J. R. Edwards Pierre Emelie Mike Logue Zhuang Wu Development of One-Dimensional Band Structure in Artificial Gold Chains J. R. Edwards Pierre Emelie Mike Logue  Zhuang Wu

1-D Band Structure in Gold Nano-Chains Background Theoretical Experimental Next Step

Background Work Atoms exhibit different properties than bulk---with a continuum of properties in-between Metal clusters exhibit absorption behavior Small metal aggregates exhibit catalytic behavior

Metal clusters exhibit absorption behavior

Small metal aggregates exhibit catalytic behavior

Problems in Background Work Preparation and analysis of well-defined nano-structures is difficult Continuum of Sizes Different Geometries

Geometric structures and Size Continuum Geometric structures near the bulk transition 2-D states on surface 1-D states in step edges Geometric structures near the atom transition Nano-clusters

Single Cu atoms evaporated at 15 K on Cu(111). The Cu atoms form an island with local hexagonal order Single Cu atoms are trapped in front of a descending step edge

Au/Ti02 catalysts prepared by deposition-precipitation Uniform clusters are difficult to prepare Both size and geometry varies

How is this work different from background work Nano-chain structure is near atom transition Nano-chains are well-defined and readily-modified geometries that provide useful analysis

What is being studied Interrelation between geometric structure, elemental composition and electronic properties in metallic nanostructures. The behavior of matter in the atom-to-bulk transition range for well defined 1-D structures. How values for the effective mass and density of states of the 1-D, 20-atom length, gold chain compare to known results from other experiments.

Why do this To be able to understand the electronic properties of metallic nanostructures on the size of a few atoms as related to geometric structure and elemental makeup. Demonstrate a strategy for studying this relationship. Use the knowledge gained to be able to control the intrinsic properties of metallic nanostructures whose size is in the atomic-to-bulk transition range.

Why gold chains on NiAl(110) The NiAl(110) structure is made up of alternating rows of Ni troughs and protruding Al rows. This structure acts as a natural template for building the 1-D gold chain. The distance between adjacent Ni bridge sites (2.89 Å) matches almost exactly the nearest neighbor distance (2.88 Å) in bulk Au.

How are the electronic properties measured STS-Scanning Tunneling Spectroscopy Uses the STM (Scanning Tunneling Microscope) to take very precise and accurate measurements of the electronic properties Why use STS? Because of its sensitivity to vibrational, optical, and magnetic properties. Because it can move atoms around as well as image atomic scale surfaces

Scanning tunneling spectroscopy (STS)

STS & STM

STS & DOS ρs is the density of the electronic state of the sample surface ρt is the density of the electronic state of the tip

How to move the atoms

The atoms are added to the chain one after an other

Conductivity changes during this process Measurements are taken in the center of the chains Peak splits due to strong coupling between atoms, and there is a downshift of the peaks Due to the overlap between neighboring peaks, conductivities become indistinguishable for chains with 4 or more atoms

1-D quantum well The energy levels are discrete For infinity 1-D quantum well, the wave function of the electron at certain energy level En is φn=sin(nπx/r0) The wave function of the electron is the superposition of a series of φn Ψ(x)=ΣAnSin(nπx/r0) The probability to find a electron at x-point is proportional to | Ψ(x)|2

Measuring at different positions The derived coefficients are c1=0.31, C2=0.29, c3=0.26, c4=0.11 for 0.78 V; C5=0.26, C6=0.50, c7=0.24 for1.51 V; and c6=0.13, C7=0.29, c8=0.39, c9=0.19 for 2.01 V. P(x)=ΣCnSin2(nπx/L)

This can be simulated very well by 1-D infinity quantum well : Ψ(x)=ΣCnSin(nπx/L) To account for a finite barrier height, the absolute length of the well (L) is treated as an adjustable parameter. For Au20, L varies from 59 to 62 Å with increasing energy. The measured dI/dV signal is high, when the sample bias matches one of the energy levels En.

Conductivity changes during this process Measurements are taken in the center of the chains Peak splits due to strong coupling between atoms, and there is a downshift of the peaks Due to the overlap between neighboring peaks, conductivities become indistinguishable for chains with 4 or more atoms

Density of States (DOS) comparison The picture to the left compares the relative DOS for a Au20 chain to that of a 60 A long quantum well and a 1D free-electron gas. Quantum well states are marked with bars along the left axis. This data corresponds well with the predictions of an E-1/2 dependence and variations from the perfect 1-D behavior is attributed to the finite length of the chain and the limited number of states on the parabolic band.

What have we learned? Scanning Tunneling Microscopy: Preparation of well-defined nanosized structures Scanning Tunneling Spectroscopy: - dispersion relation - effective mass - density of states

Challenges Can we improve this STM/STS approach? What are the alternative techniques? What is the next step?

Preparation of nanosized structures STM is a very useful tool to manipulate single atoms at low temperature It has also been used to manipulate single molecules at room temperature Problem: time required to obtain these structures by STM M. F. Crommie et al., Science, 262 218 (1993) M. T. Cuberes et al., Appl. Phys. Lett., 69 3016 (1996)

Preparation of nanosized structures Novel approach: use tip geometries combined with millisecond voltage pulses Facet terminated STM tips are employed Polarity of the field is arranged to have the tip positive Both electrodes are of the same material Field enhanced evaporation

Preparation of nanosized structures No pulse 4V/5ms 0.4 µm Au thin films with Au tips 3.8V/5ms 2.8V/5ms Halo creations: diameters around 210 Ǻ walls extend to 70 Ǻ laterally manufacturing time 106 faster P. A. Campbell et al., Nanotechnology, 13 69 (2002)

Photoelectron Spectroscopy (UPS) UPS is one of the many complementary/alternative techniques to STS to study the electronic properties of nanosized structures Based on the absorption of a photon by an electron in the valence band Applications: - electronic structure of solids - adsorption of molecules on metals

Next Step 2D and 3D systems can be analyzed by this STM/STS approach Electronic and optical properties have to be characterized to potentially develop nanosized devices with novel applications Preparation and manufacturing of these nanosized structures will be a challenge