Luận án tiến sĩ: Synthesis and characterization of functional nanostructures

The second chapter describes the synthesis and structural, elemental, and magnetic characterization of bartum-doped lanthanum manganite nanocubes.. Manganite is an attractive transition-

HARVARD UNIVERSITY

GRADUATE SCHOOL OF ARTS AND SCIENCES

DISSERTATION ACCEPTANCE CERTIFICATE The undersigned, appointed by the:

Department of Chemistry and Chemical Biology

have examined a thesis entitled:

Synthesis and Characterization of Functional Nanostructures

presented by:

Lian Ouyang

candidate for the degree of Doctor of Philosophy and hereby

certify that it is worthy of acceptance

Typed name Professor Xiaowei Zhuang, Chemistry

Date, 22 August 2006

Synthesis and Characterization of Functional Nanostructures

A thesis presented

by Lian Ouyang

to

The Department of Chemistry and Chemical Biology

in partial fulfillment of the requirements

for the degree of Doctor of Philosophy

in the subject of Chemistry

Harvard University Cambridge, Massachusetts

Submitted in August 2006

UMI Number: 3245177

Copyright 2006 by Ouyang, Lian

All rights reserved

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©2006 Lian Ouyang All rights reserved.

This thesis is dedicated to

my husband and my parents

ill

Synthesis and Characterization of Functional Nanostructures

Harvard University August 2006

Abstract

Nanostructured materials or nanomaterials are generally considered to possess at least one dimension on the nanometer scale The physical properties of nanomaterials often differ significantly from those of bulk materials, largely due to quantum size effects and surface effects Because of these unique properties and their potential applications, there

is a great deal of interest in developing methods for controlling the composition and morphology of nanostructures This thesis describes approaches to the synthesis and characterization of several functional materials in a nanostructured form, including metal oxides, metal silicides, and compound semiconductors These materials were chosen because they exhibit unique and useful electronic, magnetic, and optical properties

The first chapter provides a brief introduction to nanotechnology, nanomaterials, and the organization of this thesis The second chapter describes the synthesis and

structural, elemental, and magnetic characterization of bartum-doped lanthanum

manganite nanocubes Manganite is an attractive transition-metal-oxide system because there is an incredible variety in their electronic and magnetic properties that may be tuned

by chemical doping We developed a hydrothermal synthesis of manganite nanocubes

1V

whose chemical doping was adjusted by changing the relative abundance of chemical precursors and the solution pH levels

The third chapter presents a vapor-phase synthesis of iron monosilicide nanowires and the characterization of the product The cubic B20-type intermetallic compound ¢-

FeSi is a narrow band-gap semiconductor and a Kondo insulator The synthesis yields

straight and branched single-crystalline FeSi nanowires Magnetic and electrical

measurements were performed on the nanowire ensembles and on individual nanowires

and the results were discussed

The following two chapters focus on semiconductor nanorods and axial nanorod heterostructures Specifically, the fourth chapter explores a solution—liquid—solid method for the synthesis of cadmium chalcogenide nanorods with built-in heterojunctions

Structural, electrical and optical characterization of the heterostructures consistently demonstrated the success of this synthetic strategy The final chapter presents an

electroluminescence study on individual colloidal CdSe nanorods devices with a

transistor geometry The electrical and optical properties of the devices were measured simultaneously at low temperature The devices displayed Coulomb blockade behavior at

low bias voltages At higher bias voltages, we observed a superlinear increase in current

with bias, accompanied by electroluminescence with a broad spectral distribution These

measurements enabled us to explore the interplay between charge transport and light emission in a single-nanocrystal transistor and propose a light emission mechanism

Acknowledgement

If I can live up to the average lifespan—eighty years, I only have 16 five-years I consider myself extremely fortunate to have spent one sixteenth of my life at the graduate school of Harvard University The last five years has been the most rewarding and unimaginable period of my life In the process of ‘surviving’ graduate school, trying- failing-trying different projects, meeting all kinds of incredible people in and out of Harvard, I have had a chance to know myself better, to know this country better, and hopefully to know science better I have to thank many people for their support during

this unforgettable period of my life, without whom the research presented in this thesis

could not have been accomplished

I must first thank my advisor, Professor Hongkun Park, for his shepherding and

guidance throughout my five years as a group member I will be always indebted to him because I would have been separated from my husband for the last five years without his kind help I have learned from him not only the scientific thinking but also the effective

communication of thoughts, honesty and courage of self-criticizing

I would like to thank Professor Lieber and Professor Zhuang for serving on my Graduate Advising Committee and giving valuable suggestions to this thesis

I am deeply appreciative to my husband, Nan, who has always been supportive

and giving I am grateful for his indulgence and trust in me I am blissful to have him as

my closest friend, a “mentor”, a “cheer leader’, a “chauffeur”, and most importantly a

loving and caring husband

Dr Jeff Urban has been an integral part of my research and non-scientific

experience at Harvard He never hesitated in giving me advice and help whenever I

vi

asked him He taught me how to put septa onto flasks in the glove box (believe me, it is absolutely a critical training for someone who has no previous experience in using the glove box); how to clean schlenk line; how to seal quartz tubes; how to differentiate

‘tower’ and ‘towel’; how to learn new things—wun poco per dia

I want to thank Junqiao, Qian and Kristin who have always been around to answer

my questions no matter how trivial they are I owe a lot of thanks to all the current and former group members who have made my life in the group a memorable experience

I also want to thank Dr Chenyan Wen, Mr Xi Wang and Dr David Bell for their

selfless help with the high-resolution TEM I couldn’t get those beautiful TEM images of

the nanostructures I made without them

I have to thank many of my classmates: Yi-wen, Ked, Daina, Amethyst, Chen, Carl, Yue, Isaac, Vijay, Jake, Ping, Gengfeng and Xiaolin (If I forget any names, please believe me that I appreciate their friendship as much.) Besides having fun with them, I also learned quite different yet invaluable lessons from them outside classroom and laboratory

Finally, my greatest thanks go to my parents for their understanding and

encouragement through the way Their ineffable influence contributes a totally different

yet absolutely essential part of this thesis work They have been always worried about what I was worried about and excited about what I was excited about I don’t know how

to truly thank them

Vil

Table of Contents

Chapter 1: InfroducfÏOI 00G 0 5505.508008 68 5086 s.ssssseesse

1.1 NANOTECHNOLOGY AND NANOMATTERIALS ĩc S11 E2 1S St ivseirerrrsrrrsee 2 1.1.1 Terminology and history 0ƒ nanofechHỌOBV cà shthieiisirerrrrerereo 2 1.1.2 Nanomaterials and ways t†o make th€HN àcccctnTt Tnhh ng the 4 1.2 cá) sa a 6 1.3 QVERVIEW OF THE THESIS LÁ Ă HT TH HH nà TH Hệ 10

2.2.3 Synthesis of LBMO nanocubes with controllable doping -«-«c+« 28 2.2.4 Characterization oƒ mangqnite nahOCH€ LH hy 32 2.3 HYDROTHERMAL SYNTHESIS OF LA¡.xSRxMNOs MICROCUBES -. 41 2.4 CONCLUSION AND FUTURE DIRECTIONS nh HH Hư HH Hy 42

3.2 VAPOR-PHASE SYNTHESIS OF 8-FESI NANOWIRES Ung ng ng Hs nen ng vêy 52

3.2.1 Experimental 86 nốố.ốẻốẦốẦốỐẦỐẦỐẦồẦồẦốấ a 32

SN C2218 2 nốốố-ee 53 3.3 CHARACTERIZATION OF FESI NANOWIRES Án HH HH ng Hà HT Thy 55 3.3.1 Structural and compositional Characterization ccccccccccccccccsesscstscesscssssssessseseees 35 3.3.2 Magnetic characterization 0ƒ FeSĩ HGHOWÌF§ à Tnhh niệu 59 3.3.3 Transport measurement of individual FeSi Hañ1OWÌY€S cà cccecvvccsse2 61

3.4 CONCLUSION - HH HH HH TH TH TT TT HH HH Tà TH TT TH TH TH 64

3.5 REFERENCES Ăn Hàn TH HH TH HH Ti TT 66

Chapter 4: Synthesis and Characterization of CdE (E = S, Se) Nanorod

Heterostructures sessesse sessssse — ÔÔÔˆ

4.2 SYNTHESIS OF CDE NANOROD HETEROSTRUCTURES nhe, 73 4.2.1 Synthetic SÍYGÍCĐV nh HH HH TH TT HT HT ng HT HH 73 4.2.2 ExperimeH{Ï S€CÍÍOH Ăn TH TH KT HH Hit 74 4.3 CHARACTERIZATION OF CDE NANOROD HETEROSTRUCTURES s5c c5: 77 4.4 DISCUSSION ON THE EXPERIMENTAL RESULTS -.- 5S 33 t3 ve rriserree 83

4.6 REFERENCES TH TH KT KH TK kh cu 9 kg 6698190 11 87

Chapter 5: Electroluminescence from a Single CdSe Nanorod

Transisfor ` ¬- ¬ ssssesencesenseseesscsesssesons 91

5.1 MOTIVATIONS OF RESEARCH ON INDIVIDUAL NANOCRYSTALS - 5555 +cs+ <5 92

5.2 EXPERIMENTAL SECTION - càng TH TH Hà nh TH HH TH ni ĐH Hit 93

3.2.1 Synthesis oƒ CdSe HaHOYOđÌS SH HH Hi Hư nai 93

5.2.2 2.1.0 nan ốốốốỐe< 94 5.2.3 Low-temperature electrolumin€esC€HC€ Il€ASUFGIHGHÍ Ăn 94 3.2.4 Inelastic scattering IHOđ€Ï sàng HT ngư 100

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5.3 CONCLUSION

5.4 REFERENCES

Appendix 1: Experimental conditions assessed for work presented in Chapf€Y G0 00 9 50.6 00000 0000.0001 00000 0600004 0009.0804 0600006000 106 Appendix 2: Experimental conditions assessed for work presented in Chapf€T Ổ 0G 0 cọ cọ 0 T00 00940000900 0609080 0080080890 109

Appendix 3: Supporting information for work presented in Chapter 4

Chapter 1

Introduction

Nano means extremely small The word originated from Greek nanos, meaning dwarf Nowadays, it is used as a standard prefix in the list of physical units and stands for one billionth (10°) A size map of some representative small objects including ‘nano’-

crystals is illustrated in Figure 1 [1]

The term nanotechnology broadly refers to the science and technology on the nanometer (1 nm = 10° meter) scale It is an emerging field of research and development dedicated to increasing control over physical structures of nanoscale size (1 to 100 nm) in

at least one dimension [2, 3] This chapter serves as a brief introduction to certain aspects

of this field and provides a general overview for the research work presented in this dissertation The first section is a brief review of the history and recent development in nanotechnology as well as nanomaterials The following section explores the reasons why nanomaterials have been stunningly attractive over the past decade or so The

concluding section presents an overview of the following chapters of this thesis

1.1 Nanotechnology and nanomaterials

1.1.1 Terminology and history of nanotechnology

Nanotechnology should really be called “nanotechnologies” since there is no single field

of nanotechnology It is a “cluster of emerging techniques from solid-state technology,

biotechnology, chemical technology and scanning-probe technology that converge “top- down’ and ‘bottom-up’ to the nanoscale” [3] “Top-down” refers to the increasingly precise downsizing of macroscopic materials to nanometer scale while “bottom-up” refers to synthesis from individual molecules or atoms The latter is the core idea that guided the work presented in this dissertation

The term "nanotechnology" was defined by Professor Norio Taniguchi of Tokyo Science University in a 1974 paper [6] In the 1980s the basic idea of this definition was explored in much more depth by Dr Eric Drexler, who promoted the technological significance of nano-scale phenomena and devices through speeches and the books:

“Engines of Creation: The Coming Era of Nanotechnology” and ‘““Nanosystems:

Molecular Machinery, Manufacturing, and Computation” Nonetheless, long before the term was coined, physicist Richard Feynman mentioned some of the distinguishing concepts in nanotechnology in the lecture “There's Plenty of Room at the Bottom” at an

American Physical Society meeting in 1959 Feynman described a process by which the

ability to manipulate individual atoms and molecules might be developed, using one set

of precise tools to build and operate another proportionally smaller set, until down to the needed scale In the course of this, he noted, scaling issues would arise from the changing magnitude of various physical phenomena: gravity would become less important, surface tension and Van der Waals attraction would become more important, etc

It is noteworthy that, however, some of our ancestors unknowingly became

‘nanotechnologists’ [7] long before the Industrial Revolution Interestingly, these

medieval artisans created ‘nano’ gold spheres by mixing gold chloride into molten glass

to make red stained glass even though they had no idea about surface plasmon or

nanoparticles Figure 2 shows the stained-glass made by those ancient glass makers The current enthusiasm of nanotechnology is, of course, based on rational interests instead of serendipity, which I will address in more details in Section 1.2

The First Nanotechnolagists

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Figure 2 Stained-glass made from gold and silver nanoparticles Figure adapted from

source [7]: The New York Times; Images courtesy of the Stained Glass Museum, Britain

1.1.2 Nanomaterials and ways to make them

Nanomaterials are materials with a controlled morphology of at least one dimension on the nanometer scale The simplest example is particles with diameters between 1 and 100

nm (nanoparticles or 0-D nanostructures) such as Ceo [8], molecular clusters [9], Au or

Ag nanoparticles as shown in Figure 2 Nanotubes [10, 11], nanowires or nanorods [12] constitute the quasi 1-D nanomaterials while thin films [13] with thickness of a few atomic layers are considered as 2-D nanomaterials Besides individual nanostructures, collections of them in the form of arrays and superlattices [14-16] are of vital interest to the science and technology of nanomaterials

Since the focus of this thesis is the synthesis and characterization of functional nanomaterials, it is time to discuss the methods of making nanoscale entities As

mentioned earlier in the previous subsection, there are two general paradigms: “‘top- down” and “bottom-up” In a “top-down” approach one starts with bulk materials and sculpts downward to the nanoscale through machining, lithography, and etching methods This approach has been enormously successful especially in the microelectronic industry However, this approach will not lead to the manufacture of structures with infinitesimal dimensions due to technological and economical reasons For instance, the

photolithography process which defines the critical dimension of the electronic structures

is ultimately limited by the wavelength of the light On the other hand, in a “bottom-up” paradigm, one starts with molecules, atoms or nanostructures as building blocks and synthesizes more complicated hierarchies through various chemical or physical routes, such as solution chemistry, vapor transport, molecular beam epitaxy (MBE), etc It is possible in the future to build “virtually any kind of device or functional system, ranging from ultrasensitive medical sensors to nanocomputers” [17] The successful

implementation of the “bottom-up” model requires, in the end, the controlled growth of

nanomaterials, which serves as the motivation for many researchers including me in the field of nanomaterial synthesis

1.2 Why “nano”

There are many reasons why nanomaterials triggered intense interests among scientists

and technologists [2, 3, 7, 9, 17, 18] In this section, some of the basic interests within the “nano” field are listed

First, the structure and properties of nanomaterials differ significantly from those

of atoms and molecules as well as those of bulk materials largely due to quantum

mechanical effects at small sizes and surface effects Quantum mechanics are the basis

of the properties of atoms and molecules, but are largely hidden behind the classical behaviors in macroscopic materials Since nanomaterials have at least one spatial

dimension commensurate with the characteristic size of the specific materials (exciton Bohr radius for semiconductors [19, 20]), they have demonstrated quantum mechanical effects and remarkable electronic properties [2, 18, 21] Figure 3 shows the appearance

of new features in the electronic structures of metal and semiconductor nanocrystals in comparison to those of isolated atoms and bulk materials Surface effects are also responsible to some of the marvelous phenomena of nanomaterials The massive surface area-to-volume ratio of nanoparticles, which is proportional to the reciprocal of the nanoparticle diameter, means that a large portion of atoms are on the surface For example, 60% of the atoms in a 2 nm CdSe nanoparticle are surface atoms [22] It is not difficult to imagine that the physical properties will be dominated by the surface effects The reduced coordination number of surface atoms compared to bulk atoms makes

surface atoms less constrained in their thermal motion and thus leads to such interesting

observations as size-dependent melting [23, 24]and phase transitions [25, 26]

Bulk Nanocrystal isolated

atom

EFermi

Occupied Density of states

Figure 3 Density of states for metal (a) and semiconductor (b) nanocrystals In each

case, the density of states is discrete at the band edges The Fermi level is in the center of

a band in a metal, and so kgT may exceed the electronic energy level spacing even at room temperatures and small sizes In contrast, in semiconductors, the Fermi level lies between two bands, so that the relevant level spacing remains large even at small sizes The HOMO-LUMO gap increases in semiconductor nanocrystals of smaller sizes Figure taken from reference [21]

promising that useful insights into the formation and properties of bulk crystals will be

gained Developing a system of synthetic methods to make nanomaterials as precisely as

to make molecules and bulk materials is a wonderful challenge for chemistry

Third, self-assembled superlattices [27-30] of nanomaterials demonstrate the potential of nanocrystals as building blocks to make well-controlled structures with novel physical properties Since the physical properties of the superlattice structures depend not only on the identity of the component nanocrystals but also on the interaction and arrangement of them in the lattices, they can be controllably tuned with more flexibilities

in comparison to normal crystals based upon atoms or molecules by varying those

parameters More interestingly, assembling two different types of nanoparticles in a single material (binary nanocrystal superlattices with some examples shown in Figure 4) not only combines the properties of the building blocks but also provides model systems with which it is possible to explore the development of new collective properties that may emerge from nanoparticle interactions For example, the combinations of two different magnetic materials (one with high coercivity and the other with high magnetization) can produce higher energy density magnets [31], while semiconductor and magnetic

composites might be explored for magneto-optics Ordered films of multi-component mixtures could also be valuable as multifunctional catalysts or photocatalysts

Fourth, the emerging nanotechnology encompasses entirely new means of

investigating structures and properties besides exploiting the traditional microscopic, diffraction, and spectroscopic methods Computer-controlled scanning probe microcopy enables a real-time nanostructure manipulation [32, 33] Optical tweezers [34, 35]

provide another approach to holding and moving nanometer objects Nanotechnology

has also brought in such remarkable interdisciplinary research fields as nanotoxicology [36], DNA computing [37], drug delivery and targeting [38, 39], etc

Figure 4 (a, b) Self-assembled from LaF3 triangular nanoplates (9.0 nm side) and 5.0

nm Au nanoparticles (c) self-assembled from LaF3 triangular nanoplates and 6.2 nm PbSe nanocrystals The insets show (a) a magnified image, and (b, c) proposed unit cells

of the corresponding superlattices The structure shown in (a) forms on silicon oxide surfaces, while structures shown in (b) and (c) form preferentially on amorphous carbon substrates Figure taken from reference [29]

There are many other exciting reasons that one could list for the current frenzy in nanotechnology I would like to sum up this section with the following excerpt from an essay in Small: nanotechnology is like “a thread woven into many fields of science chemistry will play a role; whether this role is supporting or leading will depend in part

on how the field develops and what opportunities emerge, and in part on how imaginative and aggressive chemists and chemical engineers are, or become, in finding their place in

it.” [18]

1.3 Overview of the thesis

This dissertation focuses on the controlled synthesis and characterization of functional nanostructures, including transition metal oxide nanocubes (Chapter 2), transition metal silicide nanowires (Chapter 3), semiconductor nanorods (Chapter 4), and nanorod

heterostructures (Chapter 5)

Over the past few decades, scientists have demonstrated a truly incredible level of control over nanomaterials, such as semiconductor nanocrystals and nanowires [9, 12, 14- 16] However, only a small fraction of bulk materials are able to be prepared in a

nanostructured form Expanding the ability to prepare nanocrystalline forms of materials with the right stoichiometries or complex properties is one challenging issue The work

that I presented in Chapter 2 describes the synthesis and characterization of

nanostructures of one of the transition metal oxide systems: the mixed-valence

manganites [40] Manganite is an attractive material system because there is an

incredible variety in the electronic and magnetic properties of this system that may be tuned by chemical doping Moreover, the complex phase diagrams exhibited by these

materials are thought to derive from co-existence of nanoscale clusters which exhibit

distinct electronic and magnetic phases We wanted to prepare manganite nanocrystals whose dimensions were on the order of these nanoscale clusters to see if mixed-phase

state would still occur or not

The next chapter describes a vapor phase synthesis and characterization of iron monosilicide nanowires [41] Transition-metal silicides are significant to microelectronic industry largely because of their compatibility with conventional silicon fabrication and

10

the ohmic contact provided by silicides such as NiSi [42], CoSia, and T¡S1; In addition, there are many direct band-gap semiconductor silicides such as CrS1a, B-FeSi; Recently, Fe,Co;.,Si alloys were detected to be ferromagnetic semiconductors [43], which makes them promising for silicon-based spintronic [44] applications One of the parent

compounds is the cubic B20-type intermetallic compound ¢-FeSi, a narrow band-gap semiconductor and a Kondo insulator [45, 46] The chemical-vapor-deposition synthesis yields straight and branched single-crystalline FeSi nanowires with typical diameters ranging from 5 nm to 100 nm and lengths up to a few microns Magnetic measurements performed on nanowire ensembles show unusual magnetic properties consistent with those of the bulk material Electrical measurements of single nanowires from 2 K to room temperature are consistent with a narrow band-gap semiconductor model, with branch- induced doping playing an important role in low-temperature transport

Chapter 4 and 5 are focused on nanorods and nanorod heterostructures of

cadmium chalcogenides (CdE) Chapter 4 discusses the synthesis and characterization of CdE nanorod heterostructures The work presented in this chapter is currently

unpublished The synthetic strategy exploited in the CdE system is a combination of

traditional solution chemistry and catalyst-assisted growth-on-substrate technique

inspired by vapor—liquid—solid (VLS) method We have successfully synthesized two- segment and three-segment CdSe/CdS axial nanorod heterostructures with no observation

of alloying which has been a common problem encountered in gas-phase synthesis [47] Photoluminescence and preliminary electrical measurements have been performed on the individual nanorod heterostructures and will be discussed in this chapter Chapter 5 is based on the work presented in the publication [48] We were interested in studying the

11

interplay between charge transport and light emission in individual CdSe nanorods in the three-terminal transistor geometry CdSe nanorods with diameter less than 10 nm and length up to 100 nm were synthesized via a multiple-injection route [49] Electrical measurements of the single-nanocrystal transistor at low bias voltage and low

temperature show clear evidence of Coulomb blockade behavior Once the bias voltage exceeds the band gap of CdSe, devices with asymmetric tunnel barriers emit linearly polarized light

1.4 Reference

1 Whitesides, G M., The ‘'right' size in nanobiotechnology Nat Biotechnol 21, 1161-

1165 (2003)

2 The Chemsitry of Nanomaterials (eds Rao, C N R., Muller, A & Cheetham, A K.)

(WILEY-VCH Verlag GmbH & Co KgaA, Weinheim, 2004)

3 Nanotechnology: towards a molecular construction kit (ed Wolde, A t.) (Netherlands

Study Centre for Technology Trends (STT), 1998)

4 Black, C T., Murray, C B., Sandstrom, R L & Sun, S H., Spin-dependent

tunneling in self-assembled cobalt-nanocrystal superlattices Science 290, 1131-1134 (2000)

5 Love, J C., Gates, B D., Wolfe, D B., Paul, K E & Whitesides, G M.,

Fabrication and wetting properties of metallic half-shells with submicron diameters

Nano Lett, 2, 891-894 (2002)

6 Taniguchi, N On the basic concept of nanotechnolgoy Proc Int Congress Prod Eng

Part 2 (JSPE, Tokyo 1974)

1 Chang, K., Tiny Is Beautiful: Translating 'Nano' Into Practical New York Times

(February 22, 2005)

8 Park, H et al., Nano-mechanical oscillations in a single-C transistor Nature 407,

57-60 (2000)

12

Baibich, M N et al., Giant Magnetoresistance of (001)Fe/(001) Cr Magnetic

Superlattices Phys Rev Lett 61, 2472-2475 (1988)

Sun, S., Murray, C B., Weller, D., Folks, L & Moser, A., Monodisperse FePt nanoparticles and ferromagnetic FePt nanocrystal superlattices Science 287, 1989—

1992 (2000)

Gudiksen, M S., Lauhon, L J., Wang, J., Smith, D C & Lieber, C M., Growth of

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Properties of Semiconductor Microcrystallites (Zero-Dimensional Systems) and Some Quasi-2-Dimensional Systems Adv Phys 42, 173-266 (1993)

Buhro, W E & Colvin, V L., Semiconductor nanocrystals - Shape matters Nat Mater 2, 138-139 (2003)

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in the melting point of size-selected atomic clusters Nature 393, 238-240 (1998)

13

Spanier, J E et al., Ferroelectric phase transition in individual single-crystalline BaTiO; nanowires Nano Lett 6, 735-739 (2006)

Redl, F X., Cho, K S., Murray, C B & O'Brien, S., Three-dimensional binary

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968-971 (2003)

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AB(13) nanoparticle superlattices: An example of semiconductor-metal metamaterials J

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Zeng, H., Li, J., Liu, J P., Wang, Z L & Sun, S H., Exchange-coupled

nanocomposite magnets by nanoparticle self-assembly Nature 420, 395-398 (2002)

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17, 1821-1833 (2005)

Tseng, A A., Notargiacomo, A & Chen, T P., Nanofabrication by scanning probe microscope lithography: A review J Vac Sci Technol B 23, 877-894 (2005) Grier, D G., A revolution in optical manipulation Nature 424, 810-816 (2003) Ashkin, A., Optical trapping and manipulation of neutral particles using lasers Proc Natl Acad Sci U S A 94, 4853-4860 (1997)

Service, R F., Nanotoxicology: Nanotechnology grows up Science 304, 1732-1734 (2004)

Adleman, L M., Computing with DNA Sci.Am 279, 54-61 (1998)

Langer, R., Drug delivery and targeting Nature 392, 5-10 (1998)

14

Urban, J J., Ouyang, L., Jo, M.-H., Wang, D S & Park, H., Synthesis of Single-

Crystalline La} Ba,MnO; Nanocubes with Adjustable Doping Levels Nano Lett 4,

1547-1550 (2004)

Ouyang, L., Thrall, E S., Deshmukh, M M & Park, H., Vapor-Phase Synthesis and Characterization of €-FeSi Nanowires Adv Mater 18, 1437-1440 (2006)

Wu, Y., Xiang, J., Yang, C., Lu, W & Lieber, C M., Single-crystal metallic

nanowires and metal/semiconductor nanowire heterostructures Nature 430, 61-65

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Aeppli, G & Fisk, Z., Kondo Insulators Comments Condens Matter Phys 16, 155-165 (1992)

Fisk, Z et al., Kondo Insulators Physica B 224, 409-412 (1996)

Bell, D C et al., Imaging and analysis of nanowires Microsc Res Tech 64, 373-

389 (2004)

Gudiksen, M S., Maher, K N., Ouyang, L & Park, H., Electroluminescence from a

single-nanocrystal transistor Nano Lett 5, 2257-2261 (2005)

Peng, Z A & Peng, X G., Nearly monodisperse and shape-controlled CdSe

nanocrystals via alternative routes: Nucleation and growth J Am Chem Soc 124, 3343-3353 (2002)

15

Chapter 2

Synthesis and Characterization of Mixed-Valence Manganite Nanocubes

Transition metal oxides constitute one of the most fascinating classes of inorganic solids,

[1, 2] exhibiting a very wide variety of structures and interesting physical properties such

as ferromagnetism, ferroelectricity and high temperature superconductivity Recently, the transport properties of mixed-valence manganites have generated enormous interest in the scientific community with the first reports of colossal magnetoresistance (CMR) [3, 4] Ever since the discovery of CMR, great efforts have been exerted in this family of mixed valence manganites to examine the origin of the phenomenon and other interesting properties as well as the potential applications in memories and sensors

Nanocrystalline materials typically exhibit physical and chemical properties that are distinct from their bulk counterparts [5, 6] Previous investigations on thin-film manganite samples have indeed shown that their properties depend sensitively on the film thickness, leading to surface-induced phase separation[7-9] and strain-dependent metal— insulator transition (MIT) [10] However, not much work has been done on the

nanocrystals of manganites largely due to the lack of reliable methods to prepare well- isolated manganite nanocrystals with variable chemical doping To this end, this chapter

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discusses the use of a hydrothermal method to yield single crystal manganite nanocubes with control of three different chemical doping levels

2.1 Introduction to mixed-valence manganites

2.1.1 Structure of manganites

Since physical properties are closely related to the specific structures of any inorganic

solids, we need to understand the structure of manganites in order to understand their

appealing properties Manganites are a member of the perovskite family with the formula R).,A,MnQ3, where R denotes a rare earth ion (La, Pr, Nd, Sm, etc.) and A is a divalent alkaline earth ion (Ba, Sr, Ca, etc.) whose structure is shown in Figure 1a [11] MnŸ” (3¿, toe đ `) in LaMnQ; is a Jahn—Teller ion [12] (as shown in Figure 1b) LaMnO3 shows

antiferromagnetic ordering Ifa fraction (x) of the trivalent La** ions are substituted by divalent Sr**, Ca’* or Ba” ions, a fraction (x) of Mn ions become Mn** (3a”, tag°eg°) In

the case of La;.,Sr,MnQ3, the Jahn—Teller distortion vanishes and the system becomes

ferromagnetic with a Curie temperature (7c) around room temperature when x reaches 0.175 [13] Around 7c the material shows an extremely large magnetoresistive effect which has been called colossal magnetoresistance (CMR) The term ‘giant’ has been already taken In the following section, a brief summary on different types of

magnetoresistance effects is provided

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2.1.2 Magnetoresistance

Magnetoresistance (MR) is the change of a material’s resistance under the influence of a magnetic field H The magnetoresistance Ap/p is usually defined as [2(H) — p(0)]/ (0) The MR effects have been discovered in a variety of materials and various physical origins are responsible for them MR effects include ordinary MR, anisotropy MR, giant

MR and colossal MR

Ordinary Magnetoresistance (OMR)

When a magnetic field is applied to a metal or semiconductor perpendicular to an electric field, the Lorentz force affects the trajectories of the conduction electrons which

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results in a change of the transverse resistivity, Ap, (transverse magnetoresistance), A similar effect is also found when a magnetic field is applied parallel to the electric field, i.e a change of the longitudinal resistivity, Ap, (longitudinal magnetoresistance) This effect was first discovered in 1856 by Lord Kelvin Depending on the electronic orbital structures at the Fermi surface in a given material, a variety of features in the resistivity

vs magnetic field are observed The change in resistivity in a non-magnetic metal is usually positive [14] The MR effect is however quite small (< 1%) under a moderate magnetic field

Anisotropic Magnetoresistance (AMR)

The magnetoresistance of ferromagnets depends on the orientation of the magnetization with respect to the direction of the electric current in the material This effect is known

as anisotropic magnetoresistance Its origin is connected with the spin-orbit interaction

and the resulted anisotropic scattering of the conduction electrons The magnitude of AMR is typically a few percent in low fields (less than a few tens of Oe), e.g MR is ~

5% at room temperature for a NizoFe39 alloy

Giant Magnetoresistance

In the late 1980s, a large negative magnetoresistance of more than 50% was discovered at high magnetic field at low temperatures in Fe/Cr/Fe multilayers and dubbed the name giant magnetoresistance (GMR) [15] This effect is associated with samples with

magnetic Fe layers which were antiferromagnetically coupled It was found that the coupling between magnetic layers through a thin (a few nanometers) spacer layer

(nonmagnetic) oscillates in sign as the spacer thickness increases The origin of these

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oscillations is connected with the Ruderman—Kittel-—Kasuya— Yosida (RKKY) interaction (or itinerant exchange) between two localized spins separated by a distance The physical origin of GMR can be qualitatively understood in terms of the two-current model which separately considers the individual currents of electrons with spin parallel or antiparallel

to the magnetization When there is a different spin scattering rate in the two spin sub- channels, the total scattering rate depends upon the relative orientation of the

magnetization in the ferromagnetic layers, which can be tuned by the external magnetic field GMR has been used extensively in the read heads in modern hard drives and in non-volatile magnetic random access memory (MRAM)

Colossal Magnetoresistance

More recently, a much larger magnetoresistance effect was discovered in mixed-valence manganites with a perovskite structure In 1993 von Helmolt and his coworkers observed

a magnetoresistance of 60% at room temperature in Lao ¢7Bao.33MnO; thin films [4] In

the following year, Jin et al reported an MR effect in excess of a million percent (Figure

2) at 77K in a Lao67Cao33MnO; thin film [3] This effect was given the name colossal

magnetoresistance (CMR) Later, CMR was observed in other complex oxides such as layered perovskites [16], double perovskites Sr2FeMoO, [17] and pyrochlores Tl,Mn2O;

[18, 19]

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Figure 2 Resistivity of Lao 67Cao33MnOs3 thin film versus H curve after heating to 900

°C for 0.5 hour R (0) = 1.35 MQ, p (0) = 11.6 Q-em, R (6T) = 1.06 kQ, p (6T) = 9.1

mQ:cm, AR/R (H) ~ 127,000%, and AR/R (0) ~ 99.9% Figure adopted from reference [3]

The origin of the CMR effect is believed to arise from the close correlation

between the magnetic phase transition and the metal—-insulator transition near the Curie temperature (7c), which is partly connected with the phenomenon of double exchange [20, 21] Ina Mn’ ion, the crystal field splits the five d orbitals into a fy, triplet and an e,

doublet as shown in Figure 1b The on-site Coulomb repulsion is so strong that no d

orbitals can be occupied by two electrons and that all electrons spins on a given Mn are ferromagnetically aligned by a large Hunds-rule coupling [12] The three tr, electrons are tightly bound to the ion and form an electrically inert core spin of magnitude 3/2, while the e, electron can move through the crystal subject to the constraint that its spin must be parallel to the local core spin In other words, the amplitude for a carrier to hop from site i

to site 7 is modulated by the overlap between having its spin parallel to 7 core spin and having its spin parallel to 7 core spin This connection between the magnetic correlations

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and transport is called ‘double exchange’ [20] Because of the double exchange

interaction, the hopping probability of e, electrons between Mn sites is proportional to [cos (6/2)| where @ is the angle between the two Mn core spins The magnetic field aligns the core spins and therefore increases the conductivity, especially near Tc

The situation is more complicated because the electrons interact with phonons due

to the Jahn-Teller effect The strong electron-phonon coupling in these systems implies that the carriers are actually polarons above 7c, i.e electrons accompanied by a large lattice distortion These polarons are magnetic and self trapped in the lattice The

transition to the magnetic state can be regarded as an unbinding of the trapped polarons Since the CMR effect usually requires strong magnetic fields, typically in the range of several Tesla near 7c, the immediate technological application is limited so far

Nonetheless, the understanding and the potential applications of CMR offers tremendous opportunities for the development of new technologies such as read/write heads for high- capacity magnetic storage and spintronics

2.1.3 Rich phase diagrams of manganites

The CMR effect renewed enthusiasm for the perovskite manganites which served to uncover a wealth of truly remarkable chemical and physical properties inherent to these inorganic materials One of the fascinating properties of manganites is the strong

interplay of their charge, spin, orbital, and lattice degrees of freedom [12, 22] This

unique characteristic ultimately manifests itself in spectacularly rich phase diagrams, as shown in Figure 3 for La;.,Ca,MnO3 [12] and Figure 4 for La;.,Ba,MnO; [23] and Lai yot,MnO; [24]

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These phase diagrams of manganites shows that the their properties can vary over

a wide range by varying several parameters such as chemical doping level (x), pressure, temperature, or applied magnetic field However, the most powerful tuning parameter is probably the chemical doping level of the manganite By selecting a mixture of divalent

and trivalent cations of different sizes to sit in between the MnOg octahedra, it is possible

to both change the number of itinerant (i.e conduction) electrons and also alter Mn—O-

Mn bond angle and Mn—O bond distance [2, 25-27] As these structural parameters cause

significant local modifications in the strain and electronic structure (due to changes in

orbital overlap), it is expected that chemical changes may be utilized to adjust manganite properties

La1.„ Cay MnOa

Figure 3 Phase diagram of La).,Ca,MnO3 as a function of temperature and doping level

of Ca’* ions showing the magnetic and structural phase boundaries The various phases include charge-ordered (CO) [9, 25], antiferromagnet (AF), canted antiferromagnet (CAF), ferromagnetic metal (FM), ferromagnetic insulator (FI) The unlabelled region of the phase diagram is simply paramagnetic and insulating and exhibits no special magnetic

or charge ordering phenomena Figure adapted from reference [12]

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Figure 4 Phase diagrams of Laj.,Ba,MnO3 and Lay.,Sr,MnOs3 as a function of

temperature and doping levels of Ba’* and Sr** respectively (a) Phase diagram of Lay xBa,MnO; shows a variety of electronic structural and magnetic phases such as

ferromagnetic insulator (FI), ferromagnetic metal (FM), ferromagnetic multiphase (FMP) and paramagnetic insulator (PI) Figure adapted from reference [23] (b) Phase diagram

of Laj.,Sr,MnO3 shows magnetic and structural phase boundaries The crystal structures

(Jahn-Teller distorted orthorhombic: O’, orthorhombic O, orbital-ordered orthorhombic:

O”, rhombohedral: R, tetragonal: T, monoclinic: Mc, and hexagonal: H) are indicated as

well as the magnetic structures (paramagnetic: PM (green), short range order: SR, canted:

CA, A-type antiferromagnetic structure: AFM (yellow), ferromagnetic: FM (blue), phase separated: [28] PS, and AFM C-type structure) and the electronic state (insulating: J (dark), metallic: M4 (light)) Figure adapted from reference [24]

Another exciting feature of the La).,Ca,MnO; phase diagram may be noted by

looking at the x = 0.5 boundary Below 150 K, it is possible for two completely different electronic and magnetic phases (FM and AFD) to stably co-exist within the same crystal, which contradicts the expectation that a single-crystal of a compound exists in one unique thermodynamically stable phase This situation arises because of the competition

between two competing processes: double exchange and the Jahn-Teller effect [12, 28, 29] In many perovskite manganites, the energy scales associated with these two distinct interactions are comparable and as a result various types of ground states can have similar energies Consequently, slight local differences in parameters such as the chemical composition, temperature, strain, or magnetic field one may favor the existence of one

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phase over another Furthermore, several reports have provided evidence for the

coexistence of distinct magnetic, electronic, and crystallographic texture on length scales

of tens to hundreds of nanometers in La;.,Ca,MnO; crystals [7-9] Similar phase

coexistence (i.e phase separation, phase-separated state or mixed-phase state) was also

observed in Laj Sr,MnO3 systems recently [30, 31] Based upon these findings, one

might reasonably anticipate that the coexistence of multiple phases (within the same crystal) exhibiting vastly different physics might then be more noticeable in

nanocrystalline samples in which these inhomogeneities are unlikely to be “averaged” out Indeed, previous investigations on thin-film manganite samples have shown that their properties depend sensitively on film thickness, leading to surface-induced phase

separation [7-9] and strain-dependent MIT [10]

The exotic CMR effect and patterning of phase separation on sub-micrometer scales initially attracted us to the perovskite manganites We wondered if phase

separation would still occur as the size of the crystal approached the size of the phase- homogeneous clusters in bulk manganites We also wanted to explore if there would be any size-dependence to physical phenomena such as MIT and CMR, which is thought to originate from phase-separated clusters within a bulk crystal In order to investigate these

and other related issues, we worked to develop a method to synthesize nanocrystalline

manganites Although advances have been made in the preparation of nano- and

microcrystalline oxides including manganites [32-38], detailed physical investigations of nanocrystalline manganites have not been possible due to the lack of reliable methods to prepare well-isolated manganite nanocrystals with controllable chemical doping In this

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chapter, we describe the hydrothermal synthesis and characterization of nanocrystals of lanthanum barium manganite (LBMO) whose barium content is varied controllably

2.2 Hydrothermal synthesis of La,.,.Ba,MnO3 nanocubes

2.2.1 Methods of synthesizing bulk and thin-film manganites

Traditionally, the preparation of bulk manganite perovskites involved high temperature solid-state reactions [23, 24, 39] or sol-gel processes [40] Common drawbacks to these methods are the presence of secondary phases (due to oxygen contamination) and poor crystallinity (in the cases where low processing temperatures are used) Recently, mild

hydrothermal method was proposed for solid-state materials because of its relatively low

reaction temperature and autogenously produced high pressure In the thin-film

community where effectively two-dimensional samples of finely controlled thickness are desired, pulsed laser deposition (PLD), RF sputtering, and metal-organic chemical vapor deposition (MOCVD) are routinely employed [25] Although these techniques allow

access to films of excellent quality, the specific details of the substrate used for growth, the nature of the interfaces, and oxygen pressure used can substantially influence the

measured characteristics of the film [25]

2.2.2 Brief introduction to Hydrothermal methods

Hydrothermal method has been an open route to prepare zeolites and other microporous materials and to synthesize metastable phases [41, 42] Some attention has recently been

given to the synthesis of transition metal oxides by the hydrothermal method [43] The

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contemporary meaning of the word “hydrothermal” is not unanimously agreed upon, but

is generally used to refer to any heterogeneous chemical reaction carried out in the

presence of water above room temperature and at a pressure greater than 1 atm ina closed system [25, 44] It is a powerful technique thought to be responsible for a vast number of the natural mineral and ore deposits scattered about the earth

Water is an active participant in the hydrothermal reaction, and not just a solvent per se, as demonstrated in the well-studied case of quartz synthesis via hydrothermal methods Water (often accompanied by some alkali) under supercritical conditions forms soluble silicate complexes with the quartz crystal (starting material) which then

recrystallizes as perfect single quartz crystals at a specific location in the autoclave [25, 44] This action of water to solvate (or mineralize) otherwise insoluble inorganic

materials at high temperatures and pressures is fundamental to the hydrothermal process

These dissolved minerals are then recrystallized and recovered Because of the high

pressures involved and the unique dissolving potency of water under hydrothermal

conditions, hydrothermal reactions offer access to an entirely different set of reaction

conditions than is possible with other methods employed by solid-state chemists and physicists

Detailed reaction mechanisms in many hydrothermal reactions remain unsolved mysteries [44] despite the attention given to the hydrothermal method This is primarily due to the fact that the high temperatures, pressures, and sealed reaction vessels required

by these reactions leave careful examination of these same processes challenging [44] Even understanding the physical properties of water above its critical point is a rich and complex subject in its own right Above its critical point, water undergoes drastic

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changes in its density, dielectric constant, viscosity, and other related physical parameters which directly influence its solvating capabilities [44]

2.2.3 Synthesis of LBMO nanocubes with controllable doping

Chemicals and reaction instruments

Manganese(II) chloride tetrahydrate (MnCl.-4H20, 99.99%), potassium permanganate (KMn0O, , 99+%), lanthanum nitrate hexahydrate (La(NO3)36H20, 99.99%), barium hydroxide octahydrate (Ba(OH)2:8H20, 98+%), strontium nitrate (Sr(NO3)2, 99.995%), calcium nitrate tetrahydrate (Ca(NO3)2°4H20, 99%), and potassium hydroxide (KOH, 99.99%) were purchased from Aldrich The reaction vessels, the hydrothermal cells

(Moline IL) were purchased from Parr Instruments

Hydrothermal synthesis conditions

Based upon our previous experience with the synthesis of perovskite oxides, [34] we initially attempted to synthesize one of the end materials, LaMnOs, via decomposition of

a bimetallic alkoxide precursor containing both La and Mn ions Although several variations on this theme were explored (Appendix 1), difficulties in the preparation and stability of the manganese isopropoxide precursors proved insurmountable Next, the controlled oxidation of mixtures of Mn salts in the presence of La and Ba ions were

explored Although a variety of oxidants, temperatures, and other experimental

conditions were assessed (Appendix 1), this method proved ultimately futile and did not reliably lead to formation of the desired product At this time, a report of the

hydrothermal synthesis of Lap sBao.sMnO3 nanowires was published [36] and we

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