Quantum Wells, Nanowires, Nanodots, and Nanoparticles Y. Tzeng ECE Auburn University Auburn, Alabama July 2003
Trap particles and restrict their motion
Quantum confinement produces new material behavior/phenomena
“ Engineer confinement”- control for specific applications
Quantum dots (0-D) only confined states, and no freely moving ones
Nanowires (1-D) particles travel only along the wire
Quantum wells (2-D) confines particles within a thin layer
Energy-band profile of a structure containing three quantum wells, showing the confined states in each well. The structure consists of GaAs wells of thickness 11, 8, and 5 nm in Al 0.4 Ga 0.6 As barrier layers. The gaps in the lines indicating the confined state energies show the locations of nodes of the corresponding wavefunctions. Quantum well heterostructures are key components of many optoelectronic devices, because they can increase the strength of electro-optical interactions by confining the carriers to small regions. They are also used to confine electrons in 2-D conduction sheets where electron scattering by impurities is minimized to achieve high electron mobility and therefore high speed electronic operation. http://www.utdallas.edu/~frensley/technical/hetphys/node11.html#SECTION00050000000000000000 http://www.utdallas.edu/~frensley/technical/hetphys/hetphys.html
All TiNano 40 Series products are in the 30-50 nm primary particle size range. Surface treated products exhibit very littlecrystal growth or change of phase when held in an oxidizing atmosphere at 800º C for over 100 hours. Altium™ TiNano 40 Series slurry products are dispersed to primary crystallites in aqueous media and exhibit specific surface areas (BET) of 40-60 m²/g. The slurry product offers the advantage of requiring no dispersion, and also eliminates the handling of fine powders. A spray-dried product is also available that consists of readily dispersable agglomerates of primary particles. http://adserv.internetfuel.com/cgi-bin/omnidirect.cgi?SID=23&PID=2&LID=10&OSDELAY=10 Commercial TiO 2 Nanoparticles
Nanoparticles have been used in our daily life.
Carbon black ( a nanoscale carbon) is used for writing and painting and is added to rubber to make tires more wear resistance.
Nano phosphors in CRTs display colors.
Polishing compounds for smoothing silicon wafers include nanoscale alumina and silica, etc.
Hard disks in our computers contain nanoscale iron oxide magnetic particles.
Nanoscale zinc oxide and titania block UV light for sunscreens .
Nanoscale platinum particles are critical to the operation of catalytic converters.
Metallic nanoparticles make stained glass and Greek vase colorful.
Nanoscale thin films have also been the heart of our silicon chips for our computers, digital cameras, and photonic devices for quite a while.
The Altair Manufacturing Plant: http://adserv.internetfuel.com/cgi-bin/omnidirect.cgi?SID=23&PID=2&LID=10&OSDELAY=10
The Altair Crystal Phase Growth Process http://adserv.internetfuel.com/cgi-bin/omnidirect.cgi?SID=23&PID=2&LID=10&OSDELAY=10
Application Development Activities
Altair has on-going collaboration projects that specifically address commercial applications using our proprietary nanoparticle technology including:
Solid Oxide Fuel Cell and High Temperature Conductive Oxides
Catalysts and Catalyst Support / Surface Modification
Nanotechnology may help overcome current limitations of gene therapy Hybrid "nanodevice" composed of a "scaffolding" of titanium oxide nanocrystals attached with snippets of DNA may one day be used to target defective genes that play a role in cancer, neurological disease and other conditions. Nanocomposites not only retain the individual physical and biological activity of titanium oxide and of DNA, but, importantly, also possess the unique property of separating when exposed to light or x-rays. Researchers would attach to the titanium oxide scaffolding a strand of DNA that matches a defective gene within a cell and introduce the nanoparticle into the nucleus of the cell, where the DNA would bind with its "evil twin" DNA strand to form a double-helix molecule. The scientists would then expose the nanoparticle to light or x-rays, which would snip off the defective gene. http://www.atomworks.org/NUArgonne4.18.03
February 2003 The Industrial Physicist Magazine Quantum Dots for Sale Nearly 20 years after their discovery, semiconductor quantum dots are emerging as a bona fide industry with a few start-up companies poised to introduce products this year. Initially targeted at biotechnology applications, such as biological reagents and cellular imaging, quantum dots are being eyed by producers for eventual use in light-emitting diodes (LEDs), lasers, and telecommunication devices such as optical amplifiers and waveguides. The strong commercial interest has renewed fundamental research and directed it to achieving better control of quantum dot self-assembly in hopes of one day using these unique materials for quantum computing. Semiconductor quantum dots combine many of the properties of atoms, such as discrete energy spectra, with the capability of being easily embedded in solid-state systems. "Everywhere you see semiconductors used today, you could use semiconducting quantum dots," says Clint Ballinger, chief executive officer of Evident Technologies, a small start-up company based in Troy, New York... http://www.evidenttech.com/news/news.php
Quantum Dots for Sale The Industrial Physicist reports that quantum dots are emerging as a bona fide industry. http://www.evidenttech.com/index.php Evident Nanocrystals Evident's nanocrystals can be separated from the solvent to form self-assembled thin films or combined with polymers and cast into films for use in solid-state device applications. Evident's semiconductor nanocrystals can be coupled to secondary molecules including proteins or nucleic acids for biological assays or other applications. http://www.evidenttech.com/why_nano/docs.php 05-640-08 05-610-08 05-585-08 05-560-08 05-535-08 Part Number (8ml) SG-CdSe-Na-TOL 05-640-04 05-610-04 05-585-04 05-560-04 05-535-04 Part Number (4ml) SG-CdSe-Na-TOL 5.6 4.7 4.0 3.4 2.8 Crystal Diameter [nm - nominal] 627 597 572 547 522 1st Exciton Peak [nm - nominal] <40 <30 <30 <30 <30 Typical FWHM [nm] 640±10 610±10 585±10 560±10 535±10 Emission Peak[nm]
EviArray Capitalizing on the distinctive properties of EviDots™, we have devised a unique and patented microarray assembly. The EviArray™ is fabricated with nanocrystal tagged oligonucleotide probes that are also attached to a fixed substrate in such a way that the nanocrystals can only fluoresce when the DNA probe couples with the corresponding target genetic sequence. http://www.evidenttech.com/why_nano/docs.php
Comparison of Particle Size and Resulting Photoluminescence
High quantum efficiency: 50% to 60%.
Ultrabright: several times greater than that of fluorescein or other biological markers.
Long-lasting: Silicon nanoparticles fluoresce up to 100 times longer than other materials used in biological applications.
Selectable photoluminescence: The wavelength varies according to the size of the particles
Easy to use: Differently sized silicon nanoparticles can be excited with a single laser to obtain various colors.
Easy to manufacture: Large quantities of uniformly sized silicon nanoparticles can be produced by making small modifications to the fabrication process.
http://www.otm.uiuc.edu/technology/Silicon-nanoparticles.htm#benefits Properties and Advantages of Silicon Nanoparticles red (620 nm) 2.9 nm yellow (570 nm) 2.15 nm green (540 nm) 1.67 nm blue (400 nm) 1.0 nm resulting photoluminescence band peaks particle size
Targeted drug delivery,
Destruction of pathogens
This technology is an ideal alternative to
common dyes, which can be
burn out quickly, and are
difficult to use when labeling multiple biological materials.
Silicon nanoparticles are
incredibly bright, are
highly photostable, and
can be sized to emit varying photoluminescence
in response to a single light source.
Their emission brightness exceeds that of fluorescein in the blue wavelength (400 nm).
The process for making the silicon nanoparticles uses highly catalyzed electrochemical etching in hydrofluoric acid (HF) and hydrogen peroxide (H2O2) to disperse crystalline silicon into ultrasmall nanoparticles. The wafer is laterally anodized while being advanced slowly into the etchant, producing a large meniscus-like area. Because HF is highly reactive with silicon oxide, H2O2 catalyzes the etching, producing smaller particles. Moreover, the oxidative nature of the peroxides produces high-quality chemical and electronic samples. UIUC’s Electrochemical Process for Producing Silicon Nanoparticles http://www.otm.uiuc.edu/technology/Silicon-nanoparticles.htm#benefits
The pulverized wafer is then transferred to an
ultrasound bath for a brief treatment, under which
the film crumbles into colloidal suspension of ultrasmall blue particles.
Larger particles are less amenable to dispersion due to stronger interconnections.
A post-HF treatment weakens those particles, and then
an ultrasound treatment disperses the particles.
The mix is centrifuged, and the resulting residue contains the largest red particles, while
the suspension contains the green/yellow particles.
The residue is redissolved and sonicated.
The red-emitting particles stay in suspension, while
the green particles may be separated by additional sonication and/or the addition of a drop of HF.
Commercial gel permeation chromatography may be used to separate the particles further, if necessary, or to obtain additional accuracy in separation of the other particles.
The particles are separated into several vials, each containing particles of uniform size, with near 90% to 100% efficiency.
Quantum dots are nano-sized deposits of one semiconductor embedded in another semiconductor that has a larger energy bandgap than that of the core.
Since the dot material has an energy bandgap that is smaller than that of the surrounding material, it can trap charge carriers.
When a photon arrives at the first dot of two electrically connected quantum dots made of gallium arsenide and aluminum gallium arsenide, it excites an electron into the conduction band of the dot.
A strong bias voltage between these two quantum dots transfers this electron to the second quantum dot.
This dot acts as a single-electron transistor, which is switched by the electron to register the photon.
This one-way transfer of single electrons is crucial because it prevents an excited electron returning to its ground state in the first quantum dot before it can be registered.
Quantum dots for detection of low energy single photon http://physicsweb.org/article/news/6/6/1
Put peptide molecules that have very specific protein sequences into semiconductor quantum dots, which then very specifically bind to particular locations on cell surfaces. By using the molecular-recognition capabilities of peptide molecules, scientists have made selective electrical contacts to neurons. The cadmium sulfide contacts act as photodetectors, allowing researchers to communicate with the cells using precise wavelengths of light. In the past, a variety of objects have been attached to cells using biorecognition, such as fluorescent dyes, enzymes and radioactive labels. Relatively large electrode grids have also been implanted into patients to encourage neuron growth over the grid arrays. There was still about a 1-micron gap between the neurons and the electrodes. The quantum dot (Schmidt's) method slims that down to 3 nanometers. Activate neurons with quantum dots http://www.eetimes.com/story/OEG20011204S0068
Nanoparticles Well Studied in Isolation… Why Nanoparticle Arrays? A: Device Integration A: New Functionality in Ordered Ensembles
Ar inlet silicon source rod Turbo pump graphite crucible and silicon melt water cooled Cu electrodes Si nanocrystal synthesis and classification by size nanocrystal aerosol excess aerosol
Silicon evaporated in an atmosphere of ultra pure Ar, creating an aerosol
Nanocrystals synthesized in a clean environment to avoid oxidation
electric field lines particle trajectories sample flow, Q s excess flow, Q e V DMA sheath flow, Q sh
Size classification done with a radial differential mobility analyzer (RDMA)
Before entering RDMA particles are charged
After classification, particles are deposited on a Si substrate or SiO 2 film
S.-H. Zhang et al. Aerosol Science and Technology, 23:357-372(1995) aerosol flow, Q a
Details of size classification E D p1 v e1 v e2 v e2 < v e1 D p2 > D p1 N/ D p (10 10 nm -1 cm -2 ) (charge q) R.P. Camata et. al. Appl. Phys. Lett. 68 (22), 27 May 1996 Sheath flow Aerosol particles D p2 0 2 4 6 8 10 12 14 x 3 x 4 x 7 2.8 nm (0.6 nm) 5.2 nm (1.4 nm) 8.0 nm (1.6 nm) 10.8 nm (1.8 nm) 111 V 54.1 V 24.5 V 10.0 V Nanocrystal Diameter, D p (nm) 0.0 2.0 4.0 6.0 8.0
1 transistor/cell nonvolatile memory with Si nanoparticle floating gate:
thin tunnel oxide
Materials: AFM Charging of Si Nanoparticles, and Nanoscale Charge Imaging via Electrostatic Force Microscopy Devices: (Tiwari et al Appl. Phys. Lett. 68 (10), 4 March 1996) Improved Performance Nonvolatile Memory D h D h
Tip charging of Single Si nanocrystals
Scanning tip first brought to rest above particle
Tip lowered toward sample (change set point)
Voltage pulse of
-10- -25V applied
Tip set point returned to precharging value and height changes monitored
Where is the charge stored? defects in oxide? nanocrystals? nanoparticle/oxide interface states? surface states? Si/SiO 2 interface states? E fm E c E v E f
Flatband Lineup of Si Nanocrystals in SiO 2 Si AFM tip Si Substrate SiO 2 Energy Position
Performance of L eff = 0.2 m Nanoparticle Memory
Industry News about Nanoparticle Memory
Industry estimates forecast that flash memory revenue will hit $13 billion this year, up from $7.7 billion in 2002, according to Jim Handy, a memory services executive with Semico Research . By 2007, flash memory is expected to be a $43 billion industry. Chip giant Intel is experimenting with Ovonics Unified Memory , which uses the same material as DVD discs. Motorola, meanwhile, is looking at silicon nanocrystals , which replace a solid layer inside the transistor with a lattice of silicon atoms. Nanocrystal chips could hit the market by 2006. Other alternatives being developed include: magnetic RAM (MRAM), which isn't really magnetic; ferroelectric RAM (FeRAM), which involves shifting atoms in a crystal; and polymer memory, which is made from the stuff used in liquid-crystal display screens. http://news.com.com/2009-1040-994240.html By Michael Kanellos Staff Writer, CNET News.com March 27, 2003, 4:00AM PT
Nanocrystal Shape Control Boosts Efficiency of New Solar Cells Hybrid nanocrystal-polymer solar cell is made by blending CdSe nanocrystals with P3HT, a conducting polymer, to form a 200 nm thick film sandwiched between an aluminum top contact (orange) and a transparent bottom contact (blue). Nanocrystal shape affects the cell efficiency. Monochromatic quantum efficiencies of over 50% are achieved by using rod-like nanocrystals that are partially aligned with the path of current flow in the device. http://www.lbl.gov/~msd/PIs/Alivisatos/02/02-01_Nanosolar.ppt
Single crystal formation -- common crystallographic orientation along the nanowire axis
Minimal defects within wire
Minimal irregularities within nanowire arrays
“ They represent the smallest dimension for efficient transport of electrons and excitons, and thus will be used as interconnects and critical devices in nanoelectronics and nano-optoelectronics.” (CM Lieber, Harvard) http://www.me.berkeley.edu/nti/englander1.ppt
Ensures fabrication of electrically continuous wires since only takes place on conductive surfaces
Applicable to a wide range of materials
High pressure injection
Limited to elements and heterogeneously-melting compounds with low melting points
Does not ensure continuous wires
Does not work well for diameters < 30-40 nm
Laser assisted techniques
Important for storage device applications
Cobalt, gold, copper and cobalt-copper nanowire arrays have been fabricated
Electrochemical deposition is prevalent fabrication technique
<20 nm diameter nanowire arrays have been fabricated
Cobalt nanowires on Si substrate (UMass Amherst, 2000) http://www.me.berkeley.edu/nti/englander1.ppt
Silicon nanowire CVD growth techniques
With Fe/SiO 2 gel template (Liu et al, 2001)
Mixture of 10 sccm SiH 4 & 100 sccm helium, 500 0 C, 360 Torr and deposition time of 2h
Straight wires w/ diameter ~ 20nm and length ~ 1 m
With Au-Pd islands (Liu et al, 2001)
Mixture of 10 sccm SiH 4 & 100 sccm helium, 800 0 C, 150 Torr and deposition time of 1h
Amorphous Si nanowires
Decreasing catalyst size seems to improve nanowire alignment
Bifurcation is common
30-40 nm diameter and length ~ 2 m
Template assisted nanowire growth
Create a template for nanowires to grow within
Based on aluminum’s unique property of self organized pore arrays as a result of anodization to form alumina (Al 2 O 3 )
Very high aspect ratios may be achieved
Pore diameter and pore packing densities are a function of acid strength and voltage in anodization step
Pore filling – nanowire formation via various physical and chemical deposition methods
Anodization of aluminum
Start with uniform layer of ~1 m Al
Al serves as the anode, Pt may serve as the cathode, and 0.3M oxalic acid is the electrolytic solution
Low temperature process (2-5 0 C)
40V is applied
Anodization time is a function of sample size and distance between anode and cathode
Key Attributes of the process (per M. Sander)
Pore ordering increases with template thickness – pores are more ordered on bottom of template
Process always results in nearly uniform diameter pore, but not always ordered pore arrangement
Aspect ratios are reduced when process is performed when in contact with substrate (template is ~0.3-3 m thick)
Al 2 O 3 template preparation http://www.me.berkeley.edu/nti/englander1.ppt
(T. Sands/ HEMI group http://www.mse.berkeley.edu/groups/Sands/HEMI/nanoTE.html) The alumina (Al 2 O 3 ) template 100nm Si substrate alumina template (M. Sander) http://www.me.berkeley.edu/nti/englander1.ppt
Works well with thermoelectric materials and metals
Process allows to remove/dissolve oxide barrier layer so that pores are in contact with substrate
Filling rates of up to 90% have been achieved
(T. Sands/ HEMI group http://www.mse.berkeley.edu/groups/Sands/HEMI/nanoTE.html) Electrochemical deposition http://www.me.berkeley.edu/nti/englander1.ppt Bi 2 Te 3 nanowire unfilled pore alumina template
Template-assisted, Au nucleated Si nanowires
Gold evaporated (Au nanodots) into thin ~200nm alumina template on silicon substrate
Ideally reaction with silane will yield desired results
Need to identify equipment that will support this process – contamination, temp and press issues
Additional concerns include Au thickness, Au on alumina surface, template intact vs removed
100nm 1 µm Au dots template (top) Au (M. Sander) http://www.me.berkeley.edu/nti/englander1.ppt
Nanometer gap between metallic electrodes Electromigration caused by electrical current flowing through a gold nanowire yields two stable metallic electrodes separated by about 1nm with high efficiency. The gold nanowire was fabricated by electron-beam lithography and shadow evaporation. Before breaking After breaking http://www.lassp.cornell.edu/lassp_data/mceuen/homepage/Publications/EMPaper.pdf SET with a 5nm CdSe nanocrystal
Nanoscale size exhibits the following properties different from those found in the bulk :
quantized conductance in point contacts and narrow channels whose characteristics (transverse) dimensions approach the electronic wave length
Localization phenomena in low dimensional systems
Mechanical properties characterized by a reduced propensity for creation and propagation of dislocations in small metallic samples.
Conductance of nanowires depend on
state and degree of disorder and
elongation mechanism of the wire.
Quantum and localization of nanowire conductance http://dochost.rz.hu-berlin.de/conferences/conf1/PDF/Pascual.pdf
Conductance during elongation of short wires exhibits periodic quantization steps with characteristic dips, correlating with the order-disorder states of layers of atoms in the wire. The resistance of “long” wires, as long as 100-400 A exhibits localization characterization with ln R(L) ~ L 2 Short nanowire “ Long” nanowire http://dochost.rz.hu-berlin.de/conferences/conf1/PDF/Pascual.pdf
Electron localization At low temperatures, the resistivity of a metal is dominated by the elastic scattering of electrons by impurities in the system. If we treat the electrons as classical particles, we would expect their trajectories to resemble random walks after many collisions, i.e. , their motion is diffusive when observed over length scales much greater than the mean free path. This diffusion becomes slower with increasing disorder, and can be measured directly as a decrease in the electrical conductance. When the scattering is so frequent that the distance travelled by the electron between collisions is comparable to its wavelength, quantum interference becomes important. Quantum interference between different scattering paths has a drastic effect on electronic motion: the electron wavefunctions are localized inside the sample so that the system becomes an insulator . This mechanism ( Anderson localization ) is quite different from that of a band insulator for which the absence of conduction is due to the lack of any electronic states at the Fermi level. http://www.cmth.ph.ic.ac.uk/derek/research/loc.html
Molecular nanowire with negative differential resistance at room temperature http://research.chem.psu.edu/mallouk/articles/b203047k.pdf
ErSi2 nanowires on a clean surface of Si(001). Resistance of nanowire vs its length. ErSi2 nanowire self-assembled along a <110> axis of the Si(001) substrate, having sizes of 1-5nm, 1-2nm and <1000nm, in width, height, and length, respectively. The resistance per unit length is 1.2M /nm along the ErSi2 nanowire. The resistivity is around 1 cm, which is 4 orders of magnitude larger than that for known resistivity of bulk ErSi2, i.e., 35 cm. One of the reasons may be due to an elastically-elongated lattice spacing along the ErSi2 nanowire as a result of lattice mismatch between the ErSi2 and Si(001) substrate. http://www.riken.go.jp/lab-www/surf-inter/tanaka/gyouseki/ICSTM01.pdf Resistivity of ErSi2 Nanowires on Silicon
Last stages of the contact breakage during the formation of nanocontacts. Electronic conductance through nanometer-sized systems is quantized when its constriction varies, being the quantum of conductance , G o =2 e 2 /h, where e is the electron charge and h is the Planck constant, due to the change of the number of electronic levels in the constriction. The contact of two gold wire can form a small contact resulting in a relative low number of eigenstates through which the electronic ballistic transport takes place. Conductance current during the breakage of a nanocontact. Voltage difference between electrodes is 90.4 mV http://physics.arizona.edu/~stafford/costa-kraemer.pdf
Setup for conductance quantization studies in liquid metals. A micrometric screw is used to control the tip displacement. Evolution of the current and conductance at the first stages of the formation of a liquid metal contact. The contact forms between a copper wire and (a) mercury (at RT) and (b) liquid tin (at 300C). The applied bias voltage between tip and the metallic liquid reservoir is 90.4 mV. http://physics.arizona.edu/~stafford/costa-kraemer.pdf
Conductance transitions due to mechanical instabilities for gold nanocontacts in UHV at RT: Transition from nine to five and to seven quantum channels. Conductance transitions due to mechanical instabilities for gold nanocontacts in UHV at RT: (a) between 0 and 1 quantum channel. (b) between 0 and 2 quantum channels. http://physics.arizona.edu/~stafford/costa-kraemer.pdf