nanoparticle and its ligands, therefore, form a capacitor with capacitance C. When a charge e from one electron tunnels onto the nanoparticle, the energy of charging the capacitor is given by e2/(2C). If the metal surfaces have a voltage difference V, then the electron’s potential energy has dropped by eV/2 when it has moved halfway across the junction. Balancing these energies, one expects current to be suppressed when |V| < e/C, known as the threshold voltage (Vth). For a 5-nm-diameter nanoparticle with 1.5-nm ligands between two parallel metal plates,31 the predicted e/C is approximately 0.1 V. At the temperature T used in these experiments (∼300 K), the thermal energy kBT is approximately 25 meV, sufficient to cause some thermal broadening so that a sharp threshold is not expected.33−35 In our devices, the measured threshold voltage is expected to be proportional to the number of nanoparticle layers through which the current flows.7 (In the terminology that we use here, each “nanoparticle layer” contains two tunnel junctions, one from the electrode to the nanoparticle and another from the nanoparticle to the Ga droplet; see the inset of Figure 3b.) From the geometry of the device of Figure 4, we expect two nanoparticle layers so that the observed threshold voltage of 0.2 V corresponds to a threshold of 0.1 V per nanoparticle layer. That threshold voltage agrees with our previous study using millimeter-scale Ga droplets and 5-nm nanoparticles, where we measured a threshold voltage of approximately 0.1 V.21 Comparison to those data also shows that the change in conductance near threshold is much sharper in the present experiment than in the previous single-nanoparticle-layer experiment. This change in the threshold sharpness might arise from a difference in particle size distribution (a different batch of particles was used previously) or it might arise from having two nanoparticle layers in series. The I−V response is also similar to that of devices formed by lithographically defined electrodes that make contact with individual ligand-stabilized gold nanoparticles. Examples from the literature include gold nanoparticles functionalized with dodecanethiol (5-nm diameter, T = 10 K, Vth ∼ 0.1 V35 or 2−5nm-diameter, T = 300 K, Vth ∼ 0.1−0.2 V36), heptanethiol (4.6nm diameter, T = 4.2K, Vth ∼ 0.05 V37), or hexanethiol (5.8-nm diameter, T = 77 K, Vth ∼ 0.1 V38 or 5-nm diameter, T = 4 K, Vth ∼ 0.7 V39). Although the particle sizes, stabilizing ligands, and temperatures T vary, the threshold voltages are comparable to the present results. From the device geometry, we can roughly estimate the number of nanoparticles that could fit in each droplet− electrode contact region. From the image of Figure 3a, we estimate that the contact area between the Ga drop and each electrode appears to be on the order of 10 μm2, which should allow ∼105 nanoparticles if they were close-packed. Our earlier studies21 indicated that the surface coverage by nanoparticles can be an order of magnitude less than close-packed, which means that this droplet−electrode junction could potentially accommodate ∼104 nanoparticles. (The low surface coverage can be explained by the observation that the bound
Figure 4. Plot of the measured current (I) as a function of the voltage applied across two electrodes (V). The open circles show the data for the device shown in Figure 3a, which has gold nanoparticles. The smaller diamonds show data for a single Ga droplet with ligand only and no nanoparticles. Data were collected at room temperature.
(see the open circles). The response is smooth and reproducible. In particular we note the sharp threshold behavior that occurs at an applied voltage of approximately ±0.2 V. The results are repeatable; we obtained 16 I−V curves for this device and found indistinguishable results for the threshold voltage. The conductance above threshold gradually decreased from run to run, however, possibly as nanoparticles flowed out of the junction region. For comparison, the smaller diamond symbols in Figure 4 show the measured current in a device that contains a single Ga droplet stabilized by the TEG ligand only. In the absence of the nanoparticles, the current shows a smooth response that fits well to the form I = a + bV + cV3 expected for tunneling though the ligand layers. The I−V response in the presence of the nanoparticles can be readily explained using the Coulomb blockade model.31,32 When a droplet comes into contact with the gold electrode, the junction contains a layer of nanoparticles that separates the two 13643
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The measured I−V across electrodes 2−4 shows a threshold voltage that approximately equals the sum of the 2−3 and 3−4 data. These data show the potential to adjust the threshold voltage by means of the geometry of the Ga droplet array.
nanoparticles are in equilibrium with the bulk suspension of nanoparticles in the surrounding fluid where the concentration of nanoparticles is very low.) As in earlier work4 and consistent with our data, we assume that the nanoparticles in a given layer transport current in parallel. Hence, the large number of nanoparticles can explain why the above-threshold conductance values are at least an order of magnitude greater than the singlenanoparticle measurements in the literature. In our case, however, the ligand is substantially longer than those used in the literature, which should lead to a much smaller conductance per particle. Hence, a comparison of conductance values would not give an accurate estimate of the number of nanoparticles.
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CONCLUSIONS In summary, we described an approach to fabricate microscopic electronic devices using self-assembly. By this method, micrometer-scale metallic (Ga) droplets are first coated with a monolayer of ligand-stabilized gold nanoparticles and then assembled onto a patterned electrode. I−V measurements reproducibly show the Coulomb blockade effect at room temperature. The threshold voltage (Vth) increased approximately linearly with the number of nanoparticle layers in series. The results show the potential of interfacial-assembly technology in fabricating controlled microscopic electronic devices over a large area in a cost-effective fashion. In future work, it should be possible to direct the droplets onto the electrodes using more precise but straightforward processes such as electrohydrodynamic assembly.40−42 Furthermore, clusters containing pairs of nanoparticle-coated metallic droplets might be formed by suspending the metallic particles in small emulsion droplets and then evaporating the droplet fluid; this elegant approach was shown to yield clusters of colloidal spheres where the clusters could be separated by size.43 Alternatively, clusters might be formed and directed onto a substrate by electrohydrodynamic printing.44 Finally, we note that the simplicity of this method may make it attractive for studying the Coulomb-blockade effect in teaching laboratories. In such cases, proper chemical safety procedures should be established with care. The use of liquid-metal droplets offers two advantages for precise control of nanometer-scale architecture in the junction. First, the surface of the droplet is very smooth; the roughness scale is set by the amplitude of capillary fluctuations, which is typically on the order of angstroms. Therefore, when two droplets are brought into contact, the junction between the two has a smooth, well-defined shape in the steady state.45 By contrast, we have found that solid particles coated with nanoparticles sometimes form Ohmic contact because of direct contact between asperities on the particle surfaces.21 The second advantage is that the separation between the two interfaces is set spontaneously by the forces acting within the gap. In the present case, we propose that the gap width is regulated by a balance of the van der Waals attraction between the two Ga interfaces and repulsion by the disjoining pressure of the nanoparticles (which presumably comes from osmotic pressure and repulsion among the nanoparticle ligands). If droplets were squeezed into contact with a greater force or perturbed after assembly, then the contact area would change but the equilibrium separation would not. In our experiments, this effect leads to droplet-based devices with a repeatable spacing and therefore repeatable electronic performance. By contrast, in devices made from solid-state electrodes, the gap width is typically not set spontaneously; controlling the distance is difficult and devices can readily form an open circuit or a short circuit.
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MULTIDROPLET DEVICES Using the same procedure, we formed devices consisting of multiple Ga droplets. Figure 5a shows an SEM image of a
Figure 5. (a) Scanning electron microscope image of a device containing two Ga droplets stabilized by Au nanoparticles. A schematic of the measurement circuit is superimposed near the top of the image. (b) I−V data for this device.
device consisting of two Ga droplets stabilized by Au−TEG nanoparticles. Figure 5b shows the measured I−V curve for this device. The threshold voltage in this case is ∼0.3 V, which is larger than the single-droplet device. We hypothesize that there are three nanostructured electronic layers in series, with the third layer located between the two droplets. In Figure 6, we show the behavior of a device containing many Ga droplets with diameters ranging from approximately 30 nm to 5 μm. In the I−V measurements across neighboring microelectrodes 2−3 and 3−4, we observe the Coulomb blockade effect with a threshold voltage of ∼5 and 1 V, respectively. These values imply there are approximately 50 and 10 nanoparticle layers in series between the microelectrodes.
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Figure 6. (a) Scanning electron microscope image of a device containing multiple Ga droplets stabilized by Au−TEG nanoparticles. The individual microelectrodes are labeled 1−4, as shown. (b) I−V measurements across microelectrodes 2−3 (I23, filled triangles), 3−4 (I34, open squares), and 2−4 (I24, open circles).
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Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the National Science Foundation for support of this project through the Nanoscale Interdisciplinary Research Team (NIRT, CTS-0609107) and through the Materials Research Science and Engineering Center on Polymers (DMR-0820506) at UMass Amherst. The characterization of the nanoparticle assemblies at the fluid interfaces was supported by the U.S. Department of Energy, Office of Basic Energy Sciences (DEFG02-04ER46126). M.T.T. acknowledges the support of the NSF Center for Hierarchical Manufacturing (CMMR1025020). Partial support was also received from the Center for UMass/Industry Research on Polymers.
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