Nanomaterials and Analytics Semiconductor Nanocrystals and Carbon Nanotubes. - Introduction and Preparation - Characterisation - Applications

Transcription

1 Nanomaterials and Analytics Semiconductor Nanocrystals and Carbon Nanotubes - Introduction and Preparation - Characterisation - Applications

2 Simple molecules <1nm DNA proteins nm red blood cell ~5 μm (SEM) diatom 30 μm bacteria 1 μm m SOI transistor width 0.1μm semiconductor nanocrystal (CdSe) 5nm Nanometer memory element (Lieber) 10 1 bits/cm (1Tbit/cm ) control biological machines Circuit design Copper wiring width 0.μm IBM PowerPC 750 TM Microprocessor 7.56mm 8.799mm transistors

3 Low-dimensional semiconductor structures 3D: D: 1D: OD: Crystals Quantum Quantum Quantum wells (SLs) wires dots QD Substrate WL Substrate N(E) N(E) N(E) N(E) E E E E

4 Surface Area 8 N = 4096 n = 135 N = 4096 n = N = 4096 n = 3584 N = total atoms; n = surface atoms

5 Photoluminescence Spectroscopy CB E k CB ħω=e g E ex Exciton band ħω=e g- E ex VB VB A laser excites electrons from the valence band into the conduction band, creating electron-hole pairs. These electrons and holes recombine and emit a photon. We measure the number of emitted photons (intensity) as a function of energy.

6 Absorption and Emission

7 Looking for single dots Laser Beam Al Pad Apertures ZnSe capping layer (~50 nm) ZnSe buffer layer (~1 mm) QDs GaAs Substrate

8 From thousands to tens PL INTENSITY (arb. units) PL INTENSITY (arb.units) ev.7967 ev PHOTON ENERGY (ev) PHOTON ENERGY (ev)

9 Raman Experimental Set-up

10 Raman Spectroscopy Intensity / ctsmw -1 s -1 hω s =hω i +hω GaAs LO ZnSe LO Raman Shift / cm -1

11 Raman Spectroscopy 1000 laser lines hω s = hω ± hω hω s =hω i i +hω ph Information depth / nm Intensity / ctsmw -1 s -1 GaAs LO 1 1,5,0,5 3,0 3,5 Photon energy / ev buried layers ZnSe LO Raman Shift / cm -1 surface small focus intensity ω 4 high E g materials

12 Frequency Position and Lineshape Intensity / ctsmw -1 s -1 frequency shift by temperature cm -1 /100 C pressure 1cm -1 /1kbar Raman Shift / cm -1 lineshape: Intensity / ctsmw -1 s -1 asymmetric broadening and shift occurs as a result of lattice disturbance Raman Shift / cm -1

13 Determination of Surface Temperature 9 Using temperature induced shift of substrate phonon peak: Peak Position in / cm /- 0. cm -1 +/- 10 C cm -1 /100 C InSb:.1 InP:.0 GaAs: 1.8 Si:. ZnSe: Temperature / C

14 Resonance Enhancement Intensity / counts mw -1 s -1 LO ZnSe LO ZnS Raman Shift / cm LO ZnSe LO ZnSe +LO ZnS Temperature / C with increasing temperature the bandgap of ZnS 0.05 Se 0.95 approaches the photon energy of.66 ev typical gain of two orders of magnitude

15 Desorption of a Se Capping Layer Crystallisation during annealing Background due to roughness Temperature induced shift

16 Basic principles of Raman spectroscopy in crystals 1. Energy conservation: hω i = hω ± hω s hω hω k i i k s. Quasi-momentum (wavevector) conservation: ks q 0 ks q k k ± i = s q 0 q 4πn λ i ki ks q ki ki ki λ i ~ 5000 Å, a 0 ~ 5 Å q << π/a 0 only close to BZ center phonons are seen in the 1 st order Raman spectra of bulk crystals

17 General Properties of Phonons in Superlattices 400 Ω (cm-1) 300 4πn/ λ 1 4πn/ λ GaAs AlAs (GaAs) 8(AlAs) 8 GaAs AlAs 1 Reduction of symmetry: T d D d πm ( n a 1 + δ ) 0 0 Average dispersion q (units π/a 0) m = 1,, 3... d = (n 1 +n )a 0 3 q (units π/d) 3 Al Ga As Atomic displacements Optical phonons: confinement Acoustic phonons: folding

18 Structural characterisation: HRTEM Ge quantum dots: Dot base size: ~15 nm, Height: ~ 1,5 nm 100Å Period of structure is 10

19 Structural characterisation of samples: STM Ge quantum dots: Dot base size: ~15 nm, Height: ~ 1,5 nm Height, nm Density of the dots: ~ cm - Dot uniformity: ~ 0% Distance, nm

20 Typical Raman Spectrum of Ge Dot Superlattice Ge-Ge Si z(xx)-z geometry Raman intensity/ a.u. m=±1 m=± m=±3 Ge-Si local Si-Si Ge-Ge phonons, Ge/Si phonons, folded acoustic phonons m=± Raman shift/ cm -1

21 Influence of Strain, Confinement, Intermixing Raman intensity/ a.u. Ge-Ge Ge-Si Raman shift/ cm -1 Strain-induced shift Δω = ω 1 1 [ pε + q( + )] = 17cm zz εxx ε yy ω=304 cm -1, ε xx =ε yy =0.04, ε zz =C 1 /C 11. ε xx frequency shift due to atomic intermixing Confinement-induced shift

22 Ge-Ge Optical Phonons: Information about Strain Raman intensity, a.u. Ideal strained Bulk Ge 6/100 14/15 14/5 14/45 11/100 14/ Raman shift/ cm -1 z(xy)-z geometry wetting layer (no QDs): strained structure; confinement influence QDs separated by thin Si layers ( 5Å): a partial strain relaxation (strain is.8%) QDs separated by thick Si layers ( 45Å): strained QDs ( 4%)

23 Ge-Si Optical Phonons: Information about Atomic Intermixing Raman intensity, a.u. 6/100 14/15 14/5 14/45 11/100 14/15 z(xy)-z geometry wetting layer (no QDs): almost abrupt interface QDs separated by thin Ge layer ( 5Å): atomic intermixing x 0.09 QDs separated by thick Ge layer ( 45Å): atomic intermixing x 0.04 Model of atomic arrangement: Raman shift/ cm -1 Si Si Ge x Si 1-x Ge 1-x Si x Ge Ge Abrupt interface

24 Confinement of Optical Phonons Ge 1 Si ω 1 Ge Ge/Si structure with the Ge thickness of 5ML 3 q m = π m L π/l π/a Wavenumber, cm -1

25 Folded acoustical phonons ω q s Si Dispersion of folded acoustic phonons (S.M.Rytov, Akoust. Zh., 71 (1956)) Ge ωd 1 ωd k + 1 ωd 1 ωd cos( qd) = cos( )cos( ) sin( )sin( ) υ υ k υ υ 1 k = υ ρ 1 1 υ ρ d=d 1 +d ; d 1 and d, ρ 1 and ρ, υ 1 and 1 υ are the thickness, density and sound velocity in Ge and Si layers 0 π/d π/a Wave vector, cm -1

26 Folded Acoustical Phonons in Ge Dot SLs wavevector, π/d Raman intensity/a.u. 0,0 0, 0,4 0,6 0,8 1,0 14/00 11/00 x10 x wavenumber/cm -1 wavenumber/cm -1 Nominal and calculated structure periods: 14 Å 38 Å 11 Å 9 Å Elastic continuum theory is applicable for periodical structures with QDs!!!

27 Resonance Profile of Ge Optical Phonons Raman intensity/ a.u. 1,8,0,,4,6 Energy/ ev Ge phonon frequency/ cm -1 maximum of Raman intensity corresponds to resonance with E 1 exciton in Ge layers decreasing frequency position of Ge phonons manifests sizeconfinement in Ge quantum dots

28 Comparison of Strained and Relaxed Ge QDs Raman intensity/ a.u Energy/ ev Ge phonon frequency/ cm -1 Si Ge size distribution strain in QDs of particular size QD shape Si strained relaxed QDs SiO x

29 II-VI (CdS 1 x Se x, CdSe 1 x Te x, Cd 1 x Zn x S) nanocrystals in glass matrices are obtained in borosilicate (also in germanate, molibdenate, fluorophosphate) glasses by a two-step heat treatment process: 1) annealing at about 1000 C melting (co-melting); dissolution of existing nanocrystals (if any); ) rapid quenching; 3) annealing at C diffusion-limited growth Diffusion-limited growth 3 stages 1) nucleation ) normal growth 3) coalescence = Ostwald ripening = competitive growth

30 .8 nm.7 nm.5 nm.3 nm 7.5 nm 4.8 nm 3.1 nm.3 nm Average radii can be derived from the absorption spectra using effective-mass approximation A.Efros and Al.Efros (1979)

31 Raman-based determination of the glass-embedded CdS 1 x Se x nanocrystal composition Resonant Raman scattering conditions are required to record the Raman spectra of diluted quantum dots since their amount in the total scattering volume is very small (usually below 1 %) LO 1 LO LO 1 CdSe-like phonon LO CdS-like phonon

32 One-mode and two-mode behaviour of phonon spectra in mixed crystals

33 33 one-mode Cd 1-x Zn x S CdS ZnS

34 two-mode CdS 1-x Se x ±0.03

35 Specific factors affecting Raman spectra of glass-embedded II-VI nanocrystals Glass matrix pressure due to the difference of thermal expansion coefficients of the nanocrystals and the matrix Phonon confinement due to the Raman selection rules relaxation in nanocrystals Frequency increase Band asymmetry, frequency decrease Surface phonons due to higher surface-to-volume ratio in nanocrystals Compositional and size dispersion of quantum dots within the ensemble Band asymmetry, frequency decrease Linewidth increase, position fluctuation

36 Glass matrix pressure Nanocrystals grown in a glass matrix are under additional pressure due to the difference of thermal expansion coefficients of the nanocrystals and the matrix Pressure 0.5 GPa phonon frequency increase by 4 cm 1 Δa = a ν i ν i NC B 3 1 γ 1

37 I( ω) Phonon confinement r C(0, q) r ( q) = exp( q L /16π ω Δω sin ( q / 4) j j0 j I.H. Campbell, P.M. Fauchet, Sol. St. Comm. 58, 739 (1986) ω = j A j r dqc( 0, q) r 3 [ ] ω ω q ( ) j ( ) + Γ0 / ) Band asymmetry, frequency decrease r =. nm r = 4.8 nm Raman shift / cm -1

38 Surface phonons due to higher surface-to-volume ratio in nanocrystals Band asymmetry, frequency decrease 4 ω + ω ω TO1 ω ω TO TO ω xε K TO 1 + ω xε K LO1 1 ω ω TO1 ( ) ( ) ω ω + ω ω TO1 LO1 TO1 (1 x) ε + K (1 x) ε K ω LO ω ω TO TO LO = 0 TO + where K = ( l+ 1) ε + xε + ( 1 x) ε m 1 A.Roy, A.K.Sood, Phys. Rev. B 53, 117 (1996)

39 Surface phonons in glass-embedded CdS 1-x Se x nanocrystals LO with confinement taken into account surface phonon contribution

40 Variation of composition of glass-embedded CdS 1 x Se x nanocrystals under heat treatment Increase of thermal treatment duration τ or temperature T a Increase of the nanocrystal size In some cases -- a noticeable red shift of the absorption edge in other cases -- almost no shift Possible reason -- variation of the nanocrystal composition with thermal treatment parameters

41 Selenium-rich CdS 1 x Se x nanocrystals size increases τ and / or Т a increases Se content increases

42 Sulphur-rich CdS 1 x Se x nanocrystals size increases τ and / or Т a increases S content increases

43 CdS 1 x Se x nanocrystals with nearly equal S and Se content size increases τ and / or Т a increases S / Se ratio remains constant

44 Cd 1 y Zn y S 1 x Se x x=0.38 τ and / or Т a increases Cd Zn LO 1 increases LO increases CdS 1-x Se x, ZnS 1-x Se x Cd 1-y Zn y S, Cd 1-y Zn y Se two-mode one-mode Cd 1-y Zn y S 1-x Se x two-mode