One Dimensional Rare Earth Metal Silicides Applications in - - PowerPoint PPT Presentation

One Dimensional Rare Earth Metal Silicides Applications in Applied and Fundamental Science

Saban Hus University of Tennessee Oct 25 2010

Outline

• Introduction and Motivation
• Rare Earth Metal Silicide nanowires
• Applications in applied and fundamental sciences

– Nanowires for DNA sequencing – Nanowires as nano probes – Nanowires for plasmonics

• Experimental Methods
• STM and 4-probe STM
• YSi2 nanowires
• Growth and Structure
• Electrical Characterization of YSi2 nanowires

Rare-Earth Metal Silicide Nanowires

100nm

• Epitaxial RESi2 are formed via self

assembly of rare earth metal (RE)

• nto an atomically clean Si (001)

surface

• RE = Dy, Er, Y, Sm, Gd, Ho, Sc ...
• These nanowires exhibit high

aspect ratios having lengths exceeding 1 micron and widths as small as 1.15 nm

Applications in

• Applied Sciences
• as quantum devices
• interconnects for nano

electronics.

• Molecular level probing
• Fundamental Sciences
• Understanding the electronic

behavior in 1D systems

• As a probe to understand nano

structures.

Zeng et al. Nature Mat., Vol. 7, 539 (2006)

Charge ordering in YSi2 nanowires YSi2 nanowires for DNA sequencing UTK- UNC- UCSD

Nanowires for DNA sequencing

• Current DNA sequencing

technologies perform sequencing

• n amplified copies of templates
• These techniques are slow and

expensive.

• A nanowire electrode can be used

to achieve single nucleotide resolution by measuring the tunneling current as DNA pass through a nano channel.

RE Nanowires for Surface Enhanced Raman Scattering

• Arrangements of nano particles are

typically employed in SERS experiments

• A better control on nano particle

size can further enhance the Raman scattering to a level sensitive enough to detect single molecules

• Nobel metal particles (gold or

platinum) aggregate preferentially

• n top of RESi2 nanowires forming

an ordered array of noble metal nano clusters or nanowires

You et al. Nano Lett., Vol. 6, No. 9, 2006

Nanowires as nano probes

• Nanowires with thicknesses ~1 nm

can be used to probe the electronic structure of smallest nano structures.

Experimental Methods

Creating images from tunneling Creating images from tunneling current current

• 2 piezoelectric tube control the

lateral position (x,y) of the tip with ≤ 1Å accuracy

• Another piezoelectric tube

controls the height (z) of the tip with 0.05 Å accuracy

• At a given (x,y) coordinate the

tip is moved up or down (±z)to provide a predefined tunneling current (~0.1 nA)

• Z position of the tip is plotted as

a function of (x,y)

Tunneling current

• Tunneling current is also

proportional to the local density

• f states (LDOS) near Fermi

level.

• where f is the Fermi function, ρs

and ρt are the density of states in the sample and tip, respectively

• M is the tunneling matrix

between the modified wavefunctions of the tip and the sample surface

4 Probe STM

• CNMS / ORNL has a
• Ultrahigh Vacuum Cryo 4-

probe STM

• 4 STM probes can take atomic

resolution images

• A SEM is used to monitor SEM

tips while approaching to the nano wire or contact paths

• Gold coated SEM tips can also

be used for contact fabrication

YSi2 Nanowires

YSi2 Nanowires

• The wires form through self-

assembly after depositing approximately 0.5 monolayer of yttrium onto an atomically clean Si(001) surface at about 600 C.

• The silicide form wires due to

anisotropic lattice mismatch with silicon substrate

• YSi wires have higher aspect

ratios and better uniformity compared the other rare-earth silicide nanowires

A, Large-scale STM image of YSi2 nanowires B, High-resolution STM image at room temperature. C, Simulated STM image of a 1.1 nm wide wire D, Cross-sectional line profile of a 1.1 nm wide nanowire.

Zeng et al. Nature Mat., Vol. 7, 539 (2006)c

Structure of YSi2 Nanowires

• To determine the structure compare
• Experimental

– STM images – Tunneling spectra

with

• Theoretical

– image simulations – local density of states calculations,

using Density Functional Theory.

A, Structure model of bulk YSi2. B, Calculated structure of the thinnest, 1.1 nm wide YSi2 nanowires. C, dI/dV spectra of the 1.1 nm wide YSi2 nanowires, D,E, Top and cross-sectional view of the calculated wavefunction densities

• n the 1.1 nm wide YSi2 nanowire.

Zeng et al. Nature Mat., Vol. 7, 539 (2006)

Transport measurements on YSi2 Nanowires

• 4 Probe STM Measurements
• Nanowires are conducted by

very small Pt electrodes, fabricated via EBID,

• Pt electrodes are connected to

thick mesoscopic contact pads.

Nanowire between the Pt electrodes Approaching the 4 STM tips to mesoscopic contacts for I-V measurements

Thanks