Controlled Growth of High Aspect-ratio Single-crystalline Gold Platelets

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Controlled Growth of High Aspect-ratio Single-crystalline Gold Platelets Enno Krauss, René Kullock, Xiaofei Wu, Peter Geisler, Nils Lundt, Martin Kamp, and Bert Hecht Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b00849 • Publication Date (Web): 15 Feb 2018 Downloaded from http://pubs.acs.org on February 19, 2018

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Crystal Growth & Design

Controlled Growth of High Aspect-ratio Single-crystalline Gold Platelets †

Enno Krauss,

René Kullock,

†

Xiaofei Wu,

Kamp,

‡,¶

†

Peter Geisler,

and Bert Hecht

†

‡

Nils Lundt,

Martin

∗,†,¶

†NanoOptics & Biophotonics Group, Experimentelle Physik 5, Physikalisches Institut,

Universität Würzburg, Am Hubland, 97074 Würzburg, Germany ‡Technische Physik, Physikalisches Institut, Universität Würzburg, Am Hubland, 97074

Würzburg, Germany ¶Röntgen Research Center for Complex Material Systems (RCCM), Am Hubland, 97074

Würzburg, Germany E-mail: [email protected]

Abstract We describe the wet-chemical synthesis of high aspect-ratio single-crystalline gold platelets with thicknesses down to 20 nm and edge lengths up to 0.2 mm. By employing statistical analysis of a large number of platelets we investigate the eect of temperature on the growth velocities of top and side facets for constant concentrations of the three common ingredients - ethylene glycol, chloroauric acid and water. We further show that by varying the chemical environment during growth, the ratio between the growth velocities can be adjusted, and thus thickness and lateral size can be tuned independently. Very large but ultrathin single-crystalline gold platelets represent an important starting material for top-down nanofabrication and may also nd applications as transparent conducting substrates as well as substrates for high-end scanning probe and electron microscopy. 1

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Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nanoscale metal structures are rapidly gaining interest in various elds such as Plasmonics 1 and sensing, 24 surface enhanced Raman scattering 5 and solar energy conversion. 6,7 Especially single-crystalline gold, which combines superior optical properties with chemical stability in ambient conditions, has attracted some attention, as it forms the basis for precise single crystal nanoscale structures of arbitrary shape. 8,9 While rod-like gold structures can be grown with the probably highest yield of all anisotropic synthesis, 10 for large plate-like crystals there is still a lack of control and understanding of the conditions that favor high aspect ratios. 11 Recipes for the synthesis of rather thick gold platelets whose lateral size almost reaches the mm-regime have already been reported, 12,13 however, large and yet very thin platelets have only rarely been realized, 1416 and typically of insucient quality to use them as substrates for high-precision nanoscale structuring. To represent a platform for high-quality plasmonics thicknesses below 80 nm are needed to achieve the required precision during nanostructuring and lateral dimensions above 100 µm edge length provide space for large arrays of plasmonic devices allowing systematic studies. 17 Decreasing the thickness even further, below 20 nm, would result in ultra thin metal lms, which, in theory, promise strongly increased eciency in optical coupling to plasmonic modes. 18 Therefore independent control over the thickness and lateral dimensions is desirable which has not been reported so far. However, control of these geometrical parameters is complicated by a large spreading of gold platelet dimensions even under nominally identical growth-conditions. 19 Statistical analysis of the resulting platelet sizes is therefore necessary to separate random uctuations from the actual impact of changes in the recipe. Here we present a facile synthesis method for platelets with lateral sizes of hundreds of microns and thicknesses ranging down to 20 nm. To cope with the broad distribution of lateral sizes and thicknesses that occur during the synthesis, we use computer-automated analysis of all geometrical dimensions in combination with time-dependent growth studies for several temperatures. This allows us to investigate the inuence of the temperature on the growth velocities of the platelets' top and side facets, and we nd that by altering the 2


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chemical environment during growth, the velocities can be altered, resulting in platelets with adjustable aspect ratio.

Growth mechanism

Figure 1: Growth Model: I. - III.) Chemical reactions leading to the reduction of gold atoms which can form crystal seeds (IVa.) or bind to the surface of an already existing crystal (IVb.). The schematic drawing of a gold-platelet exhibiting a typical distribution of the two lattice planes, {100} and {111}, as side facets. The growth of plate-like gold crystals follows a two-step process consisting of (a) initial seed formation and (b) subsequent crystal-growth. The seed formation is induced by a supersaturation of the Au-precursor (HAuCl2− ). 20 An established model for the reducing of the chloroauric acid HAuCl4 is a 3-step chemical process 21 as described in Figure 1, where in our case ethylene glycol is thought to act as solvent and reduction agent. 22,23 Reaction I is endothermic and can be initiated e.g. by heat. The subsequent spontaneous reaction II leads to a metastable precursor HAuCl2− which is further reduced to neutral gold-atoms. Those can either attach to already existing crystals or, under conditions of supersaturation, form 3



new seeds. The specic atomic arrangement of the seeds determines the nal macroscopic shape of the resulting crystals like platelets, tetrahedrons, nanorods, spheres and so on. 20,24,25 The chemical environment and temperature have been shown to inuence the number as well as the kind of seeds 26 and therefore determine the nal yield of the dierent shapes of Au crystals. 20 The reason for the strong anisotropic growth of gold platelets are stacking faults of the

{111}-layers in the crystals, originating from twinned crystal seeds. 9,19,27 They lead to {111} top and bottom facets and a composition of alternating {100} and {111}-planes on the side facets. 28 Since an adatom has 4 neighboring atoms on the {100}-plane and only 3 on the

{111}-plane, the {100}-plane is energetically favored and thus grows much faster. Hence, the twinned crystal seeds grow into large but thin platelets. Note, for seeds with only one single twin plane the {100} side facets disappear with increasing lateral size, 29 causing anisotropic growth to stop early, while in the presence of two or more twin planes, the side {100}-planes regenerate and therefore allow large lateral sizes. While supersaturation is required in the beginning of the growth to produce seeds, it is no longer desired in later stages of the growth since the formation of seeds would lead to a contamination of the sample with newly formed small particles. This atomistic description can now be related to the macroscopic dimensions of the platelets, namely the thickness or height h and the area AT on the top (or bottom) {111}facet of the platelet. Quantitatively, during the growth the area is expected to increase linear by time according to AT (t) = AT,0 + vT ∗ t with dierent parameters for individual platelets probably depending on the arrangement and number of twin planes. 30,31 The side-facet area, a composition of {100} and {111}-facets, is proportional to the product of height h and √ √ average length l = AT of the platelet. It can be expressed as AS = c h AT , where the constant c depends on the geometrical form as well as the composition of {100} and {111}planes of the side facets and is specic for each platelet. So far kinetics of the growth of the side facets have not been considered. The dimensions of nal structures are determined by 4


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the maximal aspect ratio AR = AT /AS = l/ch of the platelets. To change this AR, the ratio of the facet growth velocities has thus to be altered.

Experimental Recipe The growth is performed in polypropylene centrifuge tubes [Greiner, 50 ml] using ethylene glycol (EG)[J.T. Baker] as solvent. Throughout the manuscript, all concentrations are given with respect to EG. Chloroauric acid HAuCl4 -salt [Sigma Aldrich], dissolved in a volume of puried water [Sigma-Aldrich], is added to the EG. Changing the order of the mixing, for example dissolving the salt in EG and adding the water afterwards, did not aect the results. This indicates that the seed formation, which strongly inuences the results, happens at a later stage, supposedly while heating. As substrates for the growth glass coverslips [24mm × 24mm #1.5 Menzel] are used since platelets directly grown on the substrates show increased quality in comparison to those dispersed in solution. 30 The glass substrate further facilitates follow-up characterization of the platelets. The cover glasses are cleaned in acetone and ethanol in an ultrasonic bath for 10 minutes each, and afterwards rinsed with ultra-pure water and exposed to an oxygen plasma for 15 minutes. Two cover slips were immersed into the growth solution back-toback, such that only one side of each is exposed to the growth solution. The cover slip pairs were positioned slightly tilted upwards in the tube. Subsequently the solution was heated in an oven and kept at a xed temperature for a certain period of time during which the growth reaction took place. The tubes were kept shut during the growth. After the growth period the slides were taken out of the solution, rinsed with ethanol and water, and dried in a nitrogen stream. Only the coverslip that was facing downwards during growth was considered for further evaluation, since it is free of unwanted sedimentation from akes and particles dispersed in the growth solution. 5



Determination of thickness and lateral area 1

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70

Figure 2: Flake Analysis: Scanning electron microscope (SEM) (a), atomic-force microscope (AFM) (b) and optical transmission (c) images of three gold platelets. The AFMline-cuts (inset in b) indicate thicknesses of 22, 28 and 61 nm. Note, the AFM image is a compilation of three measurements, whose borders are indicated by white dashed lines. d) Optical transmission of the platelets in dependence on the thickness (black points) and an exponential t (red line). A typical example of the resulting gold platelets is displayed in Figure 2a (also see SI for transmission electron microscope images verifying the crystallinity of the resulting platelets as well as the presence of twin planes). The scanning electron microscope (SEM) image shows platelets with lateral sizes of tens of micrometers and no contamination over the whole surface, however gives no information about the platelets' thickness. Atomic-force microscope (AFM) measurements (see Figure 2b) verify a at surface and are used to determine the thicknesses 22, 27 and 62 nm in the depicted case. However, since AFM is a very slow method it is unsuited for statistical analysis. We therefore developed a dierent approach. 6


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Figure 2c shows an CCD image of the optical transmission of the gold platelets [Canon EOS 100D mounted on an Olympus IX 70] using a 20x magnication objective, a 1.5x tubus lens and a halogen lamp for illumination. The transmission was obtained by evaluating the signal of the green camera channel only, subtracting dark counts and dividing by a background picture, i.e without any sample. In Figure 2c a contrast for the dierent thicknesses is clearly visible and the transmission decreases from 52% for the thinnest platelet to 45% and 18% for the thicker ones. In order to extract values for the thickness from these data, a calibration was performed as follows: Thicknesses of a broad range of individual platelets were determined with a spectral method 30 and correlated to the simple transmission obtained with the CCD green channel. The resulting data (see Figure 2d) show an exponential dependence and were tted accordingly. Hence, the thickness h of every platelet in a given image can be extracted by its integrated transmission. The method works for platelets up to a thickness of 90 nm (for thicker platelets the transmission is too low) and down to a lateral size of 20 µm (below which for the chosen magnication the transmission cannot be obtained accurately anymore due to diraction eects). The error of the transmission is derived by performing multiple measurements of the same platelet when positioned in dierent locations within the microscope's eld of view. The transmission values obtained for the three platelets in Figure 2c lead to thicknesses of

21, 26 and 61 nm, respectively. We note that there is an oset of −1 or −2 nm which is also observed between the spectral measurement and the AFM measurements. We attribute this discrepancy to the presence of a surrounding molecular layer 9 that is transparent for optical measurements but exhibits a nite thickness. The lateral scale in the transmission micrographs is calibrated using a micrometer length standard. Thus, besides the already √ discussed thickness h, the area AT and the typical length l = AT can be extracted for every platelet. To speed up this analysis the CCD images were processed by a self-written computer program.

7



Results and discussion Results 0

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Figure 3: Statistics: a, b) Height vs. typical length of platelets for a growth temperature of 90◦ C and a growth duration of 9 h (a) and 18 h (b), respectively. Corresponding length and thickness histograms are shown at the top and to the right hand side. c-e) Mean value and linear growth model of the top area AT (dots, solid line) and side area AS /c (crosses, dashed line) for 70◦ C ( c),red), 80◦ C ( d), blue) and 90◦ C ( e), black). The vertical error bars represent the standard derivation of the platelets in the corresponding sample. Note, the time axis is divided by a factor of 2 with every 10◦ C temperature step. At the beginning we optimized the basic growing recipe concentrations of incidents in order to obtain large platelets with high aspect ratios, and base all following steps on it. The basic recipe consist of a mixture of 1 mM HAuCl4 with 0.2% water in EG. In our case we used 8


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20 ml EG, mixed with 40 µl of a 0.5 M aqueous solution of HAuCl4 . Doubling or halving the volume of the overall solution did not have an impact on the obtained results. For a growth temperature of 90◦ C and durations of 9 h and 18 h, the correlation of lateral extension √ AT and thickness h is plotted in Figure 3. For the 9 h (Figure 3a) growth experiment the thicknesses are distributed homogeneously over the measurable regime, while for 18 h (Figure 3b) the thicknesses are mainly shifted into the non-transparent region above 90 nm but also the maximal length increases. In order to obtain more quantitative data for large and high-aspect ratio platelets, we use the fact that we performed measurements on individual platelets and do not deal with averaged numbers of population. Hence, it is technically possible to select specic subensembles. Similar data analysis strategies are being used with great success in the eld of single-molecule spectroscopy. 32 Note, as our measurements are restricted to thinner than

90 nm and larger than 20 µm platelets, we do not acquire the whole population but a sub-ensemble already. For the subsequent analysis of growth dynamics we pick the biggest 5%. Laterally smaller sub ensembles, e.g. the biggest 5% - 10%, show smaller AR (see SI), supporting our choice of looking only at the largest 5% for fullling our demand of large high-aspect ratio platelets. We want to point out that although the mean AR for the smaller platelets is lower and they are not taken into account for the following analysis, platelets with thicknesses between 10 nm and 20 nm occur in these sub-ensembles. With a still reasonable typical length of up to 50 µm, they might be an interesting substrate of ultra-thin structures. To investigate the time-dependent behavior more accurately we analyze samples with the same starting ingridents but dierent growth durations. These time series were acquired for three dierent growth temperatures, namely (70◦ C , 80◦ C and 90◦ C ). We want to point out that the standard deviation arising for these subensembles is not due to systematic errors caused by the measurement methods, but rather due to the fact that even for exact similar conditions, the platelets show a broad distribution in the results. This further highlights the need of statistical investigation in order to provide quantitative statements about the 9



growth kinetics. In Figure 3 c), d) and e) it is observed that for all three temperatures the top- and side areas increase linearly with time. The slope is directly linked to the growth velocities vA = dA/dt for the respective facets. In the plots the time scale was divided by a factor of two for every temperature step of 10◦ C , following a rule of thumb for rst-order chemical reactions. With this scaling, the data for the dierent temperatures indeed look very similar. More importantly, the ratio between both growth velocities vT /vS (see Table 1) does not change with temperature. This is directly visible in the similar AR of the platelets for all investigated growth temperatures (See SI). For 90◦ C and after 20 h, the top-area increase slows down (See SI). This is presumable due to the depletion of reactants, which is additionally supported by the observation of a corresponding color change of the growth solution from an initial yellow to colorless.

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Figure 4: Relling: a) Mean value of the top facet area AT (dots, solid line) and side facet area AS /c (crosses, dashed line) for the largest 5 % of the platelets with relling (0.07% water and 34 mM HAuCl4 ) every 3 h up to 15 h. The grey lines represent the control groups velocities without relling. b) Aspect ratio vT /vS with (red, crosses) and without relling for transparent (black, dots) and nontransparent platelets (black, no symbol). The horizontal lines represent the mean AR of all samples with (red, dashed) and without (black, solid) relling. c) A transmission image of two platelets grown under optimized conditions with a lateral extension up to 160 µm and a thickness of 33 nm for both. In order to prevent such depletion and to allow ongoing platelet growth, we now periodically rell a mixture of gold salt solution and water ( 0.07% water and 34 mM HAuCl4 every 10


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3 hours) at a growth temperature of 90◦ C . The rell solution was added one drop at a time to the growth solution with a temperature about 10◦ C below room temperature, since for the storing the HAuCl4 solution is kept in a fridge. The results are presented in Figure 4a where the mean top and side area of the biggest 5 % of the platelets are plotted. Again linear growth of the platelet top and side area is observed(red solid and dashed line), whereas the growth seems to start later than in the control group (grey solid and dashed line) without relling. This is indicated by the intercept with the time-axis at around 7 − 8 h for the relling experiment in comparison to 2 − 3 h for the control group. After the delayed start, however, the growth velocity of the top facet is increased compared to the control group. In contrast, the growth velocity of the side facet is similar to the control group. Thus, the ratio between both velocities has improved due to the relling. All obtained growth velocities are listed in Table 1 and in Figure 4b the mean AR with and without relling are shown. For the control group (black dots) the mean AR starts from below 2000 and then drops below 1000 after about 12 h, when thicknesses of the control group increase beyond 90 nm and thus into the nontransparent regime (See SI). In order to still obtain a reasonable range for the AR for longer times, the value of 90 nm was taken as a lower error bound and 400 nm as the upper error bound, since during further processing with additional methods like scanning electron and atomic force microscopy, thicker akes were very rarely observed. The thus acquired range for the AR is displayed as black vertical lines without symbols in Figure 4b. In contrast, the platelets of the group with constant relling (red crosses) remain transparent (See SI) and show an increase of AR over time. With relling, even after 18 h a majority of large platelets is still transparent, whereas in the control group after this time not a single large platelet was transparent anymore. Some platelets of the relling experiment reach AR of above 4000, which is an increase by an factor of at least 3 compared to the control group. This is especially very advantageous since this high AR occurs at a time where the akes are very large, with lateral dimensions of hundreds of microns. A typical result of this optimized growth is shown in Figure 4c with two platelets having lateral dimensions of up to 160 µm 11



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and a uniform transmission of 36%, corresponding to a thickness of 33 nm. This fullls the already mentioned demands of large and still optically thin single-crystalline gold platelets.

Discussion Table 1: Growth velocities: The top (vT ) and side (vS ) area growth velocities as well as their ratio vT /vS for 70◦ C , 80◦ C , and 90◦ C with and without relling are shown. T [◦ C ]/Rell 70 / 0 80 / 0 90 / 0 90 / 1

vT [µm2 ] 1.9 ∗ 102 4.4 ∗ 102 1.0 ∗ 103 1.8 ∗ 103

vS [µm ∗ nm] 1.7 ∗ 102 3.2 ∗ 102 8.5 ∗ 102 7.2 ∗ 102

vT /vS 1.1 ∗ 103 1.4 ∗ 103 1.2 ∗ 103 2.5 ∗ 103

Models for the kinetics of gold platelet growth are very rare. Two studies of individual platelets growth suggest a linear increase of the top facet area with time, 30,31 however the growth of the side facets, which might inuence the increase of the top facet area, has never been taken into account. Here we investigate lateral and thickness increase and nd a linear growth of both crystal areas for all considered temperatures. The data for the side area exhibit a bigger spreading which we attribute to the additional geometry factor

c diering from platelet to platelet. Both growth velocities double from 70◦ C to 80◦ C and further to 90◦ C . Since the change occurs for the velocities of both facets, the ratio between them is constant at about vT /vS ≈ 1.2 ∗ 103 within the temperature range, and hence the temperature cannot be used to optimize the AR of the platelets - at least not between

70◦ − 90◦ C . Also, no eect on the growth velocities could be observed by changing the composition of the starting reactants (See SI). The desired change in the vT /vS ratio was only observed when we periodically added a mixture of chloroauric acid and water during the growth, resulting in a strong increase of vT /vS by a factor of 2 − 3. For gold platelets, such an independent alteration of the growth velocities of the dierent facets without any additional capping agent has not been reported so far. To our knowledge, the only report about thickness control in gold platelets is from 12

Xu and coworkers. 33 They explain a change


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in the thickness as a

"synergistic eect" between H + and Cl− in their recipe. However,

they do not take into account the area of the platelets, and thus might have observed an eect similar to our results with change in temperature, which also aects the thicknesses at xed growth times. Hence, they have not shown an independent control of lateral extension and thickness as it is clearly demonstrated in the present study. To gain more insight we conducted further control experiments. For relling only water or chloroauric acid, no change in the AR was observed. An increase in the overall amount of the water-chloroauric-acid mixture decreased the vertical growth further, but also strongly decreased the lateral growth and particle number. By adding the whole volume otherwise relled at the beginning of the reaction, nearly no platelet growth was observed. We suppose that the added gold salt in conjunction with the water changes the chemical equilibrium of the reaction in Figure 1 directly by the amount of the reactants and also indirectly by changing the acidity of the environment. Thus, similar to Xu and coworkers, we also assume a complex non-equilibrium mechanism driving the facet growth. To gain deeper understanding is left for future work.

Conclusion We present a recipe that allows facile growth of high-quality single-crystalline gold platelets that combines smooth large lateral extension with thicknesses in the nanometer regime. For a comprehensive investigation we developed an optical method that enables statistical analysis of the geometry of the resulting platelets. By changing the chemical environment during growth, the platelets kept thicknesses that are in the optical transparent regime, which, combined with the single-crystalline gold structure, could enable transparent electrodes or novel multi-layer structures, since optical detection through the gold-substrate is possible. Although a broad distribution in the dimensions of the platelets is still observed, due to the fact that hundreds of them grow on one substrate and the procedure is very fast, we are condent that this recipe can deliver platelets of the exact wanted dimensions eciently. Our results not only enable the growth of high-quality thin gold platelets, but also might 13



inspire further investigation of the growth kinetics of crystal-facets.

Author contribution X. W. developed the original synthesis recipe, E. K. developed the relling. E. K. performed all growth series. E. K. and R. K. acquired all data discussed in the manuscript. P. G. and E. K. wrote the program for the automatized analysis. N. L., under supervision of M. K., acquired the TEM-Data. B. H. supervised the work. All authors contributed to the discussion and the writing of the manuscript.

Acknowledgement The authors thank Guilherme Stein for help with the analysis software and nancial support from DFG HE5618/4-1, HE5618/6-1 and HE5618/8-1.

Supporting Information Available The following le is available free of charge.

• SI.pdf: Transmission electron microscope images and electron diraction patterns, additional information of growth results from dierent sub-ensembles, dierent starting reactants and typical length and thickness of the platelets.

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Page 19 of 24

For Table of Content Only, Controlled Growth of High Aspect-ratio Single-crystalline Gold Platelets, Enno Krauss, René Kullock, Xiaofei Wu, Peter Geisler, Nils Lundt, Martin Kamp, Bert Hecht, Single-crystalline gold platelets showing lateral sizes up to 0.2 mm while being only a few Conventional

20 µm

70

61 nm

50 30

Refill

21 nm

26 nm

Transmission

>90

Thickness h [nm]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10 Time

tens of nanometers thick were obtained using a simple wet-chemical synthesis. Temperature and time dependent growth dynamics of the platelets facets were investigated statistically using an optical method. Extending the recipe by periodically adding reactants during growth leads to signicantly higher aspect ratios of platelets.

19


Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41


Page 20 of 24

1

2

1

0

70

3

2 62 nm

2

35 1

22 nm 100 61 nm

80 60 40

27 nm

0

d)

100 80 60 40 20

ACS Plus Environment 26 nmParagon20 0

10 20 30

40 50 60

Transmission [%]

70

Height [nm]

h [nm]

30

3

Height [nm]

0

60 Crystal Growth & Design 3

Transmission [%]

1 2 3 4 5 c)6 7 8 9 1021 nm 11 12

x [µm]

b)

a)Page 21 of 24 20 µm

b)

Crystal200 Growth & Design N 0

> 90

50


d)

70° C

0

> 90 Thickness h [nm]

Thickness h [nm]

170 2 350 430 5 10 6 20 7 8 c) 9 20 10 1115 1210 13 145 15 160 0 17

9h

200 Page 22 of 24 N

18 h

70 50 30 10 20

50


e)

80° C

90° C

10


8 6 4

ACS Paragon Plus Environment v =4.4*10 µm

vT=1.9*102 µm2 vS=1.7*102 nm*µm

20

60 40 Time [h]

80

T

2

2

2

vS=3.2*10 nm*µm

0

10

30 20 Time [h]

40

0

5

2 vT=1.0*103 µm2 vS=8.5*102 nm*µm

15 10 Time [h]

20

0

Side Area AS /c [103 µm*nm]

a)

a)

b)

8 15

6

10

4

5 0

Aspect Ratio AT/AS *103


1 2 3 4 5 6

c)

Crystal Growth & Design 10 4

20 90°/Refill

Side Area AS/c [103 µm*nm]

Page 23 of 24

3

2

1

2 ACS Paragon Plus Environment 0

5

15 10 Time [h]

20

0

0

0

5

15 10 Time [h]

20

50 µm 0

20 40 60 80 100 Transmission %

Conventional

Crystal Growth & Design 20 µm

70

Page 24 of 24

61 nm

50

1 2 30 3 4 10

RefillParagon Plus Environment ACS 21 nm

26 nm

Time

Transmission

Thickness h [nm]

>90

Controlled Growth of High Aspect-ratio Single-crystalline Gold Platelets

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