Time-resolved photoluminescence properties of CuInS2/ZnS nanocrystals:Influence of intrinsic defects and external impuritiesVamsi K. Komarala, Chuang Xie, Yongqiang Wang, Jian Xu, and Min Xiao Citation: J. Appl. Phys. 111, 124314 (2012); doi: 10.1063/1.4730345 View online: http://dx.doi.org/10.1063/1.4730345 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v111/i12 Published by the American Institute of Physics. Related ArticlesStrong two-photon fluorescence enhanced jointly by dipolar and quadrupolar modes of a single plasmonicnanostructure Appl. Phys. Lett. 101, 051109 (2012) Observation of intermediate bands in Eu3+ doped YPO4 host: Li+ ion effect and blue to pink light emitter AIP Advances 2, 032119 (2012) Silver nanoclusters decked diamond thin film as a substrate for surface-enhanced Raman scattering AIP Advances 2, 032102 (2012) Radiative and non-radiative effects of a substrate on localized plasmon resonance of particles J. Appl. Phys. 112, 013529 (2012) Stimulated Mie scattering in nanocrystals suspension Appl. Phys. Lett. 101, 011110 (2012) Additional information on J. Appl. Phys.Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors

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Time-resolved photoluminescence properties of CuInS2/ZnS nanocrystals:Influence of intrinsic defects and external impurities

Vamsi K. Komarala,1,2,a) Chuang Xie,3,4,5 Yongqiang Wang,4 Jian Xu,5 and Min Xiao2

1Centre for Energy Studies, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India2Department of Physics, University of Arkansas, Fayetteville, Arkansas 72701, USA3State Key Laboratory for Chemical Engineering, School of Chemical Engineering and Technology,Tianjin University, Tianjin 300 072, People’s Republic of China4Ocean NanoTech, 2143 Worth Lane, Springdale, Arkansas 72764, USA5Department of Engineering Science and Mechanics, The Pennsylvania State University, University Park,Pennsylvania 16802, USA

(Received 20 April 2012; accepted 22 May 2012; published online 22 June 2012)

Photoluminescence (PL) lifetime studies of CuInS2 nanocrystals (NCs) are carried out after

synthesizing core-shell and compositionally variant structures using time-resolved PL

spectroscopy. Long-lived excited state decay times are observed for the NCs, and decay times are

very much dependent on the size of the CuInS2 NCs. The emission bands are attributed to the

surface (shorter PL lifetime) and defect (longer PL lifetime) related trap states, respectively. The

decay dynamics of the CuInS2 NC’s excited-state carriers is very sensitive to the surface, intrinsic

defects, and extrinsic impurities. The observed large Stokes shifts and broad PL spectra also reveal

the involvement of the defect-related trapping sites in the emission process. VC 2012 AmericanInstitute of Physics. [http://dx.doi.org/10.1063/1.4730345]

I. INTRODUCTION

Recently, research works on copper indium disulfide

(CuInS2) nanocrystals (NCs) have gained much importance

in scientific community. Significant progress has been made

in the synthesis part and light emission at the room tempera-

ture has been achieved recently.1–5 The ever increasing inter-

est on CuInS2 NCs is due to the motivation for developing

sustainable non-toxic materials by not having any adverse

effects on health and environment, compared to the

cadmium-based NCs for various applications.4,6–9 With well

establishing synthesis procedure, many interesting applica-

tions have been demonstrated, such as solar cells having 4%

efficiency using solution processed CuInS2 NCs,6 high per-

formance CuInS2/CdS heterostructure solar cells with

CuInS2 NCs sensitization,7 and also polymer hybrid solar

cells with CuInS2 NCs.8 Further, the CuInS2/ZnS core-shell

NCs as well as Zn doped CuInS2/ZnS NCs are used in solar

cell structures as absorbing materials,9 the CuInS2/ZnS NCs

are used as Cd-free NCs for the in-vivo imaging,4 and

recently near-band-edge electroluminescence is observed

using the ZnCuInS2/ZnSe/ZnS NCs.10

Understanding of radiative and non-radiative carrier

recombination processes is very much important for biologi-

cal imaging, solid-state lighting, and photovoltaic applica-

tions. For example, in application of CuInS2 NCs for solar

cells, the separated charge carriers must be produced in high

yield near the junction and then transported efficiently to the

electrodes; for this one needs to understand the radiative

recombination processes related to the CuInS2 NCs excitons,

intrinsic defects, and surface defects, and the ternary I-III-

VI2 system is more complicated in structure and composition

than the binary II-VI semiconductors.9–13 However, the ter-

nary semiconductor’s optical and electronic properties can

be tuned by changing the composition (such as replacing

some of the Cu atoms with Zn ones),14–17 so possibilities

exist for enhancing the absorption as well as transport prop-

erties for improving certain opto-electronic devices based on

ternary semiconductor NCs. For example, the substitution of

the Zn in place of Cu in the CuInS2 NCs can result in tunable

emission from �530 to �660 nm,8 which is explained based

on redistribution of electronic states and modified confine-

ment energy, apart from the size-induced optical transitions

in the NCs.

The motivation behind our work is to understand the

charge carrier lifetimes for the excited states of the CuInS2/

ZnS NCs. We focused our attention to quantitatively under-

stand the charge carrier decay dynamics of CuInS2/ZnS NCs

having the emission peak maximum from 580 to 760 nm.

The average lifetimes of Zn-doped CuInS2/ZnS NCs and

ZnCuInS2 NCs with ZnSe/ZnS multiple shell layers are also

investigated for understanding the external impurities and

the potential barriers provided by the multiple shells to the

excited charge carriers in the core material.

II. EXPERIMENTAL DETAILS

CuInS2 NCs are prepared in the non-coordinating sol-

vent octadecene (ODE) by heating copper iodide, indium ac-

etate in dodecanethiol at 180–250 �C.12 The size of the

nanoparticles is controlled by varying the reaction time and

temperature. The resulting CuInS2 NCs are subsequently

over coated with a ZnS shell by injecting zinc precursor

(ZnO/oleci acid/ODE, 0.1 M) at 210 �C. ZnCuInS2/ZnS core/

shell NCs are prepared by growing the ZnCuInS2 core using

the procedure, 2 ml of CuI/oleyamine (OAm) solution

(45 mM) and 2 ml of InI3/OAm solution (45 mM) are mixed

a)Author to whom correspondence should be addressed. E-mail: vamsi@

ces.iitd.ac.in. Telephone: þ91-11-2659-1255. Fax: þ91-11-2659-1251.

0021-8979/2012/111(12)/124314/4/$30.00 VC 2012 American Institute of Physics111, 124314-1

JOURNAL OF APPLIED PHYSICS 111, 124314 (2012)

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with 6 ml of diethyldithiocarbamide zinc (DECZn)/tributyl-

phosphine (TBP)/ODE (16.7 mM) in the flask, the mixture is

heated to the desired temperature (100–230 �C) followed by

growth at the same temperature. Se/TBP (0.1 M), ZnO/oleci

acid/ODE (0.1 M), and DECZn/TBP/ODE (16.7 mM) are

used as the precursors for the ZnSe and ZnS shell layers,

respectively. Transmission electron microscopy image of

CuInS2/ZnS NCs is shown in the Figure 1, inset of the figure

shows the high-resolution image of NCs having the size of

�3 nm with emission around the red color. The optical

absorption spectra of the colloidal NCs are recorded using

Hewlett Packard 8453 UV-Vis spectrophotometer. Lumines-

cence spectra are recorded at room temperature using a Per-

kin Elmer LS 50B fluorescence spectrometer with the

excitation wavelength of 480 nm.

III. RESULTS AND DISCUSSION

Figure 2 shows the Photoluminescence (PL) spectra of

CuInS2/ZnS NCs having the emission peak maxima at

�600 nm, �650 nm, and �760 nm, respectively. The PL

spectra are very broad having large full-width-at-half-maxi-

mum (FWHM) values (approximately from 80 to 120 nm).

The band edge of the NCs is found using Tauc’s plot, with

the equation ah�¼A(h��Eg)1/2, by plotting the curve

between (ah�)2 and h�, which is shown in the inset of Figure

2, where Eg is optical band gap, A is a constant from the

absorbance data, a is the absorption coefficient, and h� is

the photon energy.18 We found that the approximate absorp-

tion edges of NCs are �2.66 eV (�465 nm), �2.40 eV

(�515 nm), and �2.10 eV (�590 nm) for the emission peak

maxima �600 nm, �650 nm, and �760 nm, respectively.

Compared to the bulk band gap �1.53 eV (�810 nm) of

CuInS2,11 the blue-shifted absorption edge of the CuInS2/

ZnS NCs in all the cases is an indication of quantum confine-

ment effect. The PL emission maxima for all the samples are

far from the band edge absorption, i.e., large global Stokes

shift of �130 to �170 nm, compared to the II-VI semicon-

ductor NCs, where the Stokes shift is very much minimal

due to the electron-phonon coupling. PL spectra suggest that

the emission can occur through intrinsic defects due to the

deviation from the ideal chemical formula of CuInS2 and

also surface-related trap states.1,3

The PL dynamics of CuInS2/ZnS NCs are investigated at

room temperature using a PicoQuant Microtime200 time-

resolved confocal microscope system with 150 ps time resolu-

tion to measure the predominant radiative recombination

process of electrons and holes. The samples are excited with

480 nm picosecond pulses, generated by a PicoQuant LDH-

480 laser diode. PL lifetime images are recorded by raster

scanning a sample area (80� 80 lm) over the focused spot of

the incident laser, with a total integration time of 2.81 min.

From the lifetime images, the decay curves are generated

using the SymPho Time analysis software (PicoQuant). Life-

time decays are re-convoluted using nonlinear least square

analysis (FluoFit, PicoQuant) with an equation of the form:

I (t) !X

i

ai exp (�t/si), where si are the PL decay times. The

quality of the fit is judged in terms of a v2 value (with a crite-

rion of less than 1.1 for an acceptable fit) and weighted resid-

uals for measuring the average lifetimes from decay curves.

Figure 3 shows the time-resolved PL decay curves of

CuInS2/ZnS NCs having the emission peak wavelengths of

�600,�650, and�760 nm, as well as the corresponding aver-

age PL lifetime values of �94, �125, and �200 ns, respec-

tively. The decay curves are multi-exponential in nature. The

PL decay curves are re-convoluted into three decay compo-

nents with the best possible fits. Based on the obtained life-

time values, we attribute the fast decay time component to be

the radiative recombination process of the surface defect states

(�15 ns),3,19 whereas the slower decay times to be due to the

radiative recombination processes of the donor-acceptor pairs

(�125 and �400 ns).3,13,19,20 Surface-related trap states are

usually shallow and therefore show faster decay lifetime

compared to the intrinsic defect-related trap states.16 Inset of

Figure 3 shows decay curves related to CuInS2 NCs without

FIG. 1. Transmission electron micrograph of CuInS2 NCs coated with ZnS

layer. Inset of the figure shows the high-resolution image of NCs having size

of �3 nm.

FIG. 2. Photoluminescence (with excitation wavelength: 480 nm) spectra of

CuInS2/ZnS NCs having the emission peak maxima �600 nm, �650 nm,

and �760 nm, respectively. Inset of figure shows the Tauc’s plots between

(ah�)2 and h�.

124314-2 Komarala et al. J. Appl. Phys. 111, 124314 (2012)

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and with ZnS shell layer having an emission maximum

�760 nm. The average lifetime of the CuInS2 NCs without

ZnS shell layers is significantly faster (�68 ns) than that with

the ZnS shell layers (�200 ns). Over coating with the ZnS

layer reduces the fast decay channel’s contribution from 61%

to 36%. It gives an idea that with the ZnS layer, the fast decay

channels associated with the recombination through surface-

defect-related trap states are reduced. The dangling bonds of

chalcogen atoms on NC surface and absorbents are the usual

candidates for the surface states, which can act as trap states

for both electrons and holes. Some of the surface-related non-

radiative carrier losses might have overcome the ZnS layer,

by populating the intrinsic defect-related trap states in CuInS2

NCs to have longer lifetime, which has also been revealed in a

quantitative way by the increase of the slower decay channels

related to defect trap state’s recombination process from 39%

to 64%. Slower lifetime related to the defect trap states with

ZnS layer provides a strong indication that the excited elec-

tron in the CuInS2 core can survive longer time of the increase

in carrier concentration, without undergoing possible losses at

surface boundaries due to the high-potential barrier from the

ZnS shell layer.

In the highly luminescent CdSe NCs, the major part of

the radiative recombination process is related to emission

associated to surface states with the lifetime of tens of nano-

seconds.21 However, the ternary CuInS2 NCs may have more

complicated lattice imperfections due to the tetragonal lattice

distortion than the binary lattice, and also deviation from the

stoichiometry. So, not only the surface states, but also intrin-

sic defects, are equally important and can act as potential

fluctuations to localize charge carriers during the recombina-

tion process. The donor states are due to sulfur vacancy, in-

dium interstitial and indium atom occupying the copper

vacancy; and the acceptor states are due to the copper vacan-

cies.22 Li et al.,20 from the transient absorption studies,

demonstrated that the global Stokes shift between the emis-

sion line and the band edge absorption in CuInS2 NCs is due

to the internal defects, not due to strong electron-phonon

coupling. The temperature-dependent emission study of the

CuInS2/ZnS NCs gave an evidence for the existence of

intrinsic defect-related trap states along with surface-related

defect states.19 Trapped electrons in the surface defects are

thermally active and could move to emission sites even at

low temperatures; whereas the electrons trapped in the defect

states cannot escape at low temperature due to the strong

Coulomb attraction energy between the charge carriers.

In the defect studies of the CuInS2 single crystals, distinct

double deep defect-related emission bands are observed, and

the defect bands are very sensitive to the Cu compensation.15

The observed two longer distinctive lifetimes (�125 and

�400 ns) in our lifetime analysis can be attributed to the dou-

ble defect-related recombination processes. The two lifetimes

can be assigned as due to the donor-acceptor pairs of the near-

est neighbors, and between the pairs of the next closest neigh-

bors.15 The observed emission-wavelength-dependent

behaviors of the PL decay times are shown in the Figure 2,

which can be well explained by considering the donor-

acceptor pair transition. The donor-acceptor energy levels in

the band gap will be varied due to the size induced optical

effects in the CuInS2/ZnS NCs. With the size variation, the

binding energies of the charge carriers will be affected due to

the Coulomb interaction between the donor and acceptor trap

levels. So, the transition probability between the nearby pairs

(smaller NCs) is expected to be higher than that between dis-

tance pairs (bigger NCs).23 The overlap of electron and hole

wave functions trapped in defect levels for the nearby pair in

smaller NCs is greater compared to the bigger NCs; so, the

recombination rate for a higher-energy transition is larger than

that for a lower-energy transition.

We try to further understand the role of the defect struc-

ture in the decay dynamics of excited charge carriers in the

CuInS2/ZnS NCs with the “external impurities” as compared

to the “intrinsic defects.” CuInS2/ZnS NCs doped with Zn

are synthesized. The Zn species may preferentially substitute

Cu sites since the ionic character of Zn is close to that of

Cu.2 Figure 4 shows the PL decay lifetime of ZnCuInS2/ZnS

NCs having the emission peak maximum at �580 nm. The

average decay lifetime of the ZnCuInS2/ZnS NCs is

�235 ns, compared to the lifetime of the CuInS2/ZnS NCs of

�94 ns at the emission peak maximum of �600 nm. There is

a small blue shift in the PL spectrum of ZnCuInS2/ZnS NCs

due to the presence of impurity Zn in the core material.2

With the presence of Zn, the defect-related recombination

contribution is increased to 73% from 47%. This gives an in-

dication for some inhabitation processes in the generation of

internal structural defects in the NCs due to the Zn presence

in the core material. Previously, it was observed that the Zn-

doped CuInS2 will improve the crystal structure by occupy-

ing some of the vacancy sites in the CuInS2 material.17 Zn

can introduce either a donor state by occupying either Cu

site/interstitial site or an acceptor state by occupying the sul-

fur site.14 Increased average lifetime to �235 ns from �94 ns

due to the defect-related trap states in the presence of Zn in

the CuInS2 reflects the suppression of some of the vacancies

FIG. 3. Time-resolved PL decay curves of the CuInS2/ZnS NCs having the

emission peak wavelengths of (1) �600, (2) �650, and (3) �760 nm, and

the corresponding average PL lifetime values of �94, �125, and �200 ns,

respectively. Inset of figure shows decay curves of the CuInS2 NCs (emis-

sion at �760 nm) without ZnS shell layer (�68 ns) and with the ZnS shell

layer (�200 ns), respectively.

124314-3 Komarala et al. J. Appl. Phys. 111, 124314 (2012)

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in the crystal structure. It seems that the charge carrier popu-

lation in the defect states is the most important parameter,

when considering the decay rates.

The idea of engineering both the band gap energy and

lattice parameters of core-shell NCs has shown large interest

recently, since it can be used to (1) overcome the large lattice

mismatch between the core and shell; (2) influence potential

barrier for modifying the generated charge carrier recombi-

nation; and (3) suppress the fast photo-oxidation of the NC’s

shell layers.24,25 We also measured the average lifetime of

the ZnCuInS2/ZnSe/ZnS NCs by growing a ZnSe layer

between the ZnCuInS2 core and ZnS shell. The average life-

time of the ZnCuInS2/ZnSe/ZnS NCs having the emission

around 580 nm is �140 ns (Figure 4). With the additional

ZnSe shell layer the average lifetime value comes down to

�140 ns from �235 ns. Inset of Figure 4 shows the absorp-

tion spectrum of the ZnCuInS2/ZnSe/ZnS NCs, in which a

small absorption peak appears at �500 nm, whereas the

absorption spectra of the ZnCuInS2/ZnS and CuInS2/ZnS

NCs only show gently slanting absorption feature. With the

ZnSe shell layers, the modified absorption spectrum gives a

clue that the process might be due to the excitonic absorption

transition as compared to the ZnCuInS2/ZnS and CuInS2/

ZnS NCs. Recently, Tan et al.10 has shown tunable near-

band-edge electroluminescence from the ZnCuInS2/ZnSe/

ZnS NCs. The shorter average lifetime of the ZnCuInS2/

ZnSe/ZnS NCs, compared to the ZnCuInS2/ZnS NCs, indi-

cates the modified recombination process due to the potential

barrier provided by the multiple shells to the excited charge

carriers in the ZnCuInS2. From Figure 4, it is clear that one

of the longer lifetimes, related to the defect-related recombi-

nation contribution, is affected by the multiple shell layers.

Further investigations on intra-band relaxation of the excited

charge carrier process in ZnCuInS2/ZnSe/ZnS NCs may shed

some light on the decay dynamics.

IV. CONCLUSIONS

In conclusion, we have observed longer average life-

times for the CuInS2/ZnS NCs compared to the Cd-based

NCs. The radiative recombination processes in the CuInS2/

ZnS NCs are related to both the defects and surface-related

trap states; the average lifetimes are also very sensitive to

the size of the NCs. The defect-related emission rates are re-

sponsive to the intrinsic defects as well as extrinsic impur-

ities. The longer average lifetime of the CuInS2/ZnS NCs

will enable easier performance and applications in bio-

technology to avoid the toxicity-related issues from the Cd-

based NCs. Longer lifetime of the CuInS2/ZnS NCs can also

lead to the potential applications in solar cells, since the sep-

arated charge carriers must be produced in high yield at the

junction and then transported efficiently out of the device.

ACKNOWLEDGMENTS

We acknowledge the partial financial support from the

National Science Foundation through the MRSEC Program.

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FIG. 4. Time-resolved photoluminescence decay curves of (1) CuInS2/ZnS,

(2) ZnCuInS2/ZnS, and (3) ZnCuInS2/ZnSe/ZnS NCs, and the correspond-

ing average lifetime values of �94, �235, and �140 ns, respectively. Inset

of figure shows the corresponding absorption spectra of the NCs.

124314-4 Komarala et al. J. Appl. Phys. 111, 124314 (2012)

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