ELECTRICAL CHARACTERISTIC OF CARBON NANOWIRES PRODUCED BY OXIDATIVE SHRINKING

Alfredo D. Bobadilla

Nanotechnology course – Prof Jorge M. Seminario

Final Project

Spring 2010

Texas A&M University

OUTLINE

MOTIVATION INTRODUCTION OBJECTIVE BASIC CONCEPTS REVIEW OF PREVIOUS WORKS RESEARCH PROJECT PROPOSAL FINAL CONSIDERATIONS

MOTIVATION

Current 45nm Silicon-based technology has reached its physical limit with microprocessors speed in the order of ~ 1 GHz (still far of ~ 10 GHz).

Molecule-based electronics promise advancing towards ~ 1 THz due to novel physical phenomena at molecular scale (< 10 nm) and to the possibility of an inherent much higher level of integration.

INTRODUCTION

Carbon nanostructured materials own outstanding electrical and thermal properties.

Nonlinear electrical characteristic, approaching negative differential resistance behavior, is commonly found in molecules, including carbon nanomaterials.

It is possible engineering the structure and properties of carbon nanomaterials by using ion and electron beams.

OBJECTIVE

I propose an approach to engineer molecular circuits based on carbon nanowires with carbon nanotubes serving as interconnects.

Developing a novel 10nm Carbon-based information technology.

BASIC CONCEPTS:SILICON TECHNOLOGY CMOS transistor is

the basic element for making an integrated circuit (chip).

It is a 3-terminal device in which a gate voltage (Vgs) allows controlling the level of current (Ids)) through the transistor.

http://www.cmosvlsi.com/lect3.pdf

BASIC CONCEPTS:SILICON TECHNOLOGY PHYSICAL LIMITS In digital technology the

transistor is switched between two logic states, 0 and 1, which correspond to two different voltage levels, i.e. 0V and 5V.

The 0V and 5V logic levels are always mixed with thermal noise, as it is shown in the bottom picture.

If the high logic level (5V in the example) would change to a lower level like 2V, thermal noise voltage would make it difficult distinguishing between 0V and 2V.

AnantAgarwaland Jeffrey Lang, course materials for 6.002 Circuits and Electronics, Spring 2007. MIT OpenCourseWare(http://ocw.mit.edu/), Massachusetts Institute of Technology.

BASIC CONCEPTS:SILICON TECHNOLOGY PHYSICAL LIMITS

As the CMOS transistor size is decreased, not only the number of CMOS transistors in a chip increase but also the power consumption and heat generation (thermal noise).

It makes necessary lowering the supply voltage which, in current 45nm Si technology, is approaching the thermal noise voltage.

AnantAgarwaland Jeffrey Lang, course materials for 6.002 Circuits and Electronics, Spring 2007. MIT OpenCourseWare(http://ocw.mit.edu/), Massachusetts Institute of Technology.

BASIC CONCEPTS:OPTICAL LITHOGRAPHY It is a micro-

patterning technique to selectively remove parts of a thin film photoresist.

It uses light to transfer a geometric pattern from a photo mask to a (light-sensitive) photoresist, then by a ‘development’ process the exposed area is removed.

http://cnx.org/content/m1037/latest/5.15.png

BASIC CONCEPTS:DIFFRACTION LIMIT Diffraction can be described

as the apparent bending of waves around small obstacles, and its effects are generally most pronounced for waves where the wavelength is on the order of the size of the diffracting objects.

Therefore when ligth pass through a window size (in the mask) comparable to light wavelength, the light beam will bend. This limit the minimum feature size able to be patterned in optical lithography.

http://cnx.org/content/m1037/latest/5.15.png

BASIC CONCEPTS:ELECTRON BEAM LITHOGRAPHY e-beam lithography

is a patterning technique which allow reaching feature sizes from 10nm to 100nm.

The very small electron wavelength allow reaching a very small feature size.

http://www.cnf.cornell.edu/image/spiefig1.jpg

BASIC CONCEPTS:PROXIMITY EFFECT An incident electron (purple)

produces secondary electrons (blue). Sometimes, the incident electron may itself be backscattered, as shown in the figure, and leave the surface of the resist (amber).

‘Proximity effect’ refer to scattering electrons affecting the patterning of other nearby zones.

The minimum feature size possible in e-beam lithography is not limited by the electron wavelength but by the ‘proximity effect’.

http://upload.wikimedia.org/wikipedia/en/2/22/Electron_beam_scattering.jpeg

BASIC CONCEPTS:ELECTRON BEAM LITHOGRAPHY

On it, and high energy electron beam is scanned on a resist, usually PMMA, removing selectively the exposed area.

This is followed by a development, metal deposition and a lift-off process, as shown in the figure.

http://www.aph.kit.edu/wegener/data/image/research/ebl.jpg

BASIC CONCEPTS:TRANSMISSION ELECTRON MICROSCOPY (TEM)

TEM is an imaging technique which allows reaching atomic resolution.

The transmission electron microscope (TEM) operates on the same basic principles as the light microscope but uses electrons instead of light.

What you can see with a light microscope is limited by the wavelength of light. TEMs use electrons as "light source" and their much lower wavelength makes it possible to get a resolution a thousand times better than with a light microscope.

http://nobelprize.org/educational_games/physics/microscopes/tem/index.html

BASIC CONCEPTS:TRANSMISSION ELECTRON MICROSCOPY (TEM)

On it a highly energetic beam of electrons is transmitted through an ultra thin specimen.

This stream is confined and focused using metal apertures and magnetic lenses into a thin, focused, monochromatic beam.

Interactions occur inside the irradiated sample, affecting the electron beam.

Information contained in the electron waves exiting from the sample is used to form the image.

The projector lenses allow for the correct positioning of this electron wave distribution onto the viewing system.

http://www.unl.edu/CMRAcfem/em.htm http://en.wikipedia.org/wiki/File:Scheme_TEM_en.svg

BASIC CONCEPTS: ETCHING PROCESSES Refer to the process of

removing thin films previously deposited and/or the substrate itself. A mask can be used to selectively remove a specific zone of the film.

In ‘wet etching’ the material is dissolved when immersed in a chemical solution.

In ‘dry etching’ the material is sputtered or dissolved using reactive ions or a vapor phase etchant.

With dry etching it is possible etching almost straight down without undercutting, which provides much higher resolution.

https://www.memsnet.org/mems/processes/etch.html

BASIC CONCEPTS: FOCUSED ION BEAM (FIB) Ions are larger and heavier

than electrons A high energy ion beam is

able to sputter the surface of almost any material and cause doping of the surface with atoms of the ion beam.

FIB tools are designed for site specific etching or machining of surfaces, an ideal FIB might machine away one atom layer without any disruption of the atoms in the next layer, or any residual disruptions above the surface.

http://en.wikipedia.org/wiki/Focused_ion_beam

BASIC CONCEPTS:NEGATIVE DIFFERENTIAL RESISTANCE (NDR)

A NDR device is a 2-terminal device, which shows negative resistance values, i.e. negative values of dI/dV, in the current-voltage curve.

A NDR-based device constitute the basic unit to create a complete ‘logic family’, i.e. it’s possible creating all the logic gates (AND, OR, etc) required for fabricating integrated circuits.

From the equation, I0 and I1 indicate how fast ‘V’ changes from ‘0’ state to ‘1’ state

Mathews, R. H. et al. A new RTD-FET logic family. Proc. IEEE 87, 596-605 (1999)

BASIC CONCEPTS:NEGATIVE DIFFERENTIAL RESISTANCE (NDR)

When two NDR devices work together, as shown in the picture, the two logic states, i.e. ‘0’ and ‘1’, occur at a low voltage level which imply a lower power consumption and lower heat generation.

In a switching from ‘0’ state to ‘1’ state, the currents I0 and I1 are higher than in typical CMOS logic gates, which imply the switching velocity is higher in NDR-based logic gates.

From the equation, I0 and I1 indicate how fast ‘V’ changes from ‘0’ state to ‘1’ state

Mathews, R. H. et al. A new RTD-FET logic family. Proc. IEEE 87, 596-605 (1999)

REVIEW OF PREVIOUS WORKS: CLONING CARBON NANOTUBES

The original short SWNT seed was a polymer wrapped SWNT, end-carboxylated, and further tethered with Fe salts at its ends. The Fe salts act as the growth catalysts upon subsequent reductive activation.

Smalley et al, JACS (2006)

REVIEW OF PREVIOUS WORKS: CLONING CARBON NANOTUBES

Deposition of the short SWNT-Fe tipped species upon an oxide surface was followed by heating in air to consume the polymer wrappers, then reducing the Fe salts to Fe(0) under a H2-rich atmosphere. During this heating, the Fe(0) can etch back into the short SWNT. Upon introduction of C2H4 as a carbon source the short SWNT acts as a template for new growth to a long SWNT. Analysis indicated that the templated VLS-grown long SWNT had the same diameter and surface orientation as the original short SWNT seed.

Smalley et al, JACS (2006)

REVIEW OF PREVIOUS WORKS: A BETTER WAY TO MAKE NANOTUBES

A hoop-shaped chain of benzene molecules, the shortest segment of a carbon nanotube, is synthesized.

This structure enable the growing of carbon nanotubes in a controlled way, with each nanotube of identical chirality to the next.

Jasti et al, JACS (2008)

REVIEW OF PREVIOUS WORKS:SHRINKING A CARBON NANOTUBE When carbon

nanotubes (CNT) are exposed to electron beam radiation it cause the CNT shrinking, reducing CNT diameter during the process.

At the end of the shrinking process, carbon nanowires are produced.

Yuzvinsky et al, Nano Lett (2006)

REVIEW OF PREVIOUS WORKS:NDR BEHAVIOR IN CARBON NANOWIRE CNT current-voltage

curve is monitored during the shrinking process, observing NDR behavior when a carbon nanowire is formed.

Khoo at al, Nano Lett (2008)

REVIEW OF PREVIOUS WORKS: JOINING SINGLE-WALLED CARBON NANOTUBES

Stable junctions of various geometries are created in situ in a transmission electron microscope.

Electron beam exposure at high temperatures induces structural defects which promote the joining of tubes via cross-linking of dangling bonds.

Terrones et al, PRL (2002)

REVIEW OF PREVIOUS WORKS: NANOMACHINING CARBON NANOTUBES WITH ION BEAMS

10 and 30 keV focused beams of Ga+ ions are used to thin, slice, weld, and alter the structure and composition of multiwalled carbon nanotubes at precise locations along the nanotube axis.

Harnessing ion-beam-induced defect generation and doping could be attractive for modulating chemical and electrical properties along the nanotube length, and fabricate nanotube heterostructures and networks for device applications.

(a) Prior to irradiation, and (b) after exposure to 1016 ions cm-2 of 10keV Ga+ ions; (d) An example of a nanotube network formed by several welds indicated by arrows; (e) SEM micrograph showing a CNT welded to the edge of the SiN membrane (see arrows)

Raghuveer et al, APL (2004)

REVIEW OF PREVIOUS WORKS: DERIVING CARBON ATOMIC CHAINS FROM GRAPHENE

Stable and rigid carbon atomic chains were experimentally realized by removing carbon atoms row by row from graphene through the controlled energetic electron irradiation inside a transmission electron microscope.

Jin et al, PRL (2009)

REVIEW OF PREVIOUS WORKS: DERIVING CARBON ATOMIC CHAINS FROM GRAPHENE

Consecutive HR-TEM images showing the dynamics for the formation, breakage of freestanding carbon atomic chains through continuous electron beam irradiation. (a) A GNR with a width of about 1.7 nm was formed between two holes on the graphene. (b) The GNR was thinned row-byrow under the continuous irradiation. (c) A carbon chain consisting of double strands was formed, and there was a knot remained on the left chain (marked as the black arrow). Inset is a representative scheme. The right chain broke from its bottom end (marked as white arrow) and detached with the graphene edge. (d)–(f) The broken chain (on the right) migrated along the left chain, and finally made a connection with the edge belonging to the upper graphene. (g) The carbon was found to be linear and flexible. (h) The carbon chain made a jump along the graphene edge with a changing of edge bonding. The inset is a representative scheme. (i) The carbon chain broken from its upper head. Jin et al, PRL (2009)

REVIEW OF PREVIOUS WORKS: MULTITERMINAL MOLECULAR DEVICES DFT and

nonequilibrium Keldysh theory is used to analyze electron transport through a four-terminal device.

Quantum interference between the four terminals and the central molecule originate an NDR behavior which is not present in a two terminal configuration.

Au, S, C, and H atoms are shown in yellow, red, cyan, and gray, respectively. The semi-infinite leads are built out of Au(111) nanowires.Saha et al, Phys. Rev. B (2010)

REVIEW OF PREVIOUS WORKS: THE NANOCELL The Nanocell concept is a

programmable logic circuit based on molecules, showing NDR behavior, interconnected by metal nanoparticles.

The structure of the molecular circuit is not known but its logic is programmable.

NDR devices change their states (ON to OFF or viceversa) upon application of voltage pulses from the periphery of the Nanocell.

The object in programming or training a nanocell is to take a random, fixed nanocell and turn its switches ON and OFF until it functions as a target logic device.

Seminario et al, IEEE (2006).Tour et al, IEEE (2002)

RESEARCH PROJECT PROPOSAL:GENERAL DESCRIPTION Toward a Carbon-based

Nanocell. Carbon nanowire show a NDR behavior. Carbon nanotubes serves as interconnects between nanowires.

Carbon junctions are made possible by exposure to electron or ion beams.

The molecular circuit is a suspended structure, i.e. not in contact with the substrate.

Figure adapted from Tour et al, IEEE (2002)

RESEARCH PROJECT PROPOSAL:FABRICATION PROCESS

Silicon

SiO2

Silicon

SiO2

CNT in ethanol solution

~ 0.2 ug/ml

Spin coating

FE-SEM imaging

CNT thin film

This loop need to be repeated until we find the adequate concentration level of CNT suspension to get a monolayer of CNTs.

Chromium coating

Silicon

SiO2

RESEARCH PROJECT PROPOSAL

Silicon

SiO2

CNT monolayer

Silicon

SiO2

PMMA

PMMA patterned by e-beam lithography

Silicon

SiO2

Silicon

SiO2

PMMA

Silicon

SiO2

After exposure to Reactive Ion etching

Silicon

SiO2

After PMMAdevelopment

Silicon

SiO2

CNT monolayer

RESEARCH PROJECT PROPOSAL

Silicon

SiO2

Silicon

SiO2

CNT monolayer

Au electrodes patterned by e-beam lithography

Silicon

SiO2

Silicon

SiO2

Silicon

SiO2

(Inside TEM)

Silicon

SiO2

Electron beam

Nanocell

RESEARCH PROJECT PROPOSAL

Silicon

SiO2

Silicon

SiO2

PMMA

PMMA patterned by e-beam lithography

Silicon

SiO2

Silicon

SiO2

Silicon

SiO2

After buffered HF to remove all the SiO2 which is not covered by PMMA,including the SiO2 underneath the Nanocell.

RESEARCH PROJECT PROPOSAL:FINAL CONSIDERATIONS

Carbon nanowires doesn’t own a strong NDR behavior but new configurations, like a 4 terminal device, can be explored by using molecular simulation techniques in order to look for strong NDR behavior.

The Nanocell is aimed to be designed suspended since It was found the contact to the substrate diminish the NDR behavior in CNT.

Cutting CNTs to get smaller ones (< 10 nm) is not a simple procedure. Carbon Nanohoop Structures is a novel alternative for growing small carbon nanotubes in a more reliable and controlled way.

RESEARCH PROJECT PROPOSAL:FINAL CONSIDERATIONS

The task for programming the Nanocell should take into account the quantum interference phenomena it can occur between carbon nanowires. In the original ‘Nanocell’ concept this aspect is not considered.

When ion beam is used for welding Carbon junctions, it should be taken into account that CNT interconnection would result doped with the atoms used as ion beam.