Massachusetts Institute of Technology
`Ex. 2001
`Part 2 of 4
`
`
`
`
`Ex. 2001
`Part 2 of 4
`
`
`
`
`
`PCI‘
`
`WORLD INTELLECTUAL PROPERTY ORGANIZATION
`International Bureau
`
`INTERNATIONAL APPLICATION PUBLISHED UNDER THE PATENT COOPERATION TREATY (PCT)
`(51) International Patent Classifition 5 :
`W0 93/10564
`(11) International Publication Number:
`
`7
`
`Hon’ 29/12‘
`
`(43) International Pnblimtion Date:
`
`27 May I993 (27.05.93)
`
`(21) International Applimtion Number:
`
`PCT/US92/ 10275
`
`(22) International filing Date:
`
`20 November 1992 (20.1 1.92)
`
`(74) Agents: BENZ, William, H. et al.; Morrison & Foerster,
`755 Page Mill Road, Palo Alto, CA 94304 (US).
`
`(30) Priority am:
`796,245
`
`22 November 1991 (2111.91) Us
`
`(81) Designated States: CA, JP, KR, European patent (AT, BE,
`CH. DE, DK, ES, FR, GB, GR, IE, IT. LU, MC, NL,
`se).
`
`(71)Applicant: THE REGENTS OF THE UNIVERSITY OF
`CALIFORNIA [US/US]; 300 Lakeside Drive, 22nd
`Floor, Oakland. CA 94612-3550 (US).
`
`(72) Inventors: ALIVISATOS, A., Paul ; I428 Spruce Street No.
`. B, Berkeley, CA 94709 (US). COLVIN, Vicki, L.
`; 260l
`_Warring Road, Box 222, Berkeley, CA 94720 (US).
`
`4
`
`Published
`With international search report.
`
`(54) 'l'|tle: SEMICONDUCTOR NANOCRYSTALS COVALENTLY BOUND T0 SOLID INORGANIC SURFACES US-
`ING SELF-ASSEMBLED MONOLAYERS
`
`(57) Abstract
`
`Methods are described for attaching semiconductor nanocrystals to solid inor-
`ganic surfaces, using self-assembled bifunetional organic monolayers as bridge com-
`pounds. Two different techniques are presented. One relies on the formation of self-
`assembled monolayers on these surfaces. When exposed to solutions of nanocrystals,
`these bridge compounds bind the crystals and anchor them to the surface. The sec-
`ond teehnique attaches nanocrystals already coated with bridge compounds to the
`surfaces. Analyses indicate the presence of quantum confined clusters on the sur-
`faces at the nanolayer level. These materials allow electron speetroscopies to be com-
`pleted on condensed phase clusters, and represent a first step towards synthesis of an
`organized assembly of clusters. These new products are also disclosed.
`
`EVI 004903
`
`MIT901_2001-0511
`
`
`
`FOR YHE PURPOSES OF INFORMATION ONLY
`
`Codes used to identify Sums parly In the PCT an the frunt pages of pamphlets publishing international
`applications under lhe PCI‘.
`AT
`Austria
`AU
`Australia
`BB
`Barhudus
`BE
`Bzlgium
`BF
`Burkina Faso
`BC
`Bulgaria
`Benin
`B1
`BR
`Emil
`(hnnd:
`CA
`ttnlml Alriuauu Rcpublic
`(‘ongo
`Swiuztland
`('51:: Lfivuiu:
`(‘amm-ruun
`(.'/cchusluvulin
`(‘ngzll Republic
`Denmark
`Sinin
`I-“Inland
`
`France
`Gabon
`Unxual Kingdom
`Guinea
`Gn:un:
`Hungary
`lrcl.-md'
`Italy
`Japan
`Dumocmlir: People’: RL-public
`of Rural
`Republic of Kon=I
`Kamklislan
`lJ'uchlI:nslcin
`Sri l.\nkn
`l.uxL'ml‘nu(g
`Munnca
`Madagzucar
`Mali
`Mongolia
`
`Mauritania
`Malawi
`N1.-dlcrlanda
`Norway
`New Zcalxnd
`Poland
`Portugal
`Romania
`Russian Fcdcfillion
`Sudan
`Swcdun
`Slovak Rcpubli
`Senegal
`Soviet Union
`(‘haul
`Togo
`Ukraine
`United Sula-5 of Amcriua
`Vic! Nam
`
`EVI 004904
`
`MIT901_2001-0512
`
`
`
`W0 93/10564
`
`PCT/ US92/10275
`
`SEMICONDUCTOR NANO
`
`L5
`
`V L
`
`Y BOUND TO
`
`SOLID INORGANIC SURFACES USING SELF-ASSEMBLED MONOLAYERS
`
`K
`
`Backggound of the Invention
`Field of the Invention
`This invention is in the field of semiconductor
`
`fabrication techniques. More particularly it involves
`
`the binding of semiconductor nanocrystals to solid
`
`inorganic surfaces.
`
`Backggound Information
`The ability to assemble molecules into well-
`
`defined two- and three—dimensional spatial configurations
`
`is A major goal in the field of self assembled monolayers
`
`(SAMS). Whitesides, G. M., Chimia (1990) 15:310-311.
`Since the discovery that alkane thiols will displace
`
`practically any impurity on a gold surface, Nuzzo, R. G.
`et al., J. Am. Chem. Soc.
`(1983)
`;Q§:4481—4483, and will
`
`spontaneously create an ordered monolayer of high
`quality, Porter, M.D. et al., J. Am. Chem. Soc.
`(19e‘-
`
`109, 3559-3568,
`
`interest in these systems has been
`
`SUBSTITUTE SHEET
`
`EV|O04905
`
`MIT901_2001-0513
`
`
`
`W0 93/10564
`
`PCTIUS92/ 10275
`
`-2-
`
`9
`
`extensive, Tillman, N. et al., Lamgmmim (1989) §=1020'
`1026, Reubinstein, I. et al., ymgmrg (1988) §;;:426-429,
`Bravo, 8. G. et al.,
`ggmgmmim (1989) §;1092:1095.
`Recent advances have extended sAMs beyond the prototype
`gold/thiol systems. Fatty acids on aluminum, Allara, D.
`L. et al., Langmuir (1985) ;:45-52; silanes on silicon,
`(a) Maoz, R. et al., Lgmgmmi; (1986) ;:1045-1051,
`(b)
`Maoz, R. et a1, Langmuir
`(1987) _3_:1034-1044,
`(c)
`Wasserman, S. R. et al., Langmuir
`(1989) §: 1074-1087;
`isonitriles on platinum, Hickman, J. M. et a1.) gmjgmm
`cmgm. soc.
`(1989) 1;;:7271~7272; and rigid phosphates on
`metals, Lee, H. et al.,
`J. Phys. Chem.
`(1988) 2;:2597-
`2601 are examples.
`In addition to the wide choice of the
`substrate, the chemical functionality presented at the
`top of a monolayer can be controlled by replacing
`monofunctional alkanes with difunctional organic
`
`(a) Bain, C. D. et al., J. Am. Chem. soc.
`compounds,
`(1989)
`;;;:7155-7164,
`(b) Pale-Grosdemange, C. et al.,
`J. Am. gngm, Soc.
`(1991) 11;:12-20.
`such assemblies can
`then be used to build up more complex structures in three
`
`(a) Ulman, A. et al., Langmuir (1989) §:1418-
`dimensions,
`1420 (b) Tillman, N. et al., Langmuir
`(1989) §:101—105,
`enabling chemists to engineer complex organic structures
`on top of macroscopic surfaces. This specific control
`over the microscopic details of interfaces has allowed
`
`for diverse applications of SAMs. Metals, for example,
`provide the ideal support for organic compounds with
`large non-linear optical behavior, and by using sAMs the
`molecules can be held in specific orientations with
`respect to the metal, Putvinski, T. M. et al., Langmuir
`(1990) §:1567—1571.
`In other work, the ability to
`dictate the structural details of an interface is
`
`exploited to study processes of electron transport
`between an electrode surface and an active moiety bound
`
`on top of a monolayer (a) Chidsey, C. E. D.; Science
`
`SUBSTITUTE SHEET
`
`EV|0O4906
`
`MIT901_2001-0514
`
`
`
`W0 93/10564
`
`PCTIUS92/10275
`
`_.3_
`
`(b) Chidsey, c. E. D. et al., J. m.
`(1991) g§_;,:919-922,
`Qhem. Soc.
`(1990) _1_;_2_:43o1-4305,
`(c) Chidsey, c. E. D. et
`al., Langmuir
`(1990) §:682—691.
`The invention employs
`
`the well-developed chemistry of SAMs to attach
`
`semiconductor nanocrystals to metal surfaces.
`
`The
`
`incorporation of clusters into the monolayers is a first
`
`step towards creating arrays of quantum dots, and the
`
`total assembly of clusters on metals represents a new
`
`kind of material with many potential uses. There is
`immediate application of this new assembly in the study
`of the electronic behavior of the quantum dots.
`
`Semiconductor nanocrystals have been the
`
`subject of numerous spectroscopic investigations in
`
`recent years, Alivisatos, A. P. et al.,
`
`J. Chem. Phys
`
`(a) Spanhel, L. et al, Q. Am. Chem.
`(1988) §g:4oo1-4011,
`ggg; (1987)
`;Q2:5649-5655,
`(b) Hasse, M. et al., J. Phys.
`
`fig; (1988) 9_2:482—487, the origin of the extensive
`interest is that the absorption spectrum of the clusters
`
`is a strong function of their radii, Brus, L. E., Q;
`
`Chem. Phys.
`
`(1984) §Q:4403—4409.
`
`The clusters,
`
`in that
`
`work cadmium sulfide, range in size from 10 to 100 K
`
`radius, and as their radius decreases the electronic wave
`functions are confined, causing the absorption edge to
`
`shift to the blue by as much as one volt, Brus, L. E., Q;
`
`Phys. Chem.
`
`(1986) gg:2555-2560. Despite these dramatic
`
`changes in electronic structure, only optical
`
`spectroscopies have been used to study these systems.
`other-experiments have not yet been performed because of
`
`limitations in the ability to control the environment of
`
`the clusters. Currently the nanocrystals can be isolated
`
`as powders for x~ray diffraction work, Bawendi, M. G. et
`
`(1989) g;:7282-7290, solubilized in
`al., J. Chem. Phys.
`methanol for high pressure studies,
`(a) Alvisatos, A. P.
`
`(b) Hasse, M.
`(1988) §g:5979—5982;
`et al. J Chem Ph s.
`et al. "Clusters and Cluster Assembled Materials“ MS
`
`SUBSTITUTE SHEET
`
`EV|0O4907
`
`MIT901_2001-0515
`
`
`
`W0 93/10564
`
`-4-
`
`PCTIUS92/10275
`
`symposium Proceedings, R. S. Averback, D. L. Nelson and
`J. Bernholc, editors MS Press (Pittsburgh)
`(1991),
`placed in inorganic glasses or polymers for optical
`experiments, Ekimov, A. I. et al. Journal of Luminescence
`(1990) g§:97-100, Liu, Li-Chi et al. Q4_AQE;;_gg1§;
`(1990) §§:28—32, and deposited by evaporation on graphite
`for STM imaging, Zen, Jyh-Myng et al. gangmpir (1989)
`§:1355—1358.
`A serious problem with all of these media
`is that they do not allow the clusters to dissipate
`charge. As a result, traditional probes of electronic
`structure, such as valence band photoemission, have
`
`proved impossible to perform on nanocrystals.
`
`Statement of the Invention
`
`It has now been found that semiconductor
`
`nanocrystals can be covalently bound to solid inorganic
`surfaces such as metals, oxides, or the like using self-
`
`assembled mono1ayers.as a bridge. The invention takes
`advantage of the extensive developments in SAMS to tailor
`the distance between the cluster and the substrate
`surface, and the chemical and physical properties of the
`substrate and.bridging moiety to meet spectroscopic
`
`By providing an avenue for charge
`requirements.
`dissipation, these samples on metal substrates enable
`electron spectroscopies of the density of states to be
`performed on nanocrystals for the first time.
`In
`addition, binding of the clusters to surfaces finds
`application in Raman and resonance Raman scattering
`experiments on nanocrystals which ordinarily fluoresca
`in
`strongly,
`in low temperature spectroscopy of clusters,
`far ultraviolet absorption spectroscopy to ascertain the
`highly excited electronic states of these systems, and in
`electrochemical studies.
`since the nanocrystals can now
`
`be deposited in an asymmetric environment, intact, but in
`
`SUBSTITUTE SHEET
`
`EV|0O4908
`
`MIT901_2001-0516
`
`
`
`W0 93/10564
`
`PCT/US92/10275
`
`-5-
`
`the total assembly can
`close proximity to each other,
`have collective properties of considerable interest.
`
`In one aspect, this invention provides a method
`
`for forming a monolayer of semiconductor nanocrystals on
`
`an inorganic (e.g., metal) surface.‘ This method involves
`
`covalently bonding the nanocrystals to the surface using
`
`In one embodiment the
`an organic bridging moiety.
`bridging moiety is first bound to the metal surface and
`
`then bound to the nanocrystals.
`
`In a second embodiment
`
`the bridging moiety is first bound to the a nanocrystals
`and then bound to the surface.
`
`In another aspect, this invention provides the
`semiconductor materials which include a surface with a
`
`monolayer of semiconductor nanocrystals covalently
`affixed.
`
`Detailed Description of the Invention
`
`Brief Description of the Drawings
`
`Figure 1 contains schematic illustrations of
`
`cadmium sulfide nanocrystals bound to solid inorganic
`surfaces.
`A) Cadmium sulfide from inverse micelles bound
`
`B) Cadmium sulfide
`to gold via 1,6-hexane dithiol.
`nanocrystals synthesized in water and coated with
`
`carboxylates bound to aluminum.
`
`C) Cadmium sulfide from
`
`inverse micelles bound to aluminum via a thioglycolic
`acid.
`
`Figure 2 illustrates ultraviolet visible
`
`The
`spectra of Cds clusters in heptane/micelle mixtures.
`radii listed were determined by the absorption maximum.
`
`Figure 3 illustrates TEM images of Cds clusters
`
`of different size which reveal lattice planes.
`
`The bar
`
`in panel D is 50 K, and the magnification is the same in
`
`all four panels.
`
`A statistically large enough sample of
`
`such images provides a basis for sizing.
`
`SUBSTITUTE SHEET
`
`EVl0O4909
`
`MIT901_2001-0517
`
`
`
`W0 93/10564
`
`PCTIUS92/10275
`
`-5-
`
`Figure 4 is a transmission electron micrograph
`The light
`of Cds clusters on aluminum (Figure 1C).
`mottled background is from the polycrystalline aluminum
`The
`film while the darker spots are the Cds clusters.
`average size of Cds clusters in this sample is 35 K
`radius.
`
`Figure 5 illustrates single particles of Cds on
`aluminum. This-micrograph shows several clusters
`magnified so that the lattice planes are visible.
`
`Description of Egeferred Embodiments
`This section is divided into the following
`subsections.
`
`The Semiconductor Nanocrystals and their Preparation
`
`The Inorganic Surfaces
`
`The Bridging Moieties
`
`Preparation Process and Conditions
`
`The Semiconductor Nanocrzstals and their Preparation
`This invention provides a method for bonding
`nanocrystals of semiconducting compounds to inorganic
`surfaces.
`It finds application with III-V compounds such
`as GaAs, GaP, GaAs-P, Gash, InAs, InP, Insb, A1As, A19,
`and Alsb; and with II-VI compounds such as Cds, Cdse,
`CdTe, Hgs, zns, znse and ZnTe. These compounds and their
`applications in solid state electronic devices are well
`known.
`_
`
`The above-described semiconducting compounds
`
`are employed as nanocrystals. A nanocrystal is defined
`to be a crystallite having dimensions in the nanometer
`range, that is, of less than 100 A (10 nm).
`While materials throughout this size range will
`
`work, as a general rule, materials having dimensions in
`the 1 to 6 nm range are preferred, and especially 1 to
`5 nm.
`
`SUBSTITUTE SHEET
`
`EV|00491O
`
`MIT901_2001-0518
`
`
`
`‘W0 93/10564
`
`PCT/US92/10275
`
`-7-
`
`These materials in their nanocrystalline form
`
`can be formed by various techniques designed to prevent
`
`macrocrystal formation.
`
`In a copending United States patent application
`
`USSN (Docket IB-868), filed on even date with this
`
`application by P. Alivisatos and M. olshavsky and
`
`incorporated by reference,
`
`together with their earlier
`
`publication (with A.N. Goldstein) appearing at J.A.C.s.
`
`(1990) ;;g:943s (December, 1990), a process for forming
`
`nanocrystals of III-V compounds such as GaAs is set
`forth.
`
`In this process, a group III metal source, such
`as a GaIII salt, InIII salt, or AlIII salt or
`
`corresponding metal 1-6 carbon trialkyls, is reacted
`
`directly with an arsenic, phosphorus, or antimony source
`such as arsine, phosphine, or stibine, an alkyl arsine,
`
`phcsphine or stibine, or an alkyl silyl arsine phosphine
`
`or stibine in liquid phase at an elevated temperature.
`
`Representative metal sources include GaC1,, GaBz3, Galy
`
`InCl,,
`
`InBr3, A1C1,, Ga(Me)3, Ga(Et),, Ga(Bu),, or the like.
`
`Representative arsenic, phosphorus and selinium sources
`include Asfiy PH“ Sefiy Asflfil-6 carbon alkyl), As(1-4
`
`carbon alkyl)3, P(1-4 carbon a1ky1),, As(Si(1-6 carbon
`
`alkyl)g,, P(Si(1-6 carbon a1kyl)g,, Se(Si(1-4 carbon
`alkyl)g, and the like.
`one of each of these two groups of materials
`
`are mixed together in a nonaqueous liquid reaction medium
`
`which includes a crystal growth terminator and heated to
`
`a temperature of at least about 100°C for a prolonged
`
`period of at least about 1 hour. Polar organics, such as
`
`nitrogen— and phosphorus-containing organics, can serve
`
`as crystal growth terminators. Water and air should be
`excluded from the reaction zone. This causes the desired
`
`The reaction medium can then be
`nanocrystals to form.
`removed to yield the nanocrystals in dry form.
`
`SUBSTITUTE SHEET
`
`EV|O04911
`
`MIT901_2001-0519
`
`
`
`wo 93/10554
`
`PC1'IUS92/10275
`
`-8-
`
`II-VI materials such as cds can be formed as
`nanocrystals using colloidal precipitation techniques.
`In one technique, a group II metal source such as a CdII
`salt, an HgII salt or a znII salt is dissolved in water
`and this solution suspended in an organic liquid such as
`hexane, heptane or octane or the like with a colloid
`former such as deoctylsulfosuccinate. A suitable group
`IV counterion (sulfide, selenide or telluride) source is
`dissolved in water and similarly suspended in an organic
`liquid. These two suspensions are mixed to yield a
`colloidal suspension of nanocrystals of the semiconductor
`compound. This suspension is destabilized by addition of
`a capping group, for example a thioacid such as
`thiophenol or mercaptoacetic acid. This causes the
`nanocrystals to precipitate for recovery.
`Typical metal sources include cadmium II
`sources, for example salts and dialkyls, such as Cdcly
`Bra or I2, cd(c1og,, Cd(l-6 carbon alkyl)2, as well as
`mercury II and zinc II equivalents.
`counterions can be
`provided by simple salts such as alkali metal sulfides,
`selenides or tellurides, Nazs, K23", Nazse, Kzse, Na2Te and
`Kfle. This general method for forming nanocrystals is
`referred to as a "reverse micelle" method and is
`described in more detail in M.L. steigerwald et al., J;
`Am. Chem. Soc. 11D:3046 (1988), which is incorporated
`
`herein by reference.
`other colloidal precipitation methods will work
`
`as well. For example, one can precipitate the II-VI
`materials out of acidic or basic media in the presence of
`capping agents such as mercaptoacetate ions,
`in the form
`of colloidal nanocrystals.
`
`These methods are representative, and other
`
`methods which provide nanocrystals of these materials can
`be used as well.
`
`SUBSTlTUTE SHEET
`
`Ev: 004912
`
`MIT901_2001-0520
`
`
`
`W0 93/10564
`
`PC!’IUS92ll027S
`
`-9.-
`
`The Inorganic Solid Surfaces
`
`The surface upon which the nanocrystals are
`
`deposited can be a metal or a nonmetallic inorganic
`
`materials such as metal oxide, metal sulfide, carbide, or
`the like. Nonmetals can he insulators such as silicon
`
`oxides, aluminum oxides, boron oxides,
`
`titanium oxides
`
`and the like. They also can he semiconductors.
`Metal surfaces can he made of any metal or
`often the metal is chosen for its electrical
`
`alloy.
`
`conductivity properties. Metals such as gold, silver,
`
`copper, aluminum, gallium, and the like can be used.
`
`Gold and aluminum are preferred metals.
`The nonmetal or metal surface can be a bulk
`
`surface or it can be a thin layer. Thin layers of metals
`
`or metal oxides or sulfides may be sputtered or plated or
`vapor-deposited upon a substrate.
`Best results are achieved when the surface is
`
`very clean.
`
`To this end,
`
`the surface can be plasma
`
`etched, acid etched and the like prior to coupling the
`semiconductor nanocrystals to it.
`
`The Bridging moieties
`
`The nanocrystals are covalently attached to the
`
`surface through a bridging moiety which bonds to a metal
`
`atom in the inorganic surface and to the nanocrystal.
`These materials can be homobifunctional—-that is,
`
`presenting two identical groups which can bond to the
`
`inorganic surface and to the nanocrystal. They also can
`- be heterobifunctional--that is, presenting one each of
`
`one of the groups is capable of bonding to
`two groups.
`the crystals and one bonds to the inorganic surface.
`.
`Thiol groups are capable of bonding to metal
`
`They also can bind to the nanocrystals. This
`surfaces.
`gives rise to one family of binding moieties--dithiols.
`
`These materials have the structure H-S-R-S-H, where R is
`
`SUBSTITUTE SHEET
`
`EV|O04913
`
`MIT901_2001-0521
`
`
`
`wo 93/10554
`
`PCl"lUS92/10275
`
`-10-
`
`an organic group, particularly a hydrocarbyl organic
`having a'distance of from about 8 to about 5 and
`especially about 7 to about 4 carbons between the two
`S's. These materials include 1,4-dithiol n-butane, 1,4-
`dithiol cyclohexane, 1,5-dithiol n-pentane and 1,6-
`dithiol n-hexane,
`Dithiol compounds having side chains
`It appears that materials in
`
`and the like can be used.
`
`the 4-6 carbon spacer class give good results because
`spatially there is less tendency for the spacer to loop
`and allow both ends of a single chain to both bond to the
`surface or to the nanocrystals.
`
`The dithiol can be attached to the metal first
`
`or to the nanocrystal first.
`Another family of materials are those which
`present a thiol and a carboxyl.
`The carboxyl is capable
`of binding to a metal in the surface and the thiol of
`binding to the nanocrystal. Examples of these materials
`are mercaptoacetic acid, mercaptopropionic acid, 4-
`mercapto,1—carboxyl cyclohexane, and the like.
`
`Preparation Process and Conditions
`The process of preparation involves coupling
`the nanocrystals to the surface though the bridging
`moieties.
`
`In one approach, the metal surfaces,
`scrupulously cleaned, are contacted with a solution of
`This can be carried out at room
`the bridging moiety.
`temperature for from about 15 minutes to about 24 hours
`or more. An excess of bridging moiety is used.
`The
`
`excess is rinsed away. Thereafter, the nanocrystal
`clusters are contacted with the bridging moiety-rich
`
`surface for from about an hour to as long as 48 hours or
`
`more to effect coupling. Again, good results are
`
`achieved at room temperature and higher.
`
`SUBSTITUTE SHEET
`
`EV|004914
`
`MIT901_2001-0522
`
`
`
`W0 93/10564
`
`FKIT/US92I10275
`
`-11-
`
`Similar times (1 to 48 hours) and temperatures
`
`(10 to 50°C) give good results with the other coupling
`chemistries as well.
`
`This invention will be further described by the
`
`5
`
`In them the preparation of
`following examples.
`monolayers of semiconductor nanocrystals covalently bound
`
`one technique
`to inorganic surfaces is described.
`involves building a se1f—assembled monolayer using alkane
`
`dithiol compounds.
`
`In comparison to other work which
`
`involved using thiols as coupling agents to gold,
`
`relatively short chain alkanes are used to avoid the
`
`problem of looping. The monolayers thus formed are
`stable enough to withstand further chemistry on the
`
`available thiol groups. When these thiol-rich surfaces
`
`are exposed to metal-containing nanocrystals, the sulfurs
`
`form strong bonds to the metal—containing nanocrystals,
`anchoring the clusters to the surface (Figure 1A).
`An
`
`additional method involves binding the bridging group to
`
`the clusters first, and then exposing the solution to the
`
`free metal (Figures 1B, C). Both techniques result in
`
`durable films of dispersed clusters, homogeneous on a pm
`
`scale with approximately 0.5 monolayer coverage.
`
`These examples are not to be construed as
`
`limiting the scope of the invention which is defined by
`These examples include a
`‘the appended claims.
`
`substantial discussion of the properties of the products
`
`of the invention to aid in the understanding of their
`
`application in semiconductor fabrication settings.
`
`Example 1
`
`Preparation of Semiconductor Nanocrystals by
`Inverse Micelle Method:
`
`Cadmium sulfide clusters were prepared in
`
`inverse micelles following methods developed by
`
`SUBSTITUTE SHEET
`
`EVlO04915
`
`MIT901_2001-0523
`
`
`
`W0 93/10564
`
`PC!‘/US92/10275
`
`-12-
`
`5
`
`(1988)
`steigerwald, M.L. et al., J. Am. Chem. Soc.
`11o:3046-3050, and Lianos, P. et a1., Chem. Ph s. Lett.
`(1986) 12s:299—3o2.
`Two separate solutions of 500 m1
`spectrographic grade heptane and 44.4 grams of dioctyl
`sulfosuccinate [577-11-7], AOT, were prepared under
`nitrogen.
`2.34 grams of Cd(cloQ26}QO dissolved in 12.0
`ml of deoxygenated, deionized water_were added to one
`solution, while 0.36 gtfinms of Na,s9H5o dissolved in 12.0
`ml of deoxygenated, deionized water were added to the
`other solution. Both solutions appeared clear and
`colorless after one hour of mixing.
`The cadmium solution
`was then transferred to the sulfide via a 12-gauge double
`transfer needle. The transfer process took 15 minutes
`and resulted in the formation of a clear yellow solution.
`At this point, 500 ml of this solution was reserved for
`later use, and the rest was treated with 0.45 mg of
`thiophenol, which binds to the surface of the clusters
`causing them to come out of the micelles. The resulting
`powder was vacuum filtered three times and rinsed with
`300 ml petroleum ether.
`It was redissolved in 10 ml
`The powder was heated,
`
`pyridine and filtered again.
`reprecipitated into 200 ml of petroleum ether, and
`This sample was then refluxed
`filtered again.
`Reprecipita—
`
`in 20 ml of quinoline at 240°C for 3 hours.
`tion and filtering followed this, leaving a finely
`divided yellow powder redissolvable in pyridine.
`
`Example 2
`
`Preparation of cadmium sulfide
`Nangcrgstal Clusters from an Acidic Colloid
`A 500 ml solution of 1 x 1o-3M cdc1. was
`
`prepared, and to this was added a 500 ml solution of 1.6
`X 104M sodium mercaptoacetate, resulting in a'turhid blue
`solution.
`The pH was lowered to 3.35 with Hcl, producing
`150 ml of 1 x 10'2M Na1S was then
`
`a colorless solution.
`
`SUBSTITUTE SHEET
`
`EV|0O4916
`
`MIT901_2001-0524
`
`
`
`W0 93/10564
`
`PC!‘I US92/10275
`
`-13-
`
`injected to the quickly stirring solution. This
`preparation gave nanocrystals with an absorption maximum
`at 460 nm. crystallites with absorption maxima as low as
`
`360 nm could be obtained by reducing concentrations.
`
`Example 3
`
`Preparation of Cadmium Sulfide
`
`Nanocggstgl clusters from a Basic Collgig
`
`One liter of 1 x 10’3M cdc1, was titrated with
`
`mercaptoacetic acid to pH 2.8} resulting in a turbid blue
`solution, as above. Concentrated NaOH was then added
`
`dropwise until the pH was greater than 8.5 and the
`solution was again colorless. While the solution was
`
`quickly stirred, 110 ml of 1 x 10"M Na,S was added.
`Nanocrystal particle sizes with absorption maxima between
`
`360 nm and 410 nm were produced by varying the final pH
`of the thiol titration.
`
`The colloids from Examples 2 and 3 were reduced
`
`by rotary evaporation to a redissolvable powder which
`
`contained Nacl as a reaction byproduct. Dialysis against
`
`a dilute solution of mercaptoacetic acid was used to
`
`remove the salt while maintaining the solubility of the
`colloids. Solutions of redissolved crystallites were
`stable in the dark for months. All reactions were
`
`conducted in room light using deionized, distilled water.
`
`The colloids can be grown by heating to 90°C in the
`
`presence of 0.5 ml of the thiol.
`
`Example 4
`
`The preparations of Examples 1-3 are repeated
`
`varying the starting materials.
`
`In place of CD(C1OQ;6H§)
`
`and cdc1,, Znclz can be used,
`
`leading to Zns nanocrystals.
`
`SUBSHTUTESHEET
`
`EVI 004917
`
`MIT901_2001-0525
`
`
`
`W0 93/10564
`
`PCTIUS92/10275
`
`-14-
`
`Example 5
`
`The preparations of Examples 1-3 are repeated
`varying the startingmaterials.
`In place of Nazs, Nazse
`is used,
`leading to Cdse nanocrystals.
`
`Example 6
`
`(99.99%) was purchased from Aldrich,
`Gacl,
`purified by sublimation, and stored in a dry box.
`Tris(trimethy1si1y1)arsine was prepared according to
`literature methods, Becker, G. et al., Anorg. Allg. Chem.
`(1980) 462:113, purified by vacuu distillation, and
`stored in a dry box at 0°C. Proton NMR and infrared
`
`spectra matched the literature values.
`Tris(trimethylsily1)arsine:
`IR (neat liquid) 2892 (m),
`2890 (s), 2323- (s), 2816 (s), 2785 (vs), 1446 (s), 1400
`(vs), 1306 (s), 1259 (vs), 1240 (w), 1124’ (m), 869 (w),
`‘H mm (300.1-ii-Iz, c,D,)
`5 0.35 (s, sir-:e,). Quinoline was
`purchased from Aldrich and distilled immediately prior to
`use. »Quino1ine (25 mn) containing 6.5 x 10‘ mol of
`tris(trimethylsilyl)arsine was added to 6.5 x 104 mol of
`Gacl, in 25 mL of quinoline.
`The resulting mixture was
`heated at reflux (240°C) for 3 days.
`A red powder was
`
`isolated by removal of the solvent, and the powder
`consisted of GaAs particles which are redissolvable in
`
`pyridine or quinoline.
`The quinoline-soluble GaAs particles were
`studied by TEM.
`TEM revealed prolate GaAs particles with
`an average major axis of 45 3 and minor axes of 35 A.
`
`Example 7
`
`The preparation of Example 6 is repeated using
`a corresponding phosphine, tris(trimethylsilyl)phosphine,
`in place of the arsine of Example 6. This leads to GaP
`nanocrystals as the product.
`
`SUBSTITUTE sHst-:1‘
`
`EVlOO4918
`
`MIT901_2001-0526
`
`
`
`VV()93l!0564
`
`PCT/U592/10275
`
`-15-
`
`Example 8
`
`The preparation of Example 6 is repeated using
`
`Incl, as a starting material in place of GaCl3. This
`leads to InAs nanocrystals as the product.
`
`Example 9
`
`Preparation of Metal Substrates
`
`some of the metal layers used in these
`
`10“
`
`experiments were prepared by vapor deposition of gold or
`aluminum onto glass slides.
`The vapor deposition was
`
`performed at 10” torr in a bell jar; evaporations usually
`took 10 min and resulted in films with an average
`
`thickness of 1000 A.
`
`The thickness was determined by a
`
`"quartz crystal microbalance inside the bell jar.
`Adhesion of the gold films to the glass slide was insured
`
`by use of a "molecular glue," 3-
`
`mercaptopropyltrimethoxysilane.
`
`Reproducible high~
`
`quality films were obtained when the glass slides were
`
`cleaned prior to treatment by immersion in 1:4 reagent
`
`grade 30% H53/conc.IySO4(cleaning solution) at 70°C for
`.10 min.
`
`This cleaning solution reacts violently with
`
`many organic materials and should be handled with extreme
`care.
`
`In addition to the metal evaporated films,
`metal blocks were also used as substrates to facilitate
`
`mounting of the samples to spectrometers and cryostats.
`
`for aluminum samples, solid aluminum was machined into an
`
`For a gold
`appropriate size with a satin finish.
`substrate, a micron-thick layer of gold was electroplated
`
`onto aluminum blocks;
`
`in this procedure significant
`
`etching of the aluminum produced a much smoother surface
`
`with a mirror finish. These block samples, although
`
`ideal for low temperature applications and photoemission,
`
`were more rough, and coverages for some of the samples,
`
`SUBSTITUTE SHEET
`
`EVI 004919
`
`MIT901_2001-0527
`
`
`
`wo 93/10554
`
`PCl'/US92/10275
`
`especially the water-soluble eds clusters on aluminum,
`were lower.
`
`Preparation of Nonmetallic Substrates
`Nonmetal substrates, such as silicon oxide or
`
`aluminum oxide, can also be used. These materials should
`present a scrupulously clean surface as well, which can
`be achieved by chemical cleaning as set forth above in
`this example, or by physical cleaning such as by Plasma
`etching the surface or the like.
`
`Example 1 O
`
`Pregaration of Dithiol Monolayers on Gold
`Self—assembled monolayers were prepared by immersing
`
`gold substrates in dilute solutions of hexane dithiol
`following established methods (Bain, c.D. et al., J. Am.
`Chem. Soc.
`(1989) ;;;:321-335 and ;;;:7155-7164, and
`Pale-Grosdemange, C. et al., J. Am. Chem. Soc.
`(1991)
`
`;;;:12-20). Gold substrates were p1asma—etched before
`use with an etching time of 10 minutes at 200 mtorr in N2
`
`atmosphere. Contact angles after such etching were less
`than 10°,
`indicating clean surfaces. The samples were
`placed in 5 mM ethanolic solutions of dithiol for 8 to 12
`hrs. Gold substrates were coated with 1,6—hexane dithiol
`
`the samples were removed
`(Figure 11). After immersion,
`from solution, rinsed with ethanol for 30 seconds, and‘
`
`then blown dry with argon. Contact angle measurements
`
`were performed at this time.
`
`Examgles 11 and 12
`
`Different lengths of dithiol were used with
`
`varying degrees of success. Propane dithiol monolayers
`on gold gave low contact angles and XPS showed little
`evidence of sulfur, while 1,8-octane dithiol on gold gave
`
`high Contact angles and resulted in low nanocrystal
`
`SUBSTITUTE SHEET
`
`EV|OO4920
`
`MIT901_2001-0528
`
`
`
`W0 93/10564
`
`PCT/US92/10275
`
`-17-
`
`coverages. All thiols were purchased from Aldrich; 1,6-
`
`‘hexanedithiol was 97% pure and mercaptoacetic acid was
`95% pure. Under ambient conditions,
`the dithiols will
`
`interconvert to disulfides; a disulfide impurity has
`
`little impact on the films since thiol groups are
`
`hundredfold more efficient at binding to gold (Bain, C.D.
`
`et al., Langmuir
`
`(1989) §:723-727).
`
`Example 13
`
`Preparation of Thiol Monolayers on Aluminum:
`
`Aluminum was treated with mercaptoacetic acid to make its
`
`surface thiol rich (Figure 1B)
`
`following methods
`
`developed by Nuzzo et al.
`
`(Allara, D.L. et al., Langmuir
`
`(1985) ;:45-52). Although freshly evaporated aluminum
`
`has a low contact angle, plasma etching was performed on
`the substrates prior to immersion. Etched substrates
`
`were immediately placed in solutions of 5 mm
`
`mercaptoacetic acid dissolved in ethanol and were allowed
`
`to sit for 12 hours. The substrates were removed, rinsed
`
`with ethanol for 30 seconds and blown dry with argon.
`
`Samples could be stored in a desiccator prior to coating
`with nanocrystals.
`
`Example 14
`
`Preparation of Cluster Monolayersz The
`
`aluminum and gold substrates were prepared such that
`their surfaces contained free thiols. These SANS were
`
`then exposed to solutions of cadmium sulfide clusters in
`
`micelles. These solutions contained heptane, ACT, and
`
`clusters prepared in Example 1. Exposure was completed
`much in the same way as for the original monolayers:
`the
`sulfur-rich SAMS were immersed in solutions of heptane
`containing the inverse micelles.
`The heptane solutions
`
`were used undiluted, and hence had an approximate
`
`concentration of 2.70 grams of cadmium sulfide per liter.
`
`SUBSTITUTE SHEET
`
`EVlO04921
`
`MIT901_2001-0529
`
`
`
`W0 93/10564
`
`PCI'/US92/10275
`
`-18-
`
`Typical immersion time was 12 hours, and afterwards the
`samples were rinsed with heptane for 30 seconds then
`blown dry with argon. The treatment afterwards was
`identical to the preparation of the SAMS. The films were
`indefinitely stable.
`
`Example ;5
`
`An additional method which bypasses the use of
`a preliminary monolayer was also developed.
`In this
`case, Cds nanocrystals prepared with carboxy1ate—rich
`surfaces in Examples 2 and 3 were exposed to freshly
`etched aluminum of Examples 9-13. The dialyzed powders
`of Examples 2 and 3 were dissolved in nanopure water with
`18 M-Ohm resistivity in concentrations of 4 mg/ml. The
`aluminum substrates were immersed in the water solutions
`24-48 hours.
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