Flexible deposition of nanocrystalline vanadium oxide thin films

Yuri Diaz-Fernandez , Lorenzo Malavasi * and Eliana Quartarone

J. Mater. Chem., 2008, 18, 5190-5192

Purpose

The purpose of this study is to deposit 100nm films of various vanadium oxides (V2O3, VO2, V2O5) on the same substrate and measure the differences of thickness and composition that can occur when the rf sputtering power is changed.

Methods and Procedures

The films were made by rf magnetron sputtering onto quartz and aluminum oxide substrates. By changing the substrate temperature, rf power and oxygen partial pressure, the researchers changed the ratio of the oxide composition. The crystalline nature of the films were determined by x-ray diffraction. The roughness and imaging were determined by atomic force microscopy.

Key Findings

As the rf power increases, the film density and roughness of the film goes down. This is under further investigation to be included in a subsequent full paper.

In vivo Imaging and Drug Storage by Quantum-Dot-Connugated Carbon Nanotubes

Yan Guo, Donglu Shi, Hoonsung Cho, Zhongyun Dong, Amit Kulkarni, Gioanni Pauletti, Wei Wang, Jie Lian, Wen Liu, Lei Ren, Qiping Zhang, Guokui Liu, Christopher Huth, Lumin Wang, and Rodney Ewing; Advanced Functional Materials, DOI: 10.1002/adfm.200800406, August 2008.

 

Purpose of the Study:   

 

Evalute the in vitro anti-cancer activity and in vivo imaging capabilities of a novel carbon nanotubule (CNT) functionalized with paclitaxel-loaded polymer and quantum dots (QD).

 

Methods:

 

The CNTs were coated with poly lactic-co-glycolic acid (PLGA), a biodegradable and FDA approved drug delivery polymer, by plasma polymerization in which monomers (acrylic acid, lactic acid, and glycolic acid) were introduced to the plasma reactor.  HRTEM images reveal a PLGA film was deposited (~ 7nm thick) with an acrylic acid film on top (~ 3nm).

 

Amine-functionalized quantum dots were conjugated to the CNTs via a covalent bond between the amine group and carboxyl group in the acrylic acid film on the CNTs.  HRTEM imaging and energy dispersive spectroscopy (EDS) confirmed conjugation.  TEM imaging shows QDs were not evenly distributed – preferential conjugation of QD probably due to inhomogenous CNT dispersions).  Fluorescence spectra shows emission wavelength of 73nm (shifted from 795nm with QD alone – mechanism not understood).

 

A well known chemotherapeutic agent, paclitaxel, was added by mixing PLGA-coated CNT with methanolic solution, evaporated, and then washed with distilled water. 

 

Therapeutic efficacy was estimated in vitro by adding the QD-CNTs and free paclitaxel at varying doses to human PC-3MM2 prostate cancer cells and evaluating cell viability via MTT assay.

 

In vivo imaging capabilities were evaluated by injecting the QD-CNTs in the tail vein of mice and then pictures were taken with a Kodak 4000MM whole mouse imaging system at time intervals over 6 days.

Key Findings:

 

1. HRTEM and EDS revealed that the novel CNT was fabricated as planned with a paclitaxel-loaded polymer and conjugated QDs.

 

1. In vitro dose dependence curves indicate the QD-CNTs exhibited a five-fold increase in the loading capacity when the PLGA polymer is deposited on the CNTs versus no PLGA. 

 

1. The QD-CNTs had equivalent anti-cancer effects to free paclitaxel at a concentration of 100ng/ml.

 

1. The QD’s emitted enough fluorescence for effective in vivo imaging.  After six days, the QD-CNTs were concentrated in certain organ systems (particularly liver, kidney, stomach, and intestine.

 

Definitions:

 

MTT Assay – common assay performed to test cell viability.

Photonic metamaterials by direct laser writing and silver chemical vapour deposition

Rill, Michael S.. “Photonic metamaterials by direct laser writing and silver chemical vapour deposition.” Nature materials 7.7 (2008):543-546.

Purpose

Photonic metamaterials exhibit fascinating magnetic properties at optical frequencies.  Functional devices will ultimately require large-scale three-dimensional systems.  However, current 3D device fabrication is usually limited to pattering two-dimensional systems via electron-beam lithography and metal film evaporation followed by repeated stacking.  This work investigates a faster and more direct method of laser writing and metallic chemical vapour deposition (CVD) for the purpose of rapidly prototyping 3D photonic metamaterials. 

Method

A 3D polymeric template is fabricated by standard methods of direct laser writing (DLW) a thin film of SU-8 photoresist (on glass substrate).  Post-bake and developing are performed after exposure.  Next, a thin layer of SiO2 is applied in order to increase mechanical stability and to serve as a backbone for the subsequent deposition of silver.  Following activation of the oxide layer with O2 plasma, a metal-organic precursor is added and CVD cycles are applied.  Ten cycles of silver CVD generate a silver thickness of 50nm.  Electron micrographs are used to examine the quality of the structure.  Optical properties are determined via normal-incidence transmittance measurements in a Fourier-transform microscope spectrometer.

Key findings

1. Electron imaging revealed a uniform silver coating.  This was noted to be in sharp contrast to the typical 2D evaporation method.
2. Optical transmittance measurements corresponded well with theory.
3. Overall, this method was successfully demonstrated to be a quick and reliable way of achieving 3D photonic metamaterials.  The authors note that they have approached a bottleneck in the theory of such devices.  They hope that this work spurs further theoretical investigation and research in this emerging field.

Key terms

CVD – chemical vapor deposition; DLW – direct laser writing.

Sulfur as a Novel Nanopatterning Material: An Ultrathin Resist and a Chemically Addressable Template for Nanocrystal Self-Assembly

By: Jonathan Germain, Marco Rolandi, Scott A. Backer, and Jean M. J. Fre´chet

Published in: Adv. Mater. 2008, 20, 1–4

Purpose of Study

The paper describes the development of a high-field patterning lithographic technique in which an AFM was used to induce the localized polymerization of a thin sulfur film. This technique was used to prepare a negative tone resist which may be used as a mask for substrate etching or as a chemical template for the self assembly of nanocrystals of gold.

Methods Used

A sulfur film (2.5 to 4 nm in thickness) was spin-coated onto a Si chip. A conducting AFM tip operated in tapping mode was used to apply a positive bias to the material and thereby induce chemical modifications of the sulfur. This was done in a water-free environment. After rinsing in toluene and ethanol or isopropanol, the unpatterned regions of sulfur were removed and the nanometer scale patterns in the resist were revealed.

Time of flight-secondary ion mass spectroscopy (TOF-SIMS) was used to characterize the products formed on the sulfur resist.

Key Findings

1. TOF-SIMS and etching studies proved that the patterns consist of high molecular weight sulfur. The AFM probe converts the starting material, S₈, into a polymeric sulfur material. This occurs because the high energy field causes S₈ to fragment. These highly reactive fragments subsequently polymerize.

1. The lithographic technique yielded patterns which were as narrow as 25 nm. The patterns exhibited resistance to fluorinated etchants and therefore maybe used for negative tone transfer.

1. The resist was successfully employed as a template for the directed self-assembly of gold nanocrystals. This was achieved by modifying the functional groups present on the patterns and then anchoring Au nanocrystals onto the free thiol groups in specific regions of the resist.

The effect of laser power on the formation of carbon nanotubes prepared in CO2continuous wave laser ablation at room temperature

H. Zhang et al. I Physica B 325 (2003) 224-229

Introduction

The synthesis of carbon nanotubes has generated a number of useable facile methods most of which employ the use of high temperature atmospheres.  However, the findings suggest a novel method of Carbon nanotube formation that mitigates  the concern of high temperature reactions.  Single-walled carbon nanotubes (SWCNT) were prepared by continuous wave CO₂ laser ablation without the application of additional heat to the materials.  TEM and Raman Spectra were used to assess the effects  of varied laser power on the growth and diameter distributions of the newly formed tubes.  The continuous laser source was generated by laser vaporization in laser power from 500 to 850 W and compared to previous literature.

Key Findings

1.       Carbon nanotubes are able to be formed at room temperature using infrared laser with wavelengths of 10.6 µm.

2.       Direct relationship found between SWCNT diameter and laser power. At lower laser power (below 500W) bamboo-shaped CNT’s are reported.

3.       SWCNT’s produced in different excitations wavelengths  (632.8nm v. 488nm) produce varied spectra when pulsed with continuous CO₂ laser.  Information suggest the these results are conducive to carbon nanotube diameter

4.       Spectra show diameter distribution ranging from 1.1 to 1.6nm.

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.

Prediction of effective moduli of carbon nanotube-composites with waviness and debonding

L.H.Shao, R.Y.Luo, S.L.Bai, and J.Wang. Composite Structures 87 (2009) 274-281

Purpose of the Study

To investigate the effects of waviness of CNT and interfacial debonding (between CNT and matrix) on the effective moduli. Analytical methods are presented to study the effect of these parameters on the effective moduli of the nanocomposite.

Methods

Micromechanics models are presented to analyze the influence of nanfiber waviness and debonding. In each case an RVE is considered in the analysis. For waviness, the nanofiber is projected onto the vertical and horizontal directions, and then the effective properties are calculated. In case of debonding, the total number of fibers is divided into partially debonded, fully debonded, and perfectly bonded fibers.

Key findings

1) It is shown that debonding and waviness can significantly reduce the stiffening effect of nanotubes, inspite of their high stiffness.

2) The effective elastic constants of the composites are shown to be very sensitive to the waviness in case of small waviness.

3) Other important mechanical properties including effective shear modulus and effective bulk modulus are calculated using the analytical method presented. While the effective shear modulus increases with volume fraction for partial debonding, it decreases when all the fibers are assumed to be debonded.

4) The load transferring mechanisms between the nanotube and matrix are acknowledged to be complicated when waviness and debonding simultaneously exist.

Key Terms

CNT: Carbon NanoTube

RVE: Representative Volume Element

Special Reliability Features for Hf-Based High-k Gate Dielectrics

T. P. Ma, Fellow, IEEE, Huiming M. Bu, X. W. Wang, Liyang Y. Song, W. He, Miaomiao Wang, H.-H. Tseng,
and P. J. Tobin

IEEE TRANSACTIONS ON DEVICE AND MATERIALS RELIABILITY, VOL. 5, NO. 1, MARCH 2005 (Invited Paper)

Purpose of the study:

HF-Based gate dielectrics are extensively investigated as alternatives to for future CMOS technology, but before any of these dielectrics can be selected by the semiconductor industry, its reliability must be satisfactorily demonstrated. This paper reviews some its recent reliability results.

Methods:

PolySi, TaSiN, or TiN gate MOSFETs were fabricated following a standard CMOS process on bulk silicon. ALD HfO2 of 4.4 nm in physical thickness deposited at 300 C with precursor, was used as the gate dielectric. Some of the MOSFETs had a silicon nitride cap layer of 0.5 nm thick between and gate for studying the effects of polySi/ reactions. The typical I–V and C–V characteristics for nMOSFETs are presented to show that the devices under study exhibit normal electrical characteristics. To measure the trapping-induced instability, constant voltage stress as well as pulse stress was applied to the polySi gate of MOSFETs in inversion. The threshold voltage shift, the midgap voltage shift and transconductance degradation caused by charge trapping were continuously monitored during the stress. Both pulsed voltage stress and dc constant voltage stress have been applied. Between each stressing period, there is a sensing period, in which the threshold voltage is measured.The threshold voltage shift is obtained from comparison with the initial one. Special attention is also paid to minimize the detrapping effect by using automatic program control to minimize the time interval between stressing and sensing. The density of the trapped charges, was derived from the midgap voltage shift. The interface trap density was obtained from ac conductance measurement. Voltage-dependent TDDB (time-dependent dielectric breakdown) measurements were used to determine the device lifetime. IETS spectra were taken on MOS capacitors by measuring the second harmonic signals with a standard lock-in method at liquid helium temperature (4.2 K). The modulation voltage of the excitation signal for the IETS measurements was 2 mV. A dual temperature (4.2 K, 77 K) technique was used to remove the elastic tunneling background.

Key findings:

1) The operating lifetime extracted from time-dependent-dielectric-breakdown (TDDB) is too optimistic, and the actual device lifetime is limited by the trapping-induced threshold voltage shift.

2) nMOSFETs are much more prone to trapping-induced than their pMOSFETs counterparts under normal operating conditions,due to much more electron traps than hole traps in HfO2-based gate dielectrics.

3) Metal gate yields improved reliability compared to polySi gate.

Improved nanofabrication through guided transient liquefaction

Stephen Y. Chou and Qiangfei Xia

Nature Nanotechnology Vol 3, May 2008, 295-300.

Purpose of the Study

Demonstrate a new approach called self-perfection by liquefaction, which can remove fabrication defects and improve nanostructures post-fabrication.

 

Methods

The self-perfection by liquefaction (SPEL) method involves selectively melting nanostructures for a short period of time (hundreds of nanoseconds) by laser pulse. It also applies a set of boundary conditions (like a flat plate) to guide the flow of the molten material into the desired geometry before solidification.

 

Key findings

1.     Using the open-SPEL method the 3σ line-edge roughness(LER) of 70-nm-wide chromium grating lines was reduced from 8.4 nm to less than 1.5 nm, which is well below the ‘red-zone limit’ of 3 nm discussed in the International Technology Roadmap for Semiconductors.

2.     After guided-SPEL (using a 40 nm gap and a 595 mJ cm^-2 pulse), the width of a silicon line was reduced from 285 nm to 175 nm, while its height increased from 50 nm to 90 nm.

3.     Self-perfection by liquefaction (SPEL) can also be extended to other metals and semiconductors, dielectrics and large-area wafers.

 

Important Definitions

1.     open-SPEL: the nanostructures are located on a substrate without any additional boundary conditions.

2.     guided-SPEL: a flat plate is placed above the nanostructures that are to be improved, with a gap between the plate and the nanostructure.

 

 

Multifunctional Encoded Particles for High-Throughput Biomolecule Analysis

by Daniel C. Pregibon, Mehmet Toner, Patrick S. Doyle, Science (2007) 315: 1393-1396

Purpose
To build a platform that allows for high-throughput screening of oligonucleotide analytes, with multiplexing capabilities.

Methods
Previous work by Patrick Doyle’s group allowed for the formation of microparticles in a microfluidic system by flowing a polymer/photoinitiator mixture through the chamber, and then pulsing UV irradiation through a photomask (placed on light source between condenser and objective) through a method they called “continuous flow lithography” (CFL).

In this work, the group used CFL (and its ability to rapidly form microparticles, ~100/second) to create multifunctional microparticles. This was done by exploiting laminar flow of multiple polymer chemistries and polymerizing particles the shape of pills that were perpendicular to the direction of the flow. By incorporating oligonucleotide probes into the flowed polymer mixtures, particles were created that displayed multiple probes in well-defined regions. The pattern also allowed for the incorporation of a barcode tag to help identify the oligonucleotide probes that were present on each specific microparticle.

The target sequences for the probes can be easily fluorescently labeled. Incubation of a mixture of microparticles with these fluorescently labeled targets allows the target to attach to their probes on the microparticles with high specificity. Another microfluidic device and a custom computer program was written to be able to analyze the particles for the presence of fluorescent signal at high rates.

Key Findings
-The scheme for microparticle synthesis (bar codes) and the well-written software allows for the formulation of a vast amount of different probes, each of which by themselves have multiplexing capabilities due to the fabrication method.

-Accurate and reproducible detection of analytes with sensitivity on the order of hundreds of attomoles, though this required longer incubation time between microparticles and the target mixture.

Definitions
-Oligonucleotide Probe: a single strand of DNA containing (rough estimate) 10-100 nucleotides, which can be used to probe for the presence of its complementary sequence in solution.