M. D. Upward

Translation, rotation and removal of C60 on Si(100)-2 × 1 using anisotropic molecular manipulation

We have investigated the interactions of C60 molecules adsorbed on Si(100)-(2 × 1) through their response to manipulation induced by a scanning tunneling microscope operating at room temperature. Intramolecular features are resolved which vary as a molecule is displaced showing that C60 undergoes rotation during tip-induced displacement. For translation to and from certain bonding sites, the apparent size of a molecule may change following lateral manipulation. Furthermore, reversible changes in dimer buckling are observed as a molecule is moved across the surface. Our experimental observations show that the C60Si(100)-(2 × 1) interaction is dominant over the C60C60 interaction and attempts to move a molecular pair result in the transfer of one molecule across a dimer row due to barrier lowering by the intermolecular interaction. We also show that transfer of a molecule from sample to tip (or vice versa) changes both the imaging and manipulation properties of the tip.

Structural and optical characterization of self-assembled InAs-GaAs quantum dots grown on high index surfaces

The structural and the optical properties of InAs layers grown on high index GaAs surfaces by molecular beam epitaxy are investigated in order to understand the formation and the self-organization of quantum dots (QDs) on novel index surfaces. Four different GaAs substrate orientations have been examined, namely, (111)B, (311)A, (311)B and (100). The (100) surface was used as a reference sample. STM pictures exhibit a uniform QD coverage for all the samples with the exception of (111)B, which displays a surface characterized by very large islands and where STM pictures give no evidence of QD formation. The photoluminescence (PL) spectra of GaAs (100) and (311) samples show typical QD features with PL peaks in the energy range 1.15-1.35 eV with comparable efficiency. No significant quenching of PL up to temperatures as high as 70K was observed. These results suggest that the high index substrates are promising candidates for production of high quality self-assembled QD materials for application to photonics.

Deposition of Fe clusters on Si surfaces

We have used an ultrahigh vacuum scanning tunneling microscope to investigate the deposition of mesoscopic iron clusters from a gas aggregation source. The size of the clusters was found to be in the range 1–7 nm. The effect of exposure of the iron clusters to the atmosphere results in a significant increase in cluster size, which we believe is consistent with total oxidation of the clusters. A specially designed quadrupole mass filter is incorporated inside the cluster source. We have investigated the sizes of the clusters deposited using different quadrupole settings and find that it is possible to size select the clusters prior to deposition. Finally we have studied the effect of sample surface reactivity and annealing on cluster distribution.

Measurement and manipulation of Mn clusters on clean and fullerene terminated Si(111)-7×7

We have investigated the properties of Mn clusters deposited on clean semiconductor surfaces using an ultrahigh vacuum scanning tunnelling microscope. The clusters were formed using a gas aggregation source and were deposited on Si(111)-7×7 and Si(111)-7×7 terminated by a C60 monolayer. A distribution of cluster widths was observed with a most frequent value of 2.6 nm. There was no evidence of either cluster coalescence or preferential bonding sites. The clusters had an irregular shape, but did not appear to be grossly deformed upon adsorption onto the surface. The clusters could be selectively removed from the C60 terminated surface during scanning at negative sample bias.

Passivation of Si(111)‐7×7 by a C60 monolayer

C60 monolayers are formed on a Si(111)‐7×7 surface under ultrahigh vacuum (UHV) conditions. The effects of exposure to atmosphere (for 30 min) and water (for 30 s) are assessed by comparing images of the surface acquired using an UHV scanning tunneling microscope. Following exposure and/or immersion we are able to resolve the C60 molecules exhibiting hexagonal order in an arrangement which is essentially identical to that formed prior to withdrawal from the UHV system. Our results clearly show that deposition of one monolayer of C60 on a Si surface can inhibit chemical attack by water and atmospheric oxygen.

Digital scanning probe microscope controller for molecular manipulation applications

A versatile digital controller for scanning probe microscopy capable of data acquisition during molecular manipulation has been constructed. A commercially available digital signal processor (DSP) board connected to a Pentium PC and custom-built high voltage amplifiers were used to control a commercial ultrahigh vacuum scanning tunneling microscope and to perform molecular manipulations. Use of the DSP system to produce all analog outputs resulted in an extremely flexible system with complete control of the probe tip.

C60 adsorption on the Si(110)-(16 × 2) surface

Abstract The interaction of C 60 with the Si(110)-(16 × 2) surface has been studied using a scanning tunnelling microscope (STM). For a submonolayer coverage isolated C 60 molecules are observed with no evidence for a preferential adsorption site. As the coverage is increased, a disordered monolayer is observed and at still higher coverage disordered aggregates are observed which ripen into hexagonally ordered islands. These islands could be disrupted by the STM tip and were desorbed by annealing. Our results show that the interaction between the disordered layer and C 60 islands is weak. However, the first monolayer is strongly bonded to the Si surface and forms a passivating layer which is stable to exposure to atmosphere.

Functionalized fullerenes on silicon surfaces

Abstract We have investigated the influence of phenyl addends on the packing and adsorption of C 60 on a solid substrate. On the Ag/Si(111)- ( 3 × 3 )R30° surface, the interactions of the unmodified part of the fullerene cages are not strongly affected by the addition of the phenyl groups although pronounced differences in molecular packing are observed. Phenylated C 60 may rotate to its optimum configuration and diffuse freely on this surface. In contrast it can neither rotate nor diffuse on Si(111)7×7, while on C 60 /Si(111)–7×7 it can diffuse but the presence of the phenyl groups inhibits the formation of ordered layers.

Atomic scale protection using fullerene encapsulation

The use of fullerene multilayers as a removable protective coating on a clean, reconstructed semiconductor surface has been investigated using an ultrahigh vacuum scanning tunneling microscope (UHV-STM). We have found that the Ag/Si(111)-√3×√3R30° surface, which is normally stable only under an UHV environment, can survive exposure to ambient conditions beneath a protective layer of C60. The C60 capping layer may be removed by annealing after reinsertion into UHV. For optimum protection a thickness >5 monolayers is required. The protective effect lasts for times of order several minutes.

STM investigation and manipulation of molecules adsorbed on an Si(111) surface

We have used an ultra-high-vacuum (UHV) scanning tunnelling microscope (STM) to image molecules adsorbed on a Si(111) surface. At low coverage ( monolayers) molecules are adsorbed at random sites. For coverages close to a monolayer they are partially ordered in a hexagonal arrangement. Second- and higher-layer islands, in which the molecules are clearly resolved, are observed at higher coverage. These islands may be desorbed by annealing in the range , leaving an Si surface terminated by a monolayer. This surface is stable to exposure to air and immersion in water. In addition, recent work on manipulation of molecules at various coverages is reviewed and results relating to tip alignment in UHV are discussed.