Dec 2010, Volume 5 Issue 4

Cover illustration

  • The ability to move individual atoms with the tip of a scanning microscope is a powerful first step towards building complex molecular machines at the atomic scale. For practical applications of such molecular machinery, it must be possible to construct it easily and at low cost, on a large-scale. The key satisfying these requirements is to find molecular systems that assemble themselves into the desired shapes and functions on tailor-made surfaces. Prof. Hong-jun GAO and his [Detail] ...

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    Karl-Heinz ERNST

    We discuss the occurrence of transition structures observed in molecular self-assembly at surfaces. The increasing surface coverage transitions from low coverage structures to high coverage structures are a common phenomenon. However, often observed and not perfectly understood is the formation of intermediate structures, sometimes with lower lateral density than the initial phase. We will present different examples from our recent work and discuss the possible mechanisms of intermediate phase formation. In addition, we present intermediate structures occurring due to temperature-controlled reversible phase transitions.

    Hong-wang ZHANG (张洪旺), Yi LIU (刘毅), Shou-heng SUN (孙守恒)

    This mini-review summarizes the recent advances in chemical synthesis and assembly of monodisperse magnetic nanoparticles for magnetic applications. After a brief introduction to nanomagnetism, the review focuses on recent developments in solution phase syntheses and assemblies of monodisperse Fe, CoFe, FePt and SmCo5 nanoparticles. The review further outlines the structural and magnetic properties of these nanoparticles for magnetic information and energy storage applications.

    Qin LIU (刘琴), Ke-dong WANG (王克东), Xu-dong XIAO (肖旭东)

    Scanning tunneling microscopy (STM) is not only an excellent tool for the study of static geometric structures and electronic structures of surfaces due to its high spatial and energy resolution, but also a powerful tool for the study of surface dynamic behaviors, including surface diffusion, molecular rotation, and surface chemical reactions. Because of the limitation of the scanning speed, the video-STM technique cannot study the fast dynamic processes. Alternatively, a time-dependent tunneling current, I–t curve, method is employed in the research of fast dynamic processes. Usually, this method can detect about 1000 times faster dynamic processes than the traditional video-STM method. When placing the STM tip over a certain interesting position on the sample surface, the changing of tunneling current induced by the surface dynamic phenomena can be recorded as a function of time. In this article, we review the applications of the time-dependent tunneling current method to the studies of surface dynamic phenomena in recent years, especially on surface diffusion, molecular rotation, molecular switching, and chemical reaction.

    Haiping LIN (林海平), Janosch M. C. RAUBA, Kristian S. THYGESEN, Karsten W. JACOBSEN, Michelle Y. SIMMONS, Werner A. HOFER

    The investigation of electron transport processes in nano-scale architectures plays a crucial role in the development of surface chemistry and nano-technology. Experimentally, an important driving force within this research area has been the concurrent refinements of scanning tunneling microscopy (STM) techniques. The theoretical treatment of the STM operation has traditionally been based on the Bardeen and Tersoff–Hamann methods which take as input the single-particle wave functions and eigenvalues obtained from finite cluster or slabs models of the surface-tip interface. Here, we present a novel STM simulation scheme based on non-equilibrium Green’s functions (NEGF) and Wannier functions which is both accurate and very efficient. The main novelty of the scheme compared to the Bardeen and Tersoff–Hamann approaches is that the coupling to the infinite (macroscopic) electrodes is taken into account. As an illustrating example we apply the NEGF-STM method to the Si(001)-(2×1):H surface with sub-surface P doping and discuss the results in comparison to the Bardeen and Tersoff–Hamann methods.

    Shi-xuan DU (杜世萱), Ye-liang WANG (王业亮), Qi LIU (刘奇), Hai-gang ZHANG (张海刚), Hai-ming GUO (郭海明), Hong-jun GAO (高鸿钧)

    The motion of single molecules on surfaces plays an important role in nanoscale engineering and bottom-up construction of complex devices at single molecular scale. In this article, we review the recent progress on single molecular rotors self-assembled on Au(111) surfaces. We focus on the motion of single phthalocyanine molecules on the reconstructed Au(111) surface based on the most recent results obtained by scanning tunneling microscopy (STM). An ordered array of single molecular rotors with large scale is self-assembled on Au(111) surface. Combined with first principle calculations, the mechanism of the surface-supported molecular rotor is investigated. Based on these results, phthalocyanine molecules on Au (111) are a promising candidate system for the development of adaptive molecular device structures.

    Hai-feng MA (马海峰), Mario THOMANN, Jeanette SCHMIDLIN, Silvan ROTH, Martin MORSCHER, Thomas GREBER

    Functional nano-templates enable self-assembly of otherwise impossible arrangements of molecules. A particular class of such templates is that of sp2 hybridized single layers of hexagonal boron nitride or carbon (graphene) on metal supports. If the substrate and the single layer have a lattice mismatch, superstructures are formed. On substrates like rhodium or ruthenium these superstructures have unit cells with ~3-nm lattice constant. They are corrugated and contain sub-units, which behave like traps for molecules or quantum dots, which are small enough to become operational at room temperature. For graphene on Rh(111) we emphasize a new structural element of small extra hills within the corrugation landscape. For the case of molecules like water it is shown that new phases assemble on such templates, and that they can be used as “nano-laboratories” where many individual processes are studied in parallel. Furthermore, it is shown that the h-BN/Rh(111) nanomesh displays a strong scanning tunneling microscopy-induced luminescence contrast within the 3 nm unit cell which is a way to address trapped molecules and/or quantum dots.

    Li-feng YIN, Jian SHEN

    Nature produces ferromagnetic materials based on nearest neighbor exchange interaction between atomic spins. For artificially fabricated nanomagnets, it is those “small” magnetic energies, e.g. anisotropy, dipolar interaction and indirect exchange interaction that play crucial roles against the thermal fluctuation. We have developed strong capabilities to grow nanodot assemblies in ultrahigh vacuum with controllable size and density on/in both metallic and insulating templates. Based on our novel synthesis capability, we have studied artificial nanomagnets with tunable coupling strength via dimensionality control of the mediating electrons in one-dimensional (1-D), 2-D, and 3-D. We show that such kind of dimensional confinement provides a unique way to induce novel magnetic properties and to gain control of them. The research outlined in this work provides the science base to understand, modify, and manipulate the magnetic properties through dimensional confinement.

    Min GAO (高旻), Cheng-yao LI (李成垚), Wen-liang LI (李文亮), Xiao-xian ZHANG (张小娴), Lian-mao PENG (彭练矛)

    One-dimensional (1-D) semiconductor nanostructures can effectively transport electrons and photons, and are considered to be promising building blocks for future optoelectronic nanodevices. In this review, we present our recent efforts to integrate optical techniques and in situ electron microscopy for comprehensively characterizing individual 1-D optoelectronic nanostructures and nanodevices. The technical strategies and their applications in “green” emission and optical confinement in 1-D ZnO nanostructures will be introduced. We also show in situ assembly and characterization of nanostructures for optoelectronic device purposes. Using these examples, we demonstrate that the combination of optical techniques and in situ electron microscopy can be powerful for the studies of optoelectronic nanomaterials and nanodevices.

    Ru HUANG (黄如), Run-sheng WANG (王润声)

    The gate-all-around (GAA) silicon nanowire transistor (SNWT) is considered one of the best candidates for ultimately scaled CMOS devices at the end of the technology roadmap. This paper reviews our recent work on the key issues regarding SNWTs from the top-down approach including process integration, carrier transport, and fluctuation and variability in these unique one-dimensional stronglyconfined nanowire devices. A new process integration scheme for SNWTs is discussed, which features a fully-Si-bulk substrate, an epi-free process, a self-aligned structure and a large source/drain fan-out. The physical characteristics of the fabricated devices with 10-nm-diameter nanowires are further investigated. The carrier transport performance in SNWTs is experimentally estimated, with a modified extraction methodology which takes into account the impact of temperature dependence of parasitic source resistance. SNWTs with sub-40-nm gate lengths exhibit high ballistic efficiency at room temperature, indicating great potential for SNWTs as an alternative device structure for near-ballistic transport. For heat transfer in SNWTs, the self-heating effect is also characterized. However, due to the one-dimensional (1-D) nature of nanowires and increased phonon-boundary scattering in the GAA structure, the self-heating effect in SNWTs based on the bulk substrate is comparable or even a little bit worse than SOI devices, which may limit the ultimate performance of SNWT-based circuits and thus, special design consideration is expected. On the other hand, random variation has become a practical problem at nano-scale. The characteristic variability of SNWTs is experimentally studied in detail. The results of extracted variation sources indicate that, with suppressed random dopant fluctuations in the intrinsic channel, variations in radius and metal-gate work function of SNWTs dominate both the threshold voltage and on-current fluctuations. Comparing with conventional planar MOS devices, SNWT based SRAM cells exhibit better stability due to the superior electrostatic control in SNWTs.

    Jia-ming HAO (郝加明), Min QIU (仇旻), Lei ZHOU (周磊)