Use of an electron-beam evaporator for the creation of nanostructured pits in an insulating surface

Abstract

We demonstrate a method for creating monatomic-depth rectangular pits of controlled size in an alkali halide surface by using an electron-beam evaporator. Atomic resolution noncontact atomic force microscopy is used to characterize the structure and size distribution of the pits, with mean side lengths ranging from 6.5 to 20 nm. It is also demonstrated that metal nanoparticles can be used to nucleate the growth of pits, resulting in pits with metal nanoparticles inside. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2210288͔ While surface modifications induced by electronic transitions have been of fundamental interest for several decades, 1,2 there is renewed interest in the field because of its relevance to nanofabrication. 3 Moreover, the recent development of noncontact atomic force microscopy 4 ͑NC-AFM͒ has opened up the possibility of measuring the topography of modified insulators with atomic resolution. 5 This is of great importance since the influence of surface topography on the desorption process leading to surface modification remains largely unknown. 1 Through excitation by electrons, 1 photons, 6 or ions, 7 the desorption of atoms from ionic crystal surfaces is induced by the formation of Frenkel defects in the bulk ͑F and H centers͒, which diffuse and recombine with the surface. 1 At the first stages of the typically layer-by-layer surface removal, monatomic-depth rectangular pits are formed, which are several nanometers in size. Recently, these pits have been used to trap small numbers of SubPc molecules, 3 an important step towards assembling a molecular electronic device on an insulating substrate. These well defined surface features may also serve to control the nucleation of metal nanostructures at surfaces. 6 In this letter, we report on the use of an electronbeam evaporator as a source of charged particles for creating arrays of nanostructured pits and metal nanoparticles inside pits on a KBr ͑001͒ surface.All of the sample preparation and characterization was performed in ultrahigh vacuum ͑base pressure Ͻ4 ϫ 10 −8 Pa͒. The KBr surfaces were prepared by cleaving a crystal ͑Korth Kristalle, Germany͒ in situ along the ͑001͒ plane and then heating for 1 h at 150°C, in order to remove any residual surface charge. An Oxford Applied Research EGN4 electron-beam evaporator was used for the pit creation and metal deposition. During operation, charged particles are emitted as a result of the ionization of residual gas in the vacuum chamber, the ionization of target material, and the production of low energy electrons. To reduce the amount of charge impinging on the sample during evaporation, the evaporator is fitted with a grid held at 2 kV. With this setup, extremely small current densities are accessible ͑Ͻ500 pA/ cm 2 ͒, making it possible to finely tune the irradiation dose to the sample. Tantalum ͑Alfa Aesar, 99.95%͒ was used as the evaporator target material because its high melting point enables the use of a wide range of evaporator powers, and hence current densities. The pits were cre...

Role of van der Waals forces in the adsorption and diffusion of organic molecules on an insulating surface

“…22 A coverage of 0.4 ML of PTCDA was thermally evaporated onto the KBr surface at 300°C after outgassing for 12 h at 220°C. NC-AFM measurements were performed in the same UHV system at room temperature using a JEOL JSPM 4500a.…”

High-resolution investigation of metal nanoparticle growth on an insulating surface

“…It is likely that F centers are induced during the metal deposition, as it has been recently been shown that small amounts of charge emitted during evaporation can result in the generation of surface defects which act as nucleation centers. 32,34 Some such defects may also result from the mechanical cleavage of the KBr ͑001͒ surface. 39 Ex situ TEM measurements were also performed on one of the room-temperature-prepared gold nanoparticle samples ͓0.64 ML Au, Fig.…”

“…The diffusion of the palladium under the surface may be facilitated by the presence of pitlike KBr defect sites which are known to be produced by residual charged particles during deposition and to nucleate nanoparticle growth. 32,34 Subsurface growth has previously been observed in certain metal on metal systems, although the mechanisms are only partially understood and appear to vary between systems. 58,59 Such surface modifications could potentially be employed to purposely engineer the surface structure of the substrate for nanopatterning applications, much as nanometer-scale pits have previously been used to template metal and molecule growth on alkali halide surfaces.…”

“…This reduces the amount of charge reaching the surface and hence decreases the number of charge-induced nucleation sites. 32,34 The total ͑positive+ ͉negative͉͒ current-density incident on the sample during evaporation, measured according to procedure used in Ref. 34 was 220, 680, and 250 pA/ cm 2 during the deposition of gold, tantalum, and palladium, respectively.…”

“…32,34 The total ͑positive+ ͉negative͉͒ current-density incident on the sample during evaporation, measured according to procedure used in Ref. 34 was 220, 680, and 250 pA/ cm 2 during the deposition of gold, tantalum, and palladium, respectively. The beam of charge was estimated to consist of 60-70 % positive ions ͑ionized background gas atoms and ionized evaporant atoms͒ and 30-40 % electrons.…”