CdS Nanosheets Mel - UC Homepageshomepages.uc.edu/~smithlm/documents/CdS_Nanosheets_Mel.pdf ·...
Transcript of CdS Nanosheets Mel - UC Homepageshomepages.uc.edu/~smithlm/documents/CdS_Nanosheets_Mel.pdf ·...
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