Paper NS+NC-WeM5
Atomic Scale Characterization of Charge Redistribution for Gallium Nanocluster Arrays on the Si(111)-7x7 Surface

Wednesday, October 22, 2008, 9:20 am, Room 311

In recent years, the fabrication and characterization of nanocrystals with size-dependent properties has gained interest for both fundamental studies and technological applications ranging from magnetic storage to catalysis. Self-assembled arrays of uniform nanoclusters on the Si(111)-7x7 surface from In, Ga, and Al¹ have recently been observed. These nanoclusters share common characteristics: uniform atomic structure; high thermal stability; and self-assembly into well-ordered, large-area arrays. However, the electronic properties of these nanocluster arrays are not yet well understood. Experimental investigations thus far have focused on the behavior of individual nanoclusters rather than the delocalized properties of the nanocluster array as a whole. Meanwhile, a computational study of In and Al nanocluster arrays has predicted the formation of a spatially modulated 2D electron gas (2DEG) due to surface charge redistribution.² In this study, we report the observation of atomically resolved, delocalized 2D charge redistribution associated with Ga nanocluster arrays on the Si(111)-7x7 surface.³ By using ultra-high vacuum scanning tunneling microscopy and differential tunneling conductance mapping, we correlate the topography of the Ga nanocluster array with its local density of states (LDOS). In the differential tunneling conductance maps, we observe the surface charge redistribution as distinct regions of increased LDOS forming an interconnected 2D network over the Ga nanocluster array. These results indicate that a delocalized 2DEG has been induced by the Ga nanocluster array. Furthermore, the close integration of the nanoclusters with the Si substrate can be discerned from differences in the LDOS between the faulted and unfaulted unit cell halves. Interestingly, the increased LDOS disappears abruptly over clean Si(111)-7x7 unit cells, suggesting a possible route for nanopatterning of the surface electronic structure via selective masking of the Ga nanocluster formation. These atomic-scale observations are likely to impact further fundamental studies of nanocluster arrays on Si and the development of potential nanoelectronic devices.

¹ J.F. Jia, X. Liu, et al., Phys. Rev. B, 66, 165412 (2002)
² L.X. Zhang, S.B. Zhang, et al., Phys. Rev. B, 72, 033315 (2005)
³ Q.H. Wang and M.C. Hersam, Small, in press (2008).