Silver nanoparticles were deposited on graphene flakes at a rate of 0.5 nm/min using a thermal deposition system to a thickness of 5 nm at a fixed temperature of the deposition system of 300 K. To elucidate the surface diffusion, the sample after deposition was maintained at 373 K in a vacuum for 1 h. To study the surfaces of the graphene flakes, scanning electron microscopy (SEM) images of the distribution of nanoparticles on the SiO2/Si substrate and the monolayer, bilayer, and trilayer graphene flakes were obtained, and shown on right-hand sides of Figure
a,b,c,d, respectively. The density of nanoparticles decreased as the number of graphene layers increased and was highest on the SiO2/Si substrate.
Histograms and SEM images of silver nanoparticles. (a) Substrate, (b) monolayer, (c) bilayer, and (d) trilayer graphene flakes.
To investigate the density and size of the nanoparticles, average size and density were determined by histogram analysis. The histograms, on the left-hand sides of Figure
a,b,c,d, demonstrate the distributions of nanoparticles on the SiO2/Si substrate, monolayer, bilayer, and trilayer graphene. The sizes of the nanoparticles on the monolayer graphene flake are distributed in the range of 0 to 50 nm, whereas those of the nanoparticles on the trilayer graphene flake were distributed in the range of 10 to 70 nm. Whereas the sizes of the nanoparticles on the SiO2/Si substrate were distributed in the range of 0 to 50 nm, the majority of them were in the range of 10 to 30 nm.
, zero graphene layers represent the SiO2
/Si substrate without any graphene flake. The nanoparticles on the trilayer graphene are largest, with a mean size of approximately 36 nm; as the number of graphene layers decreased, the average size fell to 32, 25, and 18 nm. The density of nanoparticles on the trilayer graphene was also the lowest, at around 3.1 × 1014
. Reducing the number of graphene layers increased the density from 3.1 × 1014
to 9.1 × 1014
. The density of nanoparticles decreased as the number of graphene layers increased, revealing a large variation in the surface diffusion strength of nanoparticles on the different substrates. The mechanisms of formation of these layer-dependent morphologies of silver on n-layer graphene are related to the surface free energy and surface diffusion of the n-layer graphene. Different morphologies of silver on the differently layered graphene are obtained when silver particles are deposited on the graphene surfaces because of the differences of surface free energy. The variation in surface free energy with number of layers is responsible for the variation in the morphologies of silver on the differently layered graphene. The particles are randomly deposited on the graphene surface, and surface diffusion causes randomly arranged particles to combine to form relative large islands. The surface diffusion coefficient D
) indicates the interaction between particles and the substrate surface, where E
denote the diffusion barrier and the Boltzmann constant, respectively. The density of the particles on the substrate (N
) can be expressed as N
for isotropic surface diffusion. The diffusion barrier specifies the interaction between particles and the substrate surface. The difference between the diffusion barriers of the substrate and monolayer can be calculated as ΔE
by combining the above equations. The difference between the barriers of the substrates with other numbers of layers can be similarly obtained, and they are 39.1 meV (E1
) between the first and second layers and 3.1 meV (E2
) between the second and third. The difference between the diffusion barriers of silver decreased as the number of graphene layers increased. According to one review
], the area density of gold nanoparticles on SiO2
/Si substrate and different graphene layers, which the most is trilayer, is in the range of 9 × 1013
to 1.25 × 1013
, and the difference between diffusion barriers of different layers is in the range 500 to 291 meV. The area density of the silver nanoparticles herein is approximately ten times this value, indicating that silver nanoparticles can be more easily prevented from moving along the graphene surface, revealing a stronger interaction between silver and graphene than between gold and graphene.
Nanoparticles on different graphene layers supported by a Si/SiO2substrate. (a) Average size and (b) density.
To determine whether the substrate effect of SiO2/Si is responsible for the variation in morphology, suspended graphene was fabricated by mechanical exfoliation of the graphene flakes onto an oxidized silicon wafer. First, ordered squares with areas of 6 μm2 were defined by photolithography on an oxidized silicon wafer with an oxide thickness of 300 nm. Reactive ion etching was then used to etch the squares to a depth of 150 nm. Micromechanical cleavage of HOPG with scotch tape was then used to deposit the suspended graphene flakes over the indents. The deposition conditions and silver nanoparticles were the same for both types of graphene. Figure
presents the SEM images of the supported and suspended graphenes which were identified as monolayer and bilayer graphene, respectively. Figure
a presents the monolayer, and Figure
b,c presents a magnified view of the suspended and supported graphene, respectively. Figure
d presents the SEM image of the bilayer, and Figure
e,f presents the SEM images of the suspended and supported graphenes. By the same analysis as above results, the average sizes of the suspended monolayer and bilayer graphenes are calculated as 25 and 34 nm. No clear differences exist between the supported and suspended graphenes, whether they be monolayer or bilayer, suggesting that trilayer graphene will similarly be unaffected. Whether the graphene is supported or suspended, the silver nanoparticles on the bilayer graphene are slightly larger than those of the monolayer, and their area density is lower. Based on the results, the use of a substrate such as SiO2 does not affect the distribution of silver nanoparticles on the surface of graphene.
SEM images of supported and suspended graphene. Monolayer (left) and bilayer (right).