Droplets to Merged Nanostructures: Evolution of Gold Nanostructures

Article pubs.acs.org/crystal

Droplets to Merged Nanostructures: Evolution of Gold Nanostructures by the Variation of Deposition Amount on Si(111) Ming-Yu Li,*,† Mao Sui,† Eun-Soo Kim,† and Jihoon Lee*,†,‡ †

College of Electronics and Information, Kwangwoon University, Nowon-gu Seoul 139-701, South Korea Institute of Nanoscale Science and Engineering, University of Arkansas, Fayetteville, Arkansas 72701, United States

‡

ABSTRACT: We studied the evolution of Au nanostructures in domeshaped Au droplets that merge into Au nanostructures on Si(111) by systematically controlling the Au deposition amount (DA) under a fixed annealing temperature and annealing duration. Even under identical growth conditions, the configuration, density, and size of Au nanostructures drastically vary depending on the amount of Au deposition in the range of 0.5−20 nm. Through systematic analysis of the resulting Au nanostructures in determining the average height, density, and surface area ratio, as well as the Fourier filter transform power spectrum and cross-sectional line profiles, we clearly demonstrate the evolution process of Au nanostructures and thus the control of the size, density, and configurations. The evolution of Au droplets on Si(111) with the increased DAs initially appears to be progressing based on the Volmer−Weber growth mode for the Au DAs up to 4 nm, but with further increased DAs up to 20 nm, it turns out that the growth occurs in the Frank−van der Merwe growth mode, resulting in a layer-by-layer growth. In addition, by the sharp comparison between preannealed samples and resulting Au nanostructures, we quantitatively present the evolution of Au nanostructures. This study can find applications in nanowire fabrication on Si(111).

1. INTRODUCTION Recently, gold particles, namely, nanoscale Au droplets, have drawn significant attention due to their unique optoelectronic properties, such as improved optical absorption at their localized surface plasmon resonance (LSPR) frequency and shift of wavelengths and local heating, through the interactions with quantum and nanostructures.1,2 Au droplets can act as catalysts for the fabrication of one-dimensional (1-D) nanostructures in several materials systems, such as nanoscale pillars and nanowires.3−5 Nonetheless, given the wide range of materials utilized, Au droplets can be successfully utilized in the fabrication of various nanowires, and many elements utilized for substrates would diffuse into the gold droplets during the fabrication process. Up to now, various applications in optoelectronic, electrochemical, and electromechanical areas have been demonstrated utilizing nanowires fabricated using Au droplet catalysts.6−19 For instance, blue LEDs, laser diodes, nanometric devices, FETs, energy-harvesting applications, and power electronic devices have been demonstrated with GaN nanowires owing to their wide bandgap and large exciton binding energy and piezoelectricity.6−13 Likewise, wide-bandgap ZnO nanowires were utilized in FETs, LEDs, piezo-actuators, and biosensors.14−17 Single-crystal triangular ZnO nanosheets were fabricated without stacking faults through catalyst-assisted growth beneath the Au droplets.18 In addition, Si nanowires based on the Au droplet catalysts exhibited great potential for thermoelectric applications, sensors, on-chip biomolecular filtering, and solar cells.19−22 The fabrication of various nanowires © 2014 American Chemical Society

through various epitaxial techniques has been extensively studied,23−26 but systematic studies on the evolution and control of Au droplets and nanostructures are still insufficient.31−35 As discussed, the Au droplets play critical roles in determining the size, density, and dimensions of droplet-assisted nanowires, which makes this study a critical research topic. We studied the detailed evolution process of self-assembled Au nanostructures including Au droplets and merged Au nanostructures by systematically varying the amount of Au deposited on Si(111). Generally, vertically standing nanowires have been extensively demonstrated on the (111) index due to the hexagonal lattice geometry.23−26 To investigate the detailed evolution process, we systematically varied the amount of Au deposited by controlling the plasma time on Si(111) with a fixed temperature and annealing duration. For example, Figure 1 illustrates the fabrication process of Au droplets and merged nanostructures on Si(111). Bare Si(111) before Au deposition is shown in Figure 1a, and the same surface after Au deposition before annealing is shown in Figure 1b. Surface morphologies are equally very smooth, as clearly evidenced with the surface line profiles in Figure 1a-1,b-1. Subsequently, after annealing at 700 °C for 150 s, distinctive nanostructures were fabricated under identical growth conditions. For instance, Figure 1c shows Au droplet formation with the deposition amount of 2 nm. Received: October 28, 2013 Revised: December 17, 2013 Published: January 21, 2014 1128

dx.doi.org/10.1021/cg401604q | Cryst. Growth Des. 2014, 14, 1128−1134

Crystal Growth & Design

Article

Figure 1. Illustration of the fabrication process of Au droplets and merged Au nanostructures by the variation of deposition amount at 700 °C annealed for 150 s on Si(111). (a) Atomic force microscope (AFM) image of bare Si(111). (b) After Au deposition. (c) Self-assembled Au droplets with 2 nm Au deposition. (d) Merged nanostructures with 6 nm Au deposition at 700 °C. AFM top views of a−c are 1 × 1 μm2, and that of (d) is 3 × 3 μm2. Panels a-1 through d-1 present cross-sectional surface line profiles indicated as white lines in panels a−d. AFM side-view of panel c-2 is 250 × 250 nm2, and that of panel d-2 is 600 × 600 nm2.

When the deposition amount was increased to 6 nm, merged Au nanostructures resulted.

2. EXPERIMENTAL PROCEDURES Si(111) wafers (singular p-type, 4 in.) with a thickness of ∼1000 μm were utilized in this study. Initially, Si wafers were diced into 1 × 1 cm2 pieces by a dicing saw and RCA cleaned. Prior to each growth, Si substrates were degassed at 850 °C for 30 min under 1 × 10−4 Torr in a pulsed laser deposition (PLD) chamber. Gold films were deposited in a plasma ion-coater chamber on Si(111) under a vacuum of 1 × 10−1 Torr. In order to investigate the comprehensive evolution process of Au nanostructures, the deposition amount of gold was systematically varied by the plasma time at a growth rate of 0.5 Å/s with an ionization current of 3 mA. Au depositions of 0.5, 1, 2, 2.5, 3, 4, 4.5, 5, 5.5, 6, 8, 16, and 20 nm were systematically performed for each sample. For example, 10 s of plasma time was used for the Au deposition of 0.5 nm. Subsequent to the gold deposition, the annealing process was performed, in which the substrate temperature was raised to 700 °C at a rate of 2.3 °C/s under a vacuum of 1 × 10−4 Torr by using a computer-controlled recipe. After reaching the target temperature of 700 °C, a dwell process for 150 s was added to ensure uniformity, followed by immediate quenching to minimize Ostwald ripening. An atomic force microscope (AFM) was used for surface morphology characterization, and XEI software (Park Systems) was used for the analysis of the acquired data. For larger-scale images, a scanning electron microscope (SEM) was utilized.

3. RESULTS AND DISCUSSION Figure 2 shows the general evolution process of Au droplets and merged nanostructures on Si(111) along with the increased Au deposition amounts of 2−16 nm at a fixed temperature of 700 °C for a fixed annealing duration of 150 s. AFM top views are presented in Figure 2a−f, and AFM side views are shown in Figure 2a-1−f-1. Figure 3 shows the summary of the average height (AH) of Au nanostructures in Figure 3a and average density (AD) in Figure 3b versus the corresponding deposition amounts. Generally, along with the increased Au deposition amount, the Au droplets became larger and the density was quite reduced, as clearly shown in Figure 2a−c. At 6 nm of Au deposition, as shown in Figure 2d, merged nanostructures were observed. With further increased Au deposition above 6 nm,

Figure 2. General evolution trend of Au droplets (a−c) and of merged nanostructures (d−f) by the variation of Au deposition amount. The corresponding Au deposition amounts are indicated with labels in (a−f). AFM top-views are presented in panels a−f, and AFM side views of the identical sizes are shown in panels a-1−f-1. Panels a−c are 1 × 1 μm2, panels d and e are 3 × 3 μm2, and panel f is 10 × 10 μm2.

merging was enhanced, and it resulted in a connected geometry of Au nanostructures, as shown in Figure 2e,f. In a more detailed analysis, on the bare Si surface in Figure 2a, with 2 nm of 1129



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when the amount of deposition was increased to 10 ML at a fixed temperature of 350 °C.27 However, the density was changed only a few times when the deposition amount was changed from 1 to 10 ML. Meanwhile, when the surface temperature was changed for a fixed deposition amount of 10 ML, the Ga droplet height and density behavior quite resembled that presented in this experiment. Figure 4 shows a more detailed evolution process of the Au droplets in Figure 4a−f and of the merged Au nanostructures in

Figure 3. Summary of the average height in the plot a and average density in plot b of Au droplets and merged nanostructures. In both plots, the x axes are the Au deposition amounts, and y axes indicate the average height in plot a and the average density in plot b.

deposition in Figure 2b, dome-shaped gold droplets were formed, and the lateral size of droplets was ∼50 nm. The average height (AH) was 16.5 nm, and the average density (AD) was 5.6 × 1010 cm−2, as plotted in Figure 3. We only plotted the average height of droplets, because the diameter is strongly affected by AFM tip convolution. Nonetheless, the same type of tip from a single batch was utilized to minimize the tip effect on the latter size. At 4 nm, as clearly seen in Figure 2c,c-1, the lateral size of Au droplets became much larger in the range of ∼130 nm, and the droplets showed a slight tendency to elongate. The AH became 48.6 nm, and AD was 6.8 × 109 cm−2, which are ×2.94 taller and ×8.2 lower in density compared with those from the 2 nm deposition. As presented in Figure 2d,d-1, with the 6 nm deposition, Au droplets completely disappeared and, instead, merged Au nanostructures with irregular shapes appeared. The AH of merged nanostructures was 68.9 nm, and the AD was reduced to 5.7 × 107 cm−2, which are ×1.4 taller and about 2 orders of magnitude lower in density compared with the 4 nm sample. In Figure 2e,f, with 8 and 16 nm depositions, Au nanostructures merged further, indicating that the region of merged nanostructures became wider, and the density was further reduced. As clearly seen in Figure 3a,b, the AH showed an overall increasing trend with the increased deposition amount, and AD showed a generally decreasing tendency when the deposition amount was increased. In the analysis of the size and density evolution, with the increased deposition amount, bigger droplets can be formed, and they have lower surface energy. Therefore, bigger droplets can attract nearby adatoms with lower surface energy and tend to grow bigger and bigger until they reach an equilibrium. As a result, the density can decrease as the dimensions become larger. The trend of decreasing density with increased nanostructure dimensions is a conventional behavior of nanostructures on various semiconductor surfaces.27−29 It is contradictory that this is happening at a fixed temperature with the variation of deposition amount, and the result is quite similar to the surface temperature variation for other metal droplets, such as with Ga droplets on GaAs. In the case of Ga droplets on various GaAs surfaces, the droplet height increased quite sharply from ∼5 nm with 1-monolayer (ML) deposition up to ∼30 nm

Figure 4. Detailed evolution process of the Au droplets (a−f) and the merged Au nanostructures (g−l) by the variation of Au deposition amount. Panels a−g are 1 × 1 μm2, panels h−j are 3 × 3 μm2, and panels k and l are 10 × 10 μm2.

Figure 4g−l with a symmetric variation of Au deposition amounts of 0.5−20 nm. Figure 5 presents the corresponding cross-sectional line profiles in Figure 5a−l along with the 2-D Fourier filter transform (FFT) power spectra in Figure 5a-1−l-1. Figure 6a−6l shows larger scale areas of Au droplets and merged nanostructures by AFM and SEM. Three size groups of Au droplets appeared: minidroplets with Au depositions below 1 nm, midsized droplets with Au depositions between 1 and 3 nm, and large droplets with Au depositions between 3 and 4.5 nm. In more detail, with 0.5 and 1 nm depositions in Figure 4a,b, the AH of droplets was only ∼4 nm, and the minidroplets were densely packed on the surface with the AD of ∼8.5 × 1010 cm−2. As the deposition was increased from 1 to 2 nm by just 1 nm, the AH showed a sharp increase from ∼4 to ∼17 nm, and the AD showed a sharp decrease to 5.6 × 1010 cm−2. Again, surprisingly, this just happened by adding 1 nm more Au while fixing the other growth parameters. With up to 3 nm, the AH was gradually increased to ∼40 nm, and AD kept decreasing to 9.6 × 109 cm−2, as clearly shown in Figure 3a,b. As the AH of droplets kept increasing with the increased deposition amount, larger-size Au droplets were observed between 3 and 4.5 nm depositions: the AH reached 1130



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Figure 5. Panels a−l show the cross-sectional surface line profiles of resulting Au nanostructures acquired from the white lines in Figure 4. The x-axes in line profiles are 500 nm in panels a and b, 1 μm in panels c−g, 3 μm in panels h−j, and 10 μm in panels k and l. The y-axes are 8 nm in panels a and b, 40 nm in panels c−f, 120 nm in panels h and i, and 160 nm in panels j and l. Insets a-1−l-1 show the corresponding 2-D Fourier filter transform (FFT) power spectra.

With high-density droplets with distinctive height, the FFT showed a brighter pattern, as in Figure 5c-1. With a decrease in the density, the FFT spectra became dimmer. With deposition above 4.5 nm, merged nanostructures were observed, as shown in Figure 4g−4l. Between 4.5 and 5 nm of deposition, there was a sharp transition from the Au droplets to the merged nanostructures, as clearly seen in Figure 4g,h. For example, at 5 nm of deposition, with 0.5 nm increased deposition, the AH was increased to ∼70 nm by ∼20 nm, and the AD was further decreased to 2.6 × 109 cm−2 by nearly 1 order of magnitude, as plotted in Figure 3. This sets the critical transition amount from Au droplets to merged nanostructures at ∼4.5 nm of deposition. Above 5 nm of deposition, the AD kept on decreasing, but the AH stopped increasing and showed a mild decreasing trend as the merging and areal expansion took over the height increase. For instance, with more than 5.5 nm of deposition, the AH stopped increasing, and the expansion of the area of merged nanostructures was dominated, as clearly shown in Figure 4i−l. With 16 and 20 nm, we could observe that the Au nanostructures were finally merged to form a layer, as shown in Figure 4l, which shows that new islands started to form on top of the layer. The FFT spectra became quite dimmer with much lower density of the nanostructures, as shown in Figure 5g-1−l-1. As seen in the larger-scale images of Figure 6a−l, the Au droplets and merged nanostructures were quite uniformly distributed over the surfaces, and the increased size of Au droplets with the increased deposition amount appeared in Figure 6a−f. Also, the merging of Au nanostructures was clearly observed in Figure 6g−l. Initially, with the Au DAs up to 4 nm, the evolution of Au droplets appeared to be in the Volmer−Weber growth mode36,37 with distinctive 3-D islands, but with further increased DAs up to 20 nm, it turned out that the growth occurred in the Frank−van der Merwe growth mode,30 resulting in the formation of a layer.

Figure 6. Large-scale images of Au droplets (a−f) and merged nanostructures (g−l), showing the uniformity over the larger surface areas. Panels a−f are 3 × 3 μm2 and show 5 × 5 μm2 of AFM images. Panels g−j are 50 × 50 μm2, and panels k−l are 70 × 70 μm2 of SEM images.

∼50 nm, and AD was 2.9 × 109 cm−2 with 4.5 nm of deposition. The height increase of Au droplets is also clearly shown with the corresponding cross-sectional line profiles in Figure 5a−f. In terms of the shape uniformity, droplets were quite uniform, as shown with the symmetric round FFT spectra in Figure 5a-1−f-1. 1131



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Figure 7. Panels a−n show the AFM images after Au deposition before annealing. Panels a-1−n-1 are the corresponding line profiles, panels a-2−n-2 are the 2-D FFT power spectra, and panels a-3−n-3 show the height distribution histograms (HDH) around zero. The AFM images in panels a−n are 1 × 1 μm2.

Figure 7a−n show the AFM images after the deposition of the corresponding amounts of gold before annealing in the range of 0−20 nm. The corresponding surface line profiles are shown in Figure 7a-1−n-1, the 2-D FFT power spectra are shown in Figure 7a-2−n-2, and height distribution histograms (HDH) are shown in Figure 7a-3−n-3. Figure 8 compares the preannealed surfaces and the annealed surfaces. The geometric area [α] is (x × y) shown in Figure 8a,d, the surface area [β] is (x × y × z), as shown in Figure 8b,e, and the surface area ratio [γ] is {(β − α)/ α} × 100 [%], as shown in Figure 8c,f. Overall, the preannealed surfaces in Figure 7a−n showed very smooth topographies, as clearly seen with the surface line profiles in the range of ±1 nm in

Figure 7a-1−n-1. The smoothness was also supported by the very small increments of surface areas in Figure 8b. The height distribution histograms in Figure 7a-3−n-3 also clearly reflect the smooth modulation of surfaces: below ±1 nm for less than 2 nm of deposition, around ±1 nm for between 2 and 16 nm of deposition, and around ±2 nm with 20 nm of deposition. Although there appeared to be an increasing trend of γ in Figure 8c, the increment was still quite minute: from 0.3% to only 2.68% when the deposition was increased even up to 20 nm. The γ was below 1% at up to 3 nm of deposition and below 1.5% even with up to 16 nm. Meanwhile, for the annealed surfaces, the γ in Figure 8f was gradually increased at up to 4 nm of deposition and 1132



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Figure 8. Comparison of the geometric areas, surface areas, and surface area ratios of preannealed surfaces in panels a−c and surfaces after annealing in panels d−f. The geometric area [α] is (x × y), the surface area [β] is (x × y × z), and the surface area ratio [γ] is {(β − α)/α] × 100 (%).

then began to decrease from there. In more detail, the γ was smaller at up to 1 nm of deposition: only 3.26% due to the minidroplet formation as discussed with Figure 4a,b. The γ was then quite sharply increased from 2 to 2.5 nm of deposition: 13.62% with 2 nm to ∼36% with 2.5 nm of deposition, which can be due to the formation of the densely packed midsized droplets as discussed with Figure 4c,d. With the further increased deposition amount, the γ kept on increasing to ∼40% with 4 nm deposition due to the large droplet formation, and then slightly decreased to ∼30% with 4.5 nm deposition as the density was sharply decreased from there. With the merged nanostructures above 5 nm, the surface ratio was further sharply decreased, as seen in Figure 8f. The increase at up to 4 nm of deposition can be due to the increased size of Au droplets, and the decrease above 4 nm could be due to the extensively reduced density of droplets, as shown with Figures 4 and 6, indicating that the expansion of surface area was overcome by the density reduction. Figure 9 shows the effects of annealing temperature and annealing duration on the Au droplets. For example, the Au droplets in Figure 9a,b were fabricated under identical growth conditions, except for the annealing temperature: 2 nm Au deposition for 150 s of annealing at (a) 350 and (b) 700 °C. As clearly seen in the AFM images, the droplets at 700 °C are somewhat larger, and the density is lower. At 350 °C, the AH was ∼15 nm with the diameter of ∼37 nm, and the AD was 6.4 × 10−10 cm−2. At 700 °C, the AH was ∼17 nm with the diameter of ∼45 nm, and the AD was 5.6 × 10−10 cm−2, showing an expansion of size with the increased height and diameter and the reduced density, as can be expected under thermodynamic equilibrium. The increased size was also observed by the slightly expanded HDH in Figure 9b-2, and both droplets show similar uniformity, as shown with the FFTs in Figure 9a-3,b-3. For the Au droplets fabricated at an extended annealing duration of 1 h

Figure 9. Annealing temperature and annealing duration effects on Au droplets. Au droplets in panel a were fabricated at 350 °C with 2 nm of Au deposition. Au droplets in panel b were fabricated under identical growth conditions except for the annealing temperature of 700 °C. (c) Au droplets resulting from an increased annealing duration of 1 h. (a−c) AFM images of 1 × 1 μm2. Panels a-1−c-1 show the cross-sectional line profiles. Panels a-2−c-2 are height distribution histograms (HDHs), and panels a-3−c-3 are the corresponding FFT power spectrum patterns. 1133



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(24-fold increase) in Figure 9c, the droplets showed quite similar size and density of samples in Figure 9b, ∼18 nm for the AH and ∼5.4 × 10−10 cm−2, but a slight increase in the height and a reduction in density were observed. It appears that the Au droplets do not respond to the extended annealing duration, and perhaps, the equilibrium was already reached for the growth conditions. In short, both the increased annealing temperature and duration showed similar effects of slightly increased size and decreased density, but the degree of variation was quite mild. In other words, the control of size, density, and dimension of Au droplets can be more effectively achieved by the DA variation.

4. CONCLUSIONS In conclusion, the evolution of self-assembled Au droplets and merged nanostructures on Si(111) has been studied by variation of the Au deposition amount in the range of 0.5−20 nm. In general, the size of Au nanostructures was increased with the increased deposition amount, while the density showed a correspondingly decreasing trend. The detailed evolution process was discussed in terms of size and density analysis, AFM images, SEM images, line profiles, and FFT spectra. There appeared three size groups of droplets: minidroplets with less than 1 nm of deposition, midsized droplets between 1 and 3 nm of deposition, and large droplets between 3 and 4.5 nm of deposition. For the Au deposition below 1 nm, the droplets are below ∼5 nm in height, and the height sharply increased with 2 nm deposition and grew up to ∼50 nm with 4.5 nm deposition. The critical deposition amount of the transition between droplets and merged nanostructures was found to be ∼4.5 nm. The surface area ratio was sharply decreased with 4.5 nm deposition, which was likely due to the formation of merged nanostructures. The expansion of surface area was overcome by the density reduction. The Au droplet evolution with the increased DAs initially appeared to progress based on the Volmer−Weber growth mode with DAs of up to 4 nm, but with further increased DAs of up to 20 nm, the growth occurred in the Frank−van der Merwe growth mode, resulting in a layer-by-layer growth. This study can help with the fabrication of nanowires on Si(111).

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation (NRF) of Korea (Grant Nos. 2011-0030821 and 2013R1A1A1007118). This research was in part supported by the research grant of Kwangwoon University in 2013.

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REFERENCES

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Droplets to Merged Nanostructures: Evolution of Gold Nanostructures

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