Bi Nanolines Research Articles

Structure and Electrical Properties of Atomic-scale In-Bi Nanowire Arrays

ABSTRACTThe 1-nm-wide Bi nanoline has been proposed as a possible template for the growth of very-high-density arrays of atomic-scale nanowires, grown epitaxially on the technologically important Si(001) surface. Indium reacts with the Bi dimers, forming a unique zigzag atomic chain structure. Simulations of the appearance in STM of the lowest-energy isomer of this structure match experimental filled-states images. Calculation of the LDOS for the single-layer islands, finds that the nanowires are semiconducting, with a band gap smaller than that of the substrate, in good agreement with STS. A delocalised LUMO state is created, which may provide a conduction pathway along the nanowire. We have performed dual-probe STM conduction measurements along the In-Bi nanowires to test this prediction.

Misoriented Bi dimers blocking Ag nanowire growth along the Bi nanoline: a first-principlesstudy

Growth of Ag nanostructures on the Bi nanoline template (a double line of Bi dimersself-assembled on the Si(001) surface) is examined at the level of the generalized gradientapproximation. The aim is to find whether Ag adsorption induces the misorientation of Bidimers (i.e. those oriented transversely), because the misoriented dimers would block Agnanowire growth along the line. The calculation finds that the Ag tetramer on misorientedBi dimers is more stable than that on oriented dimers. To grow Ag wires hencerequires suppression of the transformation to the misoriented configuration. Thecalculated energy barriers indicate that it is possible to suppress the transformationwithout inhibiting the Ag migration to the Bi nanoline. Furthermore, the greaterstability of Ag wire nuclei compared to less wetting ones such as the icosahedralAg55 cluster also supports the feasibility of Ag wire growth. On the other hand, allowing themisorientation can be useful in growing uniform Ag nanoclusters; an optimum size(∼50 atoms) is found for the pyramidal islands that are bounded by misoriented Bi dimers.

First-principles study of the geometry of Ag nanowires growing on a self-assembled Binanoline

Epitaxial Ag nanowires on a self-assembled Bi nanoline on the Si(001) surface are examinedfor their geometry and energetic stability at the level of the generalized-gradientapproximation. The orientations examined are Ag(001)[100], Ag(110)[100], Ag(111)[110],and Ag(001)[110], where the indices refer to the plane and the direction parallel to theSi(001) surface and the Bi nanoline, respectively. The wires are found to have mostlybulklike structure, except that Ag(001) monolayers undergo extensive reconstruction. Thecalculated electronic band structure indicates that the Ag wires are metallic wires.Particularly stable among the wires are the Ag(111) wires, having a coincident site latticeinterface with the Bi nanoline. The energetic stability generally improves with thickness,indicating that Ag grows through three-dimensional nucleation on the Bi nanoline.

First-principles study of the stability of atomic Ag lines epitaxial to self-assembled Binanolines

The stability of single-atom-wide Ag lines epitaxial to a Bi nanoline (a double line of Bidimers self-assembled on the Si(001) surface) is examined from first principles. Thelattice-matched Ag line, which has a coincident-site lattice (CSL) relationship with the Binanoline, is found more stable than pseudomorphic ones such as monomer and dimer lines.The greater stability of the single-row CSL line against the double-row one indicates thefeasibility of single-row lines. The greater stability of the triple-row line however suggeststhat the single-row lines should be grown under kinetically controlled conditions.

Growth of noble metal nanostructures on the Bi nanoline surface: A first-principles study

Recent experiments suggest that self-assembled Bi nanolines on the passivated Si(0 0 1) surface can be used as templates to grow one-dimensional arrays of noble-metal (Au, Ag) nanoclusters. In order to ascertain this template effect, the gold atom on the H-terminated Si(0 0 1) surface and that on the Bi line are examined by density-functional methods. The calculation indicates that the gold atom enters a Si dimer on the H-terminated surface and the Bi Si backbonds of a Bi dimer on the Bi line. The gold atom is 0.6 eV more stable on the Bi line than on the H-terminated surface. This result suggests the preferential adsorption of gold on the Bi line and confirms its template effect.

A theoretical study of Ge adsorption on Si(0 0 1) covered with Bi nanolines

The energetic stability and the equilibrium geometry of Ge adsorption on the Si(0 0 1) surface covered with Bi nanolines were examined by ab initio total energy calculations. We find that there is an energetic preference of Ge atoms lying on the Si(0 0 1) terraces, forming Si down–Ge up mixed dimers. Further investigations reveal a repulsive interaction between the mixed dimers and the Bi nanolines, suggesting that the formation of Si down–Ge up dimers can be tailored by the presence of the Bi nanolines.

Ｘ線逆格子イメージング法を用いた表界面ナノ構造評価

We have developed a nondestructive analysis method, which is named X-ray reciprocallattice space imaging, based on synchrotron diffraction for quickly characterizing a crystalline nanometer-scale structure in a non-vacuum environment. The basic idea behind the method is that the reciprocal lattice of 1 D or 2D structures are an array of sheets or rods, respectively. Thus the reciprocal-lattice space can be recorded for a fixed sample with a 2D X-ray detector fixed. We successfully demonstrated that the method was applicable to structural evaluation of ultrathin NiO wires on a sapphire surface in air, Bi nanolines buried in Si, an interfacial structure of a Au electrode in solution, and a thinfilm of Bi4Ti3O12.

First-principles study of Ag adsorption on the H-passivatedSi(001)surface with Bi nanolines

This first-principles study examines the adsorption and migration of Ag atoms on the H-passivated $\mathrm{Si}(001)$ surface with Bi nanolines, and finds that Ag nucleates preferentially at the nanolines in the following manner. An Ag atom deposited on the H-passivated surface rapidly migrates through a groove between dimer rows (the barrier is $0.14\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$), to reach a Bi nanoline. Then, the adatom enters a backbond of the Bi nanoline, bonding directly to Si and releasing the energy of $\ensuremath{\sim}0.7\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. After that, the adatom migrates along the nanoline (the barrier is $0.33\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$), and encounters another adatom to form a dimer, without leaving the backbonds. The dimer is sufficiently stable at room temperature (dissociation energy is $1.0\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$) to serve as a growth nucleus. Thus, Ag nucleates preferentially at the Bi nanolines, directly on Si.

Fe adatoms along Bi nanolines on H∕Si(001)

The stability and electronic and magnetic properties of Fe atoms adsorbed on the self-assembled Bi-line nanostructure on the H∕Si(001) surface are addressed by spin-density functional calculations. Our results show that Fe adatoms are much more stable on sites closer to the Bi nanolines suggesting that they form one-dimensional atomic arrays. The most stable structure occurs on a missing dimer line beside the Bi dimers, which corresponds to an array with distances between Fe adatoms of about 8Å. In this array the irons are coupled antiferromagnetically with spin magnetic moment of about 1.5μB per Fe atom, whereas the coupling exchange interactions is found to be of about 14.4meV. We also estimate a large magnetic anisotropy energy of 3meV∕at. originated on the structural anisotropy of the Fe-adatom site. In addition, the electronic band structure of the Fe array at the most stable structure shows a magnetic half-metal behavior.

Self-assembled nanowires on semiconductor surfaces

A number of different families of nanowires which self-assemble on semiconductor surfaces have been identified in recent years. They are particularly interesting from the standpoint of nanoelectronics, which seeks non-lithographic ways of creating interconnects at the nanometre scale (though possibly for carrying signal rather than current), as well as from the standpoint of traditional materials science and surface science. We survey these families and consider their physical and electronic structure, as well as their formation and reactivity. Particular attention is paid to rare earth nanowires and the Bi nanoline, both of which self-assemble on Si(001).

Growth of Ag nanoclusters on a 1D template

Bi nanolines, which are 1.5 nm wide and can be over 500 nm long, have been used as a 1D template for the formation of arrays of small clusters of silver. Distinct nucleation and growth phases are observed. At room temperature, silver atoms preferentially adsorb onto the Bi nanoline templates to form arrays of spheroidal nanoclusters 0.55 nm in height, up to a saturation density of 6.7 clusters per 100 nm of nanoline. Further deposition of Ag results in the growth of the clusters on the nanolines up to 1.4 nm, accompanied by the formation of 0.55 nm clusters on the background. At 400 K, however, deposition of Ag results in the formation of clusters up to 4 nm in height, with the majority formed on the nanoline template.

Comment on ‘Bi nanolines on Si(001): registry with substrate’

A recent article (Miwa et al 2005 Nanotechnology 16 2427) casts doubt on thefour-dimer-wide Haiku model for the Bi nanoline on Si(001), suggesting instead that thethree-dimer-wide Miki model (which had been ruled out) is a better fit in particular tox-ray data. The reasons why the Haiku model provides the best fit to all publisheddata among currently proposed structures are discussed, concentrating on thewidth and registry of the Bi nanoline, and mentioning new data which showsthat under appropriate conditions the two structures coexist in the same surface.

Reply to Comment on ‘Bi nanolines on Si(001): registry with the substrate’

The registry of bismuth dimers, integral components of the bismuth nanoline on Si(001), isexamined. In contrast to the currently accepted view, the bismuth dimers are found to bein registry with the two-dimensional lattice created by the silicon dimers. The consequencesof this finding are briefly explored.

One-dimensional epitaxial growth of indium on a self-assembled atomic-scale bismuthtemplate

1D indium islands have been formed by epitaxial growth on a self-assembled template.The templates are formed from Bi nanolines, which are 1.5 nm wide, and canbe over 500 nm in length. The islands conform to the template width and areover 100 nm long. The first layer of the island forms a novel chain structure ofIn and Bi atoms, which accounts for the epitaxial growth of the nanocrystalson the template. The second layer shows a hexagonal structure 19 Å or 2.5 dimerrows wide. Epitaxial growth saturates after two layers of In have been deposited.With further deposition, a transition to the growth of 3D In droplets is observed.

Identification of intermediate linear structure formed during Bi/Si(0 0 1) surface anneal

Bi nanolines form at around 570 °C during the anneal of a Bi-rich surface, which is composed of a mixture of Bi dimers, Si dimers, and missing dimer defects. High-temperature STM observations find that as well as the stable nanolines previously reported, a second Bi-related linear feature forms in the background, which has not previously been identified. Using quench experiments, and by comparison with tightbinding simulations of candidate surface structures, the structure of this linear feature has been identified. It is concluded that the linear defective feature comprises a single missing dimer defect (1DV) decorated with Bi, and forms as a result of cooperative strain relaxation between the Bi dimers and the 1DV.

Encapsulation of atomic-scale Bi wires in epitaxial silicon without loss of structure

We demonstrate, using x-ray diffraction, that we have taken one-dimensional Bi nanolines fabricated on a Si(001) surface, and buried them in crystalline silicon while retaining both their one-dimensional characters and important aspects of their structure. In particular, after burial, the nanolines retain the two-by periodicity associated with their surface structure along their length. We have used density functional theory calculations to model a structure for these buried nanolines, whose minimum length can be estimated to be 100 nm from the coherence length of the x-ray measurements.

Surface bismuth removal after Bi nanoline encapsulation in silicon

Recently, several kinds of atomic-scale wires have been reported for nano-scale device and interconnect applications. The Bi nanoline is one of the most promising candidates because of its structural perfection and the capability to be buried in an epitaxial Si layer. A surfactant layer of Bi is necessary for the encapsulation process, and this remains on the sample surface after the burial, but it needs to be removed so that the buried Bi line may be properly characterized by I– V measurements, X-ray diffraction, etc. without perturbations from the surface layer. It is shown by RHEED observation that the thermal desorption is not suitable for this purpose, because the one-dimensional structure of the buried Bi line is disrupted over the same time scale as for removal of the surfactant Bi. Chemical wet etching of the surface layer was found to be a better solution to the removal of the surfactant Bi atoms without destroying the buried Bi lines.

The equilibrium geometry and electronic structure of Bi nanolines on clean andhydrogenated Si(001) surfaces

The equilibrium geometry, electronic structure and energetic stability of Bi nanolines onclean and hydrogenated Si(001) surfaces have been examined by means of ab initio totalenergy calculations and scanning tunnelling microscopy. For the Bi nanolines on a clean Sisurface the two most plausible structural models, the Miki or M model (Miki et al 1999Phys. Rev. B 59 14868) and the Haiku or H model (Owen et al 2002 Phys. Rev. Lett. 88226104), have been examined in detail. The results of the total energy calculations supportthe stability of the H model over the M model, in agreement with previous theoreticalresults. For Bi nanolines on the hydrogenated Si(001) surface, we find that an atomicconfiguration derived from the H model is also more stable than an atomic configurationderived from the M model. However, the energetically less stable (M) model exhibitsbetter agreement with experimental measurements for equilibrium geometry. Theelectronic structures of the H and M models are very similar. Both models exhibit asemiconducting character, with the highest occupied Bi-derived bands lying at∼0.5 eV below the valence band maximum. Simulated and experimental STM images confirm thatat a low negative bias the Bi lines exhibit an ‘antiwire’ property for both structural models.

The electronic properties ofSi(001)–Bi(2 × n)

A Bi 2 × n surface net was grown on the Si(001) surface and studied with inverse photoemission,scanning tunnelling microscopy and ab initio and empirical pseudopotential calculations.The experiments demonstrated that Bi adsorption eliminates the dimer relatedπ1* and π2* surface states, produced by correlated dimer buckling, leaving the bulk bandgap clear ofunoccupied surface states. Ab initio calculations support this observation and demonstratethat the surface states derived from the formation of symmetric Bi dimers donot penetrate the fundamental bandgap of bulk Si. Since symmetric Bi dimersare an important structural component of the recently discovered Bi nanolines,that self-organize on Si(001) above the Bi desorption temperature, a connectionwill be made between our findings and the electronic structure of the nanolines.

Self-assembled Nanoline Template for Growth of Nanoparticles and Nanowires on Si(001)

AbstractLong, straight self-assembled nanolines of Bismuth are used as an atomic-scale non-lithographic template for the deposition of different metals. The Bi nanolines are 15 Å wide and can grow to lengths exceeding 1μm, limited only by the substrate terraces. We describe the method of formation of these templates, and demonstrate strongly preferential adsorption of different metals. We observe both wetting behaviour, in which elongated metal islands form nanowires along the template, and nonwetting behaviour, in which arrays of metal clusters of uniform size are produced.