Metal Colloids and Quantum Dots : Linear and Ngnlinear Optical Properties

Nanophase materials have found a wide application in a variety of technological areas which include ultrafast optical switching high density information storage and retrieval, electronics, and catalysts, to mention a few. Nanocrystal science has also drawn considerable interest from the fundamental perspective engaging physicists, chemists, and material scientists into this area of rapidly expanding and challenging research. Basic questions concerning how matter evolves from atomic like behavior to molecular and onto bulk lie at the center nanocrystal research. In addition, because of the high surface to volume ratio of the nanocrystals, the interaction potential between a nanocrystal and its surrounding environment becomes an important issue in determining its properties. While significant progress has been made in nanocrystal research, there are many problems concerned with their fabrication. In particular, the difficulty of incorporating nanocrystals into a matrix that is appropriate for ultimate device development has hindered some aspects of nanocrystal research. Ion implantation is a method that is now established as a technique for fabricating metal and semiconductor nanocrystals. It is highly versatile in that one may select nearly any host material for incorporating the nanocrystals of interest. The flexibility of being able to select the host matrix is also interesting from the point of view that it opens the opportunity to investigate matrix-nanocrystal interactions. We summarize in the following sections results on metal and semiconductor nanocrystals formed by ion implantation into dielectric hosts.


Background
Nanophase materials have found a wide appfidon in a miety of technological areas which include ultrafht optical switching high density infbrmation storage and retrieval, elektronics, and catalysts, to mention a few.. Nanoc~ystal science has also dram collsiderable interest fiom the fundmental perspective engaging physicists, chemists, and material scientists into this area of rapidly expandingand cballme research Basic pestions concerning bow matter evolves from atomic like behavior to molecular and onto bulk lie at the center nanocrystal research.In addition, because of the high surfkce to volume ratio of the nanouystals, the interaction potential between a nanonystal and its surrounding enVimmneaabecom~ an important issue in determining its properties.While sinificant progress has been made in rianocrystal research, there are many problems conoerned with their fabrication In partidar, the difiidty of incorporating nanocIystals into a matrix that is appropriate for ultimate device development has hinderred some aspects of nzj~~ocrystaI research.Ion implantation is a method that is now established as a technique for fabricating metal and semiconductor nanouystals.Et is highly versde m that one may select nearly any host material fbr incorporating the nanocrystals of interest.The flexiity of being able to select the host matrix is also interesting from the point of view that it opens the opportunity to investigate matrix-nanocrystal interactions.We summarize m the fbllowing sections results on metd and semiconductor nanonystds f o d by ion implantation into dielectric hosts.

Metal Nanocrystals Progress'
Metal nanocry&als are particularly interesting in that they possess a large polasizability.The large polarizability which otiginates from the surkce plasmon resonance is expected to enhance the third order nonlinear s o s c e p t i i r">.Enhancemetrt off) is important because the real part is responsibIe for many processes based on optical &taERy swh as optical switching.Consequently, it is essential that the linear optid response of metal nanocaystals be hlly characterized in order to understand the optical nodinearities.

Gold Narroaystals
ses between 1 ~1 0 ' ~ and lxIO" ions/cm2 w e r e implanted at room temperature at an energy of 1.1 MeV.The substrates were annealed at temperatures between 900 and 1200'C in 95% Ar -t 5YE, and 95% Ar+ 5%0, atmospheres.Optical spectra were m e a d before and after the impIanted samples showed peaks at 575 and 530 nm which are assigned to F , plasmon absorption of gold nanocrystals, respectively.atmosphere caused the peak at 530 nrr~ to shift to the same sample m a reducing atmosphere caused the peak at 560 nm to shift b 30 nm.T b process cydes as many as 30 times.We have irrtapreted that this frequency shift arises fiom the gold nanocrystals interacting w i t h the F, centers.When the sample is mealed in B reducing atmosphere, F, centers are generated.These atmosphere annihilates the Fa centers causing the surfkce plasmon hquency to return 530 nrn.The particular signifjcance of these results lies in the fact that idormation colicezning the interface between the gold nanocxystd and the MgO mafrix has been obtained.This clearly demonstrates thai e&ctive medium theories (Maxwen Garnett, Bruggeman, etc) cannot account for intexfke e&ts such as the ones just noted and care should be taken when evaluating the quality of experimental data with effective medium theo&-~.
Gold was implanted into porous Vycor substrates at 1.1 MeV and at doses from 1 ~1 0 ' ~ to IxlO" iondcm'.The sampIes were annealed in 8 95% At + 5% H, atmosphere at temperatures from 800 to 1100°C.The wirational and electronic spectra were recorded b&re and after the anmaling treatments.The surface plasmon absorption was observed at 540 nm and increased in intensity with increasing mneaiing t h e and temperature, An additional mode also appeared near 900 nm after annealing and was assigned to a second surface plasmon absorption that redted from the removal of the sphericat symmetty of the m e .This occurs by misotropic shrinkage of the glass which is larger in the radial direction than in the mid.The spectra were reproduced using Maxwell Garnett theory for prokite eEpsojds.The fit was within 8 few percent error and confirmed the particles were prolate ellipsoids.

Zinc N l u z ~s f a f s
Zinc ion implanted silid with controlled thermal annealing was investigated.Low temperature measurements indicate the presence of 251 clusters in the as-implanted sampIes.optical spectra of the samples annealed under a reducing environment suggest Zn cluster and Zn colloid fbnnation.An absorption peak at 5.3 eV may be due to the s d plasmon absorption of Zn colloids.The samples annealed in a ~x i d i atmosphere show peaks near 4.3 and 4.8 eV indicaiing ZnO quantum dot formation.The blueshift of the exciton is attriiuted to ~uantum confinement &cts.on a silicon substrate.The sequence of the implants wen found to significanty effect the nanocrystal formation and was attriiuted to ion beam mixing.The fbrmation of the nanocrystals was confinned by cross sectional TEM and by x-ray d i t i o n measurements.When the samples were implanted with Ge and then Si fbUowed by annealing at 1100°C the nanocrystals are much larger than when the sequence is Si fbnowed by Ge.It is expected that this finding is general fbr compound semicoaductm hmed bysquemtia?ion implantation and will be exploited in &e future for afkcting size control of the quantum dots.

Ghaium Amaide
Quantum dots ofGaAs were k m e d by sequential ion implantation into Alllo, and SO,.The cqstds are randomly oriented in silica and less than 15 nm in diameter.In the A&Q matrix, the oanocrystals are three dimensionally aligned with respect to the crystal lattice.Mared refiectance measurements show the surface phonon mode of GaAs nanoqstals and it agrees well with the value predicted by Frtilich's theory of Surface phonons.Also, strong inhomogeneous broadening is observed for the GaAs nanouystals isolated in SiQ as compared to the A&Q mafrix The presence of the nanocrystals was confumed by TEM, XRD and inkired spectroscopy.

Gallium Phosphide a d Indium phosphide
Both InP and Ga9 were qmthesized m silica by sequential ion implantation.Quantum confinement e k t s were observed i n the electronic spectra.Initiared spectroscopy revealed the surfirce phonons of both materds and were found to be m good agreement with Fr6EcWs theory.TEM images. of InP showed extensivetwinning among the nanoclystals.X-ray maction measurements confirmed that InP had been formed. .

Lead Su@&
then inject electrons (the F& level is lower for gold than the F centers) -into the gold nanoaystal causing the surface plasmon fkquency to bIueshift.Annealing the substrates in an oxid* Nanocrystals, Progress' Semiconductor nanocrystals intrigue scientists for a m b e r of reasons.They offer the oppoxtmity for testing theories based on quantum size effects, and they are rich candidates for optical devices.Such devices include ff at panel displays, nano ' I energy implants were used to produce an excess Si concentration of 5x102'/m3 throughout the f b and the implanted sample wsls axme 1 hour in a 96%A.r+ 4% H , atmosphere.Under these conditions, the 8v 4 nm in diameter and very few exceeded 8 xun.The nanocrystals scence (PL) in the wavelength range of 750 nm although the origin to the photoluminescence of porous silicon, the intensity and stabiity of the PL fiom the nanocrystds was greater than that of porous silicon.The formation of the nanocrystas was also confirmed by cross sectional TEM iumAUoy SiGe nanocrystal alloys w e r e fbrmed by sequential ion implantation of Si and Ge into a layer of SiO,

I
joined th and will work on Taylor should graduate in Fall 1999.-Last !3ummer, fbx students OSazR s Jones, and Angela Allen participated in a summa mtem program sponsored by NSF.An fbur students worked on nanocrystas and Osaze

NrnocryStaIs ofthis narnrw gap SefnicODductOr were Mnicated in (1 11) cut CaF, single crystals. The appearance of a peak near 600 mn for the sequentiany implanted substrate suggested quantum confinement. The peak blueshifted as the ion dose decreased which is consistent quantum mntinement theory. j 3.6 Selenium Selenium was implanted into f i ~ and annealed at 600,800, and temperature increased. Tbis absorption onset is ent with the bulk value (no quantum confinement effects) expected for trigonal selenium. er, selenium also exists m a monoclinic form which has a band gap of 1.95 eV suggesting that the absorption onset of 2.5 eV could be attributed to quantum confined mon
I 1OOO"C.The optical spectra s h a w e d an

absorption onset near 2.5 eV that redshifted as the anneaIing 4. Students Chrisptopher completed er of 1996. His s thesis was on Meharry Mkdical School where he is currently a student. A new graduate student, Taravia Taylor just
IIndium Phosphide quantum dots formed by ion implantation fiued silica.He has been accepted at .