Research Papers

Evolution of Pt Clusters on Graphene Induced by Electron Irradiation

[+] Author and Article Information
Cezhou Dong

Institute of Applied Mechanics,
Zhejiang University,
Hangzhou 310027, China

Wenpeng Zhu

AML, Department of Engineering Mechanics,
Tsinghua University,
Beijing 100084, China

Hongtao Wang

e-mail: htw@zju.edu.cn
Institute of Applied Mechanics,
Zhejiang University,
Hangzhou 310027, China

Wei Yang

Institute of Applied Mechanics,
Zhejiang University,
Hangzhou 310027, China;
AML, Department of Engineering Mechanics,
Tsinghua University,
Beijing 100084, China
e-mail: yangw@zju.edu.cn

1Corresponding authors.

Manuscript received January 8, 2013; final manuscript received February 1, 2013; accepted manuscript posted April 8, 2013; published online May 31, 2013. Editor: Yonggang Huang.

J. Appl. Mech 80(4), 040904 (May 31, 2013) (8 pages) Paper No: JAM-13-1010; doi: 10.1115/1.4024168 History: Received January 08, 2013; Revised February 01, 2013; Accepted April 08, 2013

In situ low-voltage transmission electron microscopy (TEM) was performed to study the evolution of small Pt clusters on suspended graphene. Pt clusters, trapped by the edge of holes, generally take a stable shape of truncated octahedron for sizes ranging from sub-1 to ∼5 nm. The interaction to the graphene dots takes in charge when they form composite nanostructures embedded in graphene. The Pt clusters are slowly flattened due to hole enlargement under electron irradiation. The planar structure is maintained by the peripheral Pt-C bonds and instantly collapses into a three-dimensional (3D) cluster if one side is detached from the edge. Based on the heat transfer model, the thermal effect can be excluded under the experimental condition. Atomistic evolution can be attributed to the electron irradiation. Molecular dynamics simulations revealed that the evolution kinetics was found to be dominated by the surface diffusion (characterized by the migration barrier Em), the temperature (the thermal activation energy ∼5kBT), and the scattering from electrons (the maximum transferred energy Emax). The corresponding energies are comparable for the Pt cluster system, leading to similar evolution behaviors. A different scenario in graphene systems is due to the large difference in agitations, i.e., Emax ≫ Em ∼ 5kBT at 3000 K. This unique behavior comes from TEM observation, implying that electron beam irradiation can be utilized as a unique tool in shaping carbon nanostructures.

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Grahic Jump Location
Fig. 1

(a) Raman spectra of a transferred graphene on Si substrate. (b) Reconstructed high resolution TEM image of a graphene by averaging 11 images in order to enhance the contrast and minimize the noise. Scale bar: 1 nm. (c) SEM image of a transferred monolayer graphene on a holey carbon TEM grid. Scale bar: 50 μm. The small circular regions are holes. The amorphous carbon support has a lighter contrast when covered by graphene.

Grahic Jump Location
Fig. 2

(a)–(d) Evolution of Pt clusters at the graphene edge under 60 kV electron irradiation. (e) A model of truncated octahedron in an fcc crystal coordinate system. The white curve outlines the hole edge. (f) Two snapshots of crystal 3 with the atomic model viewed along different directions.

Grahic Jump Location
Fig. 3

Image sequence shows the evolution a Pt-graphene composite. The insets to the subfigures are the atomic model of the boxed region and the corresponding TEM simulation. Scale bar: 2 nm.

Grahic Jump Location
Fig. 4

DFT simulation on the stability of the atomic model in the inset to Fig. 3(c). (a) The relaxed configuration of a Pt cluster embedded in the graphene plane. (b) The corresponding charge density map.

Grahic Jump Location
Fig. 5

A schematic shows the interaction between a Pt cluster and the graphene support. Top: atomic model. Bottom: a sketch.

Grahic Jump Location
Fig. 6

The C atomic displacement rate decreases with the increase of the threshold energy. Zero rate is approached at Ethr = Emax, which is 11.6 eV for C. The threshold energy can be estimated by the specific binding energies. Inset shows three typical configurations, i.e., a dangling atom, a zigzag edge, and an armchair edge. The labeled atoms are easily sputtered off with the rates marked on the curve.

Grahic Jump Location
Fig. 7

(a) A schematic experiment setup. The free-standing graphene is supported by the holey amorphous carbon film (thickness ∼20 nm). The center shadowed region is illuminated by the electron beam. (b) The temperature profile as a function of the distance from the center.

Grahic Jump Location
Fig. 8

Structure evolution of a 2.8 nm Pt cluster under thermal annealing and electron irradiation (marked by dashed boxes). The staircase curve is the temperature profile in simulating the annealing process. For electron irradiation, the intensity is 10 e/ps and the temperature is controlled to be in the range of 300–400 K. The structural evolution depended on the total electron dosage, i.e., the product of the beam intensity and the time, rather than each single factor.

Grahic Jump Location
Fig. 9

(a) Energy profile of a migrating Pt atom on top of an fcc Pt slat with (111) surface. The stacking sequence is distinguished by colors. (b) Energy profile of a migrating C atom on a zigzag edge. The migration path is indicated by the long arrow with characteristic locations highlighted by transparent spheres. The profiles were calculated by the nudged elastic band energy method.

Grahic Jump Location
Fig. 10

The fraction of atoms with a kinetic energy above Ek/kBT. Here we assume Boltzmann distribution to be obeyed in an equilibrium system. Thus the cumulative distribution can be written as P(Ek)=erfc(Ek/kBT)+2Ek/πkBT exp(-Ek/kBT), which equals to the fraction of atoms with kinetic energy above Ek by assuming an ergodicity motion.

Grahic Jump Location
Fig. 11

(a)–(d) TEM observation and (e)–(h) MD simulation of the structure evolution of a graphene nanoribbon under 60 keV electron irradiation. Scale bar: 1 nm. For simulation, the beam intensity is controlled to be 10 e/ps.

Grahic Jump Location
Fig. 12

(a)–(f) MD simulation for the structure evolution of a graphene nanoribbon under thermal annealing with a staircase temperature profile. The staircase temperature profile ranged from 800 to 3000 K with an increase of 200 K per step. The MD time for each step is 1 ns. The arrow traces the location of a dangling atom on the edge.



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