SPATIALLYSPATIALLY--RESOLVED RESOLVED PHOTOLUMINESCENCE MAPPINGPHOTOLUMINESCENCE MAPPINGPHOTOLUMINESCENCE MAPPING PHOTOLUMINESCENCE MAPPING

OF SINGLE CdS NANOSHEETSOF SINGLE CdS NANOSHEETSMelodie Fickenscher, Lyubov V. Titova, Thang B. Hoang, Amir Maharjan,

Ashu Mishra, Andrei Kogan, Leigh M. Smith, Howard E. Jackson

U i i f Ci i i Ci i i OhiUniversity of Cincinnati, Cincinnati, Ohio

J. M. Yarrison-RiceMiami University, Oxford, OhioMiami University, Oxford, Ohio

Heesuk Rho, Kyoung-Yeon Lee

Chonbuk National University, Jeonju, Korea

Young-Jin Choi, Lyoung-Jin Choi, Jae-Gwan Park

Korea Institute of Science and Technology, Seoul, Korea

MotivationMotivation

Semiconductor nanowires and nanosheets exhibit new material properties that are of interest to both basic and applied scientists

We are interested in exploring using single p g g gCdS nanosheets to make a sensitive biosensor for biomolecules and pathogens.

CdSCdS NanosheetNanosheet SamplesSamplesGrown by pulsed laser deposition using vapor-phase transport method (800oC; 20min)Individual nanosheets were ~50 nm thick; ~ 4 um wide and 30-100 um longSurfaces were curved & smooth with uniform thicknessSurfaces were curved & smooth with uniform thickness.

SEM images of ensemble and single nanosheets

CdS Nanosheet SamplesCdS Nanosheet Samples

•HRTEM images show variation in the orientation

[001]

[100]variation in the orientation of the c-axis

•Wurtzite structures exhibit maximum PL emmision perpendicular to C axis.p p

•Want non-destructive way to measure the orientationto measure the orientation of the C axis in these samples.

Experimental SetupExperimental SetupL - LensBS - Beam

CCDLSplitter

al

Ar+ 458nm CWDoubled Ti:Sapphire

432 (0 2 )

BS2D CCD image

L

Y

λ

laser

spat

ia432nm (0.2ps)

emission energyL λ

sample slit

X-Y-Ztranslation

3D image translation

stage 1.2um resolution

Experimental SetupExperimental Setup

11

2

33

Low Temperature PLLow Temperature PL

• PL spectra from• PL spectra from several nanosheets show A- and B- likeshow A- and B- like exciton states

St PL i t it• Strong PL intensity indicating high quantum efficiencyquantum efficiency

PL PolarizationPL Polarization

l

Circular Polarizer

Linear Analyzer

luminescence

θ

laserspectrometer

samplepPL emissions of A- and

B-like excitons are strongly polarized sstrongly polarized perpendicular to the c-axis L

ong

Axi

s

Consistent with HRTEM

Shee

t

θ

HRTEM

C axis

Low Temp PL ImagingLow Temp PL Imaging

68

105 10 15 20 25 30 35 40

Along nanobelt (μm)

belt

(μm

)

a)

b)

024

Acr

oss

321

32

1

321

4

6

8

10c)

32

1

oss

belt

(μm

)

3

2

1

3

2

1

3

3

2

1

3

2

nten

sitie

s (a

.u)

1

0

2

Acr

o

2.52 2.54 2.56

Emission energy (eV)

3 3

2.52 2.54 2.56

3

2.52 2.54 2.56

2.52 2.54 2.56

3

PL in

PL spectrum exhibits high variability bothPL spectrum exhibits high variability both along and across the sheet.

Low Temp PL ImagingLow Temp PL Imaging

68

100 5 10 15 20 25 30 35 40E = 2.547 eV

Along sheet (μm)a)A- and B-like exciton

emissions are spatially

0246

shee

t (μm

)

8

E = 2.563 eVb)

emissions are spatially separated: lower energy excitons stronger at the

Cro

ss

02468

)

)

gedges while higher energy excitons

0

c)

nsiti

es (a

.u.)

2.563 eV2.547 eV

dominate at the center

may suggest a spatial

2.52 2.54 2.56 2.58 2.60

PL in

tenmay suggest a spatial

distribution of stress in the nanosheet

Energy (eV)e os ee

TimeTime--Resolved PL ImagingResolved PL ImagingPL emission at energies 2.547 eV and 2.563 eV (I2A

and I2B) are the most intense.2

Exhibit time-decays with lifetimes of ~200 ps: h h i b lk CdS ( 1 ) b l h ishorter than in bulk CdS (~1 ns) but longer than in

CdS nanowires ( < 50ps)

(Appl. Phys. Lett., 89 053119 (2006))

PhotocurrentPhotocurrentCdS Nanosheets

bridging a gap inbridging a gap in Titanium contact pads

Initial meas rementsInitial measurements show a photo-induced current response to acurrent response to a voltage bias.

Biosensor DevelopmentBiosensor Development

We will use the CdS h k d inanosheets to make device

where the photocurrent responds to the presence of a p ppathogen.

The next step will be toThe next step will be to functionalize the nanosheet with antibodies sensitized to

ti l bi l lparticular biomolecules or bacteria.

ConclusionsConclusions

Strongly polarized PL emission perpendicular to h i i h HRTEMthe c-axis -> agrees with HRTEM

High quantum efficiency -> potential applications g q y p ppfor nanosheet-based biosensor

Spatial dependence of emission energies ->Spatial dependence of emission energies -> possible strain distribution in the nanosheets

E it d ti 200 h t th i b lkExciton decay-time ~200 ps: shorter than in bulk CdS, but much longer than in nanowires