Diamond FED diodes

Final Paper    Final Paper

Final Presentation     Final Presention

A Single Molecule Diode

Mark Elbing, Rolf Ochs, Max Koentopp, Matthias Fischer, Carsten von Hanisch, Florian Weigend, Ferdinand Evers, Heiko B. Weber, and Marcel Mayor

PNAS, June 21, 2005; volume 102 no. 25, 8815-8820

One purpose of this study was to create a single molecule bonded to two gold electrodes which acts as a diode. Another purpose was to investigate the transport mechanism across this molecular diode.

Methods:

The molecule is contacted to the gold electrodes using the mechanically controlled break junction (MCB) technique; in order to anchor the molecule to the gold, Sulfur atoms are used as the attachment atoms. Three different configurations of molecular rods were used to compare their electron transport properties; all three rods are based off a two phenyl-ethynyl-phenyl π-systems, fused by a biphenylic C-C bond. Steric repulsion of two o-methyl groups with respect to the biphenylic C-C bond induces a torsion angle which reduces the electronic coupling of π electrons. The three systems are D-A, D-D, and A-A configurations. (1) Comparisons were then made with theoretical predictions using density functional theory.

Results:

(1) The donor-acceptor molecule exhibited an I-V curve which was stepwise, and asymmetric, with a rectification ratio of 1:4.5 at U= + or -1.5 V. Conversely, the D-D and A-A arrangements showed symmetric I-V curves, with rectification ratios near 1.

(2) Similar to a p-n junction, the current and spacing between steps are biased in the forward direction.

(3) Transport mechanism can be viewed as two weakly coupled quantum dots in series. This means that the electronic levels of both dots are localized on their respective dot. Applying a voltage shifts the electronic levels with respect to one another; at certain levels, two voltages cross, and electrons are allowed to flow. This increases the current flow in a step-wise manner.

(4) D-A molecules exhibit diode like behavior and serve as a prototype for a possible molecular diode.

Definitions:

(1) D (electron donor): for this experiment: 4-floro-phenyl-ethynyl-o-methyl-phenyl

A (electron acceptor): for this experiment: phenyl-ethynyl-o-methyl-phenyl

(2) o- : ortho position

Polymer Pen Lithography

Fengwei Huo, Zijian Zheng, Gengfeng Zheng, Louise R. Giam, Hua Zhang, Chad A. Mirkin

 

19 September 2008, Science Vol 321

 

Purpose of Study

 

Lithography tools have problems generating both nano- and microscale features in a single experiment in a parallel, high-throughput, direct manner.  This paper reports a low cost, high-throughput lithography technique capable of creating feature sizes that range from 90 nm to hundreds of microns.  Polymer pen lithography (PPL) combines the feature size control of Dip pen lithography (DPN) with the large area capabilities of contact printing.

 

Method:

 

A master mold is created by conventional photolithography followed by wet etching; this mold contains thousands of pyramidal-shaped holes.  PDMS (polydimethylsiloxane) fills this mold and is attached to a glass substrate creating an array of elastomeric polymer tips.  The substrate and thin PDMS backing behind the tips improves the uniformity of the array over a large area.  The PDMS is dipped in the ink you want to use, and the ink is absorbed into the array; unlike DPN, the PDMS acts as a reservoir allowing a greater amount of polymer to be patterned.  When the array is brought into contact with a substrate, the ink is delivered at the points of contact; the amount of ink deposited is a function of time and force.  Like DPN, the deposited ink varies linearly with time; but due to the elastomeric nature of PDMS, as force is applied, the pyramids deform and become blunt against the substrate.  This increases the rate of deposition and allows for vastly different feature sizes to be produced by a single mask.  This elastic nature of PDMS also makes it easies to bring all the tips into contact with a substrate without changing your deposition of ink; the PDMS compresses before it deforms.

 

Key Findings:

 

(1)  Creation of a lithography technique capable of patterning with a large degree of freedom in the feature sizes created.  This allows for a single mask to be used, both increasing speed and lowering costs.

(2)      The dependence of the transfer rate on force applied perpendicular to the substrate, due to the elastic characteristics of PDMS, allows for precise control over feature sizes over a range that beforehand was unthinkable.

(3)      Demonstration of feature sizes ranging from 90 nm to hundreds of microns, and creation of an integrated gold circuit created by PPL.

Field Emission Characteristics of Diamond Edge-shaped Emitters Fabricated Using Nitrogen-Methsne Plasma

Field Emission Characteristics of Diamond Edge-shaped Emitters Fabricated Using Nitrogen-Methane Plasma

R.S. Takalkar, W.P. Kang, JL. Davidson, B.K. Choi, W.H. Hofmeister, K. Subramanian

Purpose of the Study

The purpose of this study was to characterize the emission spectra from an edge-shaped diamond emitter fabricated using a Nitrogen-methane deposition, and compare it to emitters fabricated using a Hydrogen-methane deposition.

Methods

A diamond edge emitter was fabricated using a photolithographic pattern etched by BOE and KOH to form a pyramidal mold. The mold is sharpened by thermal oxidation, and then the diamond is grown using a PECVD method; the plasma in this case is a 12:1 nitrogen-methane mixture. The Si and SiO₂ are removed leaving a diamond-edged cathode which is then brazed to a Mo substrate. An SEM was used to characterize the edge sharpness of a nitrogen-methane edge versus a hydrogen-methane edge. It was determined that Nitrogen-methane deposition yields sharper edges than Hydrogen-methane deposition. It was then characterized using energy dispersion spectroscopy, and Raman spectroscopy.

Findings

1) EDS confirms nitrogen is incorporated into the diamond edge, which introduces energy states inside the band gap. This reduces the work function.

2) Raman spectroscopy confirms a higher density of sp² hybridized carbon in nitrogen-methane grown diamond, 1.05:1 vs 2.43:1 sp³ to sp².

3) Nitrogen-methane deposited diamond has a lower turn-on voltage than hydrogen-methane deposited diamond, 2 V/μm vs 6 V/μm.

4) Grain sizes created during this new deposition technique are much smaller than using older techniques. The grain sizes were approximately 5-10 nm. The reduction in grain size leads to a larger grain-boundary volume, causing an increase in sp² hybridized carbon.

ELECTRONIC PROPERTIES OF METALS AND ALLOYS: Nanocontact spin-electric effect

R. N. Gurzhi, A. N. Kalinenko, A. I. Kopeliovich, and A. V. Yanovskii

B. I. Verkin Institute for Low Temperature Physics and Engineering of the National Academy of Sciences of Ukraine, pr. Lenina 47, Kharkov 61103 Ukraine

Purpose of Study

It is theorized that a spin signal can be converted into a change in electric potential. The purpose of this study is to quantize this relationship and use it as direct detection of spin signals in nonmagnetic materials and semiconductors.

Methodology

If a magnetic probe is applied to the non-magnetized conductor with spin polarity electrons penetrate into the magnetic contact. Spin up and Spin down electrons penetrate the magnetic contact at different probabilities, and an electric double layer forms along the magnetic probe and nonmagnetic conductor between the spin polarized electrons and the holes which they leave in the conductor. This generates an electric potential between probe and conductor which can be measured; the potential varies based on the difference in concentrations of the spin up and spin down electrons in the nonmagnetic conductor.

Key Findings

1) The formula:

e φ = (Π_↓ μ_↓ + Π_↑ μ_↑)/( Π_↓ + Π_↑)

describes the relationship of the electric potential generated at the contact border as a relationship of the density of states of spin up and spin down electrons.

2) The potential changes over time as the densities of the two states alter inside the external circuit attachment. This change is described by:

δ φ(t) = e^-1/2 * (Π_↓,M - Π_↑,M)/( Π_↓,M + Π_↑,M) * δ μ(t)

3) Estimates for change in potential over time correspond to experimentally gained values; giving credence to the theory that by measuring the potential jump across the magnetic probe and nonmagnetic circuit connection can be used to calculate the spin density in the nonmagnetic circuit, especially for magnets that are virtually 100% spin-selective.

Glossary

1) Π_↓,M = Density of spin down state in magnetic probe

2) φ = electric potential

3) μ_↓ = electrochemical potential of spin down state