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J. Phys. Chem. C 2008, 112, 4242-4247
Growth of Gold Fractal Nanostructures by Electrochemical Deposition in Organic Electrolytes: Morphologies and Their Transitions Jia Liu, Yunyi Fu,* Ao Guo, Chuan Wang, Ru Huang,* and Xing Zhang Department of Microelectronics, Peking UniVersity, Beijing, 100871, People’s Republic of China ReceiVed: October 16, 2007; In Final Form: December 11, 2007
We have studied the growth of gold fractal nanostructures by a novel electrochemical deposition (ECD) method. In our ECD process, instead of an aqueous solution of metallic salt, we use an organic solution (such as N,N-dimethylformamide (DMF) or acetonitrile) as the electrolyte and integrate the microscale ECD system into the SiO2/Si chip. Quasi-two-dimensional gold fractal nanostructures can be formed using the organic electrolyte. The morphology of the deposit strongly depends on the kind of organic electrolyte that has been used. A diffusion-limited aggregation (DLA) pattern can be formed in DMF. While in acetonitrile, aside from the DLA pattern, a variety of other typical morphologies can be identified, and they appear in consecutive patterns varying from a DLA fractal, to a dense branching morphology (DBM), to a ramified structure. In some cases the ramified structures can even evolve into nanoscale dendrites or nanowires. To the best of our knowledge, these phenomena have not been reported before. We propose a qualitative explanation for these phenomena based on the mass transfer equation and discuss possible applications of these nanostructures.
Introduction In recent years much effort has been devoted to understand the nonequilibrium growth phenomena from the theoretical and experimental points of view. Fractals are generally observed in the nonequilibrium growth processes1-4 and have attracted great interest in the physics and materials communities over the past two decades. The interest stems from the attempt to understand the fundamental mechanisms of their growth and harness them to use in technological applications. Up to now, a lot of methods have been developed to prepare fractal structures such as dielectric breakdown, crystallization, and electrochemical deposition (ECD).5-11 Among them, the ECD has drawn much attention since it can offer a lot of opportunities to explore the fractal morphologies.12-16 The ECD reported so far was usually conducted in a cell consisting of two parallel metallic electrodes and two glass plates.15 The two electrodes were sandwiched by the two glass plates and separated with a distance on the centi- or millimeter scale. The electrolyte for deposition was usually an aqueous solution of a metal salt and filled the cell. However, by use of the conventional ECD system, one can only realize macroscale (centimeter or millimeter) fractal structures. To explore the nonequilibrium growth phenomena in the microscale and explore fractal’s possible applications such as in micro- or nanoelectronics, we propose incorporating such an ECD system into a SiO2/Si chip and in situ preparing controllable fractal nanostructures in it. In our ECD process, we have developed a novel electrochemical method. All the deposition processes are carried out on a small piece of SiO2/Si wafer with microscale predefined electrodes on the surface. Besides this, the electrolyte for ECD in our ECD process is a pure organic solution, which has been seldom used in previous ECD process. In this paper, we focus on two issues: (1) preparing gold fractal nanostructures by the * To whom correspondence should be addressed. E-mail: yyfu@ ime.pku.edu.cn (Y.F.);
[email protected] (R.H.).
ECD using organic electrolyte and (2) exploring organic electrolyte’s possible effect on the morphologies of these nanostructures. We also discuss their possible applications in micro- or nanoelectronics. Experimental Section The ECD process was carried out on a small piece of SiO2/ Si wafer with inplane paired electrodes on the surface. These electrodes, Au (100 nm)/Ti (10 nm) bilayer films, were patterned on the SiO2 (300 nm)/Si substrate by lithography and the liftoff method (as shown in Figure 1). The size of the square electrode is about 80 µm × 80 µm, and the spacing between two adjacent electrodes is about 50 µm (Figure S1a in Supporting Information). Besides, in order to measure the electric properties of gold fractal nanostructure, the electrodes with smaller sizes are also used (width of electrode, 1 µm; spacing between two adjacent electrodes, 200-600 nm) (Figure S1b in Supporting Information). In our ECD process, instead of an aqueous solution of metal salt, we used an organic solution (such as N,N-dimethylformamide (DMF) or acetonitrile) as the electrolyte (the dielectric constants for DMF and acetonitrile are 39 and 37.3, respectively). Before the ECD, a droplet of the organic electrolyte was dropped on the surface of the SiO2/ Si wafer, covering all the pairs of in-plane electrodes. Then a pair of electrodes was selected, and a DC voltage was applied between them. The applied voltage value is 6 V, and the time of ECD depends on the kind of organic electrolyte that we used. For DMF, the time of ECD process is about 300 s, while for acetonitrile, the time of ECD is about 150 s. The process of ECD was carried out at room temperature under ambient circumstance, and the organic electrolyte was exposed to air during the whole process. After holding for several minutes, we switched off the power and blew off the residual electrolyte on the surface of wafer. In our ECD process, upon applying a DC voltage between two electrodes, gold atoms would dissolve at the anode forming cations and go into the electrolyte. Then they would deposit at the cathode and form gold crystal.
10.1021/jp7100623 CCC: $40.75 © 2008 American Chemical Society Published on Web 02/22/2008
Growth of Gold Fractal Nanostructures
Figure 1. Schematic illustration of the ECD process setup for ECD. The electrodes are covered by a droplet of organic electrolyte.
The morphologies and composition of the electrodeposits were characterized by the field emission scanning electron microscopy (SEM, scanning electron microscope, Strata DB235) and the energy dispersive X-ray spectroscopy (EDX) equipped in environmental SEM (Quanta 200 FEG, FEI), respectively. Results and Discussion If pure DMF was used as the electrolyte, we could observe quasi-two-dimensional deposits with well-defined fractal patterns near the edge of the cathode (Figure 2a). The extensions of these structures are in the range of 1 to 20 µm, which could be lengthened if the time of the ECD is extended. The fractal patterns appear in different configurations and two features are worth mentioning: (1) their growth along the electric field and (2) the screening effect that prevents the further development of some initial fractals. These fractal patterns consist of nanocrystallites with diameters ranging from about 50 nm down to 5 nm (Figure 2b). Having examined by the energy dispersive X-ray analysis (EDX), these fractal nanocrystallites are composed of gold (Supporting Information, Figure S2 and Table S1). Although the fractal nanostructure is assembled from aggregated nanocrystallites, it has high conductance (Supporting Information, Figure S3), which may have applications in nanoelectronics, such as acting as the interconnect material. The morphology of the deposit is reminiscent of the fractal pattern that is described by the diffusion-limited aggregation (DLA) model.4 The DLA is a process during which particles undergo a random walk due to Brownian motion and cluster together forming aggregate. The theory of DLA, proposed by Witten and Sander in 1981,4 is applicable to explain the aggregation in any system where diffusion is the primary transport mechanism, especially in ECD process. The screening effect is one of the typical characters of the DLA model, which also has been observed in our ECD process. Besides the morphologies of the deposits, we can also understand the fractal nanostructures from the point of fractal dimension. Fractals, as we know, are typically defined as patterns that can be described mathematically by fractal sets, which describe their self-similarity properties. Usually the boxcounting method17 is used to determine the fractal dimension D. In this method, a grid of boxes with varying sizes () is superimposed on the digital image, and the number of the boxes (N) that are filled (fully or partially) by the pattern is counted. If a structure is fractal, the count (N) and the corresponding box size () have a relationship: N ≈ -D. The log of plotted against the log of N yields a straight line. The slope (S) of this line is the negative of the fractal dimension, i.e., D ) -S. We found that the fractal dimension of the nanostructure in our ECD process is 1.7123 (Figure 2c), which is close to the theoretical value of 1.7 for the structures formed in a DLA process. That is to say, from the point of fractal dimension, we also could
J. Phys. Chem. C, Vol. 112, No. 11, 2008 4243 conclude that the formation of the fractal nanostructures in DMF could be explained by the DLA model. We also find an interesting phenomenon: the morphology of deposit strongly depends on the kind of organic electrolyte that has been used. If we choose acetonitrile, a volatilizable solvent, as the electrolyte for deposition, the morphologies of gold deposits are quite different from that in DMF. A typical result is shown in Figure 3a. The nanostructure is composed of three parts: the inner part that is near the electrode, the middle part, and the peripheral part. The configuration of the inner part resembles the DLA pattern, while the middle part appears in different configuration: much more densely packed. The middle part looks like the dense branching morphology (DBM) pattern,18-21 which has a dense array of branches with a welldefined smooth envelop. At the outermost periphery of the DBM pattern, ramified structures appear, and they can even evolve into nanoscale dendrites or nanowires (Figure 4). This is a typical consecutive pattern varying from a DLA fractal, to a DBM, to a ramified structure even into nanowires. On the basis of this phenomenon, the fabrication of gold nanowires can be realized using this method, which may be useful in nanoelectronics as the inter-element wiring. By use of the box-counting method, we calculate the fractal dimensions of the patterns in different regions. The values of fractal dimensions for inner and middle parts are 1.7066 and 1.8469, respectively, which are accordingly close to the theoretic values of DLA and DBM (shown in parts b and c of Figure 3).22 This is obvious evidence that the structure of the aggregate varies from a DLA fractal (the inner part) to a DBM pattern (the middle part).23-27 This phenomenon is analogous to the Hecker effect, which has been depicted as an abrupt change in spatiotemporal morphologies of the thin-layer metal electrochemical deposits (e.g., copper and zinc), such as in color, roughness, and the number of branches of the deposits at a certain distance between anode and cathode.28-31 Usually these transition phenomena are found in an ECD process with aqueous solution of metal salt as the electrolyte. To the best of our knowledge, however, this phenomenon has not been reported in organic electrolyte system in the literature before. Why do the morphologies of deposits appear in different patterns when we use different organic solution as the electrolyte? Why does the deposit have a morphological change when acetonitrile is used as the electrolytes? Here we try to give a qualitative explanation for the phenomena. The DLA exhibits much more convolved geometry, with a few branches screening the growth of the others. The DBM are usually described by a dense array of branches with a well-defined smooth envelope. Both DLA and DBM can be included in the family of diffusionlimited patterns, the mass transfer of which must satisfy the Nernst-Planck equation.32 Generally, ions in electrolyte move in response to the concentration gradient (a process called diffusion), to the electric field (migration) and to the bulk fluid motion (convection). The net flux of ions, Jc, is therefore the sum of the diffusion (-D3Cc) term, the migration (zcucFCcE) term, and the convection (CcV) term
Jc ) -D3Cc + zcucFCcE + CcV
(1)
where Cc is the local concentration, D the diffusivity, 3Cc the concentration gradient, zc the charge number of one ion, uc the mobility, F the Faraday constant, E the electric field, and V the velocity of the bulk fluid.33-34 In the conventional ECD process, the electrolyte used for deposition is an aqueous solution of metal salt, and the concentration of cations (Cc) is usually high. In our ECD
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Liu et al. process, however, we just use an organic electrolyte that contains no ions. The gold cations for deposition are only from the anode by sacrificially dissolving. Thus the concentration of the gold ions (Cc) is low and mainly determined by: (1) the velocity differences between the dissolving of gold at the anode and the deposition of gold at the cathode and (2) the evaporation rate of the organic electrolyte. If DMF, a solvent with a low evaporation rate and relatively high dielectric constant (36.70), was used as the electrolyte, the quantity of the electrolyte and the velocity difference between the dissolving and deposition would be nearly kept the same in the whole ECD process. Thus the Cc can be regarded very low. The last two terms of the eq 1 can also be neglected since both the migration (zcucFCcE) term and the convection (CcV) term are negligible. The transport equation can be reduced to a simple diffusion equation
Jc ) -D3Cc
Figure 2. The field-emission SEM images of the well-defined fractal nanostructures prepared in DMF electrolyte: (a) showing the fractal patterns near the cathode, where the light gray area on the right side is the cathode; (b) showing that the fractal structure is composed by nanocrystallites; (c) the plot of Log(count) ≈ Log(box size) yielding the fractal dimension of the structure in (a).
(2)
That is to say, the transport of cations in DMF is dominated by the diffusion. The deposition process is a typical Laplacian growth mode and the DLA pattern appears33-34 (as shown in Figure 2a). However, when we use acetonitrile as the electrolyte, the situation is different from that in the DMF case. The vapor pressure of acetonitrile is 69.96 mmHg at room temperature, which is nearly 26 times higher than that of DMF (2.7 mmHg).35 Thus DMF is nonvolatile, and acetonitrile vaporizes very quickly. The volatilization of acetonitrile accompanies the development of the gold fractal structures, and the quantity of electrolyte covering the surface of the wafer is decreasing during the whole ECD process. The concentration of gold cations (Cc) in the acetonitrile is therefore increasing gradually. Thus the migration term (zcucFCcE) and the convection term (CcV) cannot be negligible. We should consider the contribution of all the three terms for the growth of deposit. Some ECD processses have demonstrated that the migration term (zcucFCcE) could induce a DBM,33 particularly in the high-field region. Thus the transition from DLA to DBM occurs. Besides the migration term, the convection term, CcV, also plays an important role in the growth of DBM pattern. As the electrolyte vaporizes, the concentration of the cation becomes large and CcV may even become the dominative term in eq 1. The convection generally includes natural convection (induced by density gradient), forced convection (caused by mechanical stirring or pressure gradient)36 and local convection near certain growing deposits (driven by electrical field).37 In our case, we can ignore the contribution of forced convection since we did not apply externally mechanical stirring. We should focus on the natural convection and the local convection. As the electrolyte vaporizes, it will result in strong local concentration gradient (or density gradient). Since we apply a voltage between two electrodes and the cations move toward cathode, it is reasonable to believe that there is inflow at the cathode and outflow at the anode. Thus the convection may occur in regions near the deposit driven by electrical field and the probability of the cations attaching to the cathode becomes larger and DBM forms.38 So under the influences of the natural convection and the local convection, the DBM pattern appears. On the basis of the discussions above, we can conclude that the transition from DLA to DBM (from the inner part to the middle part) in our ECD process is mainly due to the vaporizing of the acetonitrile which induces the effect of migration and convection.33,38 Once the acetonitrile evaporates completely, the gold ions cannot be supplied by the anode any more. The gold ions in residual solution will aggregate at the outermost periphery of
Growth of Gold Fractal Nanostructures
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Figure 3. (a) The field-emission SEM image of the fractal nanostructures prepared in acetonitrile electrolyte, which shows that the morphology of the deposit varying from a DLA fractal, to a DBM, to a ramified structure. The light gray area at the bottom right corner is the cathode; (b) and (c) plots of Log(count) ≈ Log(box size) yielding the fractal dimensions of the inner part and the middle part, respectively.
the DBM pattern and grow guided by the electrical field as shown in Figure 4. Thus, the ramified structures even nanodendrites (nanowires) at the outermost periphery of DBM pattern appear. Hence, we propose that the morphologic transition of deposit in our ECD processsconsecutive pattern varying from a DLA fractal, to a DBM, to a ramified structuresis chiefly due to the effect induced by the vaporizing of the acetonitrile. Here one question may be raised. As we can see from the Figure 2a, the fractal patterns were electrodeposited at the electrode plane, while in Figure 3a, the fractal patterns were electrodeposited at one corner of the electrode. As we know, the current density of the corner is often bigger than that of the plane, and the current density may have a big effect on the shape of electrodeposits. Could we simply conclude that the different morphologies are due to the difference between the DMF and acetonitrile? The answer is yes. The current density indeed has a big influence on the morphologies of the deposits. However, the difference of current density in DMF and in acetonitrile in our ECD process, in fact, results from the difference of volatilities of these two electrolytes. As we know, the current
density is positive proportion to the net flux of ions Jc. The Jc, as stated in eq 1, is determined by the contributions from the diffusion, migration, and convection of ions. The behavior of ion transport in electrolytes will affect the morphologies of the deposits. Since we use organic electrolyte, the concentration of the gold ions (Cc) initially in electrolyte solution is very low. For DMF, which vaporizes very slowly, the contributions from zcucFCcE (migration) and CcV (convection) are very small (as Cc is small). The flux of ions Jc is determined only by the diffusion of ions, i.e., Jc ) -D3Cc. Thus a DLA pattern can be observed in DMF. While in acetonitrile, which vaporizes more quickly than DMF, the quantity of the electrolyte covering the chip decreases rapidly and the concentration of gold cations (Cc) in acetonitrile accordingly increases larger and larger. The contribution of migration term (zcucFCcE) and CcV (convection) cannot be negligible, and hence the morphologies of deposits in acetonitrile are different from that in DMF. On the basis of the analysis above, it can be inferred that the difference of current density between in DMF and in acetonitrile is due to the different volatilities of these two electrolytes. In other words,
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Liu et al. Conclusion
Figure 4. The field-emission SEM image of the nanoscale gold dendrites at the outermost periphery of some DBM patterns. It shows that in some cases the dendritic deposit will evolve into two kinds of shapes: (1) with side branches growing in definite directions and (2) branchfree like nanowires (denoted by white color arrows).
We have integrated a microscale ECD system into the SiO2/ Si ship and have investigated the growth of gold fractal nanostructures. We demonstrate that quasi-two-dimensional gold deposits with well-defined fractal nanostructures could be formed using an organic solution (DMF or acetonitrile) as the electrolyte. We find that the morphology of the deposit strongly depends on the kind of organic electrolyte that is used. A DLA pattern can be observed when DMF was used. If we use acetonitrile, a volatilizable solvent, as the electrolyte for deposition, the morphology of the deposit is quite different from that in DMF. Besides the DLA pattern, a variety of other typical morphologies can be identified: consecutive pattern varying from a DLA fractal, to a DBM, to a ramified structure. And in some cases the ramified structures can even evolve into nanoscale dendrites or nanowires, which is analogous to the Hecker effect. It is suggested that the morphologies of deposits in organic electrolyte system are as diverse as these observed in convectional ECD process using aqueous metal salt solution as the electrolyte. To the best of our knowledge, these phenomena have not been reported in the literature before. We propose a qualitative explanation for these phenomena based on the mass transfer equation. A promising aspect of this research is the possibility to prepare controllable fractal nanostructures in situ in the Si chip for the applications in future nanoelectronics. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (Nos. 60625403, 60671021, 60776053 (R.H. and Y.Y.F.)), Special Funds for Major State Basic Research (973) Projects of China (2006CB302701 (R.H.) and 2006CB302704 (X.Z.)), and the NCET program and Instrumental Analysis Fund of Peking University (Y.Y.F.). Supporting Information Available: Additional experimental procedures and X-ray analysis spectra, SEM image of typical Au/Ti electrodes, table of the chemical composition, and I-V curve of the fractal gold nanostructures. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes
Figure 5. The field-emission SEM image of the fractal deposits obtained in acetonitrile using a planar electrode, near which the current density is nearly the same as that in the Figure 2a. The morphology of this deposit is similar to that in Figure 3a, i.e., also varying from a DLA fractal, to a DBM, to a ramified structure.
the effect of the current density on the morphologies of deposits has been embedded in the influence of the organic electrolyte. In fact, we can also observe similar morphologies in acetonitrile using planar electrode (Figure 5). The current density here was nearly the same as that in the Figure 2a. The phenomenon of morphologies transition of deposits, i.e., from DLA (near the Au electrode), through DBM (middle), to a ramified structure (outermost periphery), can also be noted. This experimental evidence further proves that the morphology of the deposit mainly depends on the kind of organic electrolyte that we used.
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