Imaging the Electronic and Vibronic States of Single Semiconductor NanowiresSingle Semiconductor Nanowires

Leigh M. SmithDept. of Physics, University of Cincinnati

Univ. of CincinnatiH d E J k

Ohio UniversityAl d O G

Australian National Univ.Ch ti J di hHoward E. Jackson

Lyubov TitovaThang B. HoangAhutosh Mishra

Alexander O. Govorov

Miami UniversityJan Yarrison-Rice

Chennupati JagadishHannah JoyceH. TanY. Kim

Financially supported by University of Cincinnati, the Nano-Biotechnology

TMS-Orlando, 2007

Financially supported by University of Cincinnati, the Nano Biotechnology Initiative at Ohio University and the Australian Research Council.

Semiconductor Nanowires as Photodetectors

TMS-Orlando, 2007

LED and Laser Nanowires

TMS-Orlando, 2007

Nanowires as Single Electron Transistors

TMS-Orlando, 2007

Nanowires as single photon emitters

TMS-Orlando, 2007

Nanowires as Biosensors

TMS-Orlando, 2007

Core-Shell Nanowire GrowthPre-growth Core: GaAs

GaAsH3

Shell: AlGaAs

G A

Au

Ga3 core GaAsAsH3

450oC, 30min

GaAs

600oC, 10 min

450 C, 30min

reactantshell AlGaAs

Desorb surface contaminants and form eutectic alloy

650oC, 15 min

Wire diameter is determined by Au

t l t d h ll

TMS-Orlando, 2007

and form eutectic alloy.

Vapor-Liquid-Solid growth

catalyst and shell growth time

MotivationNanowire diameters D (~50-150 nm) > Bohr exciton’s diameter (~24 nm)

NW ┴~ bulk exciton Exciton ║

Dielectric “confinement” of EM dipole field (D<<λ):

Exciton density Photoluminescence intensities

N║ = N┴ I║ >> I┴

τ║ << τ┴We are interested in exciton spin dynamics

TMS-Orlando, 2007

We are interested in exciton spin dynamics of single nanowires

Single nanowire studies

Field-Emission Scanning gElectron Microscope (FESEM) image

Nanowires were removed from theAFM

Nanowires were removed from the growth substrate into solution and deposited onto a silicon substrate

a single nanowire:

~80nm in diameter, ~5-8 μm long

i ’ di t > B h it di t

10μm

TMS-Orlando, 2007

wire’s diameter > Bohr exciton diameter

=> expect no quantum confinement effects

Single nanowire studies

Field-Emission Scanning Electron Microscope (FESEM) image:

i h t d hnanowires have tapered shape.

Nanowires were removed from theNanowires were removed from the growth substrate into solution and deposited onto a silicon substrate

A single nanowire:

~80nm in diameter, ~5-8 μm long

C di t > B h it di t (24 )

TMS-Orlando, 2007

Core diameter > Bohr exciton diameter (24nm)

=> no quantum confinement effects

Photoluminescence ImagingSpectrometer

L - LensBS Beam Splitter

1.2 μm spatial

CCDL

BS - Beam Splitter

Tunable l

resolution

BS

2D CCD image

L

Y

λlaser

Tunable Ti:Saphire

i i

spat

ial

sample

emission energy

(a.u

)

X-Y-Ztranslation stage

tegr

ated

PL

TMS-Orlando, 2007

In Energy

Slit-confocal microscopy

Polarization studiesluminescenceθ

θ

laserpolarizer analyzer

spectrometer

200

250

Data Fit Cos2(θ)

u)

excitation/emission

πθ/σ+

250

300

.)

Data Fit Cos2(θ) σ+/π

θ

excitation/emission~50

θ

sample

100

150

nten

sity

(a.u

100

150

200

nten

sity

(a.u

.

-100-80 -60 -40 -20 0 20 40 60 80 1000

50PL in

-100-80 -60 -40 -20 0 20 40 60 80 1000

50

00

PL

in

laser polarization angle θ (degree) analyzer angle θ (degree)

polarizer = πθ ; analyzer = σ+ polarizer = σ+ ; analyzer = πθ

PL emission is strongly polarized parallel to the wire and is strongly

TMS-Orlando, 2007

PL emission is strongly polarized parallel to the wire, and is strongly enhanced when the laser excitation is polarized parallel to the wire

Polarization Imaging

8

10 1.44 1.47 1.50 1.53 Energy (eV)

a)

1.44 1.47 1.50 1.53

8

10c)

Energy (eV)

Calculate pixel by pixel

4

6

8

ng w

ire (μ

m)

4

6

8

ng w

ire (μ

m)

⊥

⊥

+

−=

IIII

P||

||

0

2

Alo

n

0

2

Alo

n

Polarization

10

~82%

4

6

8

wire

(μm

) b)

200

300

400d)

uenc

yHistogram

Strongly polarized due to the large dielectric

mismatchbetween GaAs and air

1 44 1 47 1 50 1 530

2

4

Alo

ng

50 60 70 80 90 1000

100

200

Fre

qu between GaAs and air

(Science 293 1455 (2001)

TMS-Orlando, 2007

1.44 1.47 1.50 1.53Energy (eV)

50 60 70 80 90 100Polarization (%)

(Science 293 1455 (2001),

APL. 89 173126 (2006))

Resonant Excitationcore GaAs

Tune excitation energy, E Laser , record PL intensity (PLE)

Elaser

shell AlGaAs

y ( )

GaAs

AlGaAs

AlGaAs

s e Ga s

1-LO2-LOresonances

GaAs

GaAs

E

r

1 LO

hωexcitationhωemission

PLGaAs

TMS-Orlando, 2007

r hωexcitation

real space k-space

Resonant Excitation

25000

single core-shell GaAs/AlGaAs nw

36meVT=10K

15000

20000 GaAs/AlGaAs nw

37meVes (a

.u)

5000

10000

60meV

37meV

Inte

nsiti

e

PL

1.44 1.48 1.52 1.56 1.60 1.64 1.68 1.72 1.76

0

PL PLE

Clear resonances at 36, 73 and ~133 meV

Eexcitation- EX (eV)

TMS-Orlando, 2007

Clear resonances at 36, 73 and 133 meV above free exciton energy.

Resonant Excitation

1-LO and 2-LO GaAs phonons

wire 2

Resonance at ~133 meV:

phonons

y (a

. u.)

PLEPL

Resonance at 133 meV:

1. Defect-AlGaAs related.

2 Bottom of AlGaAs band

wire 1LOLO

L In

tens

ity

2. Bottom of AlGaAs band (Low concentration of Al ~10%, instead of growth condition 26%)60 30 0 30 60 90 120 150 180 210 240

PLE

P

PL

)

H d th l i ti d d

-60 -30 0 30 60 90 120 150 180 210 240

Eexcitation - EX (meV)

TMS-Orlando, 2007

How does the polarization depend on excitation energy?

Excitation dependent polarization

(a.u

)

wire 2

PL PLE

Inte

nsity

PL PLE

Excited: 1.653 eV

Excited: 1.589 eV

Excited: 1.553 eV

1.45 1.50 1.55 1.60 1.65 1.70 1.75Excitation energy (eV)

P~76%

P~82%

nsity

(a.U

)

P~92%

1.46 1.48 1.50 1.52 1.54

x2

1.44 1.46 1.48 1.50 1.52 1.54

x2

1.44 1.46 1.48 1.50 1.52 1.54

Inte

x2

TMS-Orlando, 2007

Emission energy (eV)Emission energy (eV)Emission energy (eV)

Polarization changes with excitation energy!

PL Polarization Imaging

80•Excitation laser polarized along nanowire

40

60

80

•Analyze emission polarization

⊥− IIP ||

60

80

pixe

ls

400

82%

⊥

⊥

+=

IIP

||

||

60

80

40

200

300

# pi

xels

300 600 90040

60

pixels

Pol

50 60 70 80 90 1000

100

PL Polarization

TMS-Orlando, 2007

p

Polarization depends on excitation energy

1.68

1.71wire 2

1.62

1.65

V) ts

(a.u

)

1.56

1.59

Ener

gy (e

V

eve

n

1.50

1.53 55 60 65 70 75 80 85 90 95 100degree of polarization (%)

•Note that the emission energy does

1.47

PL Intensity (a u)

not change

•Only the energy of excitation changes

TMS-Orlando, 2007

PL Intensity (a.u) changes

•Changing polarization must result from changing exciton distributions

Polarization excitation dependence also depends on wire…

1.71

wire 3

1|| →n

1.65

1.68

uenc

e

1→⊥n

1 56

1.59

1.62

Freq

Ene

rgy

(eV)

1.50

1.53

1.56

55 60 65 70 75 80 85 90 95 100

Degree of polarization (%)

E

1|| >⊥n

n

1.44

1.47

I t iti ( )

⊥As one comes closer to resonance the relative density of excitons changes

TMS-Orlando, 2007

Intensities (a.u)g

Resonant excitation creates non-equilibrium exciton spin distributions

90100110

wire 1 ideal wire

on

on (%

)

(c)

As excitation comes closer to

4050607080

N||=Nwire 2

Emis

siPo

lariz

atio As excitation comes closer to

free exciton energy:

• Along wire: polarization increases

units

)

40 (b) wire 2

PLE

increases • Perpendicular:

polarization decreases

wire 1(a)LOLO

tens

ity (a

rb.

PL

Polarization are different for different wires

PLE

PL In

t

PLWire 2: thermal equilibrium

N║ = N┴

TMS-Orlando, 2007

-60 -30 0 30 60 90 120 150 180 210 240

Eexcitation - EX (meV)

║ ┴

Exciton Dynamics

xy zsτsτ

yτ xτ zτnsτ nsτ

I

0yG

1,||

, <<>>>> ⊥

IIandsnryzx ττττ2

, 21 s

yzxεττ ⎟

⎠⎞

⎜⎝⎛ +

=

23

3003

excexcvacy D

cωπεττ ==

⎠⎝At thermal equilibrium (highest energies) assume:

nn = yI τ=⊥

TMS-Orlando, 2007

yx nn =xI τ

=||

Spin scattering time

( )+ PI 1

0/ =dtdnαSteady state: wire 1

i 2

( )( )( )−

−−+

= ⊥

PI

forPIPI

nr

s

1

||1)11

||

τ

ττ 1 wire 2

wire 2

τ s/τnr

( )( ) ⊥−

+= ⊥ for

PIPI

nr

s 111

||ττ

0.1

S50 100 150 200 250

0.01

Eexcitation- EX (meV)

Spin relaxation time depends on excitation energy

“Non-Equilibrium Exciton Spin Dynamics in Resonantly Pumped Single Core-Shell GaAs-AlGaAs Nanowires”

Thang B Hoang L V Titova J M Yarrison-Rice H E Jackson A O Govorov Y

TMS-Orlando, 2007

Thang. B. Hoang, L.V. Titova, J. M. Yarrison-Rice , H. E. Jackson, , A. O. Govorov, Y. Kim, H. J. Joyce, H. H. Tan, C. Jagadish, L. M. Smith

Nano Letters - Web release 15 Feb ’07

Conclusions

Resonances observed at 1-LO and 2-LO and ~133meV

Single GaAs-AlGaAs NWs under resonant excitation:

Resonances observed at 1-LO and 2-LO and ~133meV(AlGaAs related) above the PL emission line

R t it ti t ilib i itResonant excitation creates non-equilibrium exciton dipole distributions

Polarization of PL is strongly enhanced as excitation energy comes closer to resonance with free exciton emission.

Rate equations: dependent of spin relaxation time on excitation energy

TMS-Orlando, 2007

Rate equations

xy zsτsτ

yτ xτ zτnsτ nsτ

0yG

,2s

z

s

y

s

x

nr

x

x

xx

x

τττττnnndn

nnnnnGdt

dn++−−−=

2

,2

zxyyyz

s

z

s

x

s

y

nr

y

y

yy

y

τττττ

nnnnndn

nnnnnG

dtdn

++

++−−−=

TMS-Orlando, 2007

,2sssnrz τττττdt

++−−−=