1.21 Energy Band diagram of Semiconductor Quantum Well ...blog.sciencenet.cn/upload/blog/file/2010/8/2010823144550174861.pdf · lattice mismatches, dangling bonds, or adsorbates at

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Nano-Chemistry, LZU, 2005

Lecture 9: Lecture 9: SemiconductingSemiconducting nanoparticlesnanoparticles

(3) core(3) core--shell nanostructuresshell nanostructures


TodayToday

SemiconductorSemiconductor-- SemiconductorSemiconductorSemiconductorSemiconductor--MetalMetalMetalMetal--Polymer Polymer MetalMetal--MetalMetalMetal Metal -- SemiconductorSemiconductor


1.21 Energy Band diagram of 1.21 Energy Band diagram of different materialsdifferent materials

Free electrons

EF

E0Vacuum level

Fermi level

(Valence band)

(conduction band)

(band gap)

E0

EF

Metal Semiconductor


Semiconductor Quantum WellSemiconductor Quantum Well


What are 2D, 1D, 0D What are 2D, 1D, 0D nanosturecturesnanosturectures??

NanoNano--crystallite of crystallite of semiconductor material semiconductor material that confines carriers that confines carriers (electrons and holes) in all (electrons and holes) in all three dimensionsthree dimensions–– Quantum Well Quantum Well –– 2D2D–– Quantum Wire Quantum Wire –– 1D1D–– Quantum Dot Quantum Dot –– 0D0D

Bohr Bohr excitonexciton radius is theradius is thecritical dimensioncritical dimension

Density of states (DOS) Nano-Chemistry, LZU, 2005

Molecule, Molecule, NanoparticleNanoparticle and a Bulk and a Bulk SemiconductorSemiconductor

MOLECULE

LUMO

HOMO

CB

VB

Eg

BULK SOLID

∆E ∆E

Ener

gy

NANOPARTICLE

2


Inorganic SemiconductorsInorganic Semiconductors

Trap states are caused by defects, such as vacancies, local lattice mismatches, dangling bonds, or adsorbates at the surface


2.2 Semiconductor Nanoparticles2.2 Semiconductor Nanoparticles

Quantum dots are semiconductors particles that has all three dimensions confined to the 1-100 nm length scale

Group 14 (old group IV) Si, Ge

III-V Materials: GaN, GaP, GaAs, InP, InAs

II-VI Materials: ZnO, ZnS, ZnSe, CdS, CdSe, CdTe

Colloidal CdSe quantum dots dispersed in hexane


3. Core-shell nanoparticles

Nanoparticle Engineering and Surface Modification

core

shell


Why Core-Shell?

Tuning of Physical PropertiesTuning of Physical Properties

Chemical and Colloidal StabilityChemical and Colloidal Stability• Nanoparticle degradation through chemical etching• Agglomeration caused by strong van der Waals attractive forces

• Collective properties of nanoparticle assemblies are influenced to a large extent by the separation between the particles.

• Coating the particles with a uniform shell of inert material could control the distance between the particles

Control of Control of InterparticleInterparticle Interactions Within AssembliesInteractions Within Assemblies

For example, the optical properties of metal nanoparticles are influenced by their environments. Controlled surface modification can alter these properties


Types of Core-Shell Nanoparticles

• Semiconductor- Semiconductor• Semiconductor-Metal• Metal-Polymer • Metal-Metal• Metal - Semiconductor


3.1 Semiconductor-Semiconductor core-shell

structures

3


Energies of Various Semiconductors

1.4

GaAs

2.25

GaP

1.7

CdSe

2.5

CdS

3.2

ZnO

3.2

WO3

3.2

TiO2

3.0

TiO2

Ener

gy (e

V)

Values at pH = 1


Semiconductor on SemiconductorTailoring optical properties

Enhancing the luminescence of the core

Core

Shell

coreshell core

shellEner

gy


Examples for Semiconductor-Semiconductor Core-Shell Nanoparticles

J. Phys. Chem. B. 1997, 101, 9463J. Phys. Chem. B. 1998, 102, 1884J. Phys. Chem. 1993, 97, 5333J. Phys. Chem. 1996, 100, 6381J. Phys. Chem. 1996, 100, 8927J. Phys. Chem. 1996, 100, 13226J. Phys. Chem. 1996, 100, 20021

Examples include:ZnS on CdSeCdS on CdSeCdSe on CdS, etc


3.1.1 CdSe Coated with ZnS Nanoparticles

Me2Cd + TOPSe CdSe

∆T

300 oC(TMS)2/Me2Zn/TOP

CdSe

ZnS

TEM picture of (CdSe)ZnS nanocrystalsJ. Phys. Chem. 1996, 100, 468


CdSe Coated with ZnS NanoparticlesJ. Phys. Chem. 1996, 100, 468

Absorption spectrum of the (CdSe)TOPO(dotted line) and the (CdSe)ZnSnanocrystals (solid line). The fluorescence of the (CdSe)ZnS is also shown (solid line)

Normalized fluorescence spectra of CdSe-TOPO (dotted line) and CdSe@ZnS (solid line) with 470 nm excitation


Observations on the Optical Characteristics of CdSe/ZnS Nanoparticles

Fluorescence of CdSe-TOPO shows the broad tail, due to surface traps.

CdSe/ZnS fluorescence spectrum has a flat baseline; this indicates that the ZnS reduces the traps present on the CdSe(TOPO) surface

Fluorescence of CdSe (CdSe/ZnS) was stable for months compared to uncapped CdSe

No reduction in the CdSe quantum yield was observed for months with the CdSe/ZnS nanoparticles

J. Phys. Chem. 1996, 100, 468

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(Top) Emission spectra of several sizes of CdSe-ZnS quantum dots, with excitation at 350 nm in all cases.

(Bottom) Idealized mixed-surface QD-protein conjugate.Antibodies labeled with biotin(生物素) bind efficiently to QD surfaces due to the great strength of their interaction with bridging avidin(抗生物素蛋白)molecules


3.1.2 Synthesis of HgS/CdS Core-Shell Nanostructures

HgScore CdSshell

HgCl2 + H2S + sodium polyphosphate → HgS

HgS + Cd(ClO4)2 + H2S → HgS/CdS

CdScore HgSshell

Cd(ClO4)2 + H2S + sodium polyphosphate → CdS

CdS + HgCl2 + H2S → CdS/HgS

J. Phys. Chem. 1993, 97, 5333

Note: Due to the much lower solubility of HgS compared with CdSparticles result in an exchange of Cd2+ by Hg2+

(CdS)n + mHgCl2 → (CdS)n-m(HgS)m + mCdCl2


CdS/HgS Mixed Colloids

HgS nanoparticles HgS coated with CdS

J. Phys. Chem. 1993, 97, 5333


Absorption Spectra of Core-Shell CdSon HgS Nanoparticles

A = HgS

J. Phys. Chem. 1993, 97, 5333

More CdS


Fluorescence Spectra of Core-Shell CdSon HgS Nanoparticles

HgS nanoparticles do not fluoresce

CdS coated HgS nanoparticles fluoresce:

Possibly due to removal of traps for nonradiativerecombinationsorFluorescence could arise from band to band recombination in HgS core

J. Phys. Chem. 1993, 97, 5333

More CdS


CdS/HgS Mixed Colloids

J. Phys. Chem. 1993, 97, 5333

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3.2 Metal core


3.2.1 Oxide on Metals

• Main reason is for nanoparticle stabilization• Could also be used to assemble nanoparticles• Examples:

– Chem. Mater. 1998, 10, 1214– J. Am. Chem. Soc. 1999, 121, 8518– Adv. Mater. 1999, 11, 34– Adv. Mater. 1998, 10, 132– Chem. Commun. 1998, 351– Adv. Mater. 1999, 11, 131– J. Am. Chem. Soc. 1999, 121, 10642– Nano Lett. 2002, 2, 3


Formation of a thin silica shell on citrate-stabilized gold particles (SiO2@Au)

Langmuir 1996, 12, 4329


15 nm SiO2@Au

The silica shell keeps on growing, but eventually small silica particles also nucleate out of the solution.

18 hours after addition of active silica

42 h after addition 5 days after addition

Langmuir 1996, 12, 4329


SiO2@Au Nanoparticles

Transmission electron micrographs of silica-coated gold particles produced during the extensive growth of the silica shell around 15 nm Au particles with TES in 4:1 ethanol/water mixtures. The shell thickness are (a, top left) 10 nm, (b, top right) 23 nm, (c, bottom left) 58 nm, and (d, bottom right) 83 nm

Langmuir 1996, 12, 4329


Influence of thin silica shells on the UV-visible spectra of aqueous gold colloids

Experimental Calculated

Langmuir 1996, 12, 4329

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Effect of Solvent Refractive Index

The solvent refractive indices (left to right) are 1.45, 1.42, 1.39, and 1.36

Langmuir 1996, 12, 4329Nano-Chemistry, LZU, 2005

Silica Coating of Silver Colloids

Langmuir 1998, 14, 3740

AgClO4 + sodium citrate 10 nm Ag nanoparticles NaBH4

Ag nanoparticles

+ 3-aminopropyltrimethoxysilane + sodium silicate Ag@SiO2

Silicate ion concentration0.02 % 0.01 % 0.005 %


3.2.2 Polymer-Coated Silver Nanoparticles

J. Am. Chem. Soc. 1999, 121, 10642

TEM images of silver particles: (A) uncoated particle, (B) polystyrene/methacrylatecoated particles, (C) polystyrene/methacrylate coated particles with a covalently bound BSA layer, and (D) the same as panel C after exposure to gold colloids. Negative staining by phosphotungstic acid used for all images


Preparation of Polymer-Coated Functionalized Silver Nanoparticles

Extinction spectra of silver particles: (A) uncoated particles and (B) polystyrene coated particles. Solid line: suspension in water. Dotted line: suspension in water, after 1 h in 1.8 M NaCl

J. Am. Chem. Soc. 1999, 121, 10642


Assembly of Nanoparticle arrays

Procedures for (A) Procedures for (A) PpyPpy--linked Au Colloidslinked Au Colloids

AlkyldithiolateAlkyldithiolate--Linked Linked Au ColloidsAu Colloids

Chem. Mater. 1998, 10, 1214

Ppy = poly(pyrrole)


Au Colloids Linked by PPy

Transmission electron microscope images of 1D and near-1D arrays of Au colloids linked by Ppy

Chem. Mater. 1998, 10, 1214

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3.2.3 Metal – Metal Core-Shell Nanostructures

Examples reported in the literature

Au/Ag : J. Chem. Phys. 1964, 41, 3357-3363Au/Cd : Ber. Bunsenges. Phys. Chem. 1994, 98, 180-189Au/Pb : Ber. Bunsenges. Phys. Chem. 1994, 98, 180-189Au/Sn : J. Phys. Chem. 1994, 98, 6931-6935Au/Tl : Ber. Bunsenges. Phys. Chem. 1994, 98, 180-189Ag/Pb : Ber. Bunsenges. Phys. Chem. 1992, 96, 754-759Ag/Cd : J. Phys. Chem. 1994, 98, 6931-6935Ag/In : Ber. Bunsenges. Phys. Chem. 1992, 96, 2411-2414Au/Pt : J. Phys. Chem. B. 2000, 104, 2201-2203


Pt/Au Core-Shell Nanoparticles

PtCl42- + sodium polyacrylate Pt nanoparticles (~12 nm)

Pt nanoparticles + K2Au(CN)2 Pt/Au

H2

γ-raysMeOH

Synthesis of Ptcore Aushell Nanoparticles

Synthesis of Aucore Ptshell Nanoparticles

J. Phys. Chem. B. 2000, 104, 2201-2203

NaAuCl4 + sodium citrate Au nanoparticles (~ 20 nm)

PtCl42- + Au nanoparticles Au/Pt

H2

High T


PtcoreAushell Nanoparticles

Pt nanoparticles 1:1 Pt/Au nanoparticles 1:2 Pt/Au nanoparticles

J. Phys. Chem. B. 2000, 104, 2201-2203


Pt/Au Core-Shell Nanoparticles

Absorption spectra of Pt nanoparticles before and after deposition of various amounts of gold. Overall Pt concentration is 1 x 10-4 M Concentration of Pt:Au is given on the curves

Absorption spectra of Au nanoparticles before and after deposition of various amounts of Pt. Overall Au concentration: 3 x 10-4 M Molar of Au:Pt is given on the curves

J. Phys. Chem. B. 2000, 104, 2201-2203


AucorePtshell Nanoparticles

Electron micrograph of Au core particles before (left) and after (right) deposition of Pt in the ratio 1:2

J. Phys. Chem. B. 2000, 104, 2201-2203Nano-Chemistry, LZU, 2005

3.3 Metal-Semiconductor Core-Shell Nanoparticles

Metals can be used as templates to make hollow semiconductor nanostructures

Fabrication of composite nanoparticles with a large electronic capacitance, i.e. a large difference in the Fermi level of the core relative to the conduction band edge of the shell will enable electrons to diffuse through the shell and be trapped in the core for a long time

Examples:Au/CdSe: J. Mater. Res. 1998, 13, 905-908Au/CdS: J. Phys. Chem. B. 1997, 101, 7675Ag/TiO2: Langmuir 2000, 16, 2731-2735Au/TiO2: J. Phys. Chem. B. 2000, 104, 10851TiO2/Ag: Langmuir 1999, 15, 7084-7087ZnO/Au: J. Phys. Chem. B. 2003, 107, 7479-7485

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3.3.1 CdS/Au Composite Nanoparticles

Au nanoparticles CdS/Au composite nanoparticles

J. Phys. Chem. B. 1997, 101, 7675Nano-Chemistry, LZU, 2005

Synthesis of CdS-Capped Au Nanoparticles

NaAuCl4 + sodium citrate Au nanoparticles (~ 20 nm)High T

MNA = 2-mercaptonicotinic acidJ. Phys. Chem. B. 1997, 101, 7675


Absorption Spectra of Au/CdSNanocomposites

Au

Au/MNA

CdS

Au/CdS

Absorption properties of Au/CdS are not the result of a simple addition of the spectra of two nanoclusters, but rather an influence of the CdS on the Au.

J. Phys. Chem. B. 1997, 101, 7675Nano-Chemistry, LZU, 2005

Emission Spectra of CdS/AuNanocomposites

J. Phys. Chem. B. 1997, 101, 7675

Emission quenching of CdS is indicative of the occurrence of electron transfer from excited CdSinto the Au core.

Conduction band energy for CdS = - 1.0 V vs. NHE

Fermi level of Au = + 0.5 V vs. NHE


结语

* 核－壳结构和组成千变万化，相应的材料化学内容极为丰富；

* 很多核－壳复合材料具有意想不到的功能，应用前景十分广阔；

* 核－壳复合材料的结构与性质或功能的关系还不是很清楚，相应的理论工作大有可为。


HomeworkWhen excited by light, what would happen to the two nanoparticles A and B ?

Core

Shell

coreshell core

shellEner

gy

BA

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Art of coreArt of core--shell nanostructuresshell nanostructures？？

Triangular and Fibonacci Number Patterns Driven by Stress on Core/Shell

Microstructures

Chaorong Li, Xiaona Zhang, Zexian Cao*Institute of Physics, Chinese Academy of Sciences,

SCIENCE 2005, 309, 909


Fibonacci number patterns on Ag core/SiOx shell structure

SCIENCE 2005, 309, 909


Fibonacci Number ??斐波纳契数斐波纳契数

The sequence begins with one. Each The sequence begins with one. Each subsequent number is the sum of the two subsequent number is the sum of the two preceding numbers.preceding numbers.Fib(n) = Fib(nFib(n) = Fib(n--1) + Fib(n1) + Fib(n--2)2)Thus the sequence begins as follows:Thus the sequence begins as follows:1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 1441, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144……..


The story of Fibonacci and rabbits…..

Long Long longlong agoago…………..–– Fibonacci applied his sequence to a problem Fibonacci applied his sequence to a problem

involving the breeding of rabbits.involving the breeding of rabbits.–– Given certain starting conditions, he mapped out the Given certain starting conditions, he mapped out the

family tree of a group of rabbits that initially started family tree of a group of rabbits that initially started with only two members.with only two members.

–– The number of rabbits at any given time was always a The number of rabbits at any given time was always a Fibonacci number.Fibonacci number.

–– Unfortunately, his application had little practical Unfortunately, his application had little practical bearing to nature, since incest and immortality was bearing to nature, since incest and immortality was required among the rabbits to complete his problem.required among the rabbits to complete his problem.


3 petals: lilies3 petals: lilies5 petals: buttercups, roses5 petals: buttercups, roses8 petals: delphinium8 petals: delphinium13 petals: marigolds13 petals: marigolds21 petals: black21 petals: black--eyed susanseyed susans34 petals: pyrethrum34 petals: pyrethrum55/89 petals: daisies55/89 petals: daisies


Leaves are also found Leaves are also found in groups of Fibonacci in groups of Fibonacci numbers.numbers.Branching plants Branching plants always branch off into always branch off into groups of Fibonacci groups of Fibonacci numbers.numbers.

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Nano-Chemistry, LZU, 2005 Nano-Chemistry, LZU, 2005

Fibonacci numbers Fibonacci numbers have geometric have geometric applications in nature applications in nature as well.as well.The most prominent The most prominent of these is the of these is the Fibonacci spiral.Fibonacci spiral.

Nano-Chemistry, LZU, 2005 Nano-Chemistry, LZU, 2005

CauliflowerCauliflower Pine ConePine Cone


Phi is defined as the limit Phi is defined as the limit of the ratio of a Fibonacci of the ratio of a Fibonacci number i and its number i and its predecessor, Fib(ipredecessor, Fib(i--1).1).Mathematically, this Mathematically, this number is equal to:number is equal to:

or approximately or approximately 1.618034. 1.618034.

251+


How about your How about your body?body?You have NO IDEA You have NO IDEA how many segments how many segments of the human body of the human body are related in size to are related in size to each other by each other by ΦΦ!!

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Remember how flowers have leaves and Remember how flowers have leaves and petals arranged in sets of Fibonacci petals arranged in sets of Fibonacci numbers?numbers?This ensures that there are This ensures that there are ΦΦ leaves and leaves and petals per turn of the stem, which allows petals per turn of the stem, which allows for maximum exposure to sunlight, rain, for maximum exposure to sunlight, rain, and insects.and insects.


That is all for today!That is all for today!

Any questions?Any questions?

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1.21 Energy Band diagram of Semiconductor Quantum Well ...blog.sciencenet.cn/upload/blog/file/2010/8/2010823144550174861.pdf · lattice mismatches, dangling bonds, or adsorbates at