Alignment of Ge nanoislands on Si(111) by Ga-induced substrate self-patterning

Activating two-dimensional semiconductors for photocatalysis: a cross-dimensional strategy

The emerging two-dimensional (2D) semiconductors substantially extend materials bases for versatile applications such as semiconductor photocatalysis demanding semiconductive matrices and large surface areas. The dimensionality, while endowing 2D semiconductors the unique properties to host photocatalytic functionality of pollutant removal and hydrogen evolution, hurdles the activation paths to form heterogenous photocatalysts where the photochemical processes are normally superior over these on the mono-compositional counterparts. In this perspective, we present a cross-dimensional strategy to employ thenD (n= 0-2) clusters or nanomaterials as activation partners to boost the photocatalytic activities of the 2D semiconductors. The formation principles of heterogenous photocatalysts are illustrated specifically for the 2D matrices, followed by selection criteria of them among the vast 2D database. The computer investigations are illustrated in the density functional theory route and machine learning benefitted from the vast samples in the 2D library. Synthetic realizations and characterizations of the 2D heterogenous systems are introduced with an emphasis on chemical methods and advanced techniques to understand materials and mechanistic studies. The perspective outlooks cross-dimensional activation strategies of the 2D materials for other applications such as CO2removal, and materials matrices in other dimensions which may inspire incoming research within these fields.

Trends in low energy electron microscopy

Low energy electron microscopy (LEEM) and spin polarized LEEM (SPLEEM) are two powerful in situ techniques for the study of surfaces, thin films and other surface-supported nanostructures. Their real-time imaging and complementary diffraction capabilities allow the study of structure, morphology, magnetism and dynamic processes with high spatial and temporal resolution. Progress in methods, instrumentation and understanding of novel contrast mechanisms that derive from the wave nature and spin degree of freedom of the electron continue to advance applications of LEEM and SPLEEM in these areas and beyond. We review here the basic imaging principles and recent developments that demonstrate the current capabilities of these techniques and suggest potential future directions.

Pd atomic chain formation as a result of submonolayer deposition of 3d metals on Pd(110)

Submonolayer deposition of 3d transition metals such as Cr, Mn, Fe, Co, and Ni on Pd(110) at room temperature causes the formation of monoatomic chains of Pd as identified with scanning tunneling microscopy and spectroscopy. In agreement with recent theoretical predictions [Phys. Rev. B 79, 155410 (2009)], the substitution of Pd substrate atoms with the deposited atoms of 3d metals is found to be responsible for the formation of Pd atomic chains. This finding clarifies the long-debated issue about the chemical composition of the atomic chains grown on Pd(110) and points out the intriguing processes in the formation of self-assembled and self-organized nanostructures.

Evolution of Ge nanoislands on Si(110)-'16 x 2' surface under thermal annealing studied using STM

The initial nucleation of Ge nanoclusters on Si(110) at room temperature (RT), annealing-induced surface roughening and the evolution of three-dimensional Ge nanoislands have been investigated using scanning tunneling microscopy (STM). A few monolayers (ML) of Ge deposited at room temperature lead to the formation of Ge clusters which are homogeneously distributed across the surface. The stripe-like patterns, characteristic of the Si(110)-'16 x 2' surface reconstruction are also retained. Increasing annealing temperatures, however, lead to significant surface diffusion and thus, disruption of the underlying '16 x 2' reconstruction. The annealing-induced removal of the stripe structures (originated from '16 x 2' reconstruction) starts at approximately 300 degrees C, whereas the terrace structures of Si(110) are thermally stable up to 500 degrees C. At approximately 650 degrees C, shallow Ge islands of pyramidal shape with (15,17,1) side facets start to form. Annealing at even higher temperatures enhances Ge island formation. Our findings are explained in terms of partial dewetting of the metastable Ge wetting layer (WL) (formed at room temperature) as well as partial relaxation of lattice strain through three-dimensional (3D) island growth.

Temperature dependent low energy electron microscopy study of Ge island growth on bare and Ga terminated Si(112)

The pre-adsorption of Ga on Si(112) leads to a drastic change of the morphology of subsequently grown Ge islands. In contrast to the case for Ge growth on bare Si(112), even nanowire growth can be achieved on Ga terminated Si(112). Employing low energy electron microscopy and low energy electron diffraction, the initial phase of Ge nucleation and Ge island growth was systematically analysed for growth temperatures between 420 and 610 °C, both on clean and on Ga terminated Si(112). In both cases the island density exhibits an Arrhenius-like behaviour, from which diffusion barrier heights of about 1.3 and 1.0 eV can be estimated for growth with and without Ga pre-adsorption, respectively. The Ge island shape on the bare Si(112) surface is found to be nearly circular over the whole temperature range, whereas the shapes of the Ge islands on the Ga terminated Si(112) become highly anisotropic for higher temperatures. Ge nanowires with sizes of up to 2 µm along the [Formula: see text] direction are observed.

Iron silicide nanostructure formation on Au induced superstructures on Si(111)

Using scanning tunnelling microscopy we have investigated the formation of low dimensional Fe-Si structures on Au covered Si(111) surfaces. The ultrathin Au layer induces a variety of surface reconstructions, depending on the coverage and temperature: Si(111)-5 x 2, [alpha-squareroot 3 x squareroot 3,beta-squareroot 3 x squareroot 3], and 6 x 6-Au. The subsequent deposition of 0.28 ML (monolayers) of Fe at 400 degrees C results in the formation of Fe-Si nanostructures whose morphological properties critically depend on the underlying surface. All Au induced reconstructions give rise to an increase in diffusion length as compared to the bare Si(111)-7 x 7 surface, thereby allowing the growth of well-separated nanostructures at considerably lower temperatures. Ultimately, the decoupling of surface diffusion and temperature, induced by the Au layer, can be exploited to tailor the island dimensions and density. With an appropriate choice of substrate, passivating layer and deposited material, nanostructures with the desired properties can be grown in a controlled way.