Structural order in ultrathin films of the monolayer protected clusters based upon 4 nm gold nanocrystals: an experimental and theoretical study

Abstract

The structural order in ultrathin films of monolayer protected clusters (MPCs) is important in a number of application areas but can be difficult to demonstrate by conventional methods, particularly when the metallic core dimension, d, is in the intermediate size-range, 1.5 < d < 5.0 nm. Here, improved techniques for the synthesis of monodisperse thiolate-protected gold nanoparticles have made possible the production of dodecane-thiolate saturated ∼4 ± 0.5 nm Au clusters with single-crystal core structure and morphology. An ultrathin ordered film or superlattice of these nanocrystal-core MPCs is prepared and investigated using aberration corrected scanning/transmission electron microscopy (STEM) which allowed imaging of long-range hexagonally ordered superlattices of the nanocrystals, separated by the thiolate groups. The lattice constants determined by direct imaging are in good agreement with those determined by small-angle electron diffraction. The STEM image revealed the characteristic grain boundary (GB) with sigma (Σ) 13 in the interface between two crystals. The formation and structures found are interpreted on the basis of theoretical calculations employing molecular dynamics (MD) simulations and coarse-grained (CG) approach.

Figure 1

TEM image and diffraction pattern of 4 ± 0.5 nm sized Au superlattice. a) TEM image showing the long range ordered Au nanocrystal. The FFT patterns of the SL and HRTEM image of representative nanostructure is inset in the Figure a where the single crystal nature is observed and most of the MPCs are in the same orientation. b) Selected area diffraction pattern for SL obtained using 20 cm camera length with several diffraction rings. The inset in Figure b represents the diffraction pattern obtained using 200 cm camera length revealing the overall crystalline structure of the SL to be hexagonally packed long range order. More examples are presented in SI (Figure S2). The FFT pattern inset in frame a and diffraction pattern inset in frame b are in good agreements and shows that the SL is long range ordered hexagonally packed.

Figure 2

a) Low magnification STEM image of Au MPC superlattice. The fast Fourier transform (FFT) pattern is inserted in the top of the image and shows six (6) strong reflection points denoted by 1 and 12 weak reflection points denoted by 2 and 3. The strong and weak reflection suggests the superlattice is made from the three lattice domains oriented along distinct directions. The magnified portion of the lattices are presented in figure b (lattice 1), figure c (lattice 2) and figure d (lattice 3). The direction for the lattice vectors a1, a2 and a3 is inserted in each lattice. The average angle measured between lattice 1 and lattice 2 is (13 ± 1)° and lattice 1 and lattice 3 is (30 ± 1)°, inferring that lattice 2 is rotated from lattice 1 by an angle of (13 ± 1)° and lattice 3 is rotated by an angle of (30 ± 1)°. e) A magnified portion of STEM images in lattice 1 for the statistics of distance between two MPCs. Three different directions are marked by 1, 2 and 3 and the intensity profile for corresponding direction is presented in the right panel and labelled as 1, 2 and 3. The average distance is obtained from the average between 20 values, the average distance along direction 1 is 6.3 nm, and direction 2 is: 6.06 nm and along direction 3 is: 6.17 nm. The FFT is inset in all cases.

Figure 3

Grain boundary interpretation in the interface between lattice 1 and lattice 3. a) Typical STEM image in the interface between lattice 1 and lattice 3. The rectangular region is chosen in the interface and magnified image of the selected region is presented in (b), where a regular sequences of atomic rings of 57 is observed. The GB with the sequence of atomic rings of 57 corresponds to sigma (Σ) = 13. c) Schematic of the atomic model for structural units with atomic rings (57).

Figure 4

Top: high resolution TEM image of the 2D Au NP superlattice showing the experimentally NP-NP distances in nm. Lower: Atomistic simulations showing the internal energy as a function of the NP-NP distance. The minimum corresponds to most stable configurations.

Figure 5

2D Au NP superlattices obtained after minimization, using the coarse-grained approach, a) with 10 % of solvent trapped during superlattice formation. b) with 90 % of solvent trapped. Yellow spheres: Au NPs, White (small) spheres: solvent particles.