Nanocharacterization SPM

1.

Quantum confinementQuantum size effectEnergy bands and electronic transitionCharge quantizationThermodynamic properties

Nanostructures

Comparison: Microstructure vs. Nanostructure Microstructure Nanostructure

Physics Semi-classical Quantum mechanical Electron Particlelike Wave-like nature E or k-space Continuous Discrete Current Continuous Quantized Decision Deterministic Probabilistic Fabrication Micro-fabrication Nano-fabrication Surface area/ Small Very large volume Packing Low Very high

What is new phenomena ?Length Scale: 10-9~10-7 M (1~100 nm)Electrical, optical, thermal, magnetic, chemical, mechanical, and biological properties ?? Quantum size confinement Wave-like transport Dominant interfacial phenomena

Electron Nature in Smaller SizesEnergy quantization d ~ Fermi wave length of electron in a metal (lF) or exciton diameter in a semionductor Charge quantization Charging energy (Ec) >> Thermal energy (kT) Ballistic d

Exciton : e-h pair bounded by attractive electrostatic interactionEEgConductionbandValence bandExciton levelsBinding energy: Eex =me4/2eh2n2Bohr (exciton) radius: r = n2eh2/me2; 1/m=1/me +1/mh Si Ge GaAs CdSe KClEex (meV) 14.7 3.8-4.1 4.2 15 400r (nm) 4.3 11.5 12.4H atom-like state EEg0Exciton bindingenergy: EexEg-Eex

n=1n =2

Quantum ConfinementExciton radiusRR

Density of State: # of states per unit energy rangeN =2pn2/L2N =8pn3/3L3dN /dE = constdN /dE ~ E 1/2k=2pn/LE = hk2/2mk =(2mE)-1/2/hN = 2xn/L= k/p = (1/ph)(2mE) 1/2dN /dE = ((2m)1/2/2ph)(E)-1/2 dN /dE ~ E-1/2N = 2n/LDOSDOSDOS1D 2D 3D

Size Effect: Energy Levels and DOSA.P. Alivisatos, Science 271, 933 (1996)3d 2d 1d 0dEnergyDOSEFBulk Nano atom particleSize controlled band gap tuningDiscrete Energy levelsCB

VB

SemiconductorLUMO

HOMOBand gap

Size Effect:1D-Quantum well statesF.J. Himpsel et al, Adv. Phys. 47, 511 (1998)

Size Effect: Optical SpectraA.P.Alivisatos, J. Phys. Chem. 100, 13227 (1996)Shift to higher energy in smaller sizeDiscrete structure of spectraIncreased absorption intensity

Size effect: Tunable Band GapOptical excitation is significantly enhanced, both, in frequency and intensity, in smaller sizes.S. Ogut et al, Phys. Rev. Lett. 79, 1770 (1997)Bulk Si = 1.14 eV GaAs =1.5 eV

Energy Bands

Energy Band Structure: Energy vs. kY = CnfnV = Cn V n(h2/2m)2Y + V Y = E Y Ej= a +2b cos 2pj/N index j = 0, 1, 2 Define a new index k = 2pj/Na: wave vector E(k) = a +2b coska, Yk = eiknafn : Bloch wave function (symmetry adapted LCAO) p/a k=0 p/aE= 2 p/k = 2a

= ....a -2ba +2ba a012

Electronic Transitionp/a k=0 p/aE fiffmifDirect transition (Dk=0)In phase Added transition dipoleElectronically allowed transition fiffmifIndirect transition (Dk 0)Out of phaseCancelled transition dipoleElectronically forbidden but vibronically allowedElectric Transition dipole moment mif = Band width: overap of wave functionsSlope dE/hdk = hk/m = vg: group velocity of electron

Absorption spectra: Direct and Indirect TransitionElectronic absorption spectra for three sizes of CdSe nanocrystals, in the wurtzite (direct) and rock salt (indirect) structures. In each instance the direct gap spectrum is structured and intense, while the indirect gap one is featureless and relatively weaker. The relative absorption efficiencies do not change, despite the concentration of oscillator strength due to quantum confinement.

Fermi Golden rulemif = < ff,vib | fi,vib >Transition rateW = (2p/h 2) | mif |2d(Ef -Ei hv) r(Efk) r(Eik)dE

Frank-Condon factorPhase factor orenvelope functionElectric dipoleTransition momentmif : Transition dipole momentr(Efk); r(Eik): density of states of final and initial states

Size Effect: Enhanced AbsorptionkEEN(E)For quantum dot,Energy levels: discreteDOS: delta function DxDp ~ h x: well defined p=hk: Not well-defined k is not an exact quantum number for QDEnvelope functions sample larger k-spaceOverlap of wave functions - Increased absorption intensityM.S. Hybersten, Phys. Rev. Lett. 72, 1514 (1994)

Photon absorption: Direct vs. Indirect TransitionSelection rule k = k (Dk = 0) k = k + q (Dk 0)Energy relationship hv = Eg hv = Eg + hv(q)Interaction electronic: two body vibronic: three bodyTransition rate fast ~ 10 -7 sec slow ~ 10-2 secRadiative efficiency high lowExample GaAs (Eg (dir.) =1.4 eV) Si (Eg (ind.) = 1.1 eV) (Eg (dir.) = 3.37 eV)EkEgqhvphonon

Size Effect: Radiative Transition RateRadiative transition rate (1/t) increases with smaller sizes.For the size >2nm, phonon assisted transitions dominate, while zero-phonon transitions allowed for smaller sizes1.5 nm Si dots: quasi direct transition

Approaches for Light Emission from SiQuantum confinement

Zone folding: quasi-direct gap

Er doped Si

Direct gap formation by alloying

Property:Polarized PL from Nanowire InPPolarization ratio: r= (I- I)/(I+ I) = 0.96Large dielectric contrast between nanowires and surroundingPolarization sensitive Nanoscale photodetectorJ. Wang et al, Science 293, 1455 (2001)

Charge QuantizationCharging energy: Ec = e2/2C >> kT At T =300K kT = 26 meV C

Tunneling Spectroscopy of InAs QDEc=0.11 eV: single electron charging energyEg=1.02 eV: nanocrystal band gapd = 32AT=4.2KU. Banin et al, Nature, 400, 926 (2000)S-like

P-likeSTMOptical

Property: Crystallization of Opals from Polydisperse NanoparticlesOhara et al, Phys. Rev. Lett. 75, 3466 (1995)Driving force for size selective crystallization and phase separation is the size dependence of the dispersional interactions.

Dispersional interactions are strong enough to dominate over entropic effects, but weak enough to allow the nanocrystals to anneal into low energy, equilibrium configuaration.

Property: Melting Temperature of NanocrystalA.P.Alivisatos, J. Phys. Chem. 100, 13227 (1996)

Property: Thermodynamic Behaviors of Metal Clusters Y.J. Lee et al, J. Comp. Chem 21, 380 (2000), Phys. Rev. Lett. 86, 999 (2001) As the cluster size decreases, the melting temperature (Tm) monotonically decreases, However, when the cluster size is small enough, Tm does not vary monotonically with cluster size. The absence of a premelting peak in heat capacity curves for some clusers. Premelting: surface melting, partial melting, orientational melting, and isomerization

2. SPM References: 1. Nanotechnology Research Directions: IWGN report (1999): http://itri.loyola.edu/nano/IWGN.Research.Directions/2. Scanning Tunneling Microscopy and related methods edited by R.J. Behm, N. Garcia, and H. Rohrer (1990)3. R.J. Hamers, Scanning Probe Microscopy in Chemistry, J. Phys. Chem.100, 13013 (1996)4. G.S. McCarty and P.S.Weiss, Scanning Probe Studies of Single Nanostructures, Chem. Rev. 99, 1983 (1999)5. PSIA, www.psia.co.kr/appnotes/apps.htm6. ThermoMicroscope, www.thermomicro.com/spmguide/contents. 7. Digital Instrument, www.di.com/app_notes/full_appnotes.htm8. H.-Y. Nie, publish.uwo.ca/~hnie/mmo/all.html

SPM STM/STS AFM SPM

Scientific Issues in Nanoscience Fundamental properties of isolated nanostructures Fundamental properties of ensemble of isolated nanostructures Assemblies of nanoscale building blocks Evaluation of concepts for devices and systems Nanomanufacturing Connecting nanoscience and biology Molecular electronics(Nanotech.Res. Directions: IWGN report,1999)Local probes with nm spatial resolution for characterization

Tools for Characterizatione, hv, ione, hv, ion Structural analysis: SEM, TEM, XRD, SAM, SPM, PEEM, LEEM, STXM, SXPEM Chemical analysis: AES, XPS, TPD, SIMS, EDX, SPM Electronic, optical analysis: UV/VIS, UPS, SPM Magnetic analysis: SQUID, SMOKE, SEMPA, SPM Vibrational analysis: IR, HREELS, Raman, SPM Local physico-chemical probe: SPMtip

1 102 104 106 106104102 1 Lateral scale (A)Vertical scaleTEMSEMSTMOM(A)

OM SEM/TEM SPM

Operation air,liquid vacuum air,liquid,UHV

Depth of field small large medium

Lateral resolution 1 mm 1-5nm:SEM 2-10nm: AFM 0.1nm:TEM 0.1nm: STMVertical resolution N/A N/A 0.1nm: AFM 0.01nm: STMMagnification 1X-2x103X 10X-106X 5x102X- 108X

Sample not completely un-chargeable surface height transparent vacuum

HM (High Resolution Optical Microscope) diffraction limit ~ 150 nm PCM (Phase Contrast Microscope) subwave length X-ray Microscope l ~1A; Synchrotron radiation and X-ray optics SEM (Scanning Electron Microscopy) Focused e-beam size > 3nm TEM (Transmission Electron Microscopy) ~1 A; LEEM (Low Electron Emission Microscopy) low energy e-beam; 20nm PEEM (Photoemission electron Microscopy) lateral resolution~100nm; chemical analysis FIM (Field Ion Microscopy) tip

Interaction of Electrons with SampleWhen 20 KV e-beam is used for Ni(Z=28). Auger electron :10-30A Secondary electron: 100A Backscattering electron : 1-2 mm X-ray ~ 5 mm Penetrated electron: 5 mm Atomic NumberAccelerating voltageIncidence angle

InteractionVolume

Principle: Scanning Electron Microscopy (SEM)http://super.gsnu.ac.kr/lecture/microscopy/em.htmlLensDetectorsSampleE-gunSecondary electronBeam size: a few 30 A Beam Voltage: 20-40kVResolution: 10-100 AMagnification: 20x-650,000xImaging radiations: Secondary electrons, backscattering electronsTopographic contrast: Inclination effect, shadowing, edge contrast, Composition contrast: backscattering yield ~ bulk compositionDetections: - Secondary electrons: topography - Backsactering electrons: atomic # and topography - X-ray fluorescence: composition

Instrumentation: SEM

Principle: Transmission Electron Microscopy (TEM)Beam size: a few 30 A Beam Voltage: 40kV- 1MVResolution: 1-2AImaging radiations: transmitted electrons,Imaging contrast: Scattering effectMagnification: 60x-15,000,000xImage Contrast: 1) Amplitude (scattering) contrast - transmitted beam only (bright field image) - diffraction beam only (dark field image) 2) Phase (interference) contrast - combination of transmitted and diffraction beam - multi-beam lattice image: atomic resolution (HRTEM)ScreenThin sampleE-gun

Condenserlens Objectivelens

Projectorlens

Example: TEM images of Co-nanoparticles60 nmCo nanoparticles = 7.8 1 nm

AABB Self-assembled2D-monolayer Self-assembled3D-multilayerSurface ModificationFrom J.I. Park and J. Cheon

Interactions used for Imaging in SPM(d) CapacitanceC(d) ~ 1/d (e) Thermal gradient (f) Ion flow f(d) ?Tunneling I~exp(kd)

(b) Forces F(d) ~ various

(c) Optical near fields E ~1/d4Resolution limitsThe property probedThe probe size

Scanning Probe Microcopy Scanning Tunneling Microscopy(STM): topography, local DOS Atomic Force Microscopy (AFM): topography, force measurement Lateral Force Microscopy (LFM): friction Magnetic Force Microscopy (MFM): magnetism Electrostatic Force Microscopy (EFM): charge distribution Nearfield Scanning Optical Microscopy (NSOM): optical properties Scanning Capacitance Microscopy (SCM): dielectric constant, doping Scanning Thermal Microscopy (SThM): temperature Ballistic Electron Emission Microscopy (BEEM): interface structure Spin-polarized STM (SP-STM): spin structure Scanning Electro-chemical Microscopy (SECM): electrochmistry Scanning Tunneling Potentiometry (SPM): potential surface Photon Emission STM (PESTM): chemical identification

Scanning Tunneling MicroscopeScanning Modes:1. Constant current2. Constant heightReal space imaging with atomic resolutionCurrent Feedback Computeramplifier controllerSample biasvoltagePiezo tube scannerCoarse positioningdeviceSampleTipX,Y,Z

Theory of STMIt ~ tip2 sample2 e-2d ~ sample2 d(E-Ef) for low voltage limit; a point tiptipsampleEfEfConstant current STM image corresponds to a surface of constant state density.d- +eVbiasJ. Tersoff and D.R. Hamann, Phys. Rev. lett. 50, 1988 (1983)Figure From J. Wintterlin

Tunneling Spectroscopy It ~ r tip (E) r sample (E-eV)T(E,V) dE (Z)IV- x,y : Topography di/dV~ rsample (E): LDOS d2i/dV2: local vibrational spectra (di/dV)/(I/V) vs VI vs VJ. A. Stroscio et al. Phys. Rev. Lett. 57, 2579 (1986)SiNiSi(111)-2x1TheoryAd: fixed

Applications of STM Surface geometry Molecular structure Local electronic structure Local spin structure Single molecular vibration Electronic transport Nano-fabrication Atom manipulation Nano-chemical reaction

Atom-resolved Surface StructureVarious Reconstructions of Ge(100)-2x1Buckled 2x1 p(2x2) c(4x2) J.Y. Maeng et al (2001)

Si(100) and Ge(100) 2x1 surfaceClean Surface Structure

Lattice ConstantDimer LengthTilt Angle - *Buckled-SymmetricSi(100)5.43 2.32 700.66 eV25 meVGe(100)5.658 2.41 1401.11 eV60 meV

Molecular Orbitals Occupied state (HOMO)tipsampleEfd - + Efd+ -Ef

EfJ.J.Boland, Adv. Phys. 42, 129(1993)Unoccupied state (LUMO)

STM Topograph of Quantum DotGe pyramid containing ~2000 Ge atoms on Si(100)Ge dome grown by PVDon a 600 C Si(100)R.S. Williams et al, Acc. Chem. Res. 32, 425 (1999)

STM Images of Ni Clusters at Different Sample Bias Voltages

STS and STM of Self-assembled Co-nanoparticlesTip on self-assembled monolayer

Tip on a Co nanoparticle I vs V

dI/dV vs VPileni et al, Appl. Surf. Sci. 162/163, 519 (2000)

Tunneling Spectroscopy of InAs QDEc=0.11 eV: single electron charging energyEg=1.02 eV: nanocrystal band gapd = 32AT=4.2KU. Banin et al, Nature, 400, 926 (2000)S-like

P-likeSTMOptical

Electronic Properties of Carbon NanotubesConstant LDOSMetallic SWNT

SemiconductingSWNT

Single Molecule Vibrational SpectroscopyVibration excitation of the molecule occurs when tunneling electrons have enough energy to excite a quantized vibrational level Inelastic tunneling channelB.C. Stipe. et. al., Science 280, 1732 (1998)

Real Space Imaging of Two-Dimensional Antiferromagnetism On the Atomic ScaleWeisendanger et al, Science 288, 1805 (2000)Nonmagnetic W tipMagnetic W tip

Atomic Manipulation by STM

Circular corral radius= 71.3 A 48 Fe atoms M.F. Crommie, C.P. Lutz, D.M. Eigler. Science 262, 218-220 (1993). Quantum-mechanical interference patterns

Iron on Copper (111):

Molecular Adsorption: 2+2 cycloaddition reaction

R.J. Hamers et al, J. Phys. Chem., 101, 1489 (1997)Fabrication of molecularly ordered organic filmsAnisotropic optical and electrical propertiesA Possible way to orient molecular devices

In-situ Monitoring of Self-assemblyWolkow et al, Nature, 406, 48 (2000)Self-directed growth of Nanostructures

Single-Bond Formation by STM H.J. Lee and W. Ho, Science, 286, 1719(1999)

IWGN: Interagency working group on nanoscience , engineering, technology: piezo tube scnner, tip, feedback control systemVoltage between tip and sample, tunneling current is measured, feed back controller compare with set current, Vary the voltage applied to PZT scanner to maintain the set currentFor meta surface, LDOS at a surface follows the corrugation of the surface atomsFor semiconductors, the suface state tend to be localized on some atoms or bondsDOS features seen inI/V curves are obscured by the fact that It depend on separation and aplied volatge.This dependence can be partially removed by plotting the ration of differential to total conductivity dI/dV(I/V) dI/dV(I/V)The normalized conductivity =surface DOS at least for a metal

STM image: Spatial location of the molecular orbitals, rather than, geometrical position of atoms-sample bais image: occupied orbitals+biais image: unoccupied orbitalsSpectroscopic information can be obtained by recording multiple images at different voltagesOr spectra can be acquired at one selected point by holding the the probe still and sweeping the bias voltage

Fig 1 a : double barrier junction configurationFig 1b shows Tunneling conductance spectrum and a series of single electron tunneling peaksDT: hexane dithiolChrging energy(addtion spectrum) and discrete energy level spacings( excitation spectrum) of the QDS-like feature: doubletP-like feature: 6 peaks; p orbital can take 6 electrons