Size-Controlled Electrochemical Growth of PbS Nanostructures into

Article pubs.acs.org/Langmuir

Size-Controlled Electrochemical Growth of PbS Nanostructures into Electrochemically Patterned Self-Assembled Monolayers Fatma Bayrakçeken Nişancı and Ü mit Demir* Faculty of Sciences, Department of Chemistry, Atatürk University, 25240 Erzurum, Turkey

ABSTRACT: 1-Hexadecanethiol self-assembled monolayers (HDT SAMs) on Au(111) were used as a molecular resist to fabricate nanosized patterns by electrochemical reductive partial desorption for subsequent electrodeposition of PbS from the same solution simultaneously. The influences of potential steps of variable pulse width and amplitude on the size and the number of patterns were investigated. The kinetics of pattern formation by reductive desorption appears to be instantaneous according to chronoamperometric and morphological investigations. PbS structures were deposited electrochemically into the patterns on HDT SAMs by a combined electrochemical technique, based on the codeposition from the same saturated PbS solution at the underpotential deposition of Pb and S. Scanning tunneling microscopy measurements showed that all of the PbS deposits were disk shaped and uniformly distributed on Au(111) surfaces. Preliminary results indicated that the diameter and the density of PbS deposits can be controlled by controlling the pulse width and amplitude of potential applied at the reductive removal stage of HDT SAMs and the deposition time during the electrochemical deposition step. chemical approach to constructive lithography,8 tip-induced electrochemical deposition,9 electrodeposition after stamping or soft lithography.10,11 There have been very few studies about the electrodeposition of semiconducting compounds into patterns fabricated using SAMs as a resist. Foresti and coworkers12,13 used electrochemical atomic layer deposition (ECALE) for electrodeposition of CdS into the patterned SAMs fabricated by microcontact printing and selective electrochemical desorption methods. In EC-ALE based on layer-bylayer growth, the underpotential deposition (UPD) was used to form the sequential atomic layers of each element of a compound semiconductor.14,15 Separate solutions and potentials are used to deposit atomic layers of each element electrochemically in a cycle in this method. We recently combined the EC-ALE method with codeposition to prepare highly crystalline nanostructured compound semiconductors such as CdS,16 CdTe,17 ZnS,18 PbS,19,20 PbTe,21 Sb2Te3,22 and Bi2Te3.23 The simplicity and applicability of this new electrochemical method makes it possible to deposit the desired materials onto/into more complex structures, which may be applied to materials that are not amenable to other fabrication techniques. Zhu et al.24 successfully applied our

1. INTRODUCTION Size-controlled synthesis of compound semiconducting nanostructures is a great challenge in materials chemistry because their intrinsic chemical and physical properties are strongly dependent on the size-induced quantum confinement effects. There has been a great deal of interest in developing lithographic methods for patterning nanostructures of a predetermined size and shape for use as building blocks in the fabrication of electronic and biomedical devices. Fabrication of nanostructures is generally carried out by lithographic patterning of substrates using optical, electron-beam (e-beam), ion beam, and X-ray lithography techniques followed by deposition or etching of materials.1,2 The size of nanostructures fabricated by conventional lithography techniques is determined by the diffraction of light, the radiation wavelength, the diameter of the electron beam etc. Therefore, alternative fabrication and patterning of nanostructures with controlled shape and size at predetermined locations on surfaces has become critically important as the size of devices reaches the nanoscale and the resolution of conventional optical lithography approaches its physical limit. Electrochemical methods are suitable for the control of the size and geometry of the prepared nanoparticles. The combination of electrochemistry and patterned self-assembled monolayers (SAMs) used as a molecular resist has been used in several nano fabrication processes, using SPM-based lithography,3−7 a bipolar electro© 2012 American Chemical Society

Received: April 3, 2012 Revised: May 11, 2012 Published: May 15, 2012 8571

dx.doi.org/10.1021/la301377r | Langmuir 2012, 28, 8571−8578

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electrochemical deposition technique to deposit CdS onto TiO2 tube walls while minimizing deposition at the tube entrances, thus preventing pore clogging. Shannon and coworkers25 reported the synthesis of Au/CuI and Au/CdS core− shell nanoparticle (NP) thin films using this method. In the present study, we applied the potential steps of variable pulse width and amplitude for partial desorption of 1hexadecanethiol self-assembled monolayers (HDT SAMs) from the Au(111) surface in order to create HDT SAMs free regions with desired sizes for electrochemical deposition of compounds. Then PbS was deposited into the HDT SAMs free regions by our combined electrochemical technique,19,20 which is based on codeposition of PbS using underpotential deposition of Pb and S from the same saturated solution of PbS containing excess PbS particles as a source of Pb2+ and S2−. Electrochemical reductive desorption and deposition conditions were determined by cyclic voltammetry and chronoamperometry. The morphology and size of electrodeposited-PbS nanostructures were analyzed by means of scanning tunneling microscopy (STM). Electrochemical and morphological studies indicated that the size and density of PbS nanoparticles could be controlled by the control of pulse width and the amplitude of potential applied for partial desorption.

3. RESULTS AND DISCUSSION 3.1. Electrochemical Patterning. The cyclic voltammogram for the HDT-covered Au (111) electrode in aqueous solution of 0.1 M KOH is shown in Figure 1. The only feature

2. EXPERIMENTAL SECTION

seen in the cathodic scan is a broad reductive current peak at around −1230 mV corresponding to the reductive desorption of HDT SAMs. These results are in good agreement with those obtained electrochemically by Morin et al.27,28 The cyclic voltammogram also shows an anodic peak at the potential of −970 mV corresponding to the readsorption of the HDT molecules after a partial desorption in the cathodic scan. It has been shown that the anodic readsorption of alkanethiols is strongly dependent on the length of the alkyl chain comprising the thiol.29 The comparison of the integrated charges obtained under the reductive and oxidative peaks indicates that some of HDT monolayer was reductively removed during the potential scan, leaving HDT-free regions on the Au(111) surface. The reductive desorption of n-alkylthiol SAMs in alkaline media has been extensively studied by means of electrochemical techniques such as cyclic voltammetry and chronoamperometry and described as shown with the following equation:

Figure 1. Cyclic voltammograms of HDT SAMs covered Au(111) electrode immersed in a aquous solution of 0.1 M KOH. The sweep rate was 100 mV/s.

The working electrode was single-crystal Au (111), used as a reference surface for comparison to determine morphological and structural changes before and after electrodeposition, prepared as previously described by Hamelin.26 To obtain elliptical (111) facets, the gold electrode (Johnson Matthey, 99.999%) was flame-annealed for 20 s, and after a short cooling time in air the electrode was quenched in Milli-Q water. This procedure was repeated at least five times. When the Au(111) was used for voltammetric and chronoamperometric measurements, the polycrystalline regions were coated with a chemically inert epoxy (Epoxy-Patch). All surface area calculations were carried out by voltammetry in 5.0 mM Fe(CN)63− aqueous solutions containing 1 M KCl for the single-crystal Au(111) electrode. HDT SAMs were prepared by immersion of the Au(111) in ethanolic solutions containing 1 mM 1-hexadecanedithiol (C16H31− SH) (AlphaAesar, purity ≥95%) for 24 h. After that it was transferred into the electrochemical cell for patterning and electrodeposition studies. The cyclic voltammetry and chronoamperometry experiments were performed with a BAS100B electrochemical workstation connected to a three-electrode cell (C3 Cell Stand, BAS). In all cases, an Ag/AgCl (3 M NaCl) electrode served as the reference electrode, and a platinum wire was used as the counter electrode. All of the electrolyte solutions used in this study were prepared from deionized water (i.e., >18 MΩ) from a Milli-Q system. PbS electrodeposition was carried out from the 0.1 M KNO3 including saturated lead sulfide at different temperatures (see ref 20 for details). The cell temperature was maintained by a thermostat system. The preparation of lead sulfide was achieved by addition of 0.01 M Na2S (Na2S·9H2 O, reagent grade, Aldrich) to 0.01 M Pb2+ (Pb(CH3COO)2·3H2O, Merck) solution with rapid stirring while oxygen was purged from the solution with nitrogen. A fast evolution of the precipitate was observed after addition of Na2S to the Pb2+ solution. After being filtered and then washed with ultrapure (Milli-Q) water, the black lead sulfide precipitate was dried at 50 °C for 4 h. Scanning tunneling microscopy images of electrodeposits were acquired in ambient conditions, with a Molecular Imaging Model PicoScan instrument. The tips for STM were made of either tungsten wire (diameter 0.25 mm) prepared by electrochemical etching in 2 M KOH or cut from Pt/Ir wires (90:10).

Au−SR + e− → Au◦ + RS−

It has been shown that the reductive desorption potential of the SAMs depends on several factors, such as the length of the alkyl chain, the degree of ordering, the number of intermolecular interactions within the organic film, and the crystallinity of the substrate (Au).30 The desorption potential is also affected by the type and number of adsorption sites on Au surfaces.31 In spite of the fact that reductive desorption has been quite widely used for a range of thiols, the oxidative desorption (destructive) has also been used to remove alkylthiol SAMs from the surface at sufficiently large positive potentials.32 Au−SR + 2H 2O → Au◦ + RSO2 H + 3e− + 3H+

To create HDT-free patterns on the HDT SAMs-modified Au (111) surface before the electrodeposition of the desired compound, we performed potential step experiments in which we applied potential steps with various pulse widths and amplitudes corresponding to the reduction potential of HDT in order to strip off the SAMs of HDT partially. Electrodeposition 8572


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while the time at which maximum current obtained (tm) decreased, indicating that that desorption kinetics involves the formation and growth of two-dimensional holes.34,36 From the analysis of the experimental current transients, it was found that the reductive desorption of HDT involved two steps: first, an initial fast current transient peak associated with the charging of the double layer (jads, Langmuir-type adsorption−desorption shown in eq 3), followed by the slow current transient due to reductive formation and growth of holes.

and dissolution of metals and organic monolayers have been described in terms of a two-dimensional (2D) nucleation and growth model. In this model, electrodeposition of monolayers starts by the formation of nuclei and continues with the growing of the edges of these nuclei. This model has been successfully applied for desorption of monolayers as well.33−35 In contrast to electrodeposition, desorption of monolayers is initiated by the formation of etching centers (holes) and continues by expansion of these etching centers in circular fashion. The size and the number of these holes depend on the rate of formation and the rate of enlargement of etching centers. If the formation rate of etch centers is large and the maximum number of etch centers is formed right after the potential step (occurs when a large potential is applied), the kinetics of this process may be treated as instantaneous nucleation and growth of 2D monolayers and could be analyzed by the following equation:36 jinst = at exp( −bt 2)

jads = k exp( −k′t )

If we consider the current density as the sum of two independent terms, the current density-time transients responsible for the reductive 2D hole formation and growth (jhole/expand) could be separated from the current density-time transients for the charging current by fitting of experimental current density-time transients with respect to the model expressed by eq 4.

(1)

jtotal = jads + jhole/expand

When the formation rate of etching centers is small and remains constant on the time scale of the experiment, the kinetics of this process follows the progressive model, and current density−time transients are described by the following equation: jprog = ct 2 exp( −dt 3)

(3)

(4)

Figure 3a shows the best numerical fits between the experimental transients with overpotential of −350 mV and calculated transients according to the model shown by eq 4.

(2)

In eqs 1 and 2, the pre-exponential contribution corresponds to current density for the step edges of the expanding nuclei at shorter times, whereas the exponential terms correct for the decreased step edge length due to the overlap of adjacent nuclei at longer times. The constants a, b, c, and d depend on several physical parameters, and explicit formulas for these constants can be found in the literature.36 Figure 2 shows typical current−time (I−t) curves for the HDT SAMs coated Au(111) obtained under the same

Figure 2. Set of experimental current−time transients recorded for reductive desorption of HDT SAMs from Au(111) electrode surface. The applied potentials are labeled in the figure. Figure 3. (a) Numerical fit of the double layer charging and 2D reductive hole formation and growth model (eq 4) to a experimental transient obtained for the potential step from −800 to −1150 mV. The individual contributions of double layer charging and 2D reductive hole formation and growth are also shown, (b) corresponding dimensionless plots of I/Imax versus t/tmax for the transients associated with 2D hole formation and growth. Theoretical curves for instantaneous and progressive kinetics are also shown for comparison.

conditions as described above for the cyclic voltammetric measurements. The potential of the electrode stepped from an initial potential of −800 mV (E1) to final potentials (Ef) of 1100, 1150, and 1200 mV at which HDT SAMs start to desorb. The current−time transients show that, when the potentials (ΔEp = Ei − Ef) increased, the current maxima (Im) increased, 8573


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After subtraction of charging current transients from the experimental transients, we constructed the dimensionless plots of I/Imax versus t/tmax for the transients associated with hole formation and growth by using the following equations: ⎛ −2(t 3 − t 3) ⎞ I t2 m ⎟ progressive = 2 exp⎜ Im tm 3tm 3 ⎠ ⎝

(5)

⎛ − (t 2 − t 2 ) ⎞ I t m ⎟ instantaneous exp⎜ = Im tm 2tm 2 ⎠ ⎝

(6)

where I is the current at time t after the potential step and tm is the time that corresponds to maximum current Im. As can be seen in Figure 3b, the experimental plot of I/Im vs t/tm for HDT reductive removal from the Au surface matches the instantaneous formation and growth of holes model at short times but it differs at longer times. It has shown that the experimental current decays more slowly than predicted by these models at longer times and at higher values of overpotentials due to the interaction between adsorbed molecules and the diffusion of desorbed species from electrode surface into solution.34,37,38 In the case of an instantaneous process, the hole formation and hole growth kinetics follow the instantaneous kinetics, meaning that the rate of etch center formation is fast and all the etch centers will form at the beginning of the potential step and the number of etch centers will stay fixed until the SAMs are completely removed. When we perform the same calculations for the current transient with smaller overpotential, we see that the kinetics of desorption shifts slightly from instantaneous to progressive mode. This behavior indicates that the number of holes could be controlled by controlling the amplitude of the potential step. 3.2. Electrodeposition of PbS into Patterns. Patterned SAMs, which are fabricated by partial electrochemical reductive desorption, could be convenient molecular templates for the electrodeposition of semiconducting compounds. For the directed electrodeposition of semiconducting compound into previously etched patterns, HDT SAMs on Au(111) must be electrochemically stable in the potential window in which PbS will be deposited. Details of the electrochemical codeposition of PbS are given in our previous studies.19,20 In principle, UPD, the electrochemical deposition of a metal onto a foreign substrate at potentials more positive than the Nernst potential, is usually restricted to the formation of one atomic layer of the deposited metal. After determination of UPD potentials of Pb2+ and S2‑ by cyclic voltammetry, we set the potential of the working electrode between the oxidative UPD wave of S2‑ and the reductive UPD wave of Pb2+. Thus Pb and S were supposed to deposit underpotentially at the electrode surface simultaneously from the same saturated solution of PbS containing excess PbS particles. Then, underpotentially codeposited Pb and S atoms react to form PbS on Au(111) substrate. This electrochemical method combines UPD and codeposition. Therefore, this electrodeposition strategy could be directly applicable to sequential patterning and codeposition of compounds. The cyclic voltammograms shown in Figure 4 indicate that HDT-covered (dashed lines) Au exhibits strong blocking behavior for the electrodeposition of PbS as evidenced by the absence of faradaic current between −0.000 and −0.800 mV compared with an unmodified gold substrate (full line). The blocking ability of HDT monolayers was also tested in the same potential range by using ferri/ferrocyanide as a redox probe.

Figure 4. Cyclic voltammograms of HDT SAMs covered Au (dotted line) and naked Au (full line) electrodes in a solution containing of 2.0 mM Pb2+. The scan rate was 100 mV s−1.

These results indicate that HDT SAMs could serve as a stable resist on Au substrates for the potential range of PbS deposition. HDT SAMs prevent electrochemical deposition on the SAMs covered regions by blocking the electron transfer through the monolayer via a tunneling process or inhibiting the permeation of electroactive species through the monolayer and react at the electrode surface. As shown in Scheme 1, the general experimental strategy employed in this study involved first applying potential steps at various pulse widths and amplitudes for the partial reductive desorption of HDT SAMs in order to create etch centers with controlled sizes and then performing electrochemical codeposition of PbS into those etch centers. 3.3. Morphological Characterization. Before electrodeposition of PbS, we intended to obtain STM images of the holes formed by the procedure described above. Our attempts to obtain clear images of etch centers (SAMs-free patterns) on HDT modified surfaces consistently failed while we managed to get high resolution STM images of SAMs modified Au(111) surfaces before the potential step (Figure 5, panels a and b). Since HDT molecule is not conductive and has a straight chain in its structure, the STM tip penetrates into the HDT SAMs and continues scanning inside SAMs. This makes it difficult to get reasonable images, thus preventing the observation of height differences in the HDT SAMs by STM imaging. Similar conclusions have been drawn for the images obtained in SAMs of n-alkanethiols.33−35 Therefore we did not intent to get STM images of HDT SAMs modified Au(111) surfaces after potential step experiments performed for the electrochemical desorption of HDT SAMs from the Au surface. However, the voltammetric response exhibiting characteristics of radial diffusion could be used to show that the HDT SAMs modified Au surface has holes created by reductive desorption. Figure 6 shows the voltammetric behavior of the HDT SAMs covered Au electrode in the solution containing 2.0 mM Pb2+ after application of the potential step with a pulse width of 20 s and overpotential of −350 mV. As shown in this Figure, the voltammogram has a typical sigmoidal shape, a characteristic feature that results from the development of radial diffusion at each microelectrode surface, giving a limiting current (Ilim). There are some excellent examples reported to study the microelectrode behavior of holes fabricated on the alkanethiol monolayer on Au electrodes.39,40 8574


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Scheme 1. Schematic Illustration of the Experimental Strategy to Fabricate PbS Nanostructures on the Electrochemically Patterned HDT SAMs on Au(111)

Figure 6. Cyclic voltammetric response for HDT SAMs modified Au(111) in an aqueous solution of 2.0 mM Pb2+ obtained after application of the potential step with pulse width of 20 ms and overpotential of −350 mV; scan rate: 100 mV/s.

have an average diameter of 60 nm and a thickness of 1.5−2.5 nm. Figure 7 shows that the number of PbS deposits increases with the increasing amplitude of the potential step. This increment in the number of PbS deposits with the increase in potential step amplitude results in the formation of more holes on HDT SAMs monolayers due to the increase in the rate of etch centers formation. The STM images of PbS deposits obtained after partial reductive desorption of HDT SAMs at constant potential step amplitude of −1150 but at different pulse widths are shown in Figure 8, panels a−c. Electrodeposition time was kept constant as 30 s and the potential pulse widths are 40, 80, and 120 ms for Figure 8. Sequential images show that the diameters of diskshaped PbS deposits increase with the increase in potential pulse width while the number of PbS deposits stays almost constant. The increase in the diameters of growth centers is probably due to the smaller nucleation rate. All of these morphological observations also support the chronoamperometric results. The shape and the number of PbS deposits depend on the pulse width and amplitude of the potential step, which indicates that the desorption kinetics of HDT SAMs follows the 2D instantaneous hole formation and growth mode. These observations are similar to those reported for the electrochemical depositions with the instantaneous nucleation and growth mode.41−43 It seems that the PbS preferentially grows on etched regions of HDT SAMs; therefore, the size and the number of PbS deposits can be controlled by choosing appropriate pulse widths and potential step amplitudes. We also tested the effect of electrochemical deposition time on the structure and the size of PbS deposits. The

Figure 5. STM images of HDT SAMs modified Au(111) surfaces before (a) and after (b) reductive desorption.

Assuming that the electrodeposition will take place in the holes only, the STM images of PbS deposits could serve as a probe in order to determine the SAMs-free regions on the Au(111) surface. In other words, the morphological investigations of PbS deposits will provide information about the desorption kinetics of SAMs from the Au(111) surfaces. Figure 7, panels a−c, shows the STM images of the PbS structures electrodeposited after partial reductive desorption of HDT SAMs with various potential steps at a constant pulse duration of 20 ms. Electrodeposition time was 30 s, and it was kept constant for all cases in Figure 7. These images reveal that all the PbS electrodeposits on SAMs/Au(111) obtained at three different potential steps are in rounded shape and distributed uniformly on the surface, but the number of deposits in per 1 μm2 is dependent on the amplitude of the applied potential step. These isolated nanostructures of PbS deposits appear to 8575


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Figure 8. STM image of the PbS deposits on HDT modified Au(111) electrode after reductive desorption of the HDT with a potential of −350 mV in 0.1 M KNO3. The applied potential pulse widths are 40 (a), 80 (b), and 120 ms (c).

Figure 7. STM image of the PbS deposits electrodeposited for 30 s on HDT modified Au(111) electrode after reductive desorption of the HDT for a 20 ms pulse durations in 0.1 M KNO3. The applied potential steps are from −800 to −1100 (a), −1250 (b), and −1350 mV (c).

4. CONCLUSIONS In summary, we have described an electrochemical route that could be used for “single-pot” fabrication of compound semiconducting nanostructures by combining the electrochemical patterning and electrochemical deposition techniques using HDT SAMs as a resist. The desorption kinetics of HDT SAMs are additionally studied as a function of applied potential step and pulse width, as well as the morphology of PbS deposits by STM. Electrochemical and morphological studies show that the electrodesorption kinetics of HDT SAMs follows the 2D instantaneous nucleation and growth mode. The deposited PbS

electrochemical deposition was carried out for various time intervals while keeping the potential pulse width and amplitude constant. Figure 9 shows that PbS deposits cover the surface almost completely after 120 s of electrodeposition of PbS. The line profiles of AFM images recorded for different deposition times indicate that the thickness of PbS deposits increases until a certain time, and then both the thickness and diameter of the PbS deposits increase and they cover the surface completely at longer times. 8576


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Figure 9. STM image of the PbS deposits on HDT modified Au(111) electrode after reductive desorption of the HDT by a constant 40 ms potential pulse and a potential of −350 mV in 0.1 M KNO3. The electrochemical deposition time is 120 s.

nanostructures are disk-shaped and distributed uniformly on the surface. Our results demonstrate that the combination of electrochemical lithography and electrodeposition is simple and fast, while providing deposition of control over the size and the number of PbS nanoparticles by using appropriate potential pulse width and amplitudes in the electrochemical patterning step.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +90-442-2314434. Fax: +90-442-2360948. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Atatürk University is gratefully acknowledged for the financial support of this work. REFERENCES

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