Molecular dynamics simulations of sputtering of surface metal nanoclusters under low energy ion bombardment Текст научной статьи по специальности «Физика»

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G. V. Kornich, K. V. Pugina, G. Betz

MOLECULAR DYNAMICS SIMULATIONS OF SPUTTERING OF SURFACE METAL NANOCLUSTERS UNDER LOW ENERGY ION BOMBARDMENT

Molecular Dynamics simulations of sputtering of copper clusters, which consisted of 13, 27, 39, 75 and 195 Cu atoms on a (0 0 01) graphite surface by 100-400 eV Ar and Xe ions have been performed. The azimuthal angular distribution of sputtered Cu cluster atoms exhibit periodic maxima every 60 degrees. The obtained sputtering yields are for a surface with a single Cu cluster deposited.

1 Introduction

An increase of activity of experimental and model researches of physical processes in nano-dimensional clusters on different surfaces occurs during last decade [1-4]. The present investigations are devoted to MD simulations of low energy ion backscattering from and sputtering of binary systems, consisting of copper nano-dimensional clusters of different sizes on a graphite substrate. General analysis are performed for clusters, consisting of 13-195 atoms on a (0 0 0 1) graphite two-layer substrate, which consisted of 15005880 carbon atoms, respectively. Bombardment was simulated for Ar and Xe ions at impact energies of 100-400 eV and normal incidence. In the present work we will summarize the results for sputtering cluster atoms obtained in our MD calculations with the aim to receive general picture of physical features of sputtering of surface metal nanoclusters under low energy ion bombardment.

Results of such a calculation for an isolated surface cluster can be compared to experimental results of sputtering a substrate with a low coverage of clusters. For example sputtering yields from an array of well separated clusters on a substrate can be deduced from these MD results as discussed in Ref. [5]. Moreover, technologically reliable arrays of practically monodisperse surface clusters on single crystals can intensify sputtering regularities of cluster atoms as found for isolated cluster in the MD calculations, that is critically for experimental examination of received model results.

2 Model

The Tersoff potential [6] splined to the Ziegler-Biersack-Littmark potential [7], was applied to the CC interactions. A tight binding many body potential, directly connected to the Born-Mayer potential [8] was used for the Cu-Cu interatomic interactions. C-Cu interactions were simulated using a Lennard-Jones potential [9]. The C-Cu potential was splined to the Ziegler-Biersack-Littmark potential. lon-Cu and ion-C interactions were simulated using pure the Ziegler-Biersack-Littmark potential. Details of the preparation

© G. V. Kornich, K. V. Pugina, G. Betz 2006 r.

of the substrate-Cu cluster system are presented and discussed in Ref. [5].

Trajectories of particles were calculated in accordance to Newtons equations of motion using the Verlet algorithm [10]. Lateral periodic boundary conditions were applied to the substrate atoms. No periodic boundary conditions were applied to the Cu cluster atoms. Every ion impact was calculated for 2 ps (Ar) or 3 ps (Xe). For each energy and each cluster size 2000 impacts were performed with random ion impact points, which were chosen in accordance to the criterion discussed in Ref. [5]. The law of energy conservation was executed with accuracy no worse than 1 % in all cases.

3 Results and discussions

Azimuthal angular distributions of sputtered Cu atoms from surface Cu clusters at energies of 100, 200 and 400 eV are presented in fig.1 for Xe ions. No results are shown for 13 atom clusters as in this case no periodic structure was observed. Almost identical results have been observed for Ar ion bombardment and for the case of 200 eV ion bombardment [5]. The distributions have six maxima with a period of 60 degrees into the <1 1 0 0> directions. While for 200 and 400 eV the periodic structure is clearly visible, at 100 eV Xe bombardment it has almost disappeared. For 100 eV Ar ion bombardment the structure is also very weak but more clearly visible as for Xe bombardment. The structure with sixfold periodicity is due to the influence of the (0 0 0 1) hexagonal graphite substrate which also exhibits this symmetry. The atomic structure of surface clusters is influenced by the graphite substrate. Finally, surface clusters (excluding 13Cu atom cluster) consist of 3-6 atomic layers parallel to substrate surface. Every copper atomic layer has six <1 1 0 0> directions, along which atomic structure is the most transparent and consists of parallel high packaged (along <1 1 0 0> directions) atomic sequences. Typical result of sputtering event is atom, which left lateral cluster surface along some <1 1 0 0> direction. We have also found in additional MD calculations for Cu cluster on a (100) Cu surface

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Fig. 1. Azimuthal angular distribution of sputtered Cu atoms from 27, 39, 75 and 195 Cu atom clusters under 100-400 eV Xe ion

bombardment

such a periodic structure, but with a fourfold symmetry corresponding to the symmetry of the substrate into the <1 1 0> directions. For the largest cluster (195 atoms) there is some indication that each peak splits up into 2 peaks, as the fine structure observed is identical for 200 and 400 eV. No such indication is observed for Ar ion bombardment.

Total sputtering yields of Cu atoms from clusters are presented in fig. 2 (a, b) for the cases of Ar and Xe ions at different impact energies. Sputtering yields of Cu atoms for Xe ion bombardment are slightly larger than for Ar ions at all energies. It is shown in Fig. 2 that the dependence of the sputtering yield from cluster sizes is similar for Ar and Xe ions. The minimum for 39 Cu atom clusters and the strong increase for 75 Cu atom clusters occur for all impact energies and both types of ions. The observed yields are much larger than for a flat surface (1 0 0) Cu as seen in Fig. 2. The explanation is that for clusters due to the possibility of sputtering from the sides no complete momentum inversion is necessary. Indeed, if we analyze into which direction most Cu atoms from a cluster are sputtered, we find that the maximum of the sputtering intensity is for a direction parallel to the substrate surface for all energies and cluster sizes. This effect will in principle decrease with increasing cluster size, however the geometrical form of the cluster plays also an important role and these two effects are overlapping. The general decrease in the yield with size is seen for 100eV bombardment with both Ar and Xe ions. Finally, the difference of sputtering yield values of cluster and flat surface (at normal ion impacts) are minimal at 100 eV and increases with ion impact energy for both Ar and Xe ions. The geometry effect also is responsible for the minimum we observe for cluster size 39 atoms for all energies. For higher ion energies (400 eV) the

variation of the sputtering yield with cluster size is prominent and no general decrease of the yield with cluster size can be observed.

The general shape of the cluster and the energy transfer to the substrate, which is strongly influenced by the shape, is the dominant factor for the yield changes in simulated surface clusters. For 200 eV ion bombardment the yield variations have been described previously by a l/ns scaling law, where l is the distance from the center of mass of a cluster to the substrate surface; ns is the number of cluster atoms in the cluster/substrate interface. The same scaling law can also be applied to the whole energy range from 100-400 eV ion bombardment. This clearly shows that the yield-cluster size dependence is to a large extent determined by the cluster shape. l/ns scale demonstrates strong relative deviation from results of simulations in the case of 13Cu atom cluster [5]. In the same time, the only critical distinguish of 27 and 39 Cu atom clusters from 75 and 195Cu atom clusters is absence of some atomic sequences in structures of small clusters in <1 1 0 0> directions as compared to large ones due to small total number of atoms in clusters. As result, structures of small clusters are more transparent in normal to substrate surface direction. It may be another one, but not the main, reason of strong difference of sputtering yield values of 39-75Cu atom clusters.

4 Conclusions

Molecular dynamics simulations were performed for scattering from and sputtering of Cu clusters, consisting of 13-195 Cu atoms on a graphite (0 0 0 1) surface by Ar and Xe ions at energies between 100- 400 eV. Sputtering

Fig. 2. The total sputtering yields of Cu atoms for 13, 27, 39, 75 and 195 Cu atom clusters at 100 eV, 200 eV and 400 eV for (a) Ar and (b) Xe ion bombardment. Also shown are the calculated results for a flat (1 0 0) Cu surface

yields for cluster are much larger than for a flat Cu target. The explanation is that for clusters due to the possibility of sputtering from the sides no complete momentum inversion is necessary. In addition, the geometrical form of the cluster plays also an important role and these two effects lead in general to an increase in the yield for the observed cluster size range, with a remarkable minimum for cluster size 39. The yield-energy dependence shows an approach to the behavior of a flat surface with increasing cluster size or decreasing of ion impact energy. The contribution of secondary copper clusters to the total sputtering yield is about 2-4 times higher as compared to sputtering of a flat copper surface. The azimuthal angular distributions of sputtered Cu atoms from copper clusters on a graphite (0 0 0 1) surface exhibits a periodic structure with six maxima. This distribution reflects the substrate crystal structure which strongly influences the structure of the clusters. The periodic structure is observed at all energies, but it is very weak under 100 eV bombardment.

References

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Поступила в редакцию 30.05.2006 г.

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Выполнено молекулярно-динамическое моделирование распыления одиночных кластеров меди, состоящих из 13, 27, 39, 75 и 195 атомов, с поверхности (0 0 0 1) графита ионами аргона и ксенона с энергиями 100-400 эВ. Азимутальное угловое распределение распыленных атомов медных кластеров представляет собой квазипериодическую зависимость с шестью максимумами через каждые 60 градусов, медной монокристаллической подложки ионами аргона c энергией 200 эВ. Получены значения коэффициентов распыления для поверхностных кластеров.

Molecular Dynamics simulations of sputtering of copper clusters, which consisted of 13, 27, 39, 75 and 195 Cu atoms on a (0 0 01) graphite surface by 100-400 eV Ar and Xe ions have been performed. The azimuthal angular distribution of sputtered Cu cluster atoms exhibit periodic maxima every 60 degrees. The obtained sputtering yields are for a surface with a single Cu cluster deposited.