Nano Res

1

Large-scale synthesis of single-source, thermally

stable, and dual-Emissive Mn-doped Zn-Cu-In-S

nanocrystals for bright white light-emitting diodes

Lucheng Peng1, Dongze Li1, Zhuolei Zhang1, Keke Huang1, Ying Zhang1, Zhan Shi1, Renguo Xie1 (), and

Wensheng Yang2

Nano Res., Just Accepted Manuscript • DOI 10.1007/s12274-015-0832-9

http://www.thenanoresearch.com on June 9, 2015

© Tsinghua University Press 2015

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Nano Research

DOI 10.1007/s12274-015-0832-9

Large-Scale Synthesis of Single-Source,

Thermally Stable, and Dual-Emissive Mn-doped

Zn-Cu-In-S Nanocrystals for Bright White

Light-emitting Diodes

Lucheng Peng1, Dongze Li1, Zhuolei Zhang1, Keke

Huang1, Ying Zhang1, Zhan Shi1, Renguo Xie1*, and

Wensheng Yang2

1State Key Laboratory of Inorganic Synthesis and

Preparative Chemistry, 2State Key Laboratory for

Supramolecular Structure and Materials, College of

Chemistry, Jilin University, Changchun 130012,

China

“Non-cadmium” dual emissive QDs have been directly synthesized

using a one-pot hot-injection technique. A white LED was successfully

fabricated using a commercial blue-LED chip combined with the

optimal QDs.

Large-Scale Synthesis of Single-Source, Thermally

Stable, and Dual-Emissive Mn-doped Zn-Cu-In-S

Nanocrystals for Bright White Light-emitting Diodes

Lucheng Peng1, Dongze Li1, Zhuolei Zhang1, Keke Huang1, Ying Zhang1, Zhan Shi1, Renguo Xie1,() and

Wensheng Yang2

Received: day month year

Revised: day month year

Accepted: day month year

(automatically inserted by

the publisher)

© Tsinghua University Press

and Springer-Verlag Berlin

Heidelberg 2014

KEYWORDS

White-LED, nanocrystals,

cadmium free, Mn doped

Zn−Cu−In−S , lighting

ABSTRACT

The global demand for resource sustainability is growing. Thus, the

development of single-source environment-friendly colloidal semiconductor

nanocrystal (NC) phosphors with broadband emission is highly desirable for

use as color converters in white light-emitting diodes (WLEDs). We report

herein the gram-scale synthesis of single-source, cadmium-free, dual-emissive

Mn-doped Zn-Cu-In-S NCs (d-dots) by a simple, non-injection, and low-cost

approach in one-pot fashion. This synthesis approach led to the formation of

NCs with continuously varying compositions in a radial direction because the

reactivity of precursors totally differed among these precursors. Consequently,

d-dots exhibited two emission bands, one of which was due to Mn-related

emissions and the other was due to the band edge of Zn-Cu-In-S NCs. Emission

peaks assigned to band edge were tunable by simple control of particle size and

composition. The prepared d-dots also exhibited the characteristic zero

self-absorption, a quantum yield of 46%, and good thermal stability. A

combination of a commercial blue LED chip with optimal d-dots as color

converters gave a high color rending index of up to 90, Commission

International de l’Eclairage color coordinates of (0.332, 0.321), and correlated

color temperature of 5680 K. These results suggested that this cadmium-free,

excellent thermally stable single-phase d-dot phosphor had potential

applications in WLEDs.

Nano Research

DOI ()

Research Article

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2 Nano Res.

1 Introduction

Recently, white light-emitting diodes (WLEDs) as

solid-state light devices are attracting considerable

interest for their merit of high energy conservation,

long lifetime, high luminous efficiency, and fast

response times, especially where the primary

objective is to replace conventional light sources in

order to minimize energy costs, and therefore, global

energy consumption for lighting[1-3]. To mimic the

blackbody radiation of an incandescent bulb, the use

of phosphors to convert WLEDs represent a

completely new technology platform for the

development of solid-state lighting devices[4-8].

As the next generation color converters, scattering-

-free/scattering-less colloidal semiconductor

nanocrystals (NCs) have higher luminescence

efficiency, larger molar absorptivity and color

tunability in comparison with traditional

phosphors[9-12], which have been proven as

promising color converters for WLEDs[13-15].

Various NCs such as ZnCdS[16-21], CdSe[22-26],

ZnCdSe[27], CuInS[28-35], and InP[36-41] NCs have

been successfully applied in the fabrication of

WLEDs to enhance the color properties of the device.

The general methods to achieve white light emission

are either by coating a yellow phosphor, by

combining green and red phosphors on a

background consisting of a blue light-emitting diode

(LED), or by using NC of the three primary colors

(red, green, blue) using multilayer structures in

LEDs[14,15, 42-44]. However, when one simply mixes

together the NCs with different color emission to

generate white light, these systems suffer from

various drawbacks including self-absorption and

re-absorption and undesired non-radiative energy

transfer among the emitters. This also contributes to

undesirable changes in the chromaticity coordinates

and photometric performance of the LEDs due to the

different relative temporal stabilities of the

components of the WLEDs[14,15]. In addition, these

NCs only have a relative narrow emission band in a

limited spectral range, which result in a low CRI after

combination with commercial blue LEDs because of

their lack of specific color in the visible region.

To address these issues, some work reported the

synthesis of single-phase NC phosphor with white

light emission such as trap-rich CdS NCs, magic-

-sized CdSe, onion-like CdSe/ZnS/CdSe/ZnS, and

alloyed ZnxCd1-xSe NCs, which have been verified to

be feasible in fabricating WLED using these

phosphors[45-49]. However, the emission of the NCs

in the above-mentioned systems was assigned to the

manipulation of NC surface-state, which is

notoriously difficult to control and/or reproduce.

Furthermore, the temporal stability of these states

varies with the environmental conditions in a

manner which is presently still not fully understood.

Recently, dual emissive NCs with broad band

emission from a single component system have been

reported, which has been considered to be a

promising candidate as color convertor for

WLED[51,52]. For example, dual emissive CdSe/

ZnS/CdSe NCs have been used to produce white

light by combination with blue and ultraviolet LEDs

[52]. However, the emission wavelength of these dual

emissive NCs can only be tuned in a limited spectral

range[51-60], which also limits the tunability of color

temperature for the NCs-based WLEDs. Generally,

these most widely used color converters are not only

highly sensitive to the temperature, but are also

restricted, especially the cadmium/lead-based,

particularly in view of recent environmental

regulations and potential application in the

fabrication of WLEDs.

The introduction of transition metal ions into NCs

with low intrinsic toxicity, such as Mn- and/or

Cu-doped ZnSe and ZnS has been widely explored

because of the unique optical properties of the doped

NCs[61-68]. The doped NCs not only retain nearly all

the advantages of intrinsic properties of NCs, but

also possess new properties, such as enhanced

thermal stability, reduced chemical sensitivity, and

elimination of luminescent self-quenching and

re-absorption. Accordingly, WLEDs have been

reported by combining commercial blue LEDs with

those doped NCs[69-74]. For example, WLEDs with

CRI of 50–60 have been reported by combining

single-phase Mn-doped ZnSe NCs[74]. Subsequently,

the dual-emissive Mn and Cu co-doped ZnS (e) have

been successfully used as color converters in

fabrication of WLED[72]. However, these Mn and/or

Cu doped II-VI NCs have a wide band gap (ZnSe:

2.78 eV; ZnS: 3.6 eV), which could not be excited

effectively by commercial blue LEDs to achieve white

light.

Recently, Mn-doped CuInS2 NCs with narrow host

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3 Nano Res.

bands have been considered as low toxic alternatives

to be used as color-tunable emitters. For example, the

Mn:CuInS2/ZnS core-shell NCs with tunable PL

peaked from 542 nm to 648 nm have been reported

by Liu group[75]. Ding et al. also reported the

CuInS2/Mn:ZnS NCs had high PL QY due to the

effective surface passivation by ZnS shell[76].

Whereas, the Mn emission instead of band edge

emission from the Zn−Cu−In−S was observed in

Mn-doped ZnCuInS3 NC system[77]. These

non-cadmium doped multinary systems only

showed one narrow emission band after introducing

Mn ions into host NCs, which would not serve as

effective color converter in fabricating WLED with

effective white light. More recently, a low toxic III-V

NC based on InP has been prepared, which has been

proved to be the ideal color converter in fabricating

WLEDs. The complex synthesis process and

expensive phosphine precursor, however, are needed

upon preparation of this phosphors with broad

emission band. Therefore, the development of a kind

of ideal NCs with broad emission band in the whole

visible region, effective absorption of blue light, and

easy synthesis approach to fabricating white LEDs

with commercial blue LED is still the challenge.

Non-injection, or heating-up method, wherein all

reagents are loaded in a single reaction pot at room

temperature and subsequently heated with reflux for

NCs nucleation and growth, is a promising route to

large-scale preparation of desired phosphorescence

due to the absence of precursor injection. In this

study, we reported the large-scale synthesis of

cadmium-free, dual-emissive Mn-doped Zn-Cu-In-S

NCs (d-dots) in a one pot fashion. With increased

reaction temperature, the resulting particles showed

continuously varying compositions due to gradual

incorporation of ZnS, indicating formation of the

grated ZnS shell structure, which have been well

characterized by transmission electron microscopy

(TEM), X-ray diffraction (XRD), and inductively

coupled plasma atomic emission spectroscopy (ICP)

measurements. These d-dots showed dual emission

consisting of both doped emission and band gap

emission; the former was attributed to d-d transition

from Mn ions, and the latter resulted from

quaternary NCs. The band gap emission peak

position of d-dots was successfully tunable from the

blue to red region by simple control of the core size

and composition, respectively, while doping emission

was almost kept unchanged during growth of the

d-dots. This specific structure may reduce

re-absorption because of its large Stokes shift.

Importantly, this kind of d-dots showed excellent

thermal stability at temperatures below 100 °C. Using

d-dots with two emission peaks at 510 and 605 nm as

color converters combined with commercial blue

LED chips, we successfully obtained WLEDs with

CRI of up to 90, and Commission International de l’

Eclairage (CIE) color coordinates of (0.332, 0.321).

These first promising results demonstrate that that

this “cadmium free”, single phase d-dot phosphor

with excellent thermal stability has potential

applications in WLEDs and correlated solid-state

lighting.

2 Experimental

Materials:

Indium acetate (In(OAc)3, 99.99%), copper (II)

acetate (Cu(OAc)2, 99.99%), copper (I) acetate

(Cu(OAc), 99.5%), zinc acetate (Zn(OAc)2, 99%),

manganese(II) acetate dihydrate (Mn(OAc)2·2H2O,

98%), 1-octadecene (ODE, 90%), dodecylthiol (DDT,

99.9%), oleic acid (OA, 70%), and myristic acid (MA,

99%) were purchased from Alfa. Oleylamine (OAm,

70%) and sulfur powder (S, 99.99%) was obtained

from Sigma-Aldrich. Toluene, acetone, chloroform

and methanol were obtained from Beijing Chemical

Reagent Ltd., China. All reagents were used as

received without further experimental purification.

Thermo-curable silicone resin OE-6630 A and

OE-6630 B were purchased from Dow Corning Co.,

Ltd. Blue-LEDs were purchased from Ruijiahong

(Shenzhen) Co., Ltd.

Synthesis of Mn doped Zn−Cu−In−S NCs (d-dots).

As a typical synthetic procedure, Mn(OAc)2

(0.00173 g, 0.01 mmol), Cu(OAc)2 (0.00182 g, 0.01

mmol), Zn(OAc)2 (0.037 g, 0.2 mmol), In(OAc)3 (0.058

g, 0.2 mmol), and sulfur powder (0.016 g, 0.5 mmol)

together with 6.0 mL of OAm (2.0 mL) and 2.0 mL

DDT and 2.0 mL of OAm were loaded in a 50-mL

flask at room temperature. The mixture was heated

to desired reaction temperature (150, 180, 210 and

240 °C) at a heating rate of 15 oC/min under air to

allow the growth of the d-dots. The entire process

was monitored by absorption and PL measurements

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4 Nano Res.

by taking aliquots from the reaction mixture at a

given temperature. After the completion of particle

growth (usually 10 min), the reaction mixture was

cooled to room temperature, For purification, 8 mL of

hexane was added, and byproducts were removed by

successive methanol extraction until the methanol

phase was clear. Finally, the obtained d-dots were

precipitated by adding acetone into the hexane

solution and purified by repeated centrifugation and

decantation. To obtain d-dots with different Cu/Zn

ratios, the Cu/Zn precursor ratio was varied while

the gross amount of all the precursors is kept

unchanged in the reaction solution.

Synthesis of quaternary Zn−Cu−In−S NCs

Zn−Cu−In−S NCs were synthesized following the

same procedure of the d-dots at the absence of

Mn(OAc)2.

Synthesis of ternary Cu−In−S NCs

Cu−In−S NCs were synthesized following the same

procedure of Zn−Cu−In−S NCs at the absence of

Zn(OAc)2.

Gram-scale production of d-dots

In a typical reaction for the preparation of the

gram-scale dual emissive NCs, Mn(OAc)2 (0.0346 g,

0.2 mmol), Cu(OAc)2 (0.0364 g, 0.2 mmol), Zn(OAc)2

(0.74 g, 4 mmol), In(OAc)3 (1.16 g, 4 mmol), and

sulfur powder (0.32 g, 10 mmol) together with 12 mL

of OAm (4.0 mL) and 4.0 mL DDT and 4.0 mL of OA

were loaded in a 50-mL flask at room temperature.

The mixture was then heated to 210 oC at a heating

rate of 10-20 oC /min with vigorous stirring under air

and kept at this temperature for 1 h. The d-dots were

then obtained and purified via a routine process

elaborated above. For the as prepared samples, more

than 1.7 g of dried products was obtained.

Fabrication of WLEDs based on d-dot phosphors

WLEDs were fabricated as follows: The purified

dual emissive d-dots were dispersed in 1.5 g

chloroform after precipitation and the corresponding

weight percentage is 10%. After that, the mixture was

further mixed with 1.5 g OE-6630A and 1.5 g

OE-6630B by sonication and stirring until the mixture

became transparent ， respectively. To remove

chloroform and bubbles in the mixture, the mixture

was heated at 70 oC for 30 min. This final NC mixture

was dispensed into the mold of a blue-LED chip and

thermally cured at 120 oC for 2.0 h.

Characterization:

UV-visible spectra were recorded on a Shimadzu

UV-2450 spectrophotometer. Luminescence excitation

and emission spectra of the samples were obtained

on Edinburgh FLS920 spectrophotometer. The PL

lifetime study was performed with an Edinburgh FL

900 single-photon counting system equipped with a

hydrogen lamp as the excitation source. The

luminescence time range was selected at 0−5ms.The

outer luminescent quantum efficiency was

determined using an integrating sphere (150 mm

diameter, BaSO4 coating) from Edinburg FLS920

phosphor meter. The quantum yield can be defined

as the integrated intensity of the luminescence signal

divided by the integrated intensity of the absorption

signal. The absorption intensity was calculated by

subtracting the integrated intensity of the light

source with the sample in the integrating sphere

from the integrated intensity of the light source with

a blank sample in the integrating sphere. EL data

including the EL spectrum, luminous efficacy,

correlated color temperature (CCT), the CIE color

coordinates, and CRI of the WLEDs based on dual

emissive NCs were collected in an integrating sphere

with a diode array rapid analyzer system (PSI Co.

Ltd). Transmission electron microscope (TEM)

observations were taken on a JEOL 100CX with an

acceleration volte of 100 kV. Carbon coated copper

grids were dipped in the toluene solutions to deposit

the NCs onto the films. X-ray powder diffraction

(XRD) patterns were obtained by using a Philips

PW1830 X-ray diffractometer. The composition for

the samples was measured by means of inductively

coupled plasma atomic emission spectroscopy

(ICP-AES, Thermo Elemental IRIS1000). The samples

were prepared by dissolving in HCl/HNO3(7/3,v/v).

3 Results and discussion

In our previous work, reaction temperature had been

found to be one of the most significant factors in

controlling the size of quaternary NCs. Here, the

reaction temperature was used to control the

synthesis of Mn-doped NCs which was chosen as an

example to demonstrate this single-step non-injection

approach by heating the mixture of all the reagents in

a single pot as described in experimental section.

Typically, Zn(OAc)2, In(OAc)3, Cu(OAc)2, Mn(OAc)2

and S dissolved in OAm were loaded in ODE media

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5 Nano Res.

containing OA at room temperature and then heated

to desired reaction temperature at air under stirring.

To monitor the particle growth process, Figure 1

shows the wide-field TEM images of three

representative samples obtained at different reaction

temperatures. The as-prepared NCs displayed a

nearly spherical shape with increased mean size from

2.6 nm (180 °C), 3.6 nm (210 °C), and 4.1 nm (240 °C),

as the reaction proceeded. This trend on the variation

in size of NCs had been confirmed by preparing

quaternary NCs in our previous report[31]. Note that

the size of particles synthesized at 150 °C was

difficult to be determined due to the limited TEM

measurement. All the samples of the as-prepared

NCs had a size distribution with a standard

deviation of 8%–10% after post-preparation fraction

or size sorting.

To further identify the crystal structure of

as-prepared samples, we investigated the

crystallographic properties by powder XRD. Figure

1D shows the XRD patterns of representative

samples corresponding to the TEM images shown in

Figure 1 (A, B and C), as well as the diffraction

patterns of bulk ZnS and Cu-In-S materials obtained

from the JCPDS database. Three samples showed a

similar zinc blende structure, and their diffraction

peaks systematically shifted toward higher angles

with increased reaction temperature from 150 °C to

240 °C. Furthermore, the position of diffraction peaks

was located between ZnS and CuInS2. The

continuous peak-shift of the diffraction patterns

revealed that no phase separation and separated

nucleation occurred with increased reaction

temperature.

To identify the chemical composition of the

products, the ratio of the reacted precursor to the

initial one (i.e. the percentage of precursor’s

conversion) for each element in samples shown in the

TEM images (Figure 1) were recorded by ICP

measurement. The percentage of the reacted

precursors based on ICP data were summarized in

Table 1. For the sample obtained at 150 °C, ICP

experiments indicated that particles were composed

of Zn, Cu and In cations. However, Mn was not

detected in the particles, although the size of samples

was difficult to determine because of the limit of

TEM measurement. The percentage of the precursor’s

conversion for Zn, Cu, and In elements was

estimated to be 2%, 86%, and 17% (Table 1),

respectively, indicating the formation of Cu-rich

Zn-Cu-In-S NCs at low reaction temperature. Thus,

the reactivity of the Cu was the highest among those

precursors, which had a precursor’s conversion of

almost 83.3% even at substantially low (150 °C)

reaction temperature. This observation had been

verified by preparing Cu-based multinary NCs,

where Cu-rich CuInS2 NCs were observable at low

reaction temperature[79]. At a reaction temperature

of 180 °C, which corresponded to a particle size of 2.6

nm, the ratio of reacted In was 10 times higher than

that of reacted Zn even if the initial concentration of

the Zn precursor was 1.3 times higher than that of the

In precursor. This showed that the reactivity of the In

precursor was higher than that of the Zn precursor,

which was consistent with the sample synthesized at

150 °C, where the ratio of reacted In was 8.5 times

higher than that of reacted Zn. Notably, no

Mn-related emission was detected in the product at

this temperature, which may be attributed to a low

doping yield in particles (the percentage of Mn

precursor’s conversion: 8.2%). By contrast, the

percentage of the Cu precursor’s conversion had

increased to 94%, indicating a nearly complete

reaction for Cu precursors at 180 °C. These results

suggested that the reactivity of the Mn precursor was

the lowest among these precursors, whereas the

reactivity of the Cu precursor was the highest among

those cationic precursors. With increased reaction

temperature, the size of particles gradually increased

to 3.6 nm at 210 °C and 4.1 nm at 240 °C. The ratio of

Zn precursor in particle composition increased

significantly, and the amount of consumed Zn was

observed to be 1.1 times higher than that of the In

precursor at 210 °C, and 1.5 times higher than that of

the In precursor at 240 °C. For the amount of In

elemental chemical composition in particles, the ratio

of the reacted In in the particles was 13.2 and 14.1

times higher than that of reacted Cu at 210 and

240 °C, respectively, which almost matched the initial

ratio of In0/Cu0 precursors. This showed that the

amount of the In precursor in particles started to

stabilize due to a nearly completed reaction at

relatively high temperature (over 210 °C). In case of

Mn elemental chemical composition, the yield of the

Mn precursor was increased gradually from 8.3%,

70%, and to 91% with increased reaction temperature

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6 Nano Res.

from 180, 210, and to 240 °C, respectively. Finally, the

amount of reacted Mn was found to be nearly similar

to that of initial Mn precursor used when reaction

was carried out at 240 °C. Note that sulfur was the

only anion used, which compositions in particles do

not need to be discussed quantitatively.

Based on these results, we estimated the

quantitative composition profile of the samples

shown in Figure 2 where the size-dependent

compositions were provided by calculation of ratio of

Zn/(Cu+In+Zn+ Mn), Cu/(Cu+In+Zn+Mn),

In/(Cu+In+Zn+Mn), and Mn/(Cu+In+Zn+Mn) in

radial direction. Results showed that Cu-, Zn- and

In-based cores were formed first at 150 and 180 °C

(Figure 2A, 2B), and Zn- and In-based shells were

formed successively along with the incorporation of

Mn into the NCs at 210 °C (Figure 2C). As the

reaction temperature was increased, the formation of

ZnS-rich Zn-In-S shell on the surface of Zn-Cu-In-S

cores was observed because of the increase in particle

size. At a reaction temperature of 240 °C, i.e. the

particles with a size of 4.1 nm, a large amount of the

Zn precursor was consumed, the composition of Zn

and Mn increased with the radius of NCs and

eventually ZnS-based layers were formed on the

particle surface (Figure 2D). Notably, gradient

alloyed structure might be formed at higher reaction

temperature. At this moment, we could not give

more data to define this assumption.

Based on the results mentioned above, Figure 3

shows the possible reaction mechanism for the

single-step synthesis of the d-dots with chemical

composition gradients. The mechanism for the

formation of d-dots with chemical composition

gradient was traced as follows: nucleation was

derived mainly by Zn, In and Cu; the continuous

growth of a shell with chemical composition gradient

was driven by Zn, In, and Mn; and finally the outer

shell of the NCs were comprised of Zn. The

single-step synthesis of d-dots with chemical

composition gradient was possible with the proper

choice of reaction temperature and different chemical

reactivity of the precursor. Notably, partial alloying

process was possible to take place between the core

and shell interface and, thus, the clear core, shell

interface was difficult to be observed by the TEM

analysis[80-85].

It has been known that the optical properties of the

Mn doped d-dots were independent of their size and

the shape of host materials. Figure 4A shows the

room temperature UV-vis absorption and

photoluminescence (PL) spectra of the d-dots

obtained under different reaction temperature. The

UV-vis and PL spectra of the samples corresponded

to the same samples shown in Figure 1. All samples

had a common feature: a UV-vis absorption spectrum

displayed featureless excitonic peaks, which was

consistent with the quaternary samples reported in

our previous work[31]. The band edges of samples

slightly red-shifted with increased reaction

temperature, indicating increased particle size as

confirmed by TEM measurement (Figure 1). By

contrast, a luminescence spectrum with a broad and

asymmetric PL peak at 550 nm was observed at

150 °C. This broad emission band results from the

corresponding band edge of Zn-Cu-In-S NCs, and

may be ascribed to the poor crystallinity of the

produced Zn-Cu-In-S NCs due to low reaction

temperature used (150 °C). With increased reaction

temperature from 150 °C to 180 °C, the emission peak

narrowed gradually along with a significant

blue-shift in its emission peak position. These shifts

reflected the formation of alloyed NCs because of the

incorporation of Zn, resulting in increased band gap

of the samples[31]. With further increased reaction

temperature from 180 °C to 210 °C and 240 °C, the

emission peak narrowed gradually along with a

slight red-shift in its emission peak position. These

shifts reflected an increased leakage of excitons into

the layers with gradual chemical composition

gradient to a certain thickness. Energy gradient shells

with lower band gap were initially formed and

energy gradient shells with larger band gap were

formed later. The variations in PL spectra of NCs

demonstrated that as-prepared NCs had not only an

increase in particle size, but also formed gradient

core/shell structure with good crystallinity at

relatively high reaction temperature (over 180 °C).

This trend in optical property change was consistent

with that of compositions of NCs determined by ICP

and TEM measurements.

Notably, a new PL peak at 605 nm appeared

besides the band edge related PL peak (512 nm) at

210 °C, and remained almost unchanged during the

whole particle growth. This new emission band

should be attributed to Mn d-state emission, which

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7 Nano Res.

had been verified by quantitatively analyzing the size

and composition of NCs. Pradhan et al.

demonstrated that no band edge emission was

observed for the Mn-doped alloyed Zn-Cu-In-S NCs.

The quenching of the band edge emission should be

ascribed to happen with energy transfer between

host and dopant. The doping location-dependent

energy transfer dynamics has been well studied in

Mn-doped II-VI NC system[86-89]. In our experiment,

we assumed that the doping position may be located

at ZnS-rich ZnInS shell as discussed in the structure

characterization part, instead of homogeneously

alloyed Zn−Cu−In−S NCs. Currently, we could not

give defined Mn location in gradient NCs. This

doping strategy was similar to dual-emissive

CdS/Mn:ZnS core/shell NCs reported by Cao’s

group[87]. This result was consistent with those

observed in typical dual-emissive Mn-doped ZnCdSe

NCs[90]. Therefore, The PL spectrum showed two

distinguished peaks, one of which was attributed to

Mn-related emissions and the other resulted from

gradient Zn-Cu-In-S NCs.

For comparison, a control experiment was

conducted by preparing Zn−Cu−In−S NCs in the

absence of the Mn precursor while other

experimental conditions were kept unchanged.

Figure 4B demonstrates the temporal evolution of

UV-vis and PL spectra of Zn−Cu−In−S NCs under the

different reaction temperatures. The absorption and

emission peak position of the Zn−Cu−In−S NCs were

totally the same with those of band edge emission

from Mn-doped gradient Zn−Cu−In−S NCs. The only

difference between these two experiments was that a

new emission band was observed in doped samples.

A comparable experiment was conducted by

preparing CuInS2 NCs in the absence of Zn

precursors, as well, by directly preparing the mixture

of Cu, In and S precursors under the same

experimental conditions[91]. As a result, the

absorption and PL spectra of two experiments were

totally different, indicating that the composition of

two samples was completely different. (not shown

here) Thus, the band edge PL of samples was

ascribed to the quaternary NCs, instead of ternary

Cu−In−S ones as conformed by ICP measurement.

It has been examined that a doping process

actually included multiple elementary steps, such as

surface adsorption/desorption, lattice

incorporation/ejection, lattice diffusion, and so on,

which was related to the quantum yield of doped

NCs. It means that the critical temperature for lattice

diffusion for this d-dots system would be

approximately 210 °C, which was substantially lower

than that of Mn:ZnSe d-dots systems. This indicated

that this lattice-diffusion step was an intrinsic step

and could be decoupled from the lattice-adsorption

steps. The quantum yield of the samples obtained at

210 °C was estimated to about 13%. With increased

reaction temperature, PL QY of the d-dots increased

to 41%, and the PL stabilized when the temperature

reached about 240 °C. The variation in emission

efficiency should be ascribed to the formation of ZnS

shell on the surface of NCs with increased reaction

temperature as shown in Figure 3. This observation

on improvement of PL QY of NCs by using ZnS shell

had been confirmed in some other Mn-doped

core/shell systems[86-89, 92-96]. Above 240 °C, the

dopant PL intensity started to decrease, whereas the

band gap PL peak position still slightly shifted to

longer wavelength along with a broader PL band.

These results indicated the occurrence of Ostwald

ripening in this high temperature range.

PL excitation (PLE) spectra indicated that the PL

peak at 608 nm was associated with the Mn d-state

and the one at 512 nm was attributed to quaternary

NCs. The PLE spectra of the two PL peaks were very

similar to the absorption spectrum of the sample,

indicating that the single particles had dual emission.

To investigate the difference between doped PL and

band gap PL, Figures 5B and 5C show typical decay

curves for the dual emissions from Mn d-state and

NCs, recorded at room temperature, respectively. For

the dual-emissive phosphors, the lifetime of doped

PL at 609 nm (τ=1.5 ms) was noticeably longer

compared to that of the band gap PL at 512 nm

(τ=170 ns), which further confirmed that the dual

emission was decoupled in one particle.

It has been proved that Zn-Cu-In-S NCs had the

tuned band gaps by changing not only the particle

size but also the compositions of particles. A number

of research groups focused on composition-

-dependent band gap tuning in alloy NCs and great

success had been achieved[97-102]. For example, the

PL peak position is tunable in the visible region by

changing the ratio of Cu/Zn[31, 100-102]. Therefore,

we can tune the composition of host materials as

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8 Nano Res.

done in our previous reports. Based on our previous

work, several experiments on preparation of d-dots

were conducted by changing the ratio of the Cu/Zn

precursors while keeping the other reaction

conditions unchanged. Figure 6A shows UV-vis

absorption and PL spectra of the d-dots under

different ratios of Cu/Zn (0.5%−2.5%) precursors with

fixed growth time of 10 min at 210 °C. In the

experiments, the nominal Cu/Zn precursor molar

ratio was varied from 1:20 to 1:4 with fixed gross

amount of Zn, In and Mn precursors at 0.41 mmol

each. Experimental results showed that, with

increased nominal Cu/Zn precursor ratio from 1:20 to

1:8 and 1:4, the band edge of gradient alloyed d-dots

red-shifted slightly from 505 nm to 535 and 568 nm,

respectively. For all samples, a broad absorption

band was observed together with an absorption tail

at longer wavelengths. Similar to previous

observations, no sharp absorption peak was observed

for any of the samples[31]. As expected, the

corresponding emission also red-shifted from 500 nm

to 570 nm as shown in Figure 6A. Notably, particle

size remained almost constant with the variation in

the Cu/Zn precursor ratio. These features of fixation

of particle size and the red-shift of excitonic

absorption peak gave strong evidence to confirm the

various compositions of as-prepared d-dots. To see

clearly, the emission spectra of the band edge PL and

the Mn d-state emission were separately provided as

shown in Figure 6B. Thus, the band edge emission

was tunable by only changing the composition of

NCs under a constant emission from Mn d-state,

which resulted in various dual-emissive phosphors.

This single-source, dual emission had a broadband

consisting of green and red region, which could be

potentially combined with LEDs for white lighting.

According to previous reports, the dopant

concentration had been found to be one of the most

key factors affecting the PL QY of doped NCs, which

has been reported by several research groups

[86,92-96]. To study the influence of the Mn ions

concentration on PL properties of d-dots, a series of

d-dots samples with different nominal (Mn/Zn)

ratios (from 1.0% to 5.0%) were prepared while

keeping the other experimental variables fixed.

ICP-AES measurements revealed that the atomic

composition of the resulting d-dots had a value

similar to that of the ratios of the precursors (not

shown here). Figure 7A demonstrates that the UV-vis

absorption of the d-dots in the presence of varying

amounts of Mn showed an almost identical spectra

profile. The fixed absorption peak positions of the

dual-emissive samples indicated that the size and

composition of the doped NCs was independent on

the amount of dopant Mn used. In comparison, the

relative PL intensity in the PL spectra of the resulting

d-dots showed a substantial dependence on the

content of Mn precursor (Figure 7B). In addition, the

emission peak position of dual-emissive NCs

remained completely unchanged. For comparison,

the intensity of the band gap PL was normalized.

With increased concentration of Mn/Zn ratio from

1.0% to 2.5%, the intensity of the doped PL rapidly

increased, and that showed a maximum value at

about 2.5% Mn/Zn ratio. With further increased Mn

content, the intensity of the doped PL gradually

decreased. This trend on the variation in the doping

level-dependent PL QY had been observed in other

doped NC system. Based on the discussion above,

the dual-emissive NCs with a PL QY of 46% were

obtained under a doping level of 2.5%. Notably, the

variation in the relative intensity of both the band

edge PL and doped PL bands dramatically changed

the emission color of samples. From the perspective

of color conversion, the control of relative emission

intensities of dual emission bands is one of the

factors in lighting field.

Several research groups have found that both the

quaternary and d-dots were almost insensitive to

temperature[61-68]. The temperature-dependent PL

intensity was investigated to examine the thermal

stability issue. The excellent thermal stability of

as-prepared NCs was highly different from that of

the un-doped NCs, where they suffered almost

complete quenching of its band edge PL at about

70–80 °C, as reported in our previous work[63]. To

quantitatively verify these visual observations, the PL

spectra of the sample solutions were also recorded at

room temperature and 75 °C. As shown in Figure 8A,

the PL spectra of the d-dots for both temperatures

did not show a significant difference for both two PL

bands. Visually, these NCs did not show a significant

change in luminescence brightness upon heating to

125 °C as shown in Figure 8B while the emission

color of the samples also showed obvious difference

upon heating to 150 °C, indicating that emission

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9 Nano Res.

intensity decreased at relative high temperature.

When the sample was cooled to room temperature,

the emission intensity was almost recovered.

Normally, the emission for quaternary NCs was

associated with the Cu energy level, and should be

less coupled with lattice vibration in comparison

with the excitonic emission of un-doped NCs[101]. In

other words, the emission mechanism of multinary is

much different from that of typical NCs with band

gap emission. For example, the emission spectra of

Zn-Cu-In-S NCs are totally same to that of Cu doped

CuInS2, indicating that they have the same

recombination mechanism after some excitation

wavelength. Therefore, the recombination path in

multiry NCs involves an intrinsic defect, most likely

associated with a copper centre, as shown in various

works reported in literature[101-104]. In contrast to

the studied Mn:ZnSe doped system, the emission of

Mn-doped NCs showed extremely high thermal

stability, which was consistent with the nature of the

emission of the inner core states (d orbitals split by

the crystal field) and not coupled with the lattice

phonons. The above results revealed that this system

behaved similar to pure dopant emission, instead of

band gap emission in terms of the dependence on

temperature. The outstanding thermal stability of the

single-source dual-emissive d-dots with zero

self-absorption characteristic further implied that the

d-dots in the current work were more capable for

specific technical applications, such as solid-state

lighting, lasers, and LEDs. Apart from excellent

thermal stability, as-prepared dual-emissive d-dots

showed good chemical stability. The samples were

substantially stable under ambient conditions, no

detectable variation was found in optical properties

of the product before and after one month.

The current synthesis mechanism is suitable for large

synthesis due to the absence of the operation of

precursor injection, which ensures the scalability of

the product. To identify the feasibility on large-scale

synthesis of the sample, we increased the amount of

all reactants up to 10-fold in a 500 mL three-necked

flask. As shown in Figure 8C, more than 1.5 g of

dual-emissive d-dots can be obtained in one pot

reaction. Therefore, the reported one pot non-

-injection synthesis route aiming at producing high

quality dual d-dots for potential industrial

applications is scalable, low cost, and reproducible.

Tunable white-light generation with combinations

of dual-emissive d-dots allows the d-dot integrated

LEDs to be used in any specific application. To

demonstrate the potential application of the

as-prepared d-dot phosphors as color convertor, we

synthesized an optimal phosphor with broadband

emission. Notably, almost no overlap between the

dual PL and UV-vis absorption spectra of d-dots

could be observed. Moreover, a slightly red-shifted

spectrum was observed for the film compared with

the d-dots dispersed in chloroform (Figure 9), which

was mainly attributed to the difference in the media

used to disperse the NCs. As a result of the

appropriate band gap, enlarged Stokes shift, and

excellent thermal stability, the single-source NCs

were quite suitable for application as phosphors in

the fabrication of white LEDs to improve their color

rendering properties.

To evaluate the chromaticity characteristics of

white light generated from the d-dots under blue

light excitation, a WLED lamp was fabricated using a

commercial blue LED chip combined with optimal

dual-emissive phosphors with broadband emission.

The detailed procedure for preparing the WLED was

described in the Experimental Section. A WLED was

simply constructed by coating dual-emissive d-dots

onto a commercial blue LED chip, as schematically

depicted in Figure 10A. Figure 10B shows the

photographs of WLED using dual-emissive d-dots

coated onto a 460 nm blue LED chip after 40 mA

current was applied. Obviously, the WLEDs based on

the dual-emissive d-dots emitted bright white light.

Figure 10C shows the electroluminescence spectra of

the WLED lamps driven by various currents (20 mA

to 60 mA). The sharp emission band at 460 nm was

attributed to blue LED chip, whereas two broad

emission bands located at 610 and 530 nm were

corresponded to the dual emissions from d-dots used.

Accordingly, the variation of CIE chromaticity color

coordinates of WLED operated under different

currents showed slight differences. As the drive

current was increased, the corresponding CIE color

coordinates changed from (0.320, 0.323) to (0.332,

0.340); CRI remained between 85 and 90, and color

temperature (CT) was kept between 5238 K and

6439 K (Table 2).The dual-emissive components

increased monotonically to almost the same degree

with increased input current from 20 to 60 mA

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10 Nano Res.

without a noticeable blue-shift or NC peak shift,

showing a relatively high spectral stability of NC

based on WLEDs. As a result of the broad emission

band of the NCs, the CRI was comparable with those

of the mixed NCs based WLED. Also, the color

properties such as the CRI and CT were comparable

to those of commercial YAG:Ce-based WLED and the

NC-based ones reported in literature[2, 27].

Therefore, the dual color d-dots on blue-emitting

LEDs is a promising approach to obtain high quality

white light. The results demonstrated the excellent

color stability of phosphors with different driving

currents. The merits of the dual d-dots including

non-cadmium, single-phase, flexibly dual emission

(related to CIE, CT and CRI), and excellent thermal

stability, make them promising alternatives to

Cd-containing NCs for application in opto-electronic

devices.

4 Conclusions

We reported an optimized non-injection method for

the synthesis of single-source cadmium-free,

dual-emissive d-dots by heating up a mixture of

corresponding metal acetate salts and sulfur powder

together with DDT in OAm/ODE media. This

synthesis route was able to produce large amounts of

high quality d-dots, usually in the gram-scale for one

batch experiment. The prepared d-dots exhibited two

emission bands under single excitation wavelength,

one of which was attributed to Mn-related emissions

and the other resulted from band edge of NCs. The

emission peaks assigned to band edge ones were

tunable by simple control of the particle size and

composition. The studies of the structures showed

that the resulting d-dots had a continuously varying

composition in the radial direction since the

reactivity of the precursors were totally different. The

prepared d-dots not only exhibited the characteristic

zero self-absorption, but also showed a quantum

yield of 46% and good thermal stability. Combining a

commercial blue LED chip with optimal d-dots as

color converters, we fabricated a warm WLED with

color rendering index of up to 90, CIE color

coordinates of (0.334, 0.321), and correlated color

temperature of 5680 K. These results demonstrated

that this cadmium-free, single-phase d-dot phosphor

with excellent thermal stability has potential

applications in WLEDs and correlated solid-state

lighting.

Acknowledgments

This work was supported by the National Natural

Science Foundation of China (21373097, 51072067)

and National Basic Research Program of China (No.

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14 Nano Res.

Figure 1 TEM images of NCs samples taken at 180 °C (A),

210 °C (B), and 240 °C (C). Powder XRD patterns (D) of the

samples labeled with a–d were corresponding to TEM images.

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15 Nano Res.

Table 1 The percentage of precursor conversion determined from

ICP-AES analysis for samples at different reaction temperatures.

(Zn0=0.2 mmol; Cu0=0.01 mmol; In0=0.15 mmol, and

M0=0.005 mmol). Tem.: Temperature.

Tem./oC Zn/Zn0 Cu/Cu0 In/In0 Mn/Mn0

150 0.02 0.86 0.17 0

180 0.07 0.94 0.69 0.08

210 0.62 0.91 0.87 0.7

240 0.91 0.92 0.88 0.92

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16 Nano Res.

Figure 2 Ratio of Zn, Cu, In, and Mn to (Zn+Cu+In+Mn) for

each shell from the center of the NC. We assumed that relative

values of each component (Zn, Cu,In, and Mn) in each shell are

shown.

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17 Nano Res.

Figure 3 Schematic of the possible reaction mechanism for the

large-scale, single-step synthesis of dual-emissive d-dots with

chemical composition gradients.

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18 Nano Res.

Figure 4 Temporal evolution of UV-vis (black lines) and PL (red

lines, λex= 450 nm) spectra of NCs with (A) and without (B) Mn

grown at different reaction temperatures.

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19 Nano Res.

Figure 5 (A) UV-vis, PL, and PLE spectra of a typical

dual-emissive NC. The excitation wavelength is 460 nm.

Time-resolved Mn related PL and the band edge decay traces of

the sample recorded at the emission wavelength of 608 nm (B)

and 512 nm (C), respectively.

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20 Nano Res.

Figure 6 (A) UV-vis and PL spectra of various NCs prepared

under various Cu/Zn precursor molar ratios. The temperature was

set at 210 oC. Excitation wavelength is 440 nm. (B) The emission

peak position of both band gap PL and doped PL spectra are

separately plotted, corresponding to the emissions from the

d-cores (Figure 6A)

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21 Nano Res.

Figure 7 UV-vis absorption (A) and PL spectra (λex=390 nm)

(B) of Mn doped Zn-Cu-In-S NCs dispersed in chloroform

prepared under various Mn/Zn precursor molar ratios from

1.0% to 5.0%

350 500 650 800

Ab

so

rb

an

ce

(a

.u.)

Wavelength (nm)400 500 600 700 800

PL

In

ten

sit

y (

a.u

.)

Wavelength (nm)

Mn concentrationA B

1.0%

1.5%

2.5%

5.0%

1.0%

1.5%

2.5%

5.0%

Mn concentration

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22 Nano Res.

Figure 8 (A) Spectroscopic demonstration of temperature

effects on PL of NCs. (B) Digital pictures of NCs under

different temperatures with 365 nm UV-light irradiation. (C)

Photographs of dried powder samples prepared in a single batch

reaction under 365 nm UV-light irradiation.

400 500 600 700 800

PL

In

ten

sit

y (

a.u

.)

Wavelength (nm)

25 ℃

75 ℃

25 oC 75 oC

A

B

C

125 oC 150 oC

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23 Nano Res.

Figure 9 PL spectra of the as-prepared NCs dispersed in

chloroform (black line) and in resin (red line), respectively. The

excitation wavelength was set at 390 nm.

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24 Nano Res.

Figure 10 (A) Schematic of NC-based WLEDs. (B)

Photographs of a WLED fabricated with dual-emissive NCs

operated at 40 mA. (C) Electroluminescence spectra of the

WLED operated under various currents of 20 mA to 60 mA.

Inset: the variation in CIE chromaticity coordinates of the

WLED under various currents.

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25 Nano Res.

Table 2 CRI, CT, and CIE color coordinates of the as-fabricated

WLEDs based on d-dots operated at difference currents: 20, 40,

and 60 mA.