Cu-deficient nanocrystals showed blue shifted PL

- Large defect concentration from nonstoichiometric composition enhanced internal defect-related emission

<M. Uehara et al., J. Chem. Phys. 129, 134709 (2008)>

Relaxation time distribution based on multiexponential decay of PL was

restored by regularization method

- Both fast decay component and slow decay component can contribute PL of CuInS2 nanocrystals - Cu-deficiency minimized the contribution of fast decay component and enhanced slow decay component Contribution of internal defect-related emission was enlarged

Surface modification on the photoluminescence of CuInS2 quantum dots

Young-Kuk Kim*, Si-Hyun Ahn, Kyu-Chae Choi, Young-Sang Cho, Chul-Jin Choi Korea Institute of Materials Science (KIMS)

Photoluminescence of CuInS2 nanocrystals

Summary

1. The effect of the copper deficiency and the surface modification on photoluminescence of CuInS2 nanocrystals was investigated with the time- and the temperature-resolved photoluminescence. 2. The copper deficiency in copper indium sulfide nanocrystals induced the internal defect-related emission and minimized the relative contribution of the surface trap-related emission. 3. Luminescence of copper indium sulfide nanocrystals was dramatically enhanced by surface modification with zinc acetate and palmitic acid. 4. The large blueshift in photoluminescence after surface modification with zinc acetate and palmitic acid was ascribed to homogenization of size/shape distribution and lattice strain through

formation of lattice mismatched inorganic shell layers.

Acknowledgment This work was supported by the basic research program of Korea Institute of Materials Science (KIMS).

CuInS2 nanocrystals with reduced toxicity

Effect of non-stoichiometry on PL

CuInS2

: a chalcopyrite-type str. Eg: 1.5eV(bulk)~2.4eV(nanocrystals)

Application to in vivo biological imaging

Highly reduced toxicity

The onset of inflammation:

10 times more concentrated dose

for CuInS2/ZnS NCs than for Cd-based NCs

<T. Pons et al., ACS Nano, 2010, 4 (5), pp 2531–2538>

CBM

VBM

CuInS2

hni hno

Defect

Defect related emission

broad PL peak (FWHM~100nm)

Internal defect-related emission <S. L. Castro et al., J. Phys. Chem. B 108, 12429 (2004)>

Surface defect-related emission <Y. Hamanaka et al.,Chem. Phys. Lett. 108, 12429 (2004)>

Explains the large Stokes shift (D~600meV)

Explains the PL enhancement by Cu-deficiency

Internal defect+surface defect-related emission <L. Li et al. J. Am. Chem. Soc. 2011, 133, 1176.)>

<M. Uehara et al., J. Chem. Phys. 129, 134709 (2008)>

Cu-deficiency in CuInS2 nanocrystals

- Defect-ordered structure (Large defect concentration)

- improve PL intensity (internal defect-related emission)

- PL peak shift to short wavelength (Blueshift)

※ Large Stokes shift (D~600meV)

ZnS capping in CuInS2 nanocrystals

- highly enhanced PL intensity

- Large PL peak shift to short wavelength (Blueshift)

due to size reduction? <L. Li et al. J. Am. Chem. Soc. 2011, 133, 1176.)>

due to Zn alloying? <M. Uehara et al., J. Chem. Phys. 129, 134709 (2008)>

500 600 700 800

x=0.8

x=0.5

x=0.2

em

iss

ion

(a

.u.)

wavelength (nm)

x=0

0 1000 2000 3000 4000

Cu1-x

InS2

inte

ns

ity

(lo

g.

sc

ale

)

time (ns)

x=0.2

5

x=0.1

x=0.8

100

101

102

103

104

x=0.2

5

x=0.1

x=0.8A

decay time, (ns)

B

Cu1-x

InS2

i

ii ttI )/exp()()(

Time decay of photoluminescence (Analyzed with multi-exponential decay)

A: =10~30ns surface trap-related emission

B: =300ns internal defect-related emission

(L. Li et al. J. Am. Chem. Soc. 2011, 133, 1176.)

Photoluminescence (DE~60meV)

Restoration of relaxation time distribution,

Copper deficiency led to blueshift

and enhanced intensity in PL spectra

Effect of surface modification on PL

250 300 350 400

Raman shift (cm-1)

Cu0.2

InS2

Cu0.2

InS2/ZnS

CH CA

ZnS Shell

Raman

As synthesized CuInS2 NCs cooled to RT

Zn(CH3COO)2 + RCOOH dissolved in 1-ODE

Refluxing for 5h.

C12H25SH

Cu/In<1 Raman

defect

ordering

Cu/In~1

Effect of surface treatment by refluxing with zinc acetate

-Surface etching: Zn(CH3COO)2 + RCOOH Zn(RCOO)2+CH3COOH

-ZnS Shell capping: Cd or Zn(RCOO)2+C12H25SH (Cd,Zn)-SC12H25

Elimination of surface states by etching & shell capping

Optical spectra (PL, PLE, Absorption)

Sharp PLE peak near band edge Smaller Stokes shift (D=650350meV)

Uniform size/shape distrib. after surface modification

0 1000 2000 3000 400010

1

102

103

Cu0.2

InS2

Cu0.2

InS2/ZnS

Cu0.2

InS2/(Cd,Zn)S

PL

in

ten

sit

y(a

.u.)

time(ns)

Minimized trap-related emission and enhanced internal defect -related emission

Effect of shell composition on PL shift

500 550 600 650 700 750 800

pristine

1hr

2hr

3hr

4hr

5hr

6hrem

iss

ion

wavelength (nm)

0 1 2 3 4 5

570

580

590

600

610

620

630

640

650

660

wa

ve

len

gth

(n

m)

refluxing time (hrs.)

[Cd]/([Cd]+[Zn])=0

[Cd]/([Cd]+[Zn])=0.05

[Cd]/([Cd]+[Zn])=0.2

[Cd]/([Cd]+[Zn])=0.5

500 550 600 650 700 750 800

pristine

1hr.

2hr.

3hr.

4hr.

5hr.

em

issio

n (

a.u

.)

wavelength (nm)

[Cd]/([Cd]+[Zn])=0.5

Cu0.2InS2/ZnS Cu0.2InS2/(Cd,Zn)S

300 400 500 600 700 800

Wavelength (nm)

Ab

so

rpti

on

PL

PL

in

ten

sit

y

PLE

Cu0.2InS2/ZnS

300 400 500 600 700 800

Wavelength (nm)

Ab

so

rpti

on PLE

PL

Cu0.2InS2/(Cd,Zn)S Cu0.2InS2

300 400 500 600 700 800

Wavelength (nm)

Ab

so

rpti

on

PLE

PL

30nm 5nm 5nm 30nm 4nm 30nm

Stokes shift (D) D=650meV D=350meV D=350meV

Time-resolved PL

100

101

102

103

104

Cu0.2

InS2

Cu0.2

InS2/ZnS

Cu0.2

InS2/(Cd,Zn)S

(

)

time (ns)

Bulk band gap CuInS2: Eg=1.42eV ZnS: Eg=3. 2eV CdS: Eg=2.42eV

- Large PL peak shift to short wavelength (Blueshift)

due to size reduction? <L. Li et al. J. Am. Chem. Soc. 2011, 133, 1176.)>

due to Zn alloying? <M. Uehara et al., J. Chem. Phys. 129, 134709 (2008)>

Size reduction during shell formation : CuInS2/ZnS blueshift (expected), CuInS2/CdS blueshift (expected)

Alloying of shell composition : CuInS2-ZnS blueshift <H. Nakamura, Chem. Mater. 18, 3330 (2006)>

: CuInS2-CdS blueshift <K. Watanabe, Mater. Res. Soc. Symp. Proc. No.1064 (2008)>

Blue shift was minimized by changing shell composition

Cu0.2InS2/ZnS: compressive stress on core blue shift

Cu0.2InS2/CdS: tensile stress on core red shift

Lattice parameter CuInS2: a=0.5517nm (Eg=1.42eV) ZnS: a=0.5345nm (Eg=3. 2eV) CdS: a=0.5820nm (Eg=2.42eV) Band deform. Potential dEg/dP>0

Cu0.2InS2/(Cd,Zn)S: minimal stress on core

minimal PL shift

Contribution of strain provided by lattice mismatch bet. core and shell