Introduction to Nanotechnology

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9.5. SINGLE-ELECTRON TUNNELING 245 showing that the exciton has a radius comparable to the dimensions of a typical nanostructure. The exciton radius can be taken as an index of the extent of confinement experienced by a nanoparticle. Two limiting regions of confinement can be identified on the basis of the ratio of the dimension d of the nanoparticle to the exciton radius aeff, namely, the weak-confinement regime with d > aeff (but not d >> aeff) and the strong-confinement regime d < aeff. The more extended limit d >> aeff corresponds to no confinement. Under weak-confinement conditions the exciton can undergo unrestricted translational motion, just as in the bulk material, but for strong confinement this translation motion becomes restricted. There is an increase in the spatial overlap of the electron and hole wavefunctions with decreasing particle size, and this has the effect of enhancing the electron-hole interaction. As a result, the energy splitting becomes greater between the radiative and nonradiative exciton states. An optical index of the confinement is the blue shift (shift to higher energies) of the optical absorption edge and the exciton energy with decreasing nanoparticle size. Another result of the confinement is the appearance at room temperature of excitonic features in the absorption spectra that are observed only at low temperatures in the bulk material. Further details on exciton spectra are provided in Sections 2.3.3 and 8.2.1. 9.5. SINGLE-ELECTRON TUNNELING We have been discussing quantum dots, wires, and wells in isolation, such as the ones depicted in Figs. 9.1-9.3. To make them useful, they need coupling to their surroundings, to each other, or to electrodes that can add or subtract electrons from them. Figure 9.16 shows an isolated quantum dot or island coupled through tunneling to two leads, a source lead that supplies electrons, and a drain lead that vsdQ Source Quantum Drain Lead Dot Lead Figure 9.16. Quantum dot coupled to an external circuit through source and a drain leads.
246 QUANTUM WELLS, WIRES, AND DOTS removes electrons for use in the external circuit. The applied voltage V,, causes direct current I to flow, with electrons tunneling into and out of the quantum dot. In accordance with Ohm’s law V = IR, the current flow I through the circuit of Fig. 9.16 equals the applied source-drain voltage V,, divided by the resistance R, and the main contribution to the value of R arises from the process of electron tunneling from source to quantum dot, and from quantum dot to drain. Figure 9.17 shows the addition to the circuit of a capacitor-coupled gate terminal. The applied gate voltage Vg provides a controlling electrode or gate that regulates the resistance R of the active region of the quantum dot, and consequently regulates the current flow I between the source and drain terminals. This device, as described, hnctions as a voltage-controlled or field-effect-controlled transistor, commonly referred to as an FET. For large or macroscopic dimensions the current flow is continuous, and the discreteness of the individual electrons passing through the device manifests itself by the presence of current fluctuations or shot noise. Our present interest is in the passage of electrons, one by one, through nanostructures based on circuitry of the type sketched in Fig. 9.17. For an FET-type nanostructure the dimensions of the quantum dot are in the low nanometer range, and the attached electrodes can have cross sections comparable in size. For disk and spherical shaped dots of radius r the capacitance is given by C = 8~~ (t) r disk (9.1 I) vsd Source Lead C = 4m0 (:) r sphere Quantum Dot 0 Drain Lead (==.- (9.12) Figure 9.17. Quantum dot coupled through source and a drain leads to an external circuit containing an applied bias voltage V,,, with an additional capacitor-coupled terminal through which the gate voltage Vg controls the resistance of the electrically active region.
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INTRODUCTION TO NANOTECHNOLOGY Char
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CONTENTS Preface xi 1 Introduction
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5.2 Carbon Molecules 103 5.2.1 Natu
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9.4 Excitons 244 9.5 Single-Electro
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PREFACE In recent years nanotechnol
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INTRODUCTION The prefix nano in the
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INTRODUCTION 3 he recognized the ex
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1000 (I) a 900 4 800 a 900 I 6 700
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INTRODUCTION 7 developed before the
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2.1. STRUCTURE 9 mechanics, the res
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X T 2.1. STRUCTURE 11 Figure 2.4. C
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2.1. STRUCTURE 13 Figure 2.6. Thirt
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Sodium Nanoparticle Na, Magic Numbe
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2.1. STRUCTURE 17 of the large anio
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2.1. STRUCTURE 19 other, and high-f
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2.2. ENERGY BANDS 21 Conduction Ban
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2.2. ENERGY BANDS 23 Figure 2.13. S
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2.2. ENERGYBANDS 25 band at point T
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A X ' Wavevector A Si c z 2.2. ENER
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2.2. ENERGY BANDS 29 bands of Figs.
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2.3. LOCALIZED PARTICLES 31 add ele
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1 meV 10 meV 100 meV FJ E W 1 eV 10
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3 METHODS OF MEASURING PROPERTIES 3
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3.2. STRUCTURE 37 Table 3.1. Crysta
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3.2. STRUCTURE 39 Figure 3.2. Two-d
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3.2. STRUCTURE 41 The widths of the
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44 METHODS OF MEASURING PROPERTIES
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3.3. MICROSCOPY 51 types of transit
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3.3. MICROSCOPY 53 Actual diverging
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Tunneling current Tunneling current
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3.3. MrCAOSCOPY 57 and the latter m
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3.4. SPECTROSCOPY 59 From Eq. (3.8)
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1 Average Panicle FWHM Size (nm) in
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CdSe colloidal NCs a = 1.2 nrn 3.4.
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El X-RAY TUBE 8000 - A 6000 - 4000
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2 N I w 3.0 2.5 2.0 1.5 1 .o 0.5 3.
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i 3.4. SPECTROSCOPY 69 NMR involves
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T=220K - 2G H FURTHER READING 71 Fi
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NUMBER OF ATOMS RADIUS (nm) 1 10 1
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I L 7 3 5 7 9 11 13 15 17 NUMBER OF
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JELLIUM MODEL OF CLUSTERS ATOMS CLU
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1.02 { 1.01 - 1 - 0.99 - 4.2. METAL
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4.2. METAL NANOCLUSTERS 81 Figure 4
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4.2. METAL NANOCLUSTERS 83 Figure 4
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-0 100 200 300 400 500 600 MASSICHA
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a 42. METAL NANOCLUSTERS 87 Figure
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. . . .. . - . D 111111111111111111
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3 4 2.5 0 cn g 2 0 z d 1.5 t a 9 1
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4.3. SEMICONDUCTING NANOPARTICLES 9
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4.4. RARE GAS AND MOLECULAR CLUSTER
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4.5. METHODS OF SYNTHESIS 97 d Figu
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4.5. METHODS OF SYNTHESIS 99 isopro
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PULSED LASER BEAM 1 1 1 1 1 I ROTAT
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5.1. INTRODUCTION CARBON NANOSTRUCT
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5.2. CARBON MOLECULES 105 methane d
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5.3. CARBON CLUSTERS 107 -0 20 40 6
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PHOTON ENERGY (electron volts) 5.3.
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Figure 5.6. Structure of the CEO fu
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1 30 0 14.1 14.2 14.3 14.4 14.5 LAT
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(c) 5.4. CARBON NANOTUBES 115 Figur
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5.4. CARBON NANOTUBES 1 17 The mech
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5.4. CARBON NANOTUBES 11 9 investig
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5.4. CARBON NANOTUBES 121 energy gr
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5.4. CARBON NANOTUBES 123 stretch c
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5.5. APPLICATIONS OF CARBON NANOTUB
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- v) 450 : 400 3 350 1 300 : 5 250
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BULK NANOSTRUCTURED MATERIALS In th
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h 8 0 6.1. SOLID DISORDERED NANOSTR
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6.1. SOLID DISORDERED NANOSTRUCTURE
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6.2. NANOSTRUCTURED CRYSTALS 153 qu
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6.2. NANOSTRUCTURED CRYSTALS 155 Fi
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158 BULK NANOSTRUCTURED MATERIALS 6
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160 BULK NANOSTRUCTURED MATERIALS w
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162 BULK NANOSTRUCTURED MATERIALS 0
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164 BULK NANOSTRUCTURED MATERIALS n
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166 NANOSTRUCTURED FERROMAGNETISM f
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168 NANOSTRUCTURED FERROMAGNETISM i
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170 NANOSTRUCTURED FERROMAGNETISM M
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172 NANOSTRUCTURED FERROMAGNETISM h
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174 NANOSTRUCTURED FERROMAGNETISM d
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176 NANOSTRUCTURED FERROMAGNETISM o
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4 h $ 3 7.5. NANOCARBON FERROMAGNET
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7.6. GIANT AND COLOSSAL MAGNETORESI
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1.4 N 1 1.2 z " 1 0 v 5 h 0.8 0 0.6
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7.6. GIANT AND COLOSSAL MAGNETORESI
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7.7. FERROFLUIDS 187 Figure 7.22. M
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7.7. FECIROFLUIDS 189 Figure 7.25.
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7.7. FERRQFLUlOS 191 FerroRuids can
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FURTHER READING FURTHER READING 193
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Page 228 and 229: Excitatior I [eV]: 2.175 2.214 2.25
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Page 236 and 237: FURTHER READING 225 P. Milani and C
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Page 270 and 271: 10.1. SELF-ASSEMBLY 259 factor of 2
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Page 292 and 293: I1 ORGANIC COMPOUNDS AND POLYMERS 1
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11.5. SUPRAMOLECULAR STRUCTURES 295
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M Et3P OTf M=Pd M=Pt M=Pd, 41 % M=P
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11.5. SVPRAMOLECUUAR STRUCTURES 2s
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11 5. SUPRAMOLECULAR STRUCTURES 303
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BiodegradaWe surface 4 Functional g
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FURTHER READING 309 F. J. Owens and
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12.2. BIOLOGICAL BUILDING BLOCKS 31
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314 BIOLOGICAL MATERIALS which we w
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316 BIOLOGICAL MATERIALS Table 12.2
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318 BIOLOGICAL MATERtALS i J / Flgu
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320 BIOLOGICAL MATERIALS Pyrimidine
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322 BlOLMjlCAL MATERiALS (a) DNA do
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324 BIOLOGICAL MATERIALS so each wo
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326 BIOLOGICAL MATERIALS 12.4.2. Mi
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328 BIOLOGICAL MATERIALS tions they
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330 BIOLOGICAL MATERIALS hardened f
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13 NANOMACHINES AND NANODEVICES In
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334 NANOMACHINES AND NANODEVICES CA
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336 NANOMACHINES AND NANODEVICES Op
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338 NANOMACHINES AND NANODEVICES ti
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340 NANOMACHINES AN0 NANODEVICES PO
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342 NANOMACHINES AND NANODEVICES X
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344 NANOMACHINES AND NANODEVICES na
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346 NANOMACHINES AND NANODEVICES 31
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348 NANOMACHINES AND NANODEVICES -
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350 NANOMACHINES AND NANODEVICES re
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352 NANOMACHINES AND NANODEVICES 1
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354 NANOMACHINES AND NANODEVICES -1
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A.l. INTRODUCTION APPENDIX A FORMUL
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A.3. PARTIAL CONFINEMENT 359 limiti
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APPENDIX B TABULATIONS OF SEMICONDU
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TABULATIONS OF SEMICONDUCTING MATER
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Table B.8. Effective masses m+ rela
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372 INDEX amoeba, 3 16 amphiphilic,
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374 INDEX CdS, 9, 130, 213, 216, 36
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376 INDEX dispersion, 277 disulfide
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378 INDEX Ga (gallium) (continued)
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380 INDEX length (continued) critic
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382 INDEX multiple ionization, 93 r
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384 INDEX pi-conjugation, 282, 292
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silica, 269 silica-alumina, 269, 27
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388 INDEX wavefimction (continued)
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Introduction to Nanotechnology

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