Macro- and atomic-scale observations of a one-dimensional heterojunction in a nickel and palladium nanowire complex.

The creation of low-dimensional heterostructures for intelligent devices is a challenging research topic; however, macro- and atomic-scale connections in one-dimensional (1D) electronic systems have not been achieved yet. Herein, we synthesize a heterostructure comprising a 1D Mott insulator [Ni(chxn)2Br]Br2 (1; chxn = 1R-2R-diaminocyclohexane) and a 1D Peierls or charge-density-wave insulator [Pd(chxn)2Br]Br2 (2) using stepwise electrochemical growth. It can be considered as the first example of electrochemical liquid-phase epitaxy applied to molecular-based heterostructures with a macroscopic scale. Moreover, atomic-resolution scanning tunneling microscopy images reveal a modulation of the electronic state in the heterojunction region with a length of five metal atoms (~ 2.5 nm), that is a direct evidence for the atomic-scale connection of 1 and 2. This is the first time that the heterojunction in the 1D chains has been shown and examined experimentally at macro- and atomic-scale. This study thus serves as proof of concept for heterojunctions in 1D electronic systems.

Real time and space observation of domain wall migration in a MX-chain complex with a quasi-2D CDW state by using STM.

In STM images of [Pd(chxn-d(4))(2)Br]Br(2), which is in a quasi-2D-CDW state, we observed domain wall migration due to fast soliton motion at neighboring chains to the domain wall.

Structural study of electronic states in quasi-one-dimensional halogen-bridged mixed-metal complexes [Ni(1-x)Pd(x)(chxn)(2)Br]Br(2).

The electronic structures of quasi-one-dimensional halogen-bridged mixed-metal compounds [Ni(1-x)Pd(x)(chxn)(2)Br]Br(2) were investigated by means of powder X-ray diffraction measurements. Unit cell lengths of all directions obeyed a linear relationship when x < 0.9, whereas those deviated when x > 0.9 especially in the case of the b and c axes. This deviation can be explained by a phase change from an M(III) Mott-Hubbard state to an M(II)-M(IV) charge-density-wave state because of chemical pressure caused by the substitution of Pd ions into [Pd(chxn)(2)Br]Br(2) with smaller Ni ions.

Effect of an in-plane ligand on the electronic structures of bromo-bridged nano-wire Ni-Pd mixed-metal complexes, [Ni(1-x)Pd(x)(bn)2Br]Br2 (bn = 2S,3S-diaminobutane).

Single crystals of quasi-one-dimensional bromo-bridged Ni-Pd mixed-metal complexes with 2S,3S-diaminobutane (bn) as an in-plane ligand, [Ni(1-x)Pd(x)(bn)(2)Br]Br(2), were obtained by using an electrochemical oxidation method involving mixed methanol/2-propanol (1:1) solutions containing different ratios of [Ni(II)(bn)(2)]Br(2) and [Pd(II)(bn)(2)]Br(2). To investigate the competition between the electron-correlation of the Ni(III) states, or Mott-Hubbard states (MH), and the electron-phonon interaction of the Pd(II)-Pd(IV) mixed valence states, or charge-density-wave states (CDW), in the Ni-Pd mixed-metal compounds, X-ray structure analyses, X-ray oscillation photograph, and Raman, IR, ESR, and single-crystal reflectance spectra were analyzed. In addition, the local electronic structures of Ni-Pd mixed-metal single crystals were directly investigated by using scanning tunneling microscopy (STM) at room temperature and ambient pressure. The oxidation states of [Ni(1-x)Pd(x)(bn)(2)Br]Br(2) changed from a M(II)-M(IV) mixed valence state to a M(III) MH state at a critical mixing ratio (x(c)) of approximately 0.8, which is lower than that of [Ni(1-x)Pd(x)(chxn)(2)Br]Br(2) (chxn = 1R,2R-diaminocyclohexane) (x(c) approximately 0.9) reported previously. The lower value of x(c) for [Ni(1-x)Pd(x)(bn)(2)Br]Br(2) can be explained by the difference in their CDW dimensionalities because the three-dimensional CDW ordering in [Pd(bn)(2)Br]Br(2) observed by using X-ray diffuse scattering stabilizes the Pd(II)-Pd(IV) mixed valence state more than two-dimensional CDW ordering in [Pd(chxn)(2)Br]Br(2) does, which has been reported previously.

Electronic structure of Co(III) doped bromo-bridged Ni complexes, [Ni1-xCox(chxn)2Br]Br2.

This article describes the electronic structure of the Co(III) doped Br bridged Ni(III) complexes, [Ni(1-x)Cox(chxn)2Br]Br2 (x = 0.01, 0.02, 0.05, and 0.11) by using a optical spectroscopy, scanning tunneling microscopy (STM), and electron spin resonance spectroscopy. In the optical reflectivity spectrum, the new band was formed at about 0.5 eV, which is reasonably recognized as the d(z2) band of doped Co(III) ions. In the STM images of [Ni(1-x)Cox(chxn)2Br]Br2, the bright spots attributable to the tunnel current from the Fermi level of the STM tip to the conduction band of the sample were observed. In addition, some brighter spots were also observed. Because the number of the brighter spots is in good agreement with that of doped Co species, the brighter spots can be assigned to doped Co(III) sites. These are reasonably explained by the tunnel current from the Fermi level of the tip to the d(z2) band of Co(III). The Curie spin concentration was gradually increased with increasing Co(III) ions, which is explained by the scissions of the S = 1/2 1D antiferromagnetic chains.

Why does the disorder of R-pn and rac-pn ligands in the quasi-one-dimensional bromo-bridged NiIII complexes, [Ni(pn)2Br]Br2 (pn=1,2-diaminopropane) afford similar STM patterns?

The disordered patterns of R- and rac-1,2-diaminopropane (pn) in quasi-one-dimensional bromo-bridged Ni(III) complexes, [NiIII(pn)2Br]Br2, have been investigated by single-crystal X-ray structure determination and scanning tunneling microscopy (STM). X-ray structure determination shows that the methyl moieties are disordered on the right- and left-hand sides with half occupancies in both compounds, while the carbon atoms of the ethylene moieties of pn ligands are disordered in [Ni(rac-pn)2Br]Br2 and not disordered in [Ni(R-pn)2Br]Br2. In the STM images of both compounds, the bright spots are not straight but fluctuated with the similar patterns. We have concluded that tunnel current from the STM tip to metal ions are detected via methyl groups of pn ligands.