The Controlled Evolution of a Polymer Single Crystal

Transcription

1 Supporting Online Material The Controlled Evolution of a Polymer Single Crystal Xiaogang Liu, 1 Yi Zhang, 1 Dipak K. Goswami, 2 John S. Okasinski, 2 Khalid Salaita, 1 Peng Sun, 1 Michael J. Bedzyk, 2 Chad A. Mirkin 1* 1 Department of Chemistry and Institute for Nanotechnology, 2 Department of Materials Science and Engineering and Nanoscale Science and Engineering Center, Northwestern University, Evanston, IL 60208, USA Materials. Poly-DL-lysine hydrobromide (PLH, molecular weight = 4000) was purchased from Sigma Chemical Co. Muscovite mica (V-1 grade) substrates were purchased from Structure Probe, Inc. Mica (KAl 2 (AlSi 3 )O 10 (OH) 2, 2M 1 -muscovite) has a monoclinic structure with lattice constants a = Å, b = Å, c = Å, α = γ = 90.00, and β = Muscovite is a stacked sandwich structure that is composed of layers of potassium ions between aluminum silicate sheets. It cleaves along the potassium ion planes, exposing the oxygen atoms of the tetrahedralsilicate layer (S1). Due to charge repulsion in the oxygen layer, the ideal hexagonal surface of the oxygen is distorted, which results in the oxygen atoms forming a two dimensional lattice with threefold symmetry (Fig. 3B). Specifically, these oxygen atoms form a pseudo hexagonal surface structure with lattice constants a H = Å and b H = Å. The angle 1

2 between a H and b H in this structure is o (Fig. 3B). The symmetry of the oxygen layer and the electrostatic interaction between the PLH molecules and the oxygen layer are likely major factors that are responsible for epitaxial growth of equilateral triangles. Methods. The silicon AFM tip (NCH-W, Veeco, spring constant = 40 N/m) was coated with PLH by immersing the cantilever into an aqueous solution of PLH (2 mg/µl) for two seconds. The cantilever was then removed from the solution and used immediately as a deposition tool in an AFM raster scanning experiment. The mica substrates were freshly cleaved using Scotch tape prior to use and were typically several tenths of a millimeter thick. Care must be taken not to contaminate the freshly exposed mica surfaces. The deposition and growth of PLH nano- and microcrystals on freshly cleaved mica surfaces were achieved by scanning the surface with a PLH-coated AFM tip. To generate PLH crystals on the mica surface, the AFM (NanoMan, Veeco/Digital Instruments, Santa Barbara, CA) was operated in Tapping Mode. Unless otherwise noted, all experiments were carried out under ambient laboratory conditions at temperature of ~20 o C. Control experiments involving bulk crystallization were performed by exposing freshly-cleaved mica substrates to aqueous PLH solutions of various concentrations at a relative humidity and temperature that was identical to those used in the AFM experiments (fig. S2). The data indicate that occasionally large truncated triangles grow, but it is very difficult to grow well-defined triangular prisms and observe early nucleation events in these experiments. The truncated triangles are never observed when the PLH concentration below 2mg/ml (fig. S2 A). These experiments suggest that the AFM tip is 2

3 important in maintaining a high local concentration of PLH at the point of nucleation and crystal growth. Time-of-flight secondary ion mass spectroscopy TOF-SIMS was performed with a Physical Electronics PHI TRIFT III, which is equipped with a pulsed Ga + liquid ion gun operated at 15kV (S4). The ion source was operated with a current of 600 pa. The charging effect was neutralized by a pulsed low-energy electron flood gun. The secondary ions were accelerated to ± 3kV by applying a bias on the sample. The total ion dose was about ions/cm 2 to ensure static conditions. Negative spectra were calibrated using the CH, C 2 H, and C 3 H peaks. Positive spectra were calibrated by using CH + 3, C 2 H + 5, and C 3 H + 7 peaks. Mica crystallographic orientation was determined using back reflection Laue and also single crystal x-ray diffraction measurements with a 4-circle diffractometer. For grazing incidence oscillation measurements, the PLH prisms were grown over an (1-by-1 mm 2 ) area situated in the middle of a freshly cleaved mica surface that was 25-by-25 mm 2. A humidity-controlled sample-mounting cell was used during the x-ray measurements. The energy of the incident x-ray beam was set at E γ = KeV (λ = Å). At this energy the critical angle for total external reflection from mica is θ c = o. The incident angle was set at θ = 0.10 o to enhance the scattering from the surface overlayer and reduce the scattering from the substrate. The incident beam slit had a horizontal width of x = 1.0 mm and vertical height of z = 0.05 mm. This made the x-ray foot print on the surface of the mica 1-mm-wide by 25-mm-long. 3

4 4

5 Figure S1. (A) TOF-SIMS ion image (Br) of the PLH prism. (B) and (C) TOF-SIMS negative ion spectra of the PLH prism shown in (A). (D) TOF-SIMS positive ion spectrum of the PLH prism shown in (A). 5

6 Figure S2. Control experiments involving the bulk deposition of PLH on freshly-cleaved mica substrates. Optical microscope ( 100) images of mica exposed to (A) 2mg/ml and (B) 0.2mg/µl PLH solutions in water and topographic AFM images of mica exposed to (C) 2mg/ml and (D) 0.2mg/µl PLH solution in water. A B C D 1 µm 6

7 Figure S3. A series of topographic AFM images (6 µm by 6 µm) of a silicon surface at room temperature (20 o C), obtained by continuously scanning an AFM tip coated with PLH molecules (Scan rate: 2 Hz). The relative humidity is ~30%. Triangles do not form under these conditions. 7

8 Figure S4. A series of topographic AFM images (3 µm by 3 µm) of an 3- aminopropyltriethoxysilane-modified mica surface at room temperature (20 o C), obtained by continuously scanning an AFM tip coated with PLH molecules (Scan rate: 2 Hz). The relative humidity is ~30%. Triangles do not form under these conditions. 8

9 Figure S5. (A) A topographic AFM image of a PLH crystal line pattern formed by scanning a 10-µm single line in tapping mode at a scan rate of 2 Hz for 20s (Image size: 15 µm; RH: ~20%; RT: ~20 o C). (B) A line profile between the two arrows shown in (A). 9

10 Table S1. Observed azimuth angles (φ), relative intensities (I) and d-spacings (d) for the PLH diffraction peaks shown in Fig. 3D. The (hkl) indexing is based on the muscovite monoclinic unit cell with basis vectors: a, b, c. See Fig. 3B. The (HKL) indexing is based on a pseudo-hexagonal unit cell, with basis vectors defined as: a H = a, b H = (-a + b)/2, c H = -(c/a)cosβ a + c. Label φ (deg) I d (Å) A B C D E F G H Monoclinic Hexagonal h k l H K L

11 References: S1. H. Plank, R. Resel, A. Andreev, N. S. Sariciftci, H. Sitter, J. Cryt. Growth 239, 2076 (2002). S2. S. W. Bailey, in Crytal Structure of Clay Minerals and their X-ray Indentification, G. W. Brindley, G. Brown, Eds. (Mineral Soc. Monograph No. 5, London, 1980). S3. P. A. Cornelio, M. B. Clark and J. A. Gardella, Jr. in Secondary Ion Mass Spectrometry (SIMS) VII, A. Benninghoven, C. A. Evans, K. D. McKeegan, H. Storms and H. W. Werner, Eds., John Wiley and Sons, New York, NY, 1990, 761. S4. H. Dosch, B. W. Batterman, D. C. Wack, Phys. Rev. Lett. 56, 1144 (1986) 11